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Paleoseismologic and slip rate studies of three major faults in southern California: understanding the complex behavior of plate boundary fault systems over millenial timescales

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Paleoseismologic and slip rate studies of three major faults in southern California: understanding the complex behavior of plate boundary fault systems over millenial timescales

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Content PALEOSEISMOLOGIC AND SLIP RATE STUDIES OF THREE MAJOR FAULTS IN SOUTHERN CALIFORNIA: UNDERSTANDING THE COMPLEX BEHAVIOR OF PLATE BOUNDARY FAULT SYSTEMS OVER MILLENNIAL TIMESCALES By Lee Joseph McAuliffe A Dissertation Presented to the FACULTY OF THE USC GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA In Partial Fulfillment of the Requirements for the Degree DOCTOR OF PHILOSOPHY (GEOLOGICAL SCIENCES) August 2014 i ACKNOWLEDGEMENTS There are many people that I need to thank for all they have done to make this work possible. First and foremost I must acknowledge my primary advisor Professor James Dolan. It was his initial confidence and belief in me that has got me to where I am today. I may never have understood his many jokes or cultural references, but through it all he has always supported me and for that I am greatly indebted to him. In addition to James, I need to thank Lorraine Leon and Plamen Ganev for first exposing me to the field of active tectonics. It was while helping the two of them with their fieldwork during the summer of 2008 that I became interested in earthquake geology and led to my decision to apply for grad school in this field. My initial lab mates Plamen Ganev, Ben Haravitch and Erik Frost helped make my transition to life as a grad student so seamless, and for that I am so very grateful. I could not have asked for a better group of guys to spend every day with. Over the years, our group has reduced and then grown again in size and I am happy to have had the opportunity most recently to work and share ideas with Chris Milliner, Robert Zinke and Jessica Grenader. I would also like to thank those members of my quals and dissertation committee, Greg Davis, Charlie Sammis, Steve Lund and Theo Hogen-Esch for their guidance throughout this whole process. Each of the individual chapters that I am presenting as part of my dissertation are standalone projects that could not have been done without the help of many individuals. Several collaborators have played a large part of each of my projects including Ed Rhodes at UCLA, Sally McGill at Cal State San Bernardino, Tom Pratt with the USGS and John Shaw at Harvard. Permitting for our trench study on the China Lake Naval Weapons station would not have been possible without the help of Andy Saban and Steve Alms at the geothermal program office. ii Chandra Chandrashaker with the City of Ventura land development office was instrumental in helping me obtain the required permits to acquire seismic reflection data and excavate boreholes within the city of Ventura. Several other people have helped me in some shape or form and deserved to be acknowledged. They include John McRaney, Cindy Waite, Vardui Ter-Simonian, Barbara Grubb, Dr. Koletty, Su Jin Lee, James Hollingsworth, Mr. Williams, Professor John Platt, Professor Josh West, John and Ros Stecker, and the entire McAuliffe clan. Finally, I have made some truly great friends over the years that have helped me through my time at USC. Alison Koop, Carlie Pietch, John Yu, Liz Petsios, Amir Allam, Rad Hallman and the entire Triathlon team each contributed so much to making the last 5 years some of the best times of my life. This dissertation includes parts of the following manuscripts: 1) McAuliffe, L.J., Dolan, J.F., Kirby, E., Rollins, C., Haravitch, B., Alm, S., and Rittenour, T.M., 2013, Paleoseismology of the southern Panamint Valley fault: Implications for regional earthquake occurrence and seismic hazard in southern California: Journal of Geophysical Research, v. 118, doi:10.1002/jgrb.50359. 2) McAuliffe, L.J., Dolan, J.F., Rhodes, E. and McGill, S.F., in prep, Extreme multi- millenial slip rate variations on the Garlock fault, California: Implications for fault strength, complete stress drop, and earthquake forecasting/seismic hazard assessment 3) McAuliffe, L.J., Dolan, J.F., Hubbard, J., Shaw, J.H., Pratt, T.L., and Rhodes, E., in prep, Characterizing the recent behavior and earthquake potential of the blind Ventura fault system iii TABLE OF CONTENTS Acknowledgements ......................................................................................................................... ii Abstract ......................................................................................................................................... vii Chapter 1: Introduction ....................................................................................................................1 1.1 Introduction .................................................................................................................1 1.2 Area of Study ...............................................................................................................4 1.2.1 Eastern California shear zone and Panamint Valley fault system ...................4 1.2.2 Garlock fault system ........................................................................................7 1.2.3 Faults of the western Transverse Ranges ........................................................8 1.3 Methods .....................................................................................................................10 1.3.1 Light Detection and Ranging (LiDAR) Mapping .........................................10 1.3.2 Paleoseismologic trenching ...........................................................................11 1.3.3 High-Resolution Seismic Reflection Data ....................................................12 1.3.4 Borehole Excavations ....................................................................................12 1.3.5 Cone Penetration Testing (CPT) ...................................................................13 1.3.6 Geochronology methods ................................................................................14 1.3.6.1 14 C Radiocarbon ...............................................................................14 1.3.6.2 Luminescence dating .......................................................................16 1.4 Research Implications ...............................................................................................21 Chapter 2: Paleoseismology of the southern Panamint Valley fault: Implications for regional earthquake occurrence and seismic hazard in southern California .............................23 2.1 Abstract .....................................................................................................................23 2.2 Introduction ...............................................................................................................24 2.2.1 Panamint Valley ............................................................................................26 2.2.2 Site Description .............................................................................................27 2.3 Paleoseismic trenching and trench stratigraphy ........................................................28 2.4 Age Control ...............................................................................................................29 2.5 Interpretation of paleo-surface ruptures ....................................................................34 2.5.1 Event 1 ...........................................................................................................34 2.5.2 Event 2 ...........................................................................................................37 2.5.3 Event 3 ...........................................................................................................39 2.5.4 Event 4 ...........................................................................................................42 2.6 Discussion .................................................................................................................42 2.6.1 Coulomb failure function (ΔCFF) modeling .................................................45 2.6.2 Implications for seismic hazard in southern California .................................48 2.7 Conclusions ...............................................................................................................50 2.8 Figure Captions .........................................................................................................52 iv Chapter 3: Extreme multi-millenial slip rate variations on the Garlock fault, California: Implications for time variable fault strength, complete stress drop, and earthquake forecasting/seismic hazard assessment .......................................................................61 3.1 Abstract .....................................................................................................................61 3.2 Introduction ...............................................................................................................62 3.2.1 Garlock fault ..................................................................................................63 3.2.2 Paleoseismology ............................................................................................64 3.2.3 Previous slip rate studies ...............................................................................65 3.3 Site description ..........................................................................................................66 3.3.1 Geomorphology .............................................................................................66 3.4 Offset geomorphic landforms ....................................................................................67 3.5 Age control ................................................................................................................69 3.6 Slip rate ......................................................................................................................70 3.7 Discussion .................................................................................................................70 3.7.1 Incremental fault slip rates ............................................................................71 3.7.2 Implications for time variable slip rates ........................................................73 3.7.3 Testing for time-, slip-, and strain-predictable behavior ...............................75 3.7.4 Fault behavior and earthquake supercycles along the Garlock fault .............77 3.7.5 Implications for probabilistic seismic hazard assessment (PSHA) ...............80 3.8 Conclusions ...............................................................................................................82 3.9 Figure Captions .........................................................................................................84 Chapter 4: Characterizing the recent behavior and earthquake potential of the blind Ventura fault system .........................................................................................................................91 4.1 Abstract .....................................................................................................................91 4.2 Introduction ...............................................................................................................92 4.2.1 Recognition of emerging thrust fault hazard .................................................92 4.2.2 Regional Geology ..........................................................................................93 4.3 Study area ..................................................................................................................97 4.4 Data ...........................................................................................................................99 4.4.1 High-resolution seismic reflection ................................................................99 4.4.2 Borehole excavations ..................................................................................101 4.4.3 Stratigraphic observations ...........................................................................102 4.5 Age control ..............................................................................................................103 4.6 Discussion ...............................................................................................................105 4.6.1 Most recent event (Event 1) ........................................................................105 4.6.2 Event 2 .........................................................................................................107 4.6.3 Possible 3 rd event .........................................................................................107 4.6.4 Uplift measurements and fault displacement estimates ..............................108 4.6.5 Implications for seismic hazard in southern California ...............................111 4.7 Briggs Road .............................................................................................................114 4.7.1 Regional geology of the central Ventura basin ...........................................114 4.7.2 Briggs Road study area ................................................................................115 v 4.7.3 High-resolution seismic reflection at Briggs Road .....................................115 4.7.4 Borehole excavations at Briggs Road .........................................................116 4.7.5 The stratigraphic cross section at Briggs Road ..........................................117 4.7.6 Age control at Briggs Road .........................................................................118 4.7.7 Event at Briggs Road ...................................................................................119 4.8 Conclusions .............................................................................................................119 4.9 Figure Captions .......................................................................................................120 Chapter 5: Conclusions ...............................................................................................................126 5.1 Summary .................................................................................................................126 5.2 Patterns of seismicity along the eastern California shear zone north of the Garlock fault .........................................................................................................................126 5.3 Temporal variations in slip rate along the Garlock fault .........................................127 5.4 Slip history and the potential for system-wide ruptures of the faults along the Ventura basin ..........................................................................................................128 5.5 Complex behavior of plate boundary fault systems over millennial timescales .....129 References ....................................................................................................................................132 Chapter 2 Figures .........................................................................................................................169 Chapter 3 Figures .........................................................................................................................193 Chapter 4 Figures .........................................................................................................................207 Appendices ...................................................................................................................................226 Appendix A: Supplementary text: Possible Evidence for a Pre-Event 1/Post-Event 2 Surface Rupture? ............................................................................................................226 Appendix B: Supplementary text: Additional analysis and maximum slip rate on the central Garlock fault .......................................................................................................230 Appendix C: Day Road CPT and borehole logs .............................................................233 Appendix D: Briggs Road CPT and borehole logs ........................................................265 vi ABSTRACT Understanding the complex behavior of plate boundary faults and fault systems has been an area of ongoing study in the geological sciences. This dissertation focuses on the spatial and temporal patterns of paleo-earthquakes at three different study sites in southern California, which exhibit non-steady slip histories over millennial timescales. Elucidating the long-term patterns of seismicity along a fault or fault system by extending the paleoseismic record of that fault allows us to better understand fault mechanics, possible fault interactions and improve next generation seismic hazard analysis. Using Light Detection and Ranging (LiDAR) digital topographic data to map and measure offset geomorphic features, paleoseismic trenching to determine paleoearthquakes recorded in the rock record, and luminescence (OSL and IRSL) and 14 C Radiocarbon dating methods, I constrain the timing of paleo-earthquakes along the Panamint Valley Fault and calculate a late-Holocene slip rate for the Garlock Fault. These new paleoseismic data from the Panamint Valley Fault support the notion that earthquake occurrence in the eastern California shear zone (ECSZ) may be spatially and temporally complex, with earthquake clusters occurring in different regions at different times. Our new slip rate for the Garlock Fault provides evidence for an elevated late-Holocene slip rate that is significantly faster than the full-Holocene/latest Pleistocene rate. This new slip rate provides insight into the behavior of the Garlock Fault on 10 3 timescales and documents the temporal variability in slip behavior during the Holocene. The results presented here further validates the suggestion that the Garlock Fault experiences periods rapid fault slip that correlate with earthquake clusters interspersed with millennia-long periods of no activity and presumably a 0 mm/yr “slip-rate”. These new data also provide a unique vii opportunity in which we can compare Holocene slip rates at multiple time intervals with a well- constrained late Holocene paleoseismologic record. In addition, I use a multi-disciplinary approach that combines high-resolution seismic reflection profiles, borehole excavations, and cone penetration testing to analyze the near-surface fault-related folding related to the blind faulting of the Ventura and southern San Cayetano Faults at seismogenic depths. Our results provide evidence for at least two temporally discrete uplift events during the Holocene, which together with events recorded by marine terraces to the west, document the possibility of system wide ruptures in the western Transverse Ranges. The results of this dissertation provide a better understanding of the paleo-earthquake behavior along the Panamint Valley, Garlock and Ventura-southern San Cayetano Fault system’s and illustrate the complexities of their spatial and temporally variable slip histories. Understanding the complex behavior of these faults has significant implications for regional seismic hazard in southern California. viii CHAPTER 1: Introduction 1.1 Introduction Plate boundaries within continental lithosphere are commonly wide regions of diffuse faulting that accommodate the overall deformation (Isacks et al., 1968; Dewey and Bird, 1970; Molnar and Tapponnier, 1975; Molnar, 1988; England and Jackson, 1989; Thatcher, 1995). Due to the relative weakness of the continental lithosphere together with its strong rheological heterogeneities, these boundaries rarely exhibit narrow, discrete zones of faulting diagnostic of boundaries within oceanic lithosphere (Isacks et al., 1968; Dewey and Bird, 1970; Molnar and Tapponnier, 1975; Molnar, 1988; England and Jackson, 1989; Thatcher, 1995). The present-day Pacific-North American plate margin along the coast of western North America is one such region of distributed deformation approximately 1500 km wide from offshore of the California coast to the eastern edge of the Basin and Range Province. Within this broad zone lie two relatively narrow belts where plate motions have been regionally localized – The San Andreas fault system, locally encompassing three or more sub-parallel faults, and the eastern California shear zone (ECSZ) – a region of seismicity and active faulting ~100km wide along the California-Nevada border. The ECSZ accommodates ~20% of the Pacific-North American plate motion and has been interpreted as a nascent primary plate boundary (Dokka and Travis, 1990a; Savage et al., 1990; Bennett et al., 1999; Dixon et al., 2000, Faulds et al., 2005; Wesnousky, 2005; Faulds and Henry, 2008). Within the greater plate boundary, the reverse and left-lateral faults of the Transverse Ranges indirectly accommodate relative plate motion by accommodating 1 the north-south compression associated with the “Big Bend” in the San Andreas fault (Dibblee, 1977; Livaccari, 1979; Luyendyk et at., 1980; Luyendyk et al., 1985; Luyendyk, 1991). Assessing and reconciling the patterns of strain accumulation and release along the faults of the western Transverse Ranges and ECSZ has been a controversial topic in the earth science community and the role of the ECSZ and the Garlock fault in accommodating North America- Pacific plate motion is highly debated. Recent studies of fault behavior have examined the basic stick-slip earthquake model and the well-established concept of the earthquake cycle has been proven to be an oversimplification over varying timescales. Comparisons of averaged long-term geologic and short-term geodetic rates of elastic strain accumulation across major faults and fault systems suggest the regularity of spatial and temporal variations in strain accumulation and release (Peltzer et al., 2001; Dawson et al., 2003; Friedrich et al., 2003; Oskin and Iriondo, 2004; Meade and Hager, 2005; Dolan et al., 2007; Oskin et al., 2007; McGill et al., 2009; Saillard et al., 2009, 2011; Ganev et al., 2012). Such millennial scale variation illustrates the complexity of fault system mechanics and play key roles in our understanding of fault mechanics and behavior, which in turn have a deep impact on probabilistic seismic hazard assessment. Short instrumental (geodetic) and historical records are inadequate to characterize the complex and multi-scale seismic behavior of major fault systems, including the faults of the western Transverse Ranges an ECSZ. A comprehensive probabilistic seismic hazard assessment should include the range of possible earthquakes and slip behavior, and misrepresenting the range of earthquakes can introduce bias into earthquake prediction models by either underestimating or overestimating the size of EQs on fault system and occurrence of EQs. The best-documented example of such varying strain accumulation and release rates (a so called “strain transient”) is across the Garlock fault, where the long-term (ka to ma) geologic 2 and short-term (0.01 ka) geodetic rates differ by an order of magnitude. Despite abundant geomorphic and paleoseismologic evidence for large-magnitude Holocene earthquakes (McGill and Sieh, 1991; McGill, 1992; McGill and Rockwell, 1998; Dawson et al., 2003), and a well- documented early Holocene/ latest Pleistocene slip rate of ~7±2 mm/yr (Clark and Lajoie, 1974; McGill and Sieh, 1993; McGill et al., 2009), modeling of geodetic data indicates northwest- southeast-oriented right-lateral shear across the ECSZ with little to no apparent left-lateral strain accumulation along the Garlock fault (Savage et al., 1981; 1990; 2001; Gan et al., 2000; Miller et al., 2001; Peltzer et al., 2001; McClusky et al., 2001; Meade and Hager, 2005). Paleoseismologic studies on the Garlock fault and faults of the ECSZ through fault trenching provide some constraints on the long-term behavior of a fault or fault system. Elucidating patterns of earthquake occurrence and the reasons for time-variable fault behavior can shed light on the role of the ECSZ and Garlock fault system within the greater plate boundary. When multiple paleoseismic sites are available along the same fault or fault system, we have the ability to extract information on the timing of past events and possible patterns of earthquake clustering as well as provide estimates on the segmentation and magnitudes of paleoevents. Within the western Transverse Ranges, where direct access to faults is not possible in many locations due to the urban built environment, we have used an advanced hollow stem coring technique and the correlation of the subsurface stratigraphy to do paleoseismology. In cases where the faults are blind and the tiplines do not reach the surface, we study the shallow deformation features that form as a result of slip on the underlying fault. This technique was implemented for our work in the western Transverse Ranges to observe the slip history of the blind Ventura and southern San Canyetano faults. 3 1.2 Area of Study 1.2.1 Eastern California shear zone and Panamint Valley fault system The eastern California shear zone is a ~100-km-wide zone of dextral shear and east-west extension that spans ~500 km long from near the northern end of the Salton Sea trough to the southern end of the Walker Lane Belt (a continuation of the dextral motion north of the ECSZ). The region accommodates approximately 20% of the North American-Pacific plate motion (Dokka and Travis, 1990a; Dixon et al., 1995; Gan et al, 2000; Dixon et al., 2000; Miller et al., 2001; McClusky et al., 2001; Dixon et al., 2003; Bennett et al., 2003) and in contrast to the San Andreas fault system, has a slight component of transtension, believed to be a reason for its diffuse network of faulting (Wesnousky, 2005). The ECSZ has two distinct regions divided by the perpendicular, throughgoing Garlock fault. Both fault geometry and topographic expression are markedly different north and south of the Garlock fault. Whereas the general pattern of faulting along the San Andreas fault system is dominated by the relatively smooth, linear and continuous trace of the San Andreas fault and associated faults, the ECSZ exhibits no such single thoroughgoing strand. Regional faulting is rather a series of discontinuous right-lateral strike-slip faults interrupted by zones of east-trending left- lateral faults and normal faults that transfer the transcurrent motion and define the broad shear zone. North of the Garlock fault, the ECSZ appears to be superimposed on the extensional central Basin and Range province, which began evolving in the early Miocene as the Pacific- North American transform boundary began to develop (Atwater and Stock, 1998). The interaction between Basin and Range extension and plate boundary dextral shear has led to the development of the complex array of faults that accommodate the intraplate strain. 4 South of the Garlock fault in the region of the ECSZ that is referred to as the Mojave block, the zone of dextral shear is accommodated across a ~100-km wide network of faults linking the San Andreas fault system to the faults of the ECSZ north of the Garlock fault (Dokka and Travis, 1990a; Faulds and Henry, 2005; Frankel et al., 2008). Northwest-striking dextral faults dominate most of the Mojave block, however a distinct zone of east-striking sinistral faults occupying the northeast part of the region accommodate the relative clockwise rotation of the Mojave Block (Ross et al., 1989). The Mojave block of the ECSZ has long been considered a region experiencing an apparent strain transient, where the cumulative geologic slip rate of 6 ± 2 mm/yr (Oskin et al., 2008) accounts for only about half of the 12 ± 2 mm/yr of elastic strain measured geodetically (Savage et al., 1990; Gan et al., 2000; McClusky et al., 2001). This discrepancy in geologic and geodetic rates has been attributed to slip on alternative fault systems (Dolan et al., 2007), earthquake cycle effects (Chuang and Johnson, 2011), transient strain weakening of ductile shear zones during earthquake clusters (Oskin et al., 2008), and more recently, due to permanent off-fault deformation that is not accounted for in the geologic rates (Dolan and Haravitch, 2014; Herbert et al., 2014) resulting in an apparent discrepancy. This slip rate discrepancy is responsible for some of the uncertainty in the ECSZ’s contribution to plate boundary deformation. North of the Garlock fault, the ECSZ consists of a ~50-120-km-wide zone of northwest striking right-lateral strike-slip and oblique-normal faults. Extending from the Garlock fault in the south to the Mina Deflection to the north, dextral shear is accommodated primarily on three laterally continuous fault zones – from west to east, the Airport Lake-Owens Valley-White Mountain fault, the Panamint Valley-Hunter Mountain-Saline Valley fault, and the 250 km-long Fish Lake Valley-Furnace Creek-Death Valley fault zone (Dokka and Travis, 1990a; Dixon et 5 al., 1995; Reheis and Sawyer, 1997; Frankel et al., 2007; McAuliffe et al., 2013). Cumulative right-lateral slip in this part of the ECSZ since the inception of the San Andreas fault appears to be ~80 km (Wesnousky, 2005). While numerous geodetic data are available for this region, geologically determined slip rates attempting to measure short- and long-term rates are limited (Reheis and Sawyer, 1997; Klinger and Piety, 2001; Kirby et al., 2006; 2008; Frankel et al 2007a; 2007b; 2011). Geologically determined slip rates along the faults north of the Garlock range from 2.3 ± 0.35 mm/yr since mid-Miocene time but 0.2-0.1 mm/yr since the early Holocene for the Stateline fault (Guest et al., 2007), 2-7 mm/yr on the Death Valley-Fish Lake Valley fault system (Frankel et al., 2007a; 2007b; 2011), ≥1.75-2 mm/yr for the Panamint Valley fault (Hoffman et al., 2009), and 2.1-3.1 mm/yr on the Owens Valley fault (Lee et al., 2001a; Kirby et al., 2008). Geodetically determined rates of elastic strain accumulation, which are based on much shorter timescales, indicate slip rates for each of the faults of: 2.8 ± 0.5 mm/yr for the Death Valley fault, 2.5 ± 0.8 mm/yr for the Panamint Valley fault and 2-6 mm/yr for the Owens Valley fault (Gan et al., 2000; McClusky et al., 2001; Lifton et al., 2013). This discrepancy between long-term and short-term slip rates on the westernmost faults, where geodetic strain rates are faster than both long and short-term geologic slip rates for the Owens Valley fault have led to suggest that the primary locus of slip/shear has shifted westwards with primary slip being transferred west towards the Owens Valley fault since the mid-Miocene (Dixon et al., 1995; Gan et al., 2003). Faulting along the eastern California shear zone is thought to have initiated during the late Miocene ~6–13 Ma (Dokka and Travis, 1990), although estimates as old as ~25 Ma (Ekren et al., 1980) and as young as ~3.5-4 Ma (Henry et al., 2007; Oskin and Iriondo, 2004) have been proposed for certain faults. Studies have shown that the localization of strain along the 6 California/Nevada border can be attributed to the formation of the big bend in the San Andreas fault (Liu et al., 2010), following the eastward/inland jump of the transform margin from the western coast of Baja California to its current location in the Gulf of California at ~6 Ma (Stock and Hodges, 1989; Atwater and Stock, 1998). Additionally, the locus of deformation within the ECSZ has been the topic of debate for some time with conflicting results from the geologic and geodetic rates for this region. Dixon et al., (1995), Reheis and Sawyer (1997), Gan et al. (2003), and Faulds and Henry (2005), have produced studies that indicate that the locus of dextral shear has shifted westward over time with the fastest slip placed on the westernmost fault in the system, the Owens Valley fault zone, while other studies indicate that the Death Valley-Fish Lake Valley fault system, the easternmost fault along the ECSZ north of the Garlock fault, shows the fastest slip (Dixon et al., 2003; Frankel et al., 2007a; 2011;). 1.2.2 Garlock Fault system Separating the Mojave desert to the South and the Sierra Nevada mountain range to the north, the Garlock fault is a ~250-km-long sub-arcuate left-lateral strike-slip fault – a relative anomaly in a predominantly dextral plate boundary system. Forming a complex junction with the San Andreas fault at its western end, the Garlock fault extends eastwards along the southern tips of the Tehachapi and Slate ranges. Though its eastern extent has been thought to extend into the Kingston Valley (Davis and Burchfiel, 1973; Plescia and Henyey, 1982), most map the active part of the fault as terminating at the southern tip of the Death Valley fault system where several geophysical and stratigraphic studies indicate that the Garlock fault is truncated by the Death Valley fault zone (Jahns and Wright, 1960; Abrams et al., 1989; Brady III et al., 1989; Brady III, 1993). 7 Two major structural features define the kinks that give the Garlock fault its arcuate geometry. These two features – a prominent ~2 km wide left step over at Koehn Lake and a ~15 degree change in strike south of the Quail Mountains – have been used to separate the Garlock fault into its western, central, and eastern segments (McGill and Sieh, 1991). The total left-lateral displacement on the Garlock fault is estimated to be between 48-64 km (Smith, 1962; Smith and Ketner, 1970; Davis and Burchfiel, 1973; Carter, 1980). Extrapolating a long-term slip rate of ~7mm/yr, as suggested by McGill et al., 2009, would require that the Garlock fault initiated at ~9 Ma in order to accommodate the 64 km of total offset. This long-term average seems reasonable given that multiple studies have suggested the initiation of the Garlock fault sometime ca.10 Ma (Hill and Dibblee, 1953; Burbank and Whistler, 1987; Loomis and Burbank, 1988; Monastero et al., 1997). Several tectonic models have been proposed that explain the regional tectonic role of the Garlock fault. These include: (1) the Garlock fault as an intracontinental transform structure that accommodates extension in the Basin and Range province to the north (Davis and Burchfiel, 1973); (2) a conjugate shear to the San Andreas fault that helps accommodate north-south convergence (Hill and Dibblee, 1953); and (3) a structure that accommodates rotation of the northeastern Mojave block (Humphreys and Weldon, 1994; Guest et al., 2003). What seems increasingly likely is the need for a combination of all three models to explain the Garlock fault’s role in the deformation of the greater plate boundary (McGill et al, 2009). 1.2.3 Faults of the western Transverse Ranges The western Transverse Ranges are dominated by several major east-west-trending faults and fold structures that truncate the general north-northwesterly structural grain of coastal California. These structures are evidence of the north-south compressive forces that have been 8 responsible for the observed deformation since the early Pliocene (Luyendyk et al., 1980; Luyendyk, 1991). The Ventura basin, within the western Transverse Ranges, is the locus of some of the most rapid north-south convergence rates in southern California (Donnellan et al., 1993a; 1993b; Hager et al., 1999; Marshall et al., 2008). This shortening is accommodated by slip on opposing thrust systems – including the south-dipping Oak Ridge system to the south, and the north-dipping Ventura-San Cayetano thrust system on the north. Thrust loading of the basin has resulted in deposition of one of the thickest Pliocene-Pleistocene sedimentary sections in the world. Such deep sedimentary basins can promote the amplification of ground motion during an earthquake – a potential seismic hazard for Ventura County. Along the Ventura basin’s northern margin, several well- and not-so-well-defined faults are responsible for the uplift of the Ventura Avenue Anticline (VAA) and Topa Topa Mountains. The two faults from the region discussed in this dissertation are the Ventura fault and the southern San Cayetano fault. The Ventura fault, together with its offshore extension the Pitas Point fault to the west (Campbell et al., 1975; Sarna-Wojcicki et al., 1976; Yerkes and Lee, 1987; Kamerling and Nicholson, 1995; Hubbard et al., 2013), form the western counterparts to a series of thrust faults that link the rapid north-south shortening accommodated by the San Cayetano fault to the east. Through the city of Ventura, the Ventura fault produces an east-west- trending, monoclinal scarp (Ogle and Hacker, 1969; Sarna-Wojcicki et al., 1976; Yeats, 1982a). The seismogenic potential of the Ventura fault has long been debated, with some studies suggesting that the fault is a bending moment fault limited to the upper few hundred meters (Yeats, 1982a; Yeats, 1982b). Others, including results from a companion study demonstrate that the fault reaches seismogenic depths and is linked to other major faults in the western Transverse Ranges (Sarna-Wojcicki et al., 1976; Hubbard et al., 2014). 9 The southern San Cayetano fault is a previously unrecognized and unmapped, gently dipping blind-thrust ramp that extends upward from the Sisar Decollement at a depth of ~7 km to the east of the Ventura fault. At the surface, the fault is expressed as an east-west-trending, south-facing monocline, however the exact nature of the fault at depth is still unclear. Hubbard et al. (2014) provide two alternative models for the southern San Cayetano fault, as either a backthrust or a continuous north-dipping blind thrust that extends into the footwall of the San Cayetano fault proper. 1.2 Methods The three projects detailed in this dissertation use several methods to determine the timing of paleoearthquakes and calculate slip rates. 1.3.1 Light Detection and Ranging (LiDAR) Mapping The understanding of neotectonic processes has been made considerably easier by the development and use of Light Detection and Ranging (LiDAR) data. The acquisition of LiDAR data along the eastern California Shear zone and Garlock fault by Airborne Laser Swath Mapping (ALSM) has been an extremely useful observational tool for geomorphologists, replacing low sun-angle aerial photographs (e.g., Oskin and Irondo, 2004; Frankel et al., 2007a; b; Oskin et al., 2008). From the raw point cloud data, one can create a Digital Elevation Model (DEM), providing an incredibly detailed digital model of the landscape. The ability to filter vegetation and obtain an image of the bare earth (Jensen, 2007), as well as the ease at which one can create contour maps and topographic profiles, are some of the key features that have made LiDAR data so appealing to geomorphologists. The sub-meter resolution (0.25-1 meter) of the 10 DEM produced is crucial for identifying small scale (<10m) offset geomorphic features, and for accurately determining displacement measurements for larger offset features. The LiDAR data of the faults of the eastern California shear zone and the Garlock fault were collected in 2005 and 2008 respectively, by the National Center for Airborne Laser Mapping (NCALM), and are now publically available through http://www.opentopograpy.org. 1.3.2 Paleoseismological trenching Paleoseismologic trenching is done to provide a record of paleoseismic events that are recorded within the trench walls. The basic objective of fault trenching is to identify particular paleoevent indication structures recorded within the trench walls. These event indicators include upward fault terminations, growth stratigraphy, fissure fills and laterally continuous layers that overlay faulted layers. These features help identify specific event horizons and combined with radiocarbon or optically stimulated luminescence (OSL) samples taken from layers above and below the event horizon, can constrain the age of that paleoevents. The Playa Verde site along the Panamint Valley fault is an ideal paleoseismic trench site because of the fine grained and oftentimes continuous stratigraphy that is deposited in playa environments. These thinly bedded deposits are more likely to expose discrete surface ruptures than will massive deposits such as debris flows. Furthermore, playas tend to concentrate the sparse, low-density detrital charcoal fragments found in arid environments. This is a crucial aspect of paleoseismologic trenching because current 14 C radiocarbon dating techniques are more precise and accurate than OSL methods (Bull, 2007), and are essential for constraining the ages of paleoevents recorded within the trench wall. For our Playa Verde site we used a standard backhoe with a three foot bucket for the excavation, and hydraulic shores to support our trench 11 walls. The trench measured 28 meters long and 0.9 meters wide with a maximum depth of ~4 meters. 1.3.3 High-Resolution Seismic Reflection Data Seismic imaging is a tool commonly used by academics and industry workers alike to image and characterize the subsurface geology. Basin sediments have proven to be extremely successful in the imaging of blind-thrust faults with a high degree of accuracy (Pratt et al., 2002; Leon et al., 2007; 2009). For our study of the blind Ventura and southern San Cayetano faults, we acquired high-resolution seismic reflection data along four transects across the Ventura basin. This was done in collaboration with Thomas L. Pratt (USGS) and Judith Hubbard (Harvard) utilizing a mini-vibe seismic source with geophone station spacing of 4 meters. Utilizing a vibration sweep from 20 to 120 hertz, we were able to image reflectors from 100 to 800 meters depth. Combining the high-resolution seismic reflection data with existing petroleum industry profiles allowed us to fully image the deformation associated with slip on the underlying blind thrust fault. 1.3.4 Borehole Excavations The sampling of the subsurface sediment, and the collection of dateable material (both charcoal for 14 C and quartz/feldspar for luminescence dating) can be attained by drilling boreholes. Following the methods applied in Leon et al., (2007; 2009) I drilled several hollow- stem, continuously cored boreholes along the Day Road and Briggs Road high-resolution seismic transects in order to study the near-surface folding associated with slip on the underlying Ventura thrust. Hollow-stem continuous coring is a technique that allows for the collection of intact sediment samples with minimal disturbance of the original fabric. 12 Stratigraphic correlations from the borehole logs, together with the CPT results allowed me to analyze the near-surface folding and determine features indicative of slip events. The subsurface stratigraphy was correlated across the different boreholes by grainsize, texture and color (Munsell color chart). The boreholes were excavated using several commercial companies. ABC Liovin Drilling Inc. was used for the boreholes along Briggs Road and both ABC Liovin Drilling Inc. and Gregg Drilling and Testing Inc. were both used for the Day Road transect. The radiocarbon and luminescence samples collected from the borehole cores were used to constrain the timing of individual uplift events. These event ages were then used to compare the timing of events recorded to the west at Pitas Point to determine the potential for system wide ruptures along the faults of the western Transverse Ranges. 1.3.5 Cone Penetration Testing (CPT) Cone penetration tests (CPT) were used together with the borehole excavations to image and analyze the subsurface stratigraphy. Cone penetration testing allows for the rapid exploration of shallow (less than 30 meters) subsurface deposits while minimizing cuttings, an inconvenient and occasionally expensive byproduct of borehole excavations. The CPT consists of a penetrometer that is pushed into the ground at a controlled rate (usually 2 centimeters/second). A sensor cone at the base of the rod takes measurements the penetration resistance (q c ) and the friction on the sleeve of the shaft (f s ). These measurements of q c and f s are continuously measured as the sensor rod is pushed into the ground in addition to the dynamic pore water pressure, which is measured at 5-cm intervals. These measurements are used to interpret a Soil Behavior Type (SBT) with 12 soil behavior zones from sensitive fine grained and clay, to sands and gravel (Robertson, 1990; Lunne et al., 1997). 13 CPTs are a faster and more cost effective approach than conventional drilling for shallow subsurface exploration. Typically, four to five 15- to 30-meter-deep soundings per day can be accomplished, in contrast to one or two boreholes per day with conventional drilling and sampling. For our CPT work we used Gregg Drilling and Testing Inc. for both the Day Road and Briggs Road transects. 1.3.6 Geochronology methods The work presented here as part of my dissertation relied on the use of two geochronology methods. The paleoseismologic trench study along the Panamint Valley fault and the borehole paleoseismology study along the Ventura and southern San Cayetano faults utilized both 14 C radiocarbon and luminescence techniques (both Optically Stimulated Luminescence [OSL] and Infra-Red Stimulated Luminescence [IRSL]). The slip rate study along the Garlock fault relied entirely on luminescence dating methods (IRSL). 1.3.6.1 14 C Radiocarbon As the most commonly used radiometric dating technique, 14 C dating has many applications for geomorphologists wanting to date Quaternary deposits (<50,000 years). Radiocarbon dating works on the premise that the 14 C radioisotope is produced in very small quantities in the atmosphere due to the interactions between cosmic rays and 14 N. This carbon radioisotope quickly combines with oxygen in the atmosphere to produce 14 CO 2 . This 14 CO 2 quickly mixes throughout the global carbon reservoir and is eventually ingested by CO 2 consuming organisms. Although the 14 C concentration (and in effect 14 C/ 12 C ratio) is constantly decaying, it is constantly being replenished in the atmosphere. Hence the amount of 14 C that is stored in organic tissue effectively remains constant through time. Once an organism dies, however, its 14 C/ 12 C ratio will begin to decline as the concentration of 14 C ceases replenishment. 14 By knowing the half-life of 14 C (5730 years), it is possible to determine the time since the organism ceased consumption of 14 C by comparing the 14 C/ 12 C ratio in the organism with the modern 14 C/ 12 C ratio. Measuring 14 C in a sample is done in one of two ways – either by counting the β emissions from a sample over a period of time, or by counting the relative number of 14 C atoms and comparing it to the number of 12 C and 13 C atoms in the sample using an accelerator mass spectrometer (AMS). The advantages of using the AMS method include the much smaller sample size required, and the speed and effectiveness with which samples can be processed (Mensing and Southon, 1999). When calculated, radiocarbon ages are presented as years before present (BP), with ‘present’ referring to the year 1950. We take the assumption that the concentration of atmospheric radiocarbon is the same as it was in 1950 and has remained constant over time. This is done to avoid confusion over when the measurement was made. Calculating a raw radiocarbon date is done using the equation: where t is the amount of time that has passed since the organism stopped consuming atmospheric carbon, N 0 is the number of atoms of the isotope in the original sample (at time t=0), N is the number of atoms left after time t, and the value 8267 is the radiocarbon mean (the average time each radiocarbon atom spends in a given sample until it decays). Several assumptions are made when calculating a radiocarbon age. These assumptions are the sources of potential error and need to be accounted for during the calculation. The assumptions include: (a) that the 14 C/ 12 C ratio within the global carbon reservoir has remained constant over time, (b) that complete rapid mixing of 14 C occurs throughout the reservoirs. This is important because 14 C is produced in greater abundance at higher latitudes. (c) That the half- 15 life of 14 C is accurately known with a reasonable level of precision, (d) that natural levels of 14 C can be measured to appropriate levels of accuracy and precision, and (e) that the ratio between the other carbon isotopes ( 13 C/ 12 C) in the samples have not been altered other than by 14 C decay. Due to the abundance of detrital charcoal in playa environments, 14 C radiocarbon dating was the primary geochronology method I used for dating stratigraphic horizons within our trench. 14 C radiocarbon dating was also used with our borehole cores, but the lack of radiocarbon resulted in us having to rely heavily on luminescence samples. I collected 35 14 C radiocarbon samples from the Playa Verde trenchsite and 11 samples from the Briggs Road and Day Road borehole transects. All 14 C samples were prepared and processed at the Keck Carbon Cycle accelerator mass spectrometer (AMS) at the University of California, Irvine. 14 C dating was used to constrain the age of our events at our Panamint Valley fault trench with great success – constraining our most recent event to within ~160 years (2σ). 1.3.6.2 Luminescence dating Optically stimulated luminescence (OSL) has revolutionized the dating of quaternary deposits over the past two decades (Huntley et al., 1985; Aitken, 1998; Singhvi and Wintle, 1999) and has become one of the more common forms of radiation exposure dating employed by geomorphologists (e.g. Ganev et al., 2010; Frankel et al., 2011 ). The method has several advantages over more traditional dating techniques. With the ability to directly date the quartz and feldspar grains that compose the geomorphic feature of interest, there is no need to search for dateable organic material – often a challenge in arid environments. The basic principle of OSL dating is that chronology is obtained by measuring the cumulative effect of nuclear radiation on the crystal structure of quartz and feldspar grains (Duller, 1996). The age of a sample is derived from the freeing and counting of electrons that 16 have become trapped within structural deficits or impurities in the crystal lattice of the mineral. The longer the mineral has been exposed to radiation, the greater the number of trapped electrons. These electrons can be freed from their traps by the samples exposure to a natural or synthetic light source. Both quartz and feldspar rich sediments can be absolutely dated within a range of 10 to 300,000+ years (Ballarini et al., 2003; Olley et al., 2004; Madsen et al., 2005; Watanuki et al., 2005). Under extremely low dose rates, grains up to 500,000 years have been accurately dated. The source of the ionizing radiation is from the radioactive decay of 40 K, 232 Th, and 238 U that are present in the ground. These radioisotopes together with cosmic radiation provide a constant flux of ionizing radiation over geologic time periods. Obtaining an estimate of age for an OSL sample is calculated with the following equation: where D E is the equivalent dose (or paleodose) measured in grays (Gy), and D R is the dose rate over time, measured in Gy/ka. The equivalent dose (D E ) is the luminescence measured in the laboratory by exciting the quartz grains and freeing the trapped electrons. When the electrons are released from their traps, a photon is emitted. The greater the number of freed electrons, the greater the number of photons released, and the stronger the luminescence signal recorded. The dose rate (DR), which is the total radiation dose received by the sample, is measured either in situ using a gamma spectrometer, or by a calculation based on measurements of the sample itself. A major assumption that is made with OSL dating is that the sample’s luminescence signal has been reset at the time of deposition by sufficient exposure to daylight. Because of this assumption that must be made, it is important to assess the sedimentological setting of the sample in the field, and assess the likelyhood that the sediment experienced sufficient exposure 17 to sunlight prior to deposition. This is of great importance at the three sites documented here where the likelyhood of incomplete bleaching is potentially high. Due to the high likelyhood of incomplete bleaching, we have employed a more sophisticated luminescence technique for our Garlock fault and Ventura basin projects – single grain Infra-red stimulated luminescence (IRSL). This is a method recently developed by our colleague Ed Rhodes at UCLA that is an improvement from the original post-IR IRSL method (Buylaert et al., 2009; 2012; Thiel et al., 2012). The single grain method allows us to be selective with the grains that are used for our age estimate. Incomplete bleaching of individual grains can lead to varying luminescence characteristics, which pose a problem when determining an age. The presence of a few bright partially bleached grains could lead to a significant overestimate of the age. With the single grain dating method, any grains with abnormal luminescence behavior can be removed and limit the spread in the data (Duller, 2008, Brown, et al., accepted for publication). In addition, the IRSL technique utilizes K-feldspar grains instead of quartz grains because the luminescence signal can be up to an order of magnitude brighter than quartz at sites like Christmas Canyon West (Lawson et al., 2012; Roder et al., 2012). This has to do with the specific lattice defects in the K-feldspar and because they contain more traps and/or recombination centers due to their ability to take on more impurities (Lawson et al., 2012; Roder et al., 2012). I collected four OSL samples from the Playa Verde trenchsite, which were prepared and processed at the Utah State University Luminescence Laboratory. The 21 luminescence samples that were collected from five separate sample pit locations at Christmas Canyon West, and the 26 samples collected from the Briggs Road and Day Road borehole transects, were prepared and processed at the UCLA Luminescence Laboratory. 18 For the Garlock fault slip rate project (Chapter 3) and Ventura fault project (Chapter 4), the samples were processed according to the following protocol. Samples were opened and prepared in the laboratory at UCLA under low-intensity red and amber lighting. Potassium feldspar grains of 175-200µm were separated from the central, unexposed, portion of each sample; following wet sieving to isolate the correct grain size range, samples were treated in dilute HCl to remove carbonate, dried, and the potassium feldspar component floated off using a lithium metatungstate (LMT) solution with a density of 2.58 g.cm-3. After rinsing, samples were treated in 10% hydrofluoric acid for 10 minutes to etch the outer surfaces of each feldspar grain, dried, and sieved at 175µm to remove small fragments. Between 200 and 600 K-feldspar grains of each sample were measured using a post-IR IRSL SAR (single aliquot regenerative-dose) protocol modified for single grains from Buylaert et al. (2009). Measurements were made in a Risø TL-DA-20CD automated luminescence reader, fitted with an XY single grain attachment incorporating a 150 mW 830 nm IR laser passed through a single RG-780 filter to reduce resonance emission at 415 nm, used at 90% power for 2.5s. All measurements were made using a BG3 and BG39 filter combination, allowing transmission around 340 - 470 nm to an EMI 9235QB photomultiplier tube. For the natural measurement, and following each regenerative-dose and test dose application, a preheat of 250ºC for 60s was administered. IRSL was measured (for 2.5s for each grain) at 50ºC, and then subsequently at 225ºC (for the post-IR determination). Following a test dose of 9Gy, an identical preheat, IRSL at 50ºC and post-IR IRSL at 225ºC were administered. Each SAR cycle was completed with a hot bleach treatment using an array of Vishay TSFF 5210 870nm IR diodes at 90% power for 40s at 290ºC. The SAR sequence incorporated measurement of the natural IRSL, 19 between four and six regenerative dose points, a zero dose point to assess thermal transfer, and a repeat of the first regenerative dose point, to assess recycling behavior. Growth curves were constructed for the post-IR IRSL signal measured at 225ºC using an integral of the background-subtracted sensitivity-corrected IRSL from the first 0.5s, fitted with an exponential plus linear function. For most samples, around 5 to 10% of measured K-feldspar grains provided a useful post-IR IRSL signal, typically providing between 20 and 60 single grain results for each sample; the upper samples, used to control slip rate, were measured using larger numbers of grains to improve statistical significance of the combined equivalent dose values. Samples typically displayed a uniform minimum equivalent dose value, with other grains displaying higher dose values, interpreted as grains incompletely zeroed before or during transport owing to rapid deposition from turbid water under high energy fluvial conditions. Most samples also displayed a small number of grains with significantly lower dose values, interpreted as intrusive grains introduced by bioturbation; these grains were excluded from the age analysis. Isolation of a population of grains for age estimation used a “discrete minimum” procedure in which higher values were excluded until the remaining grains were consistent with an overdispersion (OD) value of 15%, based on experience from quartz single grain OSL dating (e.g. Rhodes et al., 2010). Fading correction was based on detailed determination of single grain post-IR IRSL fading rates for key samples, and involve an increase in apparent age of 11%. Gamma dose rates were based on in-situ NaI spectrometer measurements; external beta dose rates were calculated from ICP-MS (U, Th) and ICP-OES (K) measurements of sediment from the end of each sample tube, internal beta dose rate was based on 12.5% internal K content, cosmic dose rates were based on measured overburden depth, and moisture correction used contemporary water content values. 20 1.4 Research implications This dissertation comprises three projects, each taking a multi-disciplinary approach to study the complex behavior of second order plate boundary faults. Specifically, two of the chapters in my dissertation present research results from a paleoseismology and slip rate study of the eastern California shear zone including the Garlock fault. The results presented here have previously published in international, peer-reviewed journals [McAuliffe et al., 2013 (Chapter 2)], or are currently in preparation for submission [McAuliffe et al., in prep (Chapter 3); McAuliffe et al., in prep (Chapter 4)] Chapter 2 reports on the first paleoseismologic data from the Panamint Valley fault in an effort to increase the paleoseismologic dataset for major faults in the eastern California shear zone north of the Garlock fault. This was done through the trenching of the southern Panamint Valley fault, and 14 C radiocarbon and optically stimulated luminescence (OSL) geochronology. Recent paleoearthquake studies conducted along many of the major right-lateral strike-slip faults in the Mojave section of the eastern California shear zone south of the Garlock fault (e.g. Rockwell et al., 2000; Oskin et al., 2008; Ganev et al., 2010) reveal patterns of earthquake occurrence that appear strongly clustered in time. Our results illustrate the complex nature of fault interactions within the entire eastern California shear zone, and the illuminate the significant stress interactions between the Panamint Valley fault, and the Garlock and southern San Andreas faults. Chapter 3 details results from our slip rate study along the central Garlock fault at Christmas Canyon West. This research was done by mapping the geomorphic landforms, calculating fault displacements with the use of high-resolution Light Detection and Ranging (LiDAR) topographic data, and determining the ages of the offset landforms with luminescence 21 (single grain IRSL) geochronology. The results provide evidence for an elevated late-Holocene slip rate of the Garlock fault that is significantly faster than the full-Holocene/latest Pleistocene rate. This new slip rate provides insight into the behavior of the Garlock fault on 10 3 timescales and further validates the suggestion that the Garlock fault experiences periods rapid fault slip that correlate with earthquake clusters interspersed with millennia-long periods of no activity and presumably a 0 mm/yr “slip-rate”. The results presented in both Chapters 2 and 3 suggest the possibility that interactions between major fault systems in eastern California that contribute to the overall plate boundary deformation, may produce the alternating periods of fault activity and seismic quiescence observed in the paleoseismic record. Chapter 4 presents results from borehole and CPT data above the blind Ventura and southern San Cayetano faults. These data, together with high resolution seismic reflection profiles taken across the faults, provide evidence for at least two temporally discrete uplift events during the Holocene. The timing of these events, together with events recorded by marine terraces to the west, document the possibility of system wide ruptures in the western Transverse Ranges. These results have significant implications for understanding how faults link together to generate large earthquakes, specifically in an area where the seismic hazard was previously not well established. Each chapter of my dissertation is designed to stand alone and thus there is some unavoidable redundancy. 22 CHAPTER 2: Paleoseismology of the southern Panamint Valley Fault: Implications for regional earthquake occurrence and seismic hazard in southern California 2.1 Abstract Paleoseismologic data from the southern Panamint Valley fault (PVF) reveal evidence of at least four surface ruptures during late Holocene time (0.33-0.48 ka, 0.9-3.0 ka, 3.3-3.6 ka and >4.1 ka). These paleo-earthquake ages indicate that the southern PVF has ruptured at least once and possibly twice during the ongoing (≤1.5ka) seismic cluster in the Mojave section of the eastern California shear zone (ECSZ). The most recent event (MRE) on the PVF is also similar in age to the 1872 Owens Valley earthquake and the geomorphically youthful MRE on the Death Valley fault. The timing of the three oldest events at our site shows that the PVF ruptured at least once and possibly thrice during the well-defined 2-5 ka seismic lull in the Mojave section of the ECSZ. Interestingly, the 3.3-3.6 ka age of Event 3 overlaps with the 3.3-3.8 ka age of the penultimate (i.e., pre-1872) rupture on the central Owens Valley fault. These new PVF data support the notion that earthquake occurrence in the ECSZ may be spatially and temporally complex, with earthquake clusters occurring in different regions at different times. Coulomb Failure Function modeling of the Panamint Valley and Garlock fault’s reveals significant stress interactions between these two faults that may influence future earthquake occurrence. Specifically, our models suggest a possible rupture sequence whereby an event on the southern Panamint Valley fault can lead to the potential triggering of an event on the Garlock fault, which in turn could trigger the Mojave section of the San Andreas fault. 23 2.2 Introduction It has long been recognized that seismic moment release is heterogeneous over short time scales. Whereas the most recognizable examples of this are aftershock sequences, a growing body of evidence has begun to show the prevalence of earthquake clusters at a wide variety of spatial and temporal scales over both individual faults and regional fault networks (e.g., Ambraseys, 1971; Marco et al., 1996; Dolan and Wald, 1998; Rockwell et al., 2000; Friedrich et al., 2003; Dawson et al., 2003; Weldon et al., 2004; Ganev et al., 2010). The documentation and understanding of these seismic patterns is of great importance for probabilistic seismic hazard assessments as well as a deeper understanding of both geodynamics and earthquake physics. Located between Death Valley and Owens Valley, Panamint Valley is an extensional basin located within the eastern California shear zone (ECSZ), a N-S belt/zone of right-lateral shear approximately 100 km wide that accommodates ~25% of the total relative motion between the North American and Pacific plates (e.g., Dokka and Travis, 1990a; Dixon et al., 1995; Gan et al, 2000; Dixon et al., 2000; Miller et al., 2001; McClusky et al., 2001; Dixon et al., 2003; Bennett et al., 2003). This zone of distributed deformation, which initiated between 20 and 6 Ma (Dokka and Travis, 1990b), extends northward from near the Salton trough to the Mina Deflection, where it steps ~50 km eastwards and continues northwards as the Walker Lane belt (Figure 1a). North of the Garlock fault, right-lateral shear is accommodated primarily by three large strike-slip and oblique-normal fault systems, the Death Valley-Fish Lake Valley, Panamint Valley-Hunter Mountain-Saline Valley, and Airport Lake-Owens Valley-White Mountains fault systems, from east to west (Figure 1a). Geologic slip rate studies of these three fault systems show that north of the Garlock fault and south of the Townes Pass fault (latitude ~36.27°N), the Panamint Valley fault (PVF) has the fastest slip rate (≥1.75-2 mm/yr) of any fault in the northern part of the ECSZ (Hoffman et al., 2009); north of the latitude of the Townes Pass fault, strike- 24 slip motion is concentrated primarily on the Death Valley-Fish Lake Valley fault system (Frankel et al., 2007; 2011), with slip transferred northeastward from the Owens Valley-White Mountains fault system and from the Panamint Valley-Hunter Mountain-Saline Valley fault system by way of a series of northeast-striking normal and sinistral faults (McKenzie and Jackson, 1986; Oldow et al., 1994; Lee et al., 2001b; Frankel et al., 2011). Although little is known about the rupture histories of the major ECSZ fault systems north of the Garlock fault, there is an extensive and growing body of work on the paleoseismology of the faults in the Mojave section of the ECSZ south of the Garlock fault. Studies by Rockwell et al. (2000) and Ganev et al. (2010) on the faults in the Mojave region of the ECSZ have shown that seismic strain release over the past 12,000 years has occurred primarily during clusters of large events. Specifically, paleoseismologic data from several major ECSZ faults documented in Rockwell et al. (2000) reveal earthquake clusters at ~8-9.5 ka and 5- 6 ka, as well as an ongoing cluster during the past 1.0-1.5 ka that includes the 1992 M w 7.3 Landers and 1999 M w 7.1 Hector Mine earthquakes. Recent work by Ganev et al. (2010) on the Calico fault, the longest and fastest-slipping fault in the Mojave section of the ECSZ (Oskin et al., 2007), shows that this fault ruptured at least four times during the clusters identified by Rockwell et al. (2000). These data support the evidence for temporal clustering of earthquakes in the Mojave region. These results invite an obvious question: Do these earthquake clusters characterize the entire ECSZ, from the Mojave northwards across the Garlock fault? Or are they a localized phenomenon associated with the structural complexities that mark the individual blocks of the ECSZ. In this paper we describe the results from two trenches excavated across the southern section of the PVF. Radiocarbon dating of charcoal found within key stratigraphic units 25 constrains the ages of the four most recent surface-rupturing earthquakes along this section of the fault. By studying the patterns of seismic strain release along the ECSZ north of the Garlock fault, we can begin to make assessments on the nature of the Garlock fault as a possible structural barrier within the ECSZ. 2.2.1 Panamint Valley Panamint Valley is thought to have developed since ca. 15 Ma along an oblique, west- dipping, low-angle detachment fault system (the Emigrant detachment system) that probably linked extension in Panamint Valley with the Death Valley fault zone (Hodges et al., 1989). Since 4 Ma, Panamint Valley has been kinematically linked to dextral strike-slip displacement on the Hunter Mountain fault (Burchfiel et al., 1987; Cichanski, 2000; Walker et al., 2005; Andrew and Walker, 2009). Consistent with a two-stage model for the development of Death Valley-area basins, basin extension is thought to have initiated on a low angle normal fault between ~15 and 4.2 Ma, before switching to the more north-northwest-trending Panamint Valley fault zone during the last few million years (Dixon et al., 1995; Reheis and Dixon, 1996, Cichanski, 2000). Studies of total displacement along the PVF yield values of 9±1 km since 4 Ma (Burchfiel et al., 1987) and ~17 km since the initiation of the basin ~14 Ma (Andrew and Walker, 2009). The latter offset is based on the reconstruction of the Argus and Panamint Ranges along a displacement vector with an azimuth of 300°, and total dextral slip on the southern PVF is estimated at closer to 10.5 km (Andrew and Walker, 2009). Several geologic slip rate studies on the southern PVF indicate a slip rate of ≥1.75-2 mm/yr (Burchfiel et al., 1987; Zhang et al., 1990; Hoffman et al., 2009). 26 2.2.2 Site description The trench site lies along the southern part of the PVF within the U.S. Navy China Lake Naval Air Weapons Station South Range, ~50 km east of the town of Ridgecrest, CA (Figure 1a). Through this section of the valley, the fault projects southward into a large (1 x 3 km) playa, which we refer to as Playa Verde, within which the fault trace cannot be discerned due to the young sedimentary cover (Figures 1b and 1c). We excavated two trenches across the projected fault trace near the northern end of Playa Verde. As a result of annual to decadal resurfacing of the playa with deposition of fine-grained fluvial and lucustrine sediment, no fault scarps are visible on the playa itself. However, geomorphically prominent fault scarps in older alluvium (Unit Qa2) are present directly north of our site (Figure 1c). Although it is clear from field relations that the playa stratigraphy onlaps the coarse-gravel alluvium being deposited by the large, active fan to the northwest of the site, in determining our trench locations we needed to balance the likely depth of the base of the playa deposits at the trench site with the requirement that we be able to precisely locate the fault traces beneath the extensive playa. Although the playa itself is actively aggrading, receiving sediment from the large alluvial fan to the northwest, there is also limited outflow from the playa around the eastern edge of the prominent ridge of uplifted older alluvium that lies due north of our site (Figure 1c). In the area of our trenches, the playa outflow channels are only a few centimeters deep, but they become progressively deeper towards the north, and along the eastern edge of the uplifted older alluvium, the channels are as much as 1.5 m deep, with steep to vertical channel walls. These channels are incised into the same playa deposits exposed in our trenches. 27 2.3 Paleoseismic trenching and trench stratigraphy In order to determine the exact location of the primary fault strand(s) through the playa, we excavated a preliminary trench close to the northern edge of the playa (Figures 1c and 2). This 12-m long northern test trench (T-1) was excavated first to locate the fault and to determine the thickness of the playa deposits. Trench T-1 revealed about 2 meters of playa silts and clays overlying coarse-grained, pebble-to-cobble size alluvial gravels deposited by the major fan to the northwest of the site, as well as the locations of several well-defined fault zones (see data repository for photo mosaic of T-1). A 0.5- to 1.0–m-thick, moderately well-developed incipient argillic horizon developed through the thin-bedded playa deposits extended the length of the trench. Based on the relatively thin stratigraphic section exposed in trench T-1, we excavated a second trench (T-2) approximately 35 meters south of T-1. We used the fault locations from T-1 to guide the location of the 28-m-long trench T-2. As in T-1, trench T-2 revealed a section of thin-bedded playa silts, clays and minor sands overlying coarse-grained alluvial gravels deposited by the large fan to the northwest of the site (Figure 3). The playa stratigraphy was much thicker in T-2, with a maximum thickness of ~3.6 m in a structural depression near the central part of the trench. More than 20 layers can be traced the entire length of T-2, and many of these beds can be correlated between trenches T-1 and T-2, attesting to the lateral continuity of strata deposited on the near-horizontal playa surface. The stratigraphic units observed within the two trenches are described in the stratigraphic column (Figure 3c). Trench T-2 exposed the same moderately well-developed, incipient argillic horizon observed in T-1 (Paleosol in Figure 3), indicating this soil is laterally extensive. In T-2, the soil thickens from 40 cm at the east end of the trench to 1.1 m in the central and western parts of the trench. Soil development varies, with 28 local areas in which the original playa stratigraphy can be discerned through the soil overprint. Above the argillic horizon, fine grained playa silts dominate the stratigraphy (Units A, B and G in Figure 3). Below the argillic horizon, the playa sands and silts (Units H through L in Figure 3) thin towards the east and onlap onto the underlying coarse-grained alluvial gravels. The well- preserved stratigraphy indicates a lack of bioturbation throughout much of the trench (outside of the argillic horizon), likely a result of the frequent inundations of the playa during wet years. 2.4 Age Control Playa environments commonly provide a favorable location for the accumulation of dateable detrital charcoal, and this was the case at the Playa Verde site. We sampled charcoal fragments from throughout the stratigraphic section exposed in our trench walls, with particularly dense sampling above and below the paleo-earthquake event horizons we identified (discussed below). We submitted 39 samples to the University of California, Irvine, Keck Carbon Cycle accelerator mass spectrometer (AMS) for radiocarbon dating. Radiocarbon ages for the 35 samples that survived pre-treatment were calibrated in OxCal v.4.1 (Ramsey, 2001). Throughout this paper all ages are discussed as calibrated years before present (cal. yr. BP), with “present” defined as 1950 AD. All ages are expressed at the 95.4% confidence level. All but one of the radiocarbon samples (PVT-53 was collected from organic rich layer) are detrital charcoal fragments, likely derived from brushfires from the surrounding slopes. Almost all of the radiocarbon ages are in correct stratigraphic order, with only limited evidence for reworking of the material. Of the 35 radiocarbon dates from trench T-2, only two samples yielded problematic results. Sample PVT-403, which was collected from 27 cm depth at m 20 on the south wall, contained excess 14 C that most likely came from the mid-20 th century 29 atmospheric thermonuclear weapons tests. The anomalously young age of this sample relative to other samples from that depth range suggests that it may have been bioturbated into the section. The large uncertainty recorded for sample PVT-49 is due to the very small sample size. In addition to reporting the calibrated ages in Table 1, in the column following each calibrated sample age we also report refined age estimates based on an OxCal stratigraphic ordering model. The stratigraphic ordering model is used to better constrain the 14 C dates for each of the samples. This is done when the stratigraphic sequence is known, allowing for the elimination of some overlapping ages. The OxCal stratigraphic ordering model that was created from the dated samples includes only 20 of the 35 total samples because: (a) four samples yielded dates that were slightly older than underlying samples, indicating minor reworking of older carbon (PVT-206, PVT-208, PVT-49, PVT-214) (Figure 3). Specifically, PVT-206 and PVT-208 seem too old given the age of PVT-17 (from the same Unit G), which was collected from a well-bedded silt layer that did not show any signs of bioturbation, indicating that samples PVT-206 and PVT-208 had significant pre-burial ages before they were deposited in Units G and A respectively (Figure 4). Sample PVT-214 from meter 21.5 on the south wall was located in a fissure that we believe opened up in Event 3 (discussed below). The older date of this sample does not fit with the stratigraphy determined by samples PVT-88 and PVT-86, which lie in undisturbed clay and silt units beneath the fissure (Figure 3); (b) four samples that were much younger than surrounding sediment were likely introduced into the section during bioturbation (PVT-205, PVT-46, PVT-47, PVT-403). As shown with our sediment accumulation rate curve, accepting sample PVT-205 would require extremely high sedimentation accumulation rates during development of the Paleosol (Figure 5). This seems highly unlikely given that development of this cumulate soil likely occurred during periods of slow/intermittent sediment 30 accumulation. Samples PVT-46 and PVT-47 from meter 10 on the north wall were not included in the ordering model because of their proximity to the base of the Paleosol and the resulting uncertainty about the presence or absence of bioturbation in these layers. The patchy and irregular base of the cumulate Paleosol, together with the wide spread of the radiocarbon ages, despite their proximity to each other, made it difficult to discern whether PVT-46 is much too old, or whether PVT-47 has been bioturbated into the deposit; (c) three samples from Unit A could not be confidently located with respect to the exact event horizon for the MRE (PVT-53, PVT-52, PVT-208 [sample PVT-208 was also likely bioturbated into the section, as noted above]). Moreover, sample PVT-53 was not included in the ordering model because it came from an unusual, 2-cm-thick charcoal-rich layer observed in Unit A at meter 14 of the south wall from which we took a bulk sample. The age of the material appears too old for its stratigraphic depth and we decided to rely on dates from larger charcoal samples within well-defined beds with undisturbed upper and lower bedding contacts to minimize the possibility of bioturbation effects; and (d) five samples collected from a large fissure that opened up in the MRE produced dates that were too old or could not be uniquely correlated with strata outside the fissure (PVT-44, PVT-97c, PVT-302, PVT-200, PVT-300). An important factor in correctly interpreting charcoal ages at our trench site is that charcoal samples collected from strata filling the large fissure at meter 10 to 13 cannot be used as reliable indicators of the actual ages of those deposits because they were likely at least partially eroded from older stratigraphy in the exposed walls of the large fissure as it was being filled. Calibrated radiocarbon ages of our samples range from 0.3-0.5 ka to 4.1-4.3 ka, indicating that the trenches exposed a mid-late Holocene stratigraphic section (Figure 3; Table 1). The oldest samples, PVT-11, PVT-211, PVT-70 and PVT-88 (OxCal calibrated unmodeled 31 age of ~3892-4240 cal. yr. BP), were collected from Unit L on the south wall of trench T-2, and the youngest sample, PVT-402 (dated at 305-455 cal. yr. BP [OxCal calibrated unmodeled]) was collected from Unit A on the south wall of T-2 at a depth only 15 cm below the playa surface. Although these ages indicate an average sediment accumulation rate over the past 4.3 ka of approximately 0.85-0.9 mm/yr, the rate probably varied during this period as shown in Figure 5. Between ca. 950-2450 cal. yr. BP the sediment accumulation rate is unknown because we did not date any charcoal samples recovered from within the Paleosol out of concern for possible reworking within the generally massive unit. Interestingly, we observe a slight increase in the sediment accumulation rate after both Event 1 and Event 3. This seems logical with large earthquakes producing rockfalls, which can in turn lead to a greater sediment supply to the playa. We were unable to determine whether a similar sediment accumulation rate increase occurred after Event 2 because of inadequate age control within the Paleosol. The radiocarbon data show relatively continuous sediment accumulation over the past ~4ka, with the exception of a period of reduced sediment accumulation rate associated with the development of the prominent soil observed in the trench. Conversations with soil expert Eric McDonald in the field, based solely on his visual examination of the Paleosol exposed in the trench, suggested a duration of ~1000 years for development of the Paleosol (E. McDonald, pers. comm., 2010). This field assessment of the degree of soil development supports our preferred interpretation of the radiocarbon data, which is that the soil developed between ~950-2550 cal. yr. BP. In addition to the charcoal samples, we collected four optically stimulated luminescence (OSL) samples from the layers above and below Events 1 and 3 near fault strands 1 and 2 exposed in T-2 (Figure 3). Samples were collected using steel pipes pounded into the layers of 32 interest and shipped to the Utah State University Luminescence Laboratory for analysis. Samples for soil moisture content and ICP-MS analysis of environmental dose rate were collected from around each sample tube and sample depth noted for cosmic contribution to the dose rate (Prescott and Hutton, 1994). In the lab, the samples were processed using heavy liquid (2.7 g/m 3 sodium polytungstate) and acid (10% HCl and three 30-minute 47% HF) treatments to isolate the 63-125 μm quartz and sand component (PVT-OSL-4 sieved to 63-212 μm). OSL samples were analyzed using the single-aliquot regenerative-dose method (Murray and Wintle, 2000, 2003; Wintle and Murray, 2008) and early background subtraction method (Cunningham and Wallinga, 2010). OSL ages of the older samples (PVT-OSL-1 and PVT-OSL-2) were calculated using the weighted mean of at least 18 accepted aliquots. Ages of the younger samples (PVT-OSL-3 and PVT-OSL-4), were calculated using a minimum age model (Galbraith et al., 1999) due to signs of incomplete bleaching of the luminescence signal at deposition (significant skew and high over dispersion). The resultant OSL ages are consistent with the radiocarbon chronology (Figures 3 and 4), suggesting that this methodology has accounted for partial bleaching and that OSL ages are reliable. Specifically, all four samples are within 2σ error of the independent radiocarbon age measurements from the same stratigraphic units, though the event ages still remain better constrained using the radiocarbon ages. Nevertheless, the success of the OSL dates and their consistency with the 14 C ages at the PVT provides confidence in using OSL dating methods in playa sediments. Information regarding the dose rates for each sample can be found in the data repository. 33 2.5 Interpretation of paleo-surface ruptures Trench T-2 revealed five main fault strands, referred to as faults 1 through 5 (Figure 3), from east to west, with most slip occurring on strand 3 near the middle of the trench. A prominent two-meter-deep, upward-widening fissure marks the trace of fault 3. The multiple- stranded nature of the fault zone in T-2 was also observed in T-1 and is similar to the geomorphic expression of the fault zone to the north of our trench site. Multiple lines of mutually consistent evidence along these five fault strands allowed us to identify three well-defined earthquake horizons and a poorly constrained fourth faulting event. We refer to the most-recent earthquake (MRE) as Event 1 and the oldest as Event 4. In addition to fissure fills, the geometry of growth strata, and upward fault terminations, our identification of these events was facilitated by incremental structural reconstruction of the strata deformed by fault 3 (Figure 6). The ages of each event reported in Figure 7 are based on calculations using OxCal’s stratigraphic ordering model. The overall agreement index (an OxCal-based measure of the internal consistency of the probability distribution function relative to stratigraphic order) for our final model (Figure 7; Table 1) is 99.6%. 2.5.1 Event 1 Event 1, the most recent event, is best expressed by a large (>1-m-wide, 3-m-deep) fissure that formed along fault 3. Multiple lines of evidence indicate an event horizon within ~40 cm of the present-day ground surface. Although less pronounced, this feature was also observed along this same fault in T-1, 45 m to the north, indicating that it is laterally extensive. Following the MRE, the fissure was filled with two sedimentary deposits: an initial deposit of fragmented, colluvial sand and silt blocks derived from strata exposed in the fissure walls up to a depth of 1.5 m, and a later sequence of the playa silts and clays deposited above the colluvium, which exhibit 34 growth stratal geometries and onlap onto the eroded remnants of the fissure walls. The fissure widens upward, reflecting gradual collapse and erosion of the originally near-vertical walls (including the upward extent of fault 3). Within the top 20 cm of the trench, the horizontally stratified playa deposits are continuous across the buried fissure. Well-bedded and flat-lying silt Units B and G are vertically separated by ~20 cm across the fissure, indicating that approximately 20 cm of east-side-up vertical separation occurred during the MRE along fault 3 (Figure 6). On the north wall of the trench T-2 a small-displacement fault with 2-4 cm of vertical separation across several thin-bedded silty clay layers and a crack with no discernible offset are exposed near the base of the colluvial fissure fill at meters 11.5 and 11.0, respectively. The presence of this minor fault within the lower part of the fissure fill suggests either: (1) that minor fault slip has occurred since the MRE, possibly triggered by large aftershocks or by earthquakes on other, nearby faults; (2) that this small-displacement fault occurred during post-earthquake settling, or as a result of shrink and swell processes (desiccation cracks) that may have played a role in the early stages of post-event deposition; or (3) that the lowermost part of the large fissure actually formed during the penultimate event and was subsequently reactivated and enlarged in the MRE. Insofar as this one small-displacement fault is the only structural evidence suggestive of pre-Event 1 faulting in the trench, we consider this third scenario unlikely (see additional discussion in supplementary data). Additional evidence for the MRE includes offset strata at fault 1 in T-2 (meter 3 – north wall, meter 5.4 – south wall), where a particularly well-defined down-dropped block of well- bedded playa sediments (Units G and B) is overlain by laterally extensive, unbroken playa clays and silts (Unit A) at 25 cm depth (Figure 4). Specifically, there are four to five thin silty clay 35 beds within the upper 40 cm of the trench that can be traced for most of the length of the trench (green beds in Unit A of Figure 3). The lowermost of these silty clays was not faulted during the MRE. Thus the event horizon lies within the lowermost part of Unit A, below the lowest silty clay interbed. Another possible indication of a young event within the top few 10s of cm comes from fault strand 4, which exhibits a vertical separation of 1-2 cm and terminates upward at a similar position in Unit A. Based on the amount of structural separation evident along these three fault strands, most slip in the MRE appears to have occurred on fault strand 3. The timing of the MRE was determined using samples PVT-402 from Unit A, a package of silty clay layers that forms the current playa surface, and samples PVT-30 and PVT-17. PVT- 30 was collected from the faulted base of Unit A at fault strand 4 and PVT-17 was collected from the top of Unit G, a 7-cm-thick, light olive brown silt layer that is the shallowest definable layer displaced by fault strand 1. As explained above, we are unable to use the samples within the fissure to constrain the MRE because they are part of the unidentifiable colluvial deposits filling the base of the fissure and were likely derived largely from erosion and collapse of the fissure walls following its opening during the MRE. Similarly, we do not use the samples from the growth stratigraphy at the base of the fissure because they are 200-400 years older than PVT-17 from just below the MRE horizon. This indicates that the samples within the growth strata at the base of the fissure must have been eroded off the side of the fissure walls from deposits that were between 200 to 400 years older than the playa surface at time if Event 1. PVT-302 yields an age range that is similar to the ages of samples above the event horizon collected from other locations within the trench, suggesting that it may record partial infilling of the fissure by typical playa sediment accumulation, however, we did not include this sample in our final OxCal model because its exact stratigraphic position relative to the event horizon is not clear. 36 Using samples PVT-402, PVT-30 and PVT-17 as our constraining dates, we obtain a calibrated calendric age for the MRE of 315-483 cal. yr. BP. This range provides a maximum age range for this event. Our OxCal 4.1 stratigraphic ordering model yields a preferred range for the MRE of 328-485 cal. yr. BP (95.4% confidence interval) with a peak probability at ~425 cal. yr. BP (Figures 7 and 8). Sample PVT-52 was found within the well bedded sands and silts that comprised the fill material in the top half of the large fissure at fault strand 3. We did not include this sample in our final model because of the possibility that the sample was eroded off the wall of the fissure. However, when this sample is included into the stratigraphic ordering model (and assumed to be deposited after the MRE), we obtain an age of 364-498 cal. yr. BP for our MRE. The PDF for this alternative reconstruction is found in the data repository. 2.5.2 Event 2 Multiple lines of mutually supportive evidence indicate that the event horizon for Event 2, the penultimate event, lies within the Paleosol. The strongest evidence comes from incremental reconstructions of the strata around the large fissure in the middle part of the trench that formed during the MRE. As shown in Figure 6, the MRE resulted in a total of ~20 cm east side up vertical separation for Units G and B (directly on top of the Paleosol). Units L to F (directly below the Paleosol) show ~70 cm of total east side up vertical separation. The reconstructions show that no vertical separation was produced along fault 3 at the Event 3 horizon (Unit P), indicating that ~50 cm of east-side-up vertical separation along fault strand 3 had to have occurred during one or more events between deposition of Units L-F and B-G, with the MRE providing an additional ~20 cm of east-side-up vertical separation, after the deposition of Units G, B and the lower part of Unit A. 37 Other evidence for Event 2 comes from fault 5 and another minor fault strand observed on the south wall of T-2 at meter 3. At both of these locations, the fault strand extends upward and terminates within the Paleosol. On both the north and south wall of the trench, fault 5 displaces the base of the prominent Paleosol by ~ 3 cm, but beds above the meter-thick Paleosol are clearly not displaced. This indicates that either the penultimate event occurred during development of the soil, or that fault 5 represents a strand that slipped in the MRE, but which did not reach the surface. On the south wall of T-2, an additional fault strand at meter 3.5 extends up through the Paleosol with several centimeters of displacement shown on several thin-bedded, laterally discontinuous coarse sand units just below the Paleosol. These sand layers cannot be identified on the north wall. The silt beds above the Paleosol near meter 3.5 show no signs of faulting, indicating that the event horizon is within the Paleosol. The fault at meter 3.5 can be traced up to near the top of the Paleosol, and this suggests that the penultimate event may have occurred after the sedimentary layers within which the paleosol was developed had accumulated, perhaps as part of a temporal “couplet” of closely timed earthquakes together with the MRE. Because Event 2 occurred sometime during the deposition of the strata within which the Paleosol later formed, we cannot precisely constrain the age of this earthquake. The only reliable ages that we can use come from the samples directly below (PVT-28) and above (OxCal phase cluster of PVT-207 and PVT-7; Figure 3) the units that have been overprinted by the Paleosol. The phase cluster option in OxCal 4.1 allows us to group samples that are all from one coherent group but for which there is insufficient or no information on their internal ordering. This becomes important when we have samples from the same unit at different ends of the trench and where the exact stratigraphic relationship between the two samples cannot be determined, as is the case with samples PVT-207 and PVT-7. Our preferred interpretation of the radiocarbon data 38 suggests that at least one surface rupture occurred between 910 and 2553 cal. yr. BP, during deposition of the units that were overprinted by the paleosol (Figures 6, 7 and 8). Although it is possible that more than one surface rupture occurred during deposition of these units and subsequent period of soil development, our preferred interpretation is of a single surface rupturing event. We have no structural observations to suggest the occurrence of more than one surface rupture in this interval. 2.5.3 Event 3 Event 3 is best recorded on faults 2 and 5, with very little vertical separation evident on fault strand 3 (Figures 3 and 6). On fault strand 2, Unit O, a white silt bed, and Unit X, an alluvial gravel and sand deposit, exhibit ~15 cm of west-side-down vertical separation across a 60-to 80- cm-wide fissure filled with colluvium and thinly and locally cross-bedded sands. In both the north and south walls at fault 2, Unit E lies unconformably across the fault trace and fissure fill. Small-scale cracking and faulting of Unit E indicates minor reactivation during a later event or events. The presence of well-bedded strata overlaying down-dropped blocks of Unit O through L at fault strand 2 suggests that the event horizon at this fault can be placed above the top of Unit O, prior to the deposition of the bedded strata, and below Unit E (Figure 4). Fault strand 5 exhibits similar relationships indicating a stratigraphic position of the Event 3 horizon above Unit O and below Unit E. Specifically, in the north wall, Unit O is clearly vertically separated more than the base of Unit E (which was subsequently faulted during Event 2 as described above). Unit P is the growth stratum that fills in the accommodation space that resulted from down-dropping Unit E, onlapping older strata above a gentle west-facing scarp. Unit P, which is only observed along the west side of the trench, provides more precise stratigraphic resolution for the Event 3 horizon because it lies between Unit E and Unit O. We attribute faulting of Unit 39 P on the north wall at fault 5 to be displacement during younger Event 2, as described above. At fault 5 on the south wall there is 12 cm of west-side-down vertical separation of Unit X. This fault strand terminates upward at the base of Unit P, which onlaps older strata and pinches out 1m east of fault 5. The unbroken nature of Unit P indicates that the event horizon is at the base of Unit P. Additional evidence for Event 3 includes a small fault strand at meter 13 of the north wall that vertically separates Units Y, L, R, S, U and X by several centimeters, but which terminates upwards at the base of Unit E. The observations described above indicate a well-defined Event 3 horizon at the base of Unit P. However, detailed stratigraphic and structural observations at fault 5 on the south wall suggest the possibility that “Event 3” may actually record two closely spaced earthquakes. As described above, in three of the four exposures of the Event 3 horizon, at faults 2 and 5, there are well-bedded deposits overlying the down-faulted blocks of Units O through L. These do not appear to be faulted, suggesting that the event preceded deposition of the well-bedded strata. At fault 5 on the south wall, however, these well-bedded deposits are clearly displaced by a fault strand that exhibits a small fissure extending up to the base of Unit P that is filled with Unit P silts (Figure 9). This observation appears to indicate that there are actually two closely spaced event horizons – one that created fissures along faults 2 and 5, and a slightly younger event horizon in which the fissure-fill deposits were offset along fault 5, prior to deposition of unit P. We note, however, that the fault 5 strand that cuts the well-bedded deposits and which exhibits the Unit P fissure fill can be observed on both walls to extend upward into the Paleosol, albeit discontinuously on the south wall. This strand therefore clearly ruptured during Event 2, and it is possible that the fissure fill along this strand at the base of Unit P does not record deformation of the ground surface during a post-Event 3 surface rupture. In light of the dearth of supporting 40 evidence for two surface rupturing events between units O and P, and the possible alternative explanation for the fissure fill at the base of Unit P, we suspect that this is not a separate event horizon, but rather a strand of the fault that ruptured during Event 2. In light of the highly cohesive nature of the playa silts exposed in our trench we suspect that there may have been open fissures below the ground surface that may have been filled by downward infiltration of Unit P material following Event 2. The timing of Event 3 is constrained by the 3169-3337 cal. yr. BP 14 C age from sample PVT-36, collected from Unit E about 1 m west of fault 2 in the north wall, and the 3470-3582 cal. yr. BP age from sample PVT-75 collected from Unit O about 1 m west of fault 4 in the south wall. Using only samples PVT-36 and PVT-75 as inputs to an OxCal model yields a maximum- possible age range for Event 3 of 3246-3546 cal. yr. BP. Placing the ages of these samples together with the other samples in our OxCal model yielded a slightly more constrained age range for Event 3 of 3271-3549 cal. yr. BP (95.4% confidence interval), with the peak of the OxCal probability distribution function at between ca. 3320-3480 cal. yr. BP (Figures 7 and 8). In addition to the possibility of a second event spaced closely in time with Event 3, two structures observed along the south wall of T-1 suggest the possibility that Event 3 was preceded by an additional slightly older event with an event horizon located ~25 cm deeper than the well- defined Event 3 horizon at fault strand 5. At meter 20.5 on the south wall, a small fault vertically separates Unit R by ~3 cm (east-side-down), and terminates within the lower half of Unit S (Figure 9). At meter 14, a small isolated fault strand splays off of the main fault of strand 4 and terminates at the same position within Unit S. Unit R is also vertically separated a few centimeters (west-side-down) at this location. Though we cannot discount the possibility that these fault terminations provide evidence for an additional pre-Event 3 surface rupture, we 41 suggest the alternative possibility that these small displacement faults may record slip during Event 3, with displacement on these strands not reaching the surface (e.g. Bonilla and Lienkaemper, 1990). If, however, these two upward fault terminations do record a pre-Event 3 surface rupture, the age of that earthquake would be constrained by sample PVT-75 and a phase cluster including samples PVT-86, PVT-14 and PVT-76. Constraining the event with these samples yields a maximum possible age range of 3524-3960 cal. yr. BP. (95.4% confidence interval). The PDF for this potential pre-Event 3 rupture can be found in the data repository. 2.5.4 Event 4 Our fault strand 3 reconstruction revealed evidence for a fourth event (Figure 6). When reconstructing the stratigraphic units for Event 2, Unit Y (the deepest unit throughout most of the trench and likely part of the alluvial fan to the northwest) shows ~30 cm of east side up vertical separation. Because all of the events above Unit Y lie flat when reconstructed for pre-Event 3, this suggests that the vertical separation of Unit Y records a west-facing fault scarp that formed before Event 3. Samples PVT-70 and PVT-88, the deepest dated charcoal samples, provide a minimum-possible age for Event 4 of ~4000 cal. yr. BP. Placing these samples in our OxCal stratigraphic ordering model, we obtain a minimum age of 4094 cal. yr. BP for Event 4 (Figures 7 and 8). No samples from within Unit Y were recovered thus we are not able to constrain the maximum age for Event 4. No other evidence for a fourth event was observed in the trenches. 2.6 Discussion The occurrence of multiple event indicators within our trench has allowed us to identify at least 3 well-defined surface rupturing events as well as a poorly constrained fourth event in the last 4 ka at our site along the southern Panamint Valley fault. The identification of multiple 42 paleoearthquakes at the Playa Verde trench site adds to the growing paleoseismological dataset for the region and comparison of these results with previous data from other faults reveals several interesting patterns of earthquake occurrence (Figures 10 and 11). The 328-485 cal. yr. BP age of the MRE on the Panamint Valley fault falls within the ongoing seismic cluster of earthquakes since ~1.0-1.5 ka in the Mojave region to the south of the Garlock fault (Rockwell et al., 2000; Rymer et al., 2002; Ganev et al., 2010). Other recent earthquakes that have occurred during the current Mojave ECSZ cluster include the MRE on the Owens Valley fault (the historical 1872 rupture), and a young (<300 yr) surface rupture on the Death Valley fault system revealed by geomorphically youthful fault scarps and offset of Ubehebe Crater tephra (Slate, 1999; Klinger, 2002). In addition, the well-constrained 3.27-3.55 ka age of our Event 3 overlaps with the 3.3-3.8 ka age of the penultimate surface rupture on the Central Owens Valley fault (Lee et al., 2001a). While the MREs on the Panamint Valley and Owens Valley faults illustrate a possible coupling between the two faults, the occurrence of Event 2 at our trench site demonstrates that the Panamint Valley fault also sometimes ruptures independently of the Owens Valley fault (Figure 11). The ongoing seismic cluster in the Mojave section of the ECSZ over the past ~1000-1500 years was preceded by a long seismic lull from 2-5 ka (Rockwell et al., 2000). Thus although the MREs on the Panamint Valley fault, Owens Valley fault, and probably the Death Valley fault occurred during the ongoing Mojave cluster, the ~3.5 ka age of Event 3 on the Panamint Valley fault (as well as the 3.3-3.8 ka penultimate earthquake on the Owens Valley fault) occurred in the middle of the 2-5 ka Mojave lull. Together, these data suggest complex fault behavior in which ECSZ faults north of the Garlock fault sometimes rupture in clusters together with the faults in the Mojave region. The timing of these events along the Panamint Valley fault, together with the 43 events in the Mojave region of the ECSZ, suggests a mechanism that operates at the scale of the entire plate boundary with stress and strain capable of being transferred across the Garlock fault. How such strain is transferred across a truncating >200 km long fault system that is clearly not cut by any other fault is still very much debated. An even more complex spatio-temporal pattern of earthquakes is revealed when we consider the ages of Garlock fault earthquakes. Specifically, the 328-485 cal. yr. BP age of the MRE on the Panamint Valley fault is very similar to the well-constrained 1450-1640 A.D. age of the MRE on the central Garlock fault (Dawson et al., 2003); Madugo et al. (2012) document a similar MRE age on the western Garlock fault. Moreover, the age range of Panamint Valley fault Event 2 overlaps with the 1000-1925 cal. yr. BP ages of a cluster of three surface ruptures observed by Dawson et al. (2003) at the Garlock fault trench site. In contrast, the 3.27-3.55 ka age of Panamint Valley fault Event 3 occurred within a long lull (2-5 ka) in earthquake activity on the central Garlock fault (Dawson et al., 2003) indicating that the Panamint Valley fault does not always rupture within a short time of the Garlock fault. The recurrence of earthquakes along the central Garlock fault revealed by these paleoseismologic data shows periods of seismic clustering at 0.5-2 ka and at 5-7 ka. This pattern, however, does not fit perfectly with either the pattern of earthquake recurrence observed within the Mojave region of the ECSZ or the ECSZ north of the Garlock fault. The importance of understanding patterns of earthquake occurrence on these fault systems increases significantly if we introduce the possible triggering effects from static stress changes following a PVF earthquake. By applying Coulomb Function Failure (ΔCFF) modeling to our study area, we can use our paleoseismological data to determine the likelihood of triggered 44 events and assess the implications that an event on the Panamint Valley fault would have for the rest of southern California. 2.6.1 Coulomb failure function ( ΔCFF) modeling Coulomb Failure Function change (ΔCFF) models can be used to model changes in static stress on a target fault with a particular orientation caused by an earthquake on another fault or a different section of the same fault (e.g., King et al., 1994; Harris and Simpson, 1996; Stein et al., 1997). Increases in Coulomb failure stresses on a fault are thought to advance the earthquake cycle of the target fault and bring it closer to failure. Whereas the triggering of an earthquake due to static stress changes has been observed over short time scales (e.g., King et al., 1994; King and Cocco, 2001; Doser and Robinson, 2002; Kilb et al., 2002), what is less well understood is whether earthquake clusters on longer timescales could be caused by Coulomb Failure Function stress changes, which are commonly much smaller (less than a few bars) than typical stress drops in earthquakes (10-100 bars; Kanamori and Anderson, 1975; Hanks, 1977; Kanamori and Allen, 1986; Kanamori, 1994). Because the magnitudes of the Coulomb stress transfer presumed to cause the triggering of an adjacent fault are commonly orders of magnitude smaller than that of the earthquake stress drop, it would require that the second fault be close to failure, in order for that rupture to have had any effect from the static stress changes (e.g., Sammis and Dolan, 2003; Sammis et al., 2003; Dolan et al., 2003; Scholz, 2010). As first noted by Sammis and Dolan (2003) and Sammis et al., (2003), and subsequently by Scholz (2010), regional clusters of earthquakes may be related to rather subtle changes in failure stresses acting on the faults. These researchers noted that the small ΔCFF changes typical of regional earthquake interactions have a greater impact on the probability of future earthquake occurrence if the ΔCFF is imposed on a target fault that is late in its strain accumulation cycle. Sammis and Dolan (2003) and Sammis et 45 al., (2003) took this one step further by demonstrating that if these ΔCFF changes are consistently positive over the course of many earthquake cycles, the various interacting faults will lock into phase with one another. Using a simple oscillator model, Sammis and Dolan (2003) and Sammis et al., (2003) demonstrated that even if the faults start 180° out of phase, they will gradually lock into phase as long as the stress interactions are consistently positive from earthquake cycle to earthquake cycle. Conversely, they showed that if stress interactions amongst faults are consistently negative, then even if the faults start in phase, earthquake occurrence will gradually lock 180° out of phase over the course of many earthquake cycles. These models offer a likely explanation for the temporal and spatial clustering observed in the ECSZ, both north and south of the Garlock fault. Alternative explanations that may be mechanically complementary to the "phase lock" and "anti-phase-lock" models of Sammis and Dolan (2003) and Sammis et al., (2003) include: (1) A kinematic model whereby two plate-boundary accommodating fault systems (San Andreas fault system and eastern California shear zone) alternate activity and in doing so, suppress activity on the other fault system (Dolan et al., 2007); and (2) a feedback mechanism whereby coseismic slip can introduce fluids into the ductile roots of fault zones, thereby increasing creep rates and therefore loading rates (Oskin et al., 2008). While it is difficult to determine the exact mechanism responsible for the observed temporal cluster, it is worth noting that all of these processes may be mechanically complementary, and all could potentially be occurring to control earthquake clustering. The similarity in ages between the MREs on the Garlock and Panamint Valley faults is intriguing, and given the long recurrence interval for PVF events, suggests a possible causative relationship. To investigate this specific relationship, and more generally the implications of PVF 46 earthquakes on regional seismic hazard, we conducted ΔCFF modeling of various combinations of Panamint Valley fault and Garlock fault earthquakes. The results, shown in Figures 12 and 13, reveal several interesting patterns. For example, our models show that a rupture on just the southern Panamint Valley fault increases the Coulomb stresses along the central section of the Garlock fault, but produces a slight stress shadow along the eastern part of the fault (Figure 12, a and b). Rupture of the entire Panamint Valley fault produces the same effects but with a larger magnitude (Figure 12, c and d). These results suggest that if the MRE on the Garlock fault occurred after and was triggered by rupture of the southern PVF, then the eastern Garlock fault may not have participated in the MRE on the Garlock fault. This is consistent with the results of the only paleoseismologic study of the Eastern Garlock fault, which indicates that the most recent event on the Leach Lake strand in the northern Avawatz Mountains occurred sometime after 150-590 A.D. (McGill, 1993), considerably earlier than the ~1500 A.D. MRE on the central Garlock fault (Dawson et al, 2003). We tested variations in the slip, rupture length, and shear modulus in our ΔCFF models. The results shown in Figure 12, indicate that greater slip along the length of the Panamint Valley-Brown Mountain fault system places the Garlock fault in a state of greatest Coulomb static stress increase but places the eastern Garlock in a stress shadow. Conversely, rupture of the central Garlock fault increases the Coulomb stress at the southern end of the PVF and decreases the Coulomb stress farther north, with the length of fault for which Coulomb stress increases being dependent on the shear modulus used in the model (Figure 13, a, b and c). Rupture of both the western and central Garlock fault increases the Coulomb failure stresses along the southern Panamint Valley fault, whereas rupture of the eastern section of the Garlock fault and rupture of the entire Garlock fault places the southern Panamint Valley fault in a stress shadow (Figure 13, 47 d, e and f). These ΔCFF patterns suggest that there have been and may continue to be interactions between the Garlock fault and the Panamint Valley fault. We were interested in whether the ΔCFF modeling could discriminate between triggering of the Garlock fault MRE by a Panamint Valley fault rupture, or vice versa. It is clear from the model results, however, that longer ruptures of the Panamint Valley fault (± Brown Mountain segment) encourages rupture of the central Garlock fault, and that under certain conditions, rupture of the central (± western) Garlock fault strongly encourages rupture of the Panamint Valley fault. If the model of Sammis and Dolan (2003) and Sammis et al. (2003) is correct, these mutually positive ΔCFF changes would suggest that ruptures of the Panamint Valley fault and central (± western) Garlock fault would tend to be in phase with one another. This appears to fit existing data for Event 1 and perhaps Event 2 on the Panamint Valley fault but not Event 3, which seems to have occurred during a seismic lull on the Garlock fault. 2.6.2 Implications for seismic hazard in southern California The Panamint Valley fault traverses a very sparsely populated region, with Ridgecrest (pop. 27,600) and Trona (pop. ~2740) the only population centers of > 2000 people within 100 km of the PVF (Figure 1a). Thus, although the seismic hazard associated with this fault is significant, the local seismic risk from future PVF earthquakes is relatively low. Yet our ΔCFF modeling suggests that an event on the PVF has the potential to trigger a large event on the central and western Garlock fault by increasing its Coulomb failure stress. Exploring this line of reasoning still further, ΔCFF modeling by Rollins et al. (2010) and Rollins, (2011) indicates that one possible scenario of concern would be a large central + western Garlock rupture that leads to an increase in ΔCFF stresses on the Mojave section of the San Andreas fault, which extends closest to the Los Angeles metropolitan region. Thus, although the Panamint Valley fault 48 traverses a remote and sparsely populated area, the occurrence of earthquakes on this relatively remote fault could potentially act as a trigger for a cascade of failures that could eventually include a large-magnitude earthquake on the SAF. Indeed the paleoseismological data indicate that at least sometimes these three faults all rupture within a brief period. The most recent event on the central Garlock fault, dated to between 1450-1640 AD (Dawson et al., 2003; McGill, 1992) produced a maximum of 7+ meters of surface slip (McGill and Sieh, 1991). The 3 rd event back on the San Andreas fault at Pallett Creek is currently dated at a preferred (“fully constrained”) age of AD 1508 (95% range=1457-1568 AD) based on revised dates by Scharer et al. (2011). The ages of these events, together with the age of the MRE on the Panamint Valley fault, suggest a potential link in the possible rupture pattern of all three faults over a relatively short time period in the 15 th -17 th centuries. Our ΔCFF models illustrate that a chain of events beginning along the southern Panamint Valley fault has the potential to trigger a rupture along the central Garlock fault (Figure 14). Rollins et al., (2010) and Rollins, (2011) show that such a Garlock fault event would in turn increase the Coulomb failure stresses on the Mojave section of the San Andreas fault, enhancing the likelihood of potential triggering of a rupture along this section of the southern San Andreas fault (Figure 13). The sequential ages of the three events dramatically changes the short-term seismic hazard assessment for southern California. Although paleo-earthquake data cannot prove this sequence of events, the available paleoseismologic data are consistent with the failure of all three of these faults within a brief period of time during the 15 th -16 th centuries (This study; McGill 1992; Biasi et al., 2002; Dawson et al., 2003; Sharer et al 2011; Madugo et al., 2012). 49 2.7 Conclusions Three surface ruptures have occurred on the southern Panamint Valley fault at our trench site during the past 3600 years, with an additional event occurring at some time before 4.1 ka. The MRE and the third event back are well defined by multiple, mutually consistent lines of evidence exposed in the trench walls, and calibrated radiocarbon dates of 20 charcoal samples tightly constrain these surface ruptures to the 15 th century-early 16 th century (328-485 cal. yr. BP) and at 3.27-3.55 ka, respectively. The penultimate event occurred during a period of slow deposition and soil development spanning 0.91-2.55 ka. Future studies at this site should focus on excavation of a trench further south into the playa, where the fine-grained playa deposits are likely considerably thicker. Such a trench could potentially yield a longer-record of earthquake occurrence on this part of the Panamint Valley fault. In addition, excavation of an additional, more southerly trench could potentially help resolve the ambiguities involving the exact timing of Event 2. The well-defined 328-485 cal. yr. BP MRE occurred within a brief time of the MRE on the central Garlock fault. Both of these earthquakes occurred during an ongoing cluster of large earthquakes in the Mojave section of the ECSZ, suggesting that the faults of the ECSZ north of the Garlock fault and the Garlock fault itself, may rupture together with the Mojave faults to the south during “megaclusters” that may affect large sections of the ECSZ section of the Pacific- North America plate boundary. In contrast, the well-defined 3.27-3.55 ka Event 3 occurred during the pronounced lull in ECSZ activity from 2-5 ka, indicating that the PVF sometimes ruptures out of phase with the Mojave faults to the south. Interestingly, the 3.27-3.55 ka age of our Event 3 coincides with the 3.3-3.8 ka age of the penultimate event on the Owens Valley fault (Lee et al., 2001a). Similarly, the MRE on the Panamint Valley fault occurred within a few 50 hundred years of the 1872 earthquake on the Owens Valley fault and the MRE on the Death Valley fault. These observations suggest the intriguing possibility that the major faults of the ECSZ north of the Garlock fault may rupture together during relatively brief clusters that occur sometimes independently of clusters on ECSZ Mojave faults to the south of the Garlock fault. The southern Panamint Valley fault earthquake ages from our trenches, together with previously published paleo-earthquake ages from the Garlock and San Andreas faults allow us to compare patterns of regional earthquake occurrence with the results of the ΔCFF modeling to assess the implications of an event on the Panamint Valley fault for the rest of southern California. Using Coulomb failure function modeling, we considered several scenarios for stress interactions between the Panamint Valley fault system and nearby faults, including the Garlock fault and Brown Mountain fault. Our modeling results indicate that a rupture along the southern Panamint Valley fault places the central Garlock fault in a state of increased Coulomb failure stress, with the potential to trigger an event along that part of the fault. This has been shown to in turn, increase ΔCFF stresses along the Mojave section of the San Andreas fault. Thus although the Panamint Valley fault is removed from major population centers, and thus may be considered to represent a low risk, the potential interactions between this fault and the much larger Garlock and San Andreas fault suggest that we must consider a cascade failure when assessing the threats from different faults in southern California. These results together with the earthquake dates obtained from our trench have significant implications towards assessing the probabilistic seismic hazard in southern California of a rupture on the Panamint Valley fault. 51 2.8 Figure Captions Figure 1. (a) Map showing faults of the ECSZ north and south of the Garlock fault. The black star indicates the location of our trench along the Panamint Valley fault. AHF - Ash Hill fault, ALF - Airport Lake fault, BF - Blackwater fault, BLF - Bicycle Lake fault, BMF - Black Mountain fault, BrF - Brown Mountain fault, BSF - Benton Springs fault, CF - Calico fault, CLF - Coyote Lake fault, CRF - Camp Rock fault, DSF - Deep Springs fault, EPF - Emigrant Peak fault, EVF - Eureka Valley fault, FIF - Fort Irwin fault, FLVF - Fish Lake Valley fault, GF - Garlock fault, GLF - Goldstone Lake fault, HCF - Hilton Creek fault, HF - Helendale fault, HLF - Harper Lake fault, HMSVF - Hunter Mountain - Saline Valley fault, LF - Lenwood fault, LLF - Lavic Lake fault, LMF - Lone Mountain fault, LoF - Lockhart fault, LuF - Ludlow fault, MF - Manix fault, NDVF - northern Death Valley fault, OVF - Owens Valley fault, PF - Pisgah fault, PSF - Petrified Springs fault, PVF - Panamint Valley fault, R - Ridgecrest, RF - Rattlesnake Flat fault, RVF - Round Valley fault, SAF - San Andreas fault, SNF - Sierra Nevada frontal fault, SDVF - southern Death Valley fault, SVF - Saline Valley fault, T - Trona, TF - Tiefort Mountain fault, TMF - Tin Mountain fault, TPF - Towne Pass fault, QVF - Queen Valley fault, WF - Warm Springs fault, WMF - White Mountains fault. (b) Map showing southern extent of the Panamint Valley fault and central Garlock fault. The Playa Verde trench site is located at white box (enlarged area in (c)) just east of the Slate Range. White circle indicates slip rate site on the southern Panamint Valley fault (Hoffman et al., 2009). BrF – Brown Mountain fault. (c) Map of the trench site in southern Panamint Valley, based on GeoEarthScope LiDAR imagery collected in 2008. Enlarged area indicated in (b). Faults shown in red. Extent of playa deposit shown in green. Active alluvial fans are shown in brown. T-1 and T-2 denote paleoseismologic trenches discussed in this paper. Short red lines on T-2 show locations and trends of major faults exposed 52 in that trench. Qa2, Qa3 and Qa4 are older offset alluvial surfaces. White circle indicates Figure 2 photo vantage point. The coordinates of the trench T-1 end points were 35.75070° N, 117.11626° W, and 35.75077 N, 117.11619° W. The end points of trench T-2 were located at 35.75034° N, 117.11609° W, and 35.75050° N, 117.11584° W. Figure 2. View towards the SSE looking at the playa trench site. The Panamint Valley fault is traced in white together with a down-dropped block formed along a secondary normal fault. Trenches T-1 and T-2 are shown by the black boxes. The main alluvial input from the west is visible in the image. Figure 3. Trench logs of the north (a) and south wall (b) of trench T-2. Charcoal and OSL (yellow circles) samples collected from the trench walls are located on the logs with their calibrated unmodeled ages in years before 1950. 5 main fault strands are shown at the base of each log. Photomosaics (shown below the digitized logs) of the south wall and north wall of T-2 show the actual trench wall surface. Due to the slightly undulating surface topography, all depths are recorded from an arbitrary datum ~30 cm below the surface. Faults are drawn as thick black lines. Grey lines indicate eroded surfaces and thin black lines indicate fractures or cracks. Colored blocks within the fissure are coherent but unidentifiable units. (c) Composite stratigraphic column of playa units identified in our trenches. Stratigraphic section is representative of thicknesses at meter 20 on south wall of trench; Units Y and Z are shown projected schematically at their proper stratigraphic positions. The dates on the left show unmodeled calibrated dates based on charcoal 14C samples collected in each corresponding unit. 53 The bold red lines on the left indicate our four proposed event horizons. Unit color descriptions are based on the Munsell soil color chart. Table 1. Results of radiocarbon dating from the Playa Verde site. Footnote: The charcoal samples received a standard acid-alkali-acid (AAA) pre-treatment. All results were corrected for isotopic fractionation according to the conventions of Stuiver and Polach (1977), with δ 13 C values measured on prepared graphite using the AMS. Of the 39 samples sent to the AMS, four samples did not survive the pre-treatment and could not be measured. Figure 4. North wall (trench T-2) from meter 2-8. Digital log is draped over the photo mosaic. Faults 1 (meter 3) and 2 (meter 6) are clearly identifiable. Event horizons are indicated by red lines on the left. Figure 5. Sediment accumulation rate curve showing probability density functions of OxCal v.4.1 (Ramsey, 2001) unmodeled calibrated calendric radiocarbon ages. The depth below ground surface for each sample has been corrected by projecting all samples to a common reference point (south wall at meter 20 of trench T-2). The green swath encompasses the sediment accumulation rate within the 95.4% confidence interval. The average sediment accumulation rate of ~0.86mm/yr is based on the depth and age of sample PVT-70. The numbers next to the PDF’s represent the corresponding sample numbers in the adjacent table. The circled PDF’s represent samples that we believe either have been reworked and are too old (21, 22, 7, 5) or have been bioturbated into place and are too young (10, 12, 14). The six samples 54 not included in the figure (PVT-44, PVT-97c, PVT-302, PVT-200, PVT-300, PVT-403) form part of the older colluvial material that fell into the fissure after the MRE. Their exact stratigraphic depth is not known and thus could not be projected to a common reference point. Figure 6. Diagram of the sequential development of stratigraphy across fault strand 3 in the south wall of trench T-2 at m 9-14 adjacent to the large fissure that formed during the most recent event (MRE). (A) Restoring Units L to F by ~0.7 m vertically illustrates stratigraphic relationships just prior to Event 2. Fault strand 3 exhibits little to no deformation during Event 3. Most of the deformation from Event 3 appears to have occurred on fault strands 2 and 4, along which Event 3 is well recorded. Along fault 3 the scarp produced during Event 4 is recorded by a 20-30 cm step at the base of Unit L. (B) Deformation during Event 2 is characterized by 0.5 m of vertical separation and the development of an east-facing fault scarp. (C) Additional silt may have been deposited across the Event 2 scarp. The Paleosol continued to form incrementally during slow sediment accumulation. (D) Deposition of the units above the Paleosol through Unit G. Unit G is deposited flat throughout the trench. (E) MRE opens up a fissure and Unit G now shows 0.2m of vertical separation. Units L to F are vertically separated a total of 0.7 m. (F) The large fissure is filled in with colluvium from the fissure walls as they erode back. Clays and silts are deposited in the interstices between colluvial blocks in the lowest parts of the fissure fill. Post-earthquake settling may cause small cracks/fractures to form at the base of the fissure. (G) The upper part of the fissure continued to widen during gradual post-MRE collapse and erosion of the walls. The fissure eventually filled completely with stratified silt and clay. Soil began to form within the upper 1 meter of fissure silts. (H) Current trench stratigraphy. 55 Figure 7. Chronological ordering model created in OxCal v4.1 (Ramsey, 2001) with probability density functions for individual radiocarbon samples. Calibrated unmodeled radiocarbon distributions (prior probability distributions) are shown with the light grey curves and modeled distributions (posterior stratigraphically ordered distributions) are shown with the black curves. Modeled distributions of our four events are highlighted. Samples that are arranged in phases represent coherent units from which each of those samples was taken. Organizing them into phases indicates that the samples internal ordering is not known and the samples are dated as a package. Figure 8. Modeled probability density function of the four paleoseismic events identified in T-2 using the web based OxCal v4.1. The age range shown in each graph is the 2σ confidence interval. Figure 9. Fault strand 5 on south wall. Digital log is draped over photomosaic between meters 19 and 23. Fault 5 at this location shows evidence for Event 3 and a possible pre-Event 3. Figure 10. Map showing all of the paleoseismologic sites in the ECSZ and Garlock fault region. Red lines indicate Quaternary faults. Bold yellow lines indicate surface ruptures of the largest earthquakes within the historical record. Figure 11. Chart showing the timing of past earthquakes at all trench sites in the ECSZ and on the Garlock fault. Bars with fading ends indicate no maximum age constraints are available. Black circles with lines through indicate earthquakes with known dates due to historical record. 56 Colored bars indicate earthquake clusters (yellow: 0-1.5 ka, red: 5-6 ka, green: 8-9 ka, blue: ~15 ka) reported by Rockwell (2000) and Ganev et al., (2010). Note that some earthquakes depicted at different sites are probably common events. Chart including data from Rockwell, (2000). Figure 12. ΔCFF imparted by simulated earthquakes on the Panamint Valley fault (PVF) to the central Garlock fault. In the panels, ΔCFF is resolved on faults with strike 75°, dip 90° and rake 0°, the inferred orientation and rake of the central Garlock fault (CGF) in Pilot Knob Valley (PKV), its closest approach to the PVF (if the Brown Mountain fault is not included as part of the PVF). In the values given below, ΔCFF is resolved on the changing orientation of the Garlock along its length, with location-dependent strike, assumed dip 90° and assumed rake 0°. Slip at each point on the PVF is uniform between 0 and 10 km depth and zero below that. Calculation depth is 5 km in all panels and for the values reported below. a) Assuming, µ’ = 0.2, a M=6.38 earthquake with 0.5 m of slip on the southern PVF induces ΔCFF = -0.2 bar on the CGF south of Searles Valley, ΔCFF = +0.4 bar on the CGF in Pilot Knob Valley, and ΔCFF = - 0.3 bar on the EGF. b) Assuming µ’ = 0.6, the same event induces ΔCFF = -0.1 bar south of Searles Valley, ΔCFF = +0.6 bar in Pilot Knob Valley and ΔCFF = -0.5 bar on the EGF. c) Assuming, µ’ = 0.2, a M=7.27 earthquake with 3 m of slip on the entire PVF from Hunter Mountain to the trench site induces ΔCFF = -1.3 bars on the CGF south of Searles Valley, ΔCFF = +4.4 bar on the CGF in Pilot Knob Valley, and ΔCFF = -3.1 bar on the eastern Garlock fault (EGF). d) Assuming µ’ = 0.6, the same event induces ΔCFF = -0.3 bar SW of Searles Valley, ΔCFF ≤ +6.1 bar in Pilot Knob Valley and ΔCFF ≥ -4.8 bars on the EGF. e-f) These stress changes are amplified if 3 m of right-lateral slip on the Brown Mountain fault (BMF) is added to 57 the events c and d. In addition, the decrease in ΔCFF southwest of Searles Valley is also less powerful: ΔCFF there is -0.4 bar for µ’ = 0.2 and -0.1 bar for µ’ = 0.6 in this scenario. Figure 13. Coulomb static stress changes (ΔCFF) imparted by simulated earthquakes on the left- lateral Garlock fault and resolved on faults with strike 162°, dip 90° and rake 180°, the inferred orientation and rake of the Panamint Valley fault (PVF) at the trench site. Slip at each point on the Garlock is uniform between 0 and 15 km depth and zero below 15 km. Calculation depth is 5 km in all panels and in the numerical ΔCFF values reported below. a) A M=7.39 earthquake on the CGF induces ΔCFF = -0.7 bar on the PVF at the trench site, assuming µ’ = 0.2. McGill and Sieh (1991) inferred ~7 m of offset in the apparent MRE on the CGF south of El Paso Peaks, dated to 1450-1640 C.E. by Dawson et al (2003). Slip is 0 m on the western Garlock fault (WGF) in this CGF-only source, and so we artificially set slip south of El Paso Peaks to 3 m to avoid an unrealistic slip discontinuity at Koehn Lake. Otherwise, slip values on the CGF are smoothed McGill and Sieh (1991) offsets. Slip is 0 m on the EGF. b) Assuming µ’ = 0.4, the same CGF event induces ΔCFF = +0.4 bar at the trench site. c) Assuming µ’ = 0.6, the same CGF event induces ΔCFF = +1.5 bars at the trench site. d) Assuming µ’ = 0.4, a M=7.72 event on the WGF and CGF induces ΔCFF = +1.5 bars at the trench site. Slip values in this event are 4 m on the WGF, smoothed McGill and Sieh (1991) offsets on the CGF, and 0 m on the EGF. e) Assuming µ’ = 0.4, a M=7.23 event on the EGF induces ΔCFF = -6.7 bars at the trench site. Slip values in this event are based on McGill and Sieh (1991) offsets on the EGF and set to 0 m on the WGF and CGF. f) Assuming µ’ = 0.4, a M=7.77 event rupturing the entire Garlock fault induces ΔCFF = -5.2 bars at the trench site. Slip values in this event are set to 4 m on the WGF and based on McGill and Sieh (1991) offsets on the CGF and EGF. 58 Figure 14. Possible interactions between excavated earthquakes ca. 1500 AD on the Panamint Valley fault, the Garlock fault, and the Mojave section of the San Andreas. The black boxes are our trench site on the southern PVF, the El Paso Peaks site on the central Garlock (McGill and Sieh, 1991; Dawson et al, 2003), the Twin Lakes site on the western Garlock (Madugo et al, 2012), and Pallet Creek on the SAF. Due to the observation of 7+ m of slip in the 1450-1640 AD MRE at the El Paso Peaks site, we postulate that that event and the >1450 AD MRE on the western Garlock are the same event. a) Second panel: A M=7.6 event rupturing the Mojave section of the SAF would likely increase CFF on the western Garlock (Rollins et al, unpublished). First panel: The 1455-1622 AD event on the Mojave section of the SAF promotes the MRE on the western and central Garlock, which in turn promotes the MRE on the PVF (Figure 13). b) Second panel: A M=7.8 end-to-end event on the Garlock would likely increase CFF on the Mojave section of the SAF (Rollins et al, unpublished). An event rupturing only the western and central Garlock would have a similar effect because the eastern Garlock does not greatly affect the SAF. First panel: The postulated single MRE on the western and central Garlock increases CFF both on the Mojave section of the SAF and on the PVF. c) The MRE on the PVF promotes the MRE on the Garlock (Figure 12), which in turn promotes the ca. 1500 AD event on the Mojave section of the SAF. d) Schematic of the likely stress interactions between the central Garlock, the eastern Garlock, and the PVF based on Figures 12 and 13. Data Repository Figure Captions Supplementary figure S1. Photomosaic of trench T-1 north wall 59 Supplementary figure S2. Photomosaic of trench T-1 south wall Supplementary figure S3. Fault strands 3 and 4 on the south wall. Digital log has been draped over the photomosaic between meters 9 and 16. The fissure shows evidence of 4 events illustrated in Figure 6. Supplementary figure S4. Fault strands 1 and 2 between meters 2 and 8 on the south wall. Digital log has been draped over the photomosaic. Supplementary figure S5. Fault strand 5 on north wall. Digital log is draped over photomosaic between meters 18 and 21. Fault strand 5 on the north wall shows evidence for an event within the Paleosol. Supplementary figure S6. Probability density function of Event 1 when PVT-52 is included into the OxCal model. Only the age of the MRE changes when sample PVT-52 is included. Supplementary figure S7. Modeled probability density function of a pre-Event 3 from the web based OxCal 4.1. The age range shown is the 95.4% confidence interval. Samples PVT-75, PVT- 86, PVT-14 and PVT-76 were used to model this age. Supplementary figure S8. Luminescence dating results 60 CHAPTER 3: Extreme multi-millenial slip rate variations on the Garlock fault, California: Implications for time-variable fault strength and seismic hazard 3.1 Abstract Detailed measurements of small geomorphic offsets and luminescence dating of incised alluvial fan strata reveal a late Holocene slip rate of the Garlock fault that is significantly faster than the longer-term late Pleistocene/early Holocene rate. At the Christmas Canyon West study site along the central Garlock fault more than a dozen different incised gullies and intervening alluvial bars can all be laterally restored with a consistent 23.5 ± 2.5 meters of left-lateral slip along a 1.7 km stretch of the fault. Dating of 21 samples of offset alluvial strata from five sample pits with a newly developed single-grain feldspar IRSL (Infra-Red Stimulated Luminescence) dating technique yields a minimum slip rate of ≥12.8 ± 2.4 mm/yr relative to longer-term rates of 5-8 mm/yr. The period of late Holocene elevated slip rate corresponds to a cluster of four surface ruptures observed previously at a nearby trench, which was preceded by a paleoseismologically defined lull lastly ~ 3 ky that presumably corresponded to a 0 mm/yr “slip rate”. These data indicate that the Garlock fault experiences extreme variations in slip rate lasting several millennia. These incremental slip rate data with published measurements of small offsets in the past few surface ruptures suggests that the Garlock fault may experience strain “super-cycles” during which the elastic strain energy released during clusters of several events may represent much or perhaps even all of the strain energy stored during the preceding lulls. These results have fundamentally important implications for our understanding of earthquake physics, the possibility of time-variable fault strength, and the state of stress in the crust. Moreover, these 61 results obviate probabilistic seismic hazard assessment strategies based on single earthquake cycle (e.g., slip- and time-predictable models) but may provide a path towards more accurate future probabilistic assessments in settings where detailed incremental fault slip rates can be measured over long time intervals. 3.2 Introduction Comparisons of averaged long-term geologic and short-term geodetic rates of elastic strain accumulation across major faults and fault systems suggest the regularity of spatial and temporal variations in strain accumulation and release (Peltzer et al., 2001; Dawson et al., 2003; Friedrich et al., 2003; Oskin and Iriondo, 2004; Meade and Hager, 2005; Dolan et al., 2007; Oskin et al., 2007; McGill et al., 2009; Saillard et al., 2009, 2011; Ganev et al., 2012). Such millennial scale variation illustrates the complexity of fault system mechanics and play key roles in our understanding of fault mechanics and behavior, which in turn have a deep impact on probabilistic seismic hazard assessment. The best-documented example of such varying strain accumulation and release rates (a so called “strain transient”) is across the Garlock fault, where the long-term (ka to ma) geologic and short-term (0.01 ka) geodetic rates differ by an order of magnitude. Despite abundant geomorphic and paleoseismologic evidence for large-magnitude Holocene earthquakes (McGill and Sieh, 1991; McGill, 1992; McGill and Rockwell, 1998; Dawson et al., 2003), and a well-documented early Holocene/ latest Pleistocene slip rate of ~7±2 mm/yr (Clark and Lajoie, 1974; McGill and Sieh, 1993; McGill et al., 2009), geodetic data suggest little or no left-lateral strain accumulation across the Garlock fault over the past several decades (Savage et al., 1981; 1990; 2001; Gan et al., 2000; Miller et al., 2001; Peltzer et al., 2001; McClusky et al., 2001; Meade and Hager, 2005; but see Chuang and Johnson, 2011 and 62 Platt and Becker, 2013 for an alternative assessment). Instead, the short-term geodetic data demonstrate that the Garlock region is presently dominated by northwest-oriented right-lateral shear at rates of 9-11 mm/yr north of the Garlock fault (Dixon et al., 2000; Meade and Hager, 2005) and 12 ± 2 mm/yr south of the Garlock fault (Sauber et al., 1986; 1994; Bennett et al., 2003; Meade and Hager, 2005). While this is of no surprise given the orientation and geometry of the northwest-oriented North America-Pacific plate boundary fault system, it is the reconciliation of the varying short-term strain and long-term slip rates across the Garlock fault that continue to puzzle both geologists and geodecists. This paper documents the late Holocene (~2 ka) slip rate along the central Garlock fault and discusses the result in terms of its implications for understanding the constancy of strain accumulation and release and time- variable fault strength along this structurally simple yet major strike-slip fault. 3.2.1 Garlock fault Separating the Mojave desert to the south and the Sierra Nevada mountain range to the north, the Garlock fault is a ~250 km sub-arcuate left-lateral strike-slip fault – a relative anomaly in a predominantly dextral plate boundary system (Figure 1). Forming a complex junction with the San Andreas fault at its western end, the active portion of the Garlock fault extends eastwards along the southern tips of the Tehachapi and Slate ranges, terminating near the southern tip of the Death Valley fault system within the Avawatz Mountains (Jahns and Wright, 1960; Davis and Burchfiel, 1973; Plescia and Henyey, 1982; Abrams et al., 1989; Brady III et al., 1989; Brady III, 1993). The total left-lateral displacement on the Garlock fault is estimated to be between 48-64 km (Smith, 1962; Smith and Ketner, 1970; Davis and Burchfiel, 1973; Carter, 1980). Extrapolating a long-term slip rate of ~5-7mm/yr would require that the Garlock fault initiated at ~9-12 Ma in order to accommodate the 64 km of total offset. This long-term average seems 63 reasonable given that multiple studies have suggested the initiation of the Garlock fault sometime ~10 Ma (Hill and Dibblee, 1953; Burbank and Whistler, 1987; Loomis and Burbank, 1988; Monastero et al., 1997). This result suggests that the long-term slip rate of the Garlock fault is consistent with the full Holocene/latest Pleistocene slip rate (McGill et al., 2009) and when looked at by itself, misrepresents the apparent constancy of strain storage and release on the fault. 3.2.2 Paleoseismology The Garlock fault has not experienced a major rupture during the historical period and the activity of the fault has been determined solely from paleoseismologic trenching. If the fault does experience such fluctuations in loading rate, a basic question arises as to whether such periods of “fast” or “slow” slip correlate with clusters of large-magnitude, large-displacement earthquakes. Results from multiple paleoseismological studies on the Garlock fault (Part C in figure 5) reveal a highly clustered nature of earthquake occurrence with periods of increased activity following periods of seismic quiescence (McGill and Rockwell, 1998; Dawson et al., 2003; Madugo et al., 2012). Despite abundant geomorphic and paleoseismologic evidence for large-magnitude Holocene earthquakes (McGill and Sieh, 1991; McGill, 1992; McGill and Rockwell, 1998; Dawson et al., 2003), and a well-documented early Holocene/ latest Pleistocene slip rate of ~7 ± 2 mm/yr (Clark and Lajoie, 1974; McGill and Sieh, 1993; McGill et al., 2009), geodetic data suggest little or no left-lateral strain accumulation across the Garlock fault over the past several decades (Savage et al., 1981; 1990; 2001; Gan et al., 2000; Miller et al., 2001; Peltzer et al., 2001; McClusky et al., 2001; Meade and Hager, 2005; but see Chuang and Johnson, 2011 and Platt and Becker, 2013 for an alternative assessment). Instead, the short-term geodetic data demonstrate that the Garlock region is presently dominated by northwest-oriented right-lateral 64 shear at rates of 9-11 mm/yr north and south of the Garlock fault (Dixon et al., 2000; Meade and Hager, 2005) and 12±2 mm/yr south of the Garlock fault (Sauber et al., 1986; 1994; Bennett et al., 2003; Meade and Hager, 2005). Records of possible variations in strain accumulation and release along such major faults are of critical importance for understanding earthquake occurrence in southern California and elsewhere. 3.2.3 Previous slip rate studies Both geologic slip rate and geodetic strain accumulation rate estimates along the Garlock fault vary along strike with some geodetic estimates suggesting slightly higher values towards the west (McClusky et al., 2001; Meade and Hager, 2005), while others suggest essentially zero strain accumulation along the western Garlock fault (e.g., Miller et al., 2001; Peltzer et al., 2001; Savage et al., 2001). Early geologic studies (LaViolette et al., 1980; Clark and Lajoie, 1974; Clark et al., 1984 and McGill and Sieh, 1993) have suggested a decreasing slip gradient towards the west. This is consistent with the conjugate fault model for the Garlock fault (Hill and Dibblee, 1953) where convergence at the big bend is accommodated by a greater component of crustal thickening in the west and greater eastward extrusion to the east, as well as the differential extension model across the Basin and Range to the north (Troxel et al., 1972). Both models predict a decrease in slip rate towards the west, consistent with these early studies. More recently however, McGill et al. (2009) concluded that slip rates across the Koehn Lake stepover which separates the central from the western segments are consistent with latest Pleistocene- Holocene minimum slip rates for the central part of the Garlock fault (Ganev et al., 2012) suggesting that slip rates do not decrease towards the west. These results seem to support the transform model for the Garlock fault described by Davis and Burchfiel (1973) whereby the Garlock fault accommodates Basin and Range extension to the north and predicts increasing slip 65 rates to the west as more cumulative slip is being accommodated with the addition of more faults in the Basin and Range. Part A of figure 5 shows the available geologic and geodetically derived slip rate data for the central Garlock fault. Where available, the preferred slip rate is indicated by the open circles and the uncertainties in age and displacement are shown. The slip rate ranges for the most current and reliable studies are highlighted with the orange boxes. The average Holocene-latest Pleistocene slip rate of 5-8 mm/yr is shown with the yellow box. 3.3 Site description Our Christmas Canyon West study site is located approximately 30 kilometers southeast of Ridgecrest, CA and adjacent to the China Lake Naval Weapons Station Christmas Canyon gate (N35.5213°, W117.383°). The study area focusses on four northward flowing, low-relief alluvial fans (sites 1 – 4 in figure 2), each with similar geometry and composition. These sand- rich alluvial fans exhibit typical bar and swale topography and are all incised by small 1-3 meter wide, and 0.25-0.5 meter deep channels. The Garlock fault is well exposed at this site and is identifiable in both the airborne LiDAR imagery and on the ground. Geomorphic structures characteristic of left-lateral strike-slip faulting include linear scarps on Quaternary alluvium, sinistrally offset drainages and alluvial fans, and shutter ridges. 3.3.1 Geomorphology Within the Christmas Canyon West study area, the Garlock fault is well exposed with several offset geomorphic landforms identifiable in both the airborne LiDAR imagery and on the ground. Offset drainages and shutter ridges are two of the more prominent features that display left-lateral motion along the 1.7 km stretch of this part of the fault. Along much of the central segment, the Garlock fault strikes east-northeast and is primarily expressed as a sub-linear scarp 66 along the northern base of the Summit Range. At our study site, several secondary strands are detectable with the LiDAR imagery (Figure 2). These secondary strands produce grabens that appear to primarily accommodate local dip-slip motion. Based on the surface morphology of the alluvial fans, most of the fans appear of similar age. This is not unexpected as the formation of these features was likely climatically driven and alluvial fan aggradation within the Mojave Desert has been limited to periodic pulses of activity. The formation of these geomorphic features that are used to determine fault displacements, and calculate slip rates are highly variable through time. There are several known climatic episodes largely consistent with pulses of fan aggradation observed elsewhere in the Mojave. The most recent pulse of fan aggradation occurred between 4.3-3.5 ka (Harvey and Wells, 2003) and is likely the source of most of these top fan surfaces. 3.4 Offsets geomorphic landforms Numerous well-defined offset gullies and intervening alluvial bars are particularly well expressed in the landscape along a 1.7 km stretch of the central Garlock fault at the Christmas Canyon West site (Figure 2). Our slip rate calculation, which is heavily dependent on the measurement of these offset features, is estimated based on both reconstructing individual channels within the alluvial fans, as well as the overall fan geometry itself. Site 1 provides the best-preserved reconstruction, with multiple drainages restoring at 23 m of fault slip. Analysis of the LiDAR data, together with field measurements, provides a preferred left-lateral displacement of 23.5 ± 2.5 m (Figure 3). This site includes two younger alluvial fan surfaces that have been heavily incised (yellow and red surface), as well as a slightly older-looking surface to the west (green colored surface). Our preferred restoration uses both the alignment of swales within the 67 fan, in addition to the overall fan geometry of the yellow surface and the eroded eastern edge of the red surface. The preferred reconstruction at site 2 is based on the consistent offset of several channels that are incised into the brown colored surface, and which all restore well at ~24 m of left-lateral slip (Figure 4). Specifically, the bar into which sampling pits 12A and 12B were excavated are interpreted to be the same feature, and the heavily incised western edge of the active channel on the brown colored surface (* on Figure 4) can be restored with the corresponding western channel on the opposite side of the fault. This channel is not as pronounced on the north side of the fault because of a component of dip slip motion that has down-dropped the north side of the fault at this location. This western channel wall provides a minimum offset due to the western edge of the channel acting as the eroded bank. Furthermore, the western half of the same brown colored surface displays several small swales that all restore confidently with a ~24 m reconstruction. The ability to restore the entire landscape with our preferred lateral displacement suggests that all of the fans have experienced the same number of earthquakes at this site (Supplementary figure S2). Small differences in lateral displacement along strike are common due to the natural variability of individual ruptures (McGill and Sieh, 1991; McGill and Rubin, 1999) and likely account for the slight variation in preferred displacement values at the two sites. Due to the similarity in fan age (discussed below), it is unlikely that the fan at site 2 experienced any additional events to Site 1. We use our preferred displacement of 23.5 ± 2.5 m to calculate a maximum and minimum late Holocene slip rate for the Garlock fault. By measuring and collating all of the small scale offset features along the Garlock fault, McGill and Sieh (1991) detected displacement clusters at 7, 14 and 18 meters, which were then 68 used to interpret that the left-lateral displacement during the past three earthquakes on the Garlock fault were 7 m, 7 m and 4 m respectively. Given the lateral consistency of 23.5 ± 2.5 m offsets along the stretch of the fault (Figures 3 and 4), we interpret that these are surfaces that have experienced the past four surface rupturing events with slip during the 4 th event back being 5±4 m (on the same order as the previous three events). 3.5 Age control At the Christmas Canyon West site, we excavated five 1m 3 pits from two alluvial fans that have been left laterally offset 23.5 ± 2.5 m along a 1.7 km stretch of the fault (Figure 3 and Figure 4). The sediments exposed within these pits in all cases comprised weakly stratified sand and sandy gravel, without sharp boundaries between units of different grain size. From these pits we collected 21 luminescence dating samples, in each case as a sequence of four samples (five for pit 11A) at different depths down to ~80 cm. Samples were collected in steel tubes tapped into the more sand-rich horizon, and in-situ gamma spectrometer measurements were conducted at each sample position. All samples were prepared and processed at the UCLA luminescence laboratory. At all of our sample sites, the IR 50 -IRSL 225 ages reveal a layered fan structure composed of multiple alluvial deposits of mid- to late Holocene age. Age estimates show a high degree of internal consistency, providing confidence in these results (Table 1). Specifically, most sites had a 4-5 ka deposit at ~0.6-0.85 m depth overlain by a much younger 1.86-2.5 ka deposit (Table 1). A trench excavated through pit 12A and extending fault-parallel to the incised, active drainage to the east revealed the sub-horizontal and laterally continuous deposits that demonstrate that the incision of these drainages occurred after the deposition of the youngest sheet-like deposit 69 (supplementary figure S3). Moreover, the consistent 23.5 ± 2.5 m offset across all of our sites indicates that the entire landscape at the Christmas Canyon West has been offset the same amount, and thus has experienced the same number of surface ruptures. Thus, the youngest age of this youngest incised deposit (~1.9 ka in pit 12B) can be used as a maximum age of incision, and the resulting slip rate represents the minimum rate for this stretch of the Garlock fault over the past ~2 ka. The fact that we see the same ~23 meter offset in slightly older layers at certain sites within the younger 1.9-2.5 ka deposit fits with the notion that all of this displacement occurred after deposition of the youngest, 1.9 ka part of this sequence of fan deposits. 3.6 Slip rate Using an age of 1.86 ± 0.15 ka for the youngest alluvial fan surfaces offset a preferred 23.5 ± 2.5 m, we are able to calculate a minimum slip rate of 12.8 ± 2.4 mm/yr, a value that is significantly faster than the full Holocene/latest Pleistocene slip rate calculated along the Garlock fault (McGill et al., 2009; Ganev et al., 2012). This new result elucidates the non-uniform tectonic behavior of the Garlock fault over millennial timescales and allows us to resolve its fault slip history in great detail. 3.7 Discussion The extreme multi-millenial slip rate variations along the Garlock fault have basic implications for our understanding of earthquake occurrence, fault strength and seismic hazard assessment. Most basically, the availability of slip rates averaged over different timescales, together with a detailed paleoseismological record over the past 7 ka allows for a detailed analysis of the slip history of the Garlock fault. 70 3.7.1 Incremental fault slip rates The ~1.9 ka age of the youngest alluvial fan surfaces that exhibits the consistent 23.5 ± 2.5 m offset yields a minimum slip rate of 12.8 ± 2.4 mm/yr; this rate is a minimum because incision happened after deposition of the youngest fan deposit at 1.9 ka. The paleo-earthquake ages from the nearby El Paso Peaks site, however, indicate that the 23.5 ± 2.5 m offset at Christmas Canyon West likely spans the four most recent earthquakes on this stretch of the Garlock fault, the earliest of which occurred between 25-225 AD. Thus, the time gap between the fourth earthquake back and incision at Christmas Canyon West was likely brief. The minimum 12.8 ± 2.4 mm/yr rate is faster than the longer-term (8-13 ka) ~6.5 ± 1.5mm/yr rate of the fault (Clark and Lajoie, 1974; McGill and Sieh, 1993; McGill et al., 2009; Ganev et al., 2012), indicating that the fault was slipping much faster than its average rate during the 0.5-2.0 ka four-event earthquake cluster observed at the Dawson et al. (2003) El Paso Peaks trench site. Moreover, if the slip rate at Christmas Canyon West is similar to that at the El Paso Peaks site 30 km to the west, as seems likely given the structural simplicity of the fault along this stretch of the Garlock fault, then we can use small geomorphic offsets near that site to further refine the incremental rate. Specifically, near El Paso Peaks, McGill and Sieh (1991) measured sets of offsets at 7 m, 14 m, and 18 m, which have been interpreted to record slip in the three most-recent earthquakes (McGill and Sieh, 1991; Dawson et al., 2003; Ganev et al., 2012; i.e., 7 m, 7 m, and 4 m); we attribute the additional ~5-6 m of displacement at Christmas Canyon West to the fourth event back. Since the ≥12.8 ± 2.4 mm/yr slip rate from Christmas Canyon West includes the ~500 year-long lull since the MRE on this part of the Garlock fault, the slip rate during the 1.5 ka cluster would have been faster than this rate. Exclusion of this period of no fault slip indicates that during the four-event cluster between 0.5-2.0 ka the fault was slipping at 71 14.8-20.4 mm/yr (24 m/1175 to 1615 year span). This variable short-term slip rate during the past 4 earthquakes is shown by the red line in figure 5 part a, with the dashed red lines indicating the minimum and maximum values. Both the minimum rate and the maximum rate calculated from these four events are greater than the slip rate calculated at our Christmas Canyon West site, illustrating that our calculated rate is as expected, a minimum slip rate. Such periods of rapid slip must be balanced over the 8-13 ka time range of the longer-term slip rates by periods of slow or no slip. For example, in addition to the current 500-year-long lull since the MRE, we interpret the 2 – 5 ka gap in earthquakes observed at the El Paso Peaks trench to represent one such period of no slip. Collectively, these data reveal extreme variations in the Garlock fault slip rate that span thousands of years and multiple earthquake cycles. Interestingly, in two of the only other sites where similar comparisons can be made between incremental fault slip rates and detailed paleo-earthquake ages, both the Awatere fault at Saxton River in New Zealand (Mason et al., 2006; Gold and Cowgill, 2011) and the San Andreas fault at Wrightwood (Weldon et al., 2004) exhibit similar behavior, with clustering of earthquakes and large variations in slip rate that span multiple earthquake cycles. Although these two sites suffer from drawbacks that do not affect the Garlock fault data – the Awatere fault slip- rate data are partially based on potentially unreliable weathering rind ages and the Wrightwood incremental rate data are derived from a secondary fault, rather than the San Andreas fault itself – the presence of similar, large (factor of 2-10x) variations in rate during and between earthquake clusters raises the possibility that this is a common behavior on strike-slip faults, perhaps masked until now because of the paucity of such combined earthquake age-incremental slip rate data sets. If true, such behavior has basic implications both for our understanding of time-variable fault strength and for probabilistic seismic hazard analysis, as discussed below. 72 3.7.2 Implications for time variable slip rates The combination of the Christmas Canyon West incremental rate data, the El Paso Peaks paleo-earthquake ages, and analyses of the current geodetic velocity field allow us to construct detailed time-displacement histories for the central Garlock fault spanning mid- to late Holocene time (Figure 5). We consider two end-member situations based on the constancy (or lack thereof) of elastic strain accumulation rates along the fault, each of which has very different implications for our understanding of fault mechanics and earthquake behavior. Recent geodetically constrained models of elastic strain accumulation have suggested to many previous researchers that the central Garlock fault is storing elastic strain energy at less than half of the latest Pleistocene-Holocene slip rate of 5-8 mm/yr (e.g., McClusky et al., 2001; Miller et al., 2001; Peltzer et al., 2001; Meade and Hager, 2005; Dolan et al., 2007; McGill et al., 2009). Specifically, current geodetic velocity field data show primarily fault-perpendicular, northwest-southeast right-lateral shear (e.g., Savage et al., 1990; Peltzer et al., 2001; McGill et al., 2009), rather than obvious east-west, left-lateral elastic strain accumulation, as would be expected along the sinistral Garlock fault. If the apparent geologic-geodetic rate discrepancy is real (and some researchers have suggested that it is not [e.g., Chuang and Johnson; 2011; Platt and Becker, 2013]), the elastic strain accumulation rate must vary significantly over the timescales of one to a few earthquakes, since these rates must balance when averaged over numerous earthquakes. This suggests that the Garlock fault may experience two different modes of behavior, with alternating periods of slower-than-average strain accumulation (during lulls in earthquake activity) balanced by periods of faster-than-average rates (during earthquake clusters) (e.g., Dolan et al., 2007). If the ~3,000-year-long lull in earthquake activity between 2 ka and 5 ka was characterized by an elastic strain accumulation rate that is much slower than the long- 73 term average, then strain accumulation rates must have been much faster than average during and/or immediately preceding the cluster of four earthquakes 0.5-2 ka observed at the El Paso Peaks site. Yet the transiently reduced rate of elastic strain accumulation along the Garlock fault suggested by our current understanding of the geodetic velocity field is difficult to reconcile with observations from other faults where the geodetic strain accumulation field can be observed in isolation from other faults near the end of a strain accumulation cycle. Many of these examples indicate that strain accumulation rates and fault slip rates are broadly similar (e.g., central San Andreas fault [e.g., Sieh and Jahns, 1984; Thatcher, 1990; WGCEP, 1995] and North Anatolian fault (Kozaci et al., 2007; 2009), even in cases where transients had previously been suggested to exist (e.g., Hergert and Heidbach, 2010; Dolan and Meade, 2010; Dolan and Haravitch, 2014; Herbert et al., 2014). Thus, although our current understanding of the geodetic data suggests that the fault is storing energy at a much slower rate than its long-term average, this “strain transient” is becoming an increasingly rare outlier in comparison to other faults around the world. We therefore consider the alternative possibility that elastic strain accumulation along the central Garlock fault has been relatively constant at the preferred 5 – 8 mm/yr range of preferred Holocene-latest Pleistocene rates, except for earthquake-cycle effects (e.g., Meade and Hager; 2004), even during the long dormant periods (e.g. 2-5ka). A basic point that emerges from this scenario is that, at these rates, the Garlock fault would have stored ~15-24 m of elastic strain during the 2-5 ka lull in earthquakes, together with an additional 7.5–12 m during the 1500-year- long earthquake cluster at 0.5–2.0 ka, for a total of 22.5-36 m of elastic strain stored during the ~4500-year-long period between the beginning of the earthquake lull at 5 ka and the date of the MRE (~1500 AD). Comparison of these values with the ages and inferred displacements in 74 Garlock fault earthquakes yields an intriguing result. As shown in figure 6, if average elastic strain accumulation rates were near the low end of the range of possible values (i.e., ~5.1 – 5.3 mm/yr, as suggested by Ganev et al., (2012), the amount of strain stored between 0.5 ka and 5 ka would be indistinguishable from the 23.5 ± 2.5 m of fault slip that occurred during the 0.5-2 ka cluster of earthquakes. Alternatively, if the rate of strain accumulation has been closer to the preferred maximum rate of 8 mm/yr, then ~36 m of strain would have been stored relative to the ~24 m that has been released during the four-event cluster, resulting in a deficit of strain release during this current cycle of ~12 m. 3.7.3 Testing for time-, slip-, and strain-predictable behavior Models of fault behavior based on the variability in earthquake recurrence intervals and displacements have long been used as the basis for renewal models used in probabilistic seismic hazard assessment (PSHA). Specifically, earthquakes have been suggested to be either time- or slip-predictable. That is, that the time to the next earthquake will depend on displacement in the previous event, or the slip in an earthquake depends on the time since the previous earthquake, respectively (e.g., Shimazaki and Nakata, 1980). The reliability of these seismicity models remains a key unknown in current PSHA. One caveat to the applicability of these models is that they both assume constant elastic strain accumulation. Thus, if the Garlock fault experiences varying loading rates through time, as discussed above, then these models will not accurately predict the occurrence of future events along that fault. Considering the alternative possibility that elastic strain accumulation rates have been relatively constant through time (a key assumption implicit in time- and slip-predictable models), we can use the paleo-earthquake ages from the El Paso Peaks trench, the Christmas Canyon West slip rate, the small geomorphic offset data from near the El Paso Peaks site, and inferences about 75 reasonable maximum and minimum displacements in the 5 ka and 7 ka events to examine whether the Garlock fault exhibits time- or slip-predictable behavior. As shown in figure 7a and b, although the record is rather brief (only six events and five earthquake cycles), it is sufficiently long to demonstrate that the surface ruptures along the central Garlock fault do not appear to be either time- or slip-predictable. We also consider the possibility that the Garlock fault may be ‘strain-predictable” (Weldon et al., 2004). That is, that the likelihood of a future earthquake on the fault depends on the relative level of elastic strain that has accumulated during the duration of the available record. As with the time- and slip-predictable models, strain-predictable behavior is based on the assumption of constant rate of strain accumulation. If this assumption is valid, the relatively brief central Garlock fault record suggests that earthquake occurrence is weakly strain predictable (Figure 8). Specifically, regardless of the displacement during the two events for which we have very little slip constraints (Events 5 and 6), five of the events generally fit the strain predictable behavior, with the length of the subsequent interseismic period inversely related to the relative strain accumulation. Event 5 is the lone outlier in the strain predictable plot (Figure 8). When calculating the relative strain accumulation leading up to each event for the strain predictable model, we used a constant strain accumulation rate of 6.45 mm/yr. This is the average between the McGill et al., (2009) rate of 7.6 mm/yr and the Ganev et al., (2012) rate of 5.3 mm/yr. Interestingly, Weldon et al. (2004) suggested the same behavior at the Wrightwood site along the San Andreas fault. It is important to recall that the displacement data used in the Weldon et al. (2004) analysis come from a secondary fault, rather than the San Andreas fault proper, thus calling into question the absolute values of their suggested rate changes. Nevertheless, both the Garlock fault data discussed here and the Wrightwood San Andreas fault 76 data of Weldon et al. (2004) indicate that earthquake occurrence is neither time- or slip- predictable and the similarity in their behavior suggests that strain-predictable behavior may be common on strike-slip faults, and may thus provide a means of better forecasting the timing of future earthquakes. These observations suggest that strain-predictable models based on long paleo-earthquake age and displacement records may be worth pursuing for next-generation probabilistic seismic hazard assessment efforts. 3.7.4 Fault behavior and earthquake supercycles along the Garlock fault The strain-release record shown in figure 7c suggests that the Garlock fault experiences strain “supercycles” comprising multiple earthquakes and periods of strain accumulation considerably longer (or faster) than the simple, single-event strain accumulation–release earthquake cycles envisaged by classical elastic rebound theory. Specifically, at a constant strain accumulation rate, the release of ~23-24 m of elastic strain during the four-event earthquake cluster at 0.5-2.0 ka required much more strain than accumulated in any individual earthquake cycle, or indeed during the 1500-year-long cluster. Similar behavior has been suggested for other faults for which long and detailed paleoseismologic records are available, including the San Andreas (Weldon et al., 2004), the Cascadia subduction megathrust (Goldfinger et al., 2013; Kulkarni et al., 2013), the Haiyuan fault (Yongkang et al., 1997), and the Himalayan front (Kumar et al., 2006). Such behavior would seem to require significant changes in fault strength through time. For the Garlock fault, if elastic strain accumulation rates were constant through time, then 15 – 24 m of elastic strain energy would have accumulated during the 3,000-year-long seismic lull prior to the earliest earthquake in the 0.5-2 ka cluster. This is 2 – 4 times the displacement during any of the earthquakes in the cluster, and indeed more than has been observed in other strike-slip 77 ruptures around the world (with the possible exception of one 18 m displacement observed along the 1855 M~8 Wairarapa earthquake, which may have involved rupture downward onto the subduction megathrust or a large listric thrust fault within the subduction complex). Thus, it appears that the Garlock fault must have been stronger during the 2–5 ka seismic lull, and then much weaker during the subsequent cluster. The problem is even more extreme if we consider time-variable strain accumulation rates. If the fault accumulates little to no elastic strain energy during earthquake lulls, as implied by our current understanding the geodetic velocity field, then the strain must be stored on other faults in the plate boundary system (e.g., Peltzer et al., 2001; Dolan et al., 2007), implying both long-term regional fault interactions and large variations in relative fault strength. Various mechanisms have been suggested to explain such behavior, focusing either on ways to alternately strengthen and/or weaken the fault. In addition to the regional “switching” hypothesis, for example, one possibility is that the ductile roots of the fault directly beneath the brittle-ductile transition strain harden during periods of rapid slip along the seismogenic parts of the fault (e.g., an earthquake cluster) and subsequently anneal during lulls in earthquake activity, during which the fault does not slip at depth (Dolan et al., 2007). This behavior could, in turn, result in the relative strength of regional fault networks switching such that strain is accommodated on the weakest network in the system at any given time (Dolan et al., 2007). Alternatively, the behavior of the system may be driven by the (random) occurrence of the first event in a cluster, which may serve to somehow weaken the lower crustal shear zone below the fault, allowing continued slip driven by elastic strain energy stored during the preceding lull. For example, Oskin et al., (2008) suggested that the first earthquake may somehow release fluids downward into the ductile roots of the fault zone, weakening it and allowing faster slip. This 78 suggestion does not, however, explain why the fault would store such large strains before rupturing if there were not also some attendant fault strengthening during the lulls. Models of simulated patterns of seismicity suggest a third possibility in which this behavior may be caused by episodic, fundamental reorganizations of the mode of strain energy release that are driven by changes in the entropy of stress distributions along the fault; small total variations in stress state along the fault (i.e., a relatively coherent stress field) will favor temporal clusters of large-magnitude events, whereas large variations in the state of stress (a highly irregular, disordered stress field) will favor periods of much lower strain release and a more random distribution of smaller earthquake magnitudes (Dahmen et al., 1998; Ben-Zion et al., 1999). Although this may be an attractive model for explaining situations such as the occurrence of the 2010 M w 9.1 Tohuku earthquake in a region historically dominated by smaller-magnitude (though still large) earthquakes, it fails to explain the behavior of the Garlock fault discussed here. Specifically, it cannot explain why the fault failed to generate any paleoseismologically or geomorphically detectable evidence for slip in a series of small-moderate events despite the fact that constant loading rate of ~ 6.5 mm/yr would have stored ~20 m of elastic strain energy during the 3,000-year-long lull between the ~ 5 ka earthquake and the first event in the 0.5-2.0 ka cluster of large earthquakes. These observations again suggest to us that faults are mechanically stronger during lulls and abruptly weaken at the onset of a cluster, allowing accumulated elastic strain energy to be released. An additional complexity arises from the geologically complex nature of plate boundary deformation in southern California. The storage and release of elastic strain energy on the Garlock fault in large earthquakes does not occur in isolation, and stress interactions from 79 earthquakes on nearby faults will influence the behavior of the Garlock fault. For example, although the Weldon et al. (2004) proxy slip-rate record for Mojave section of the San Andreas fault at Wrightwood overlaps with only the most recent 1500 years of the Garlock fault record, this allows comparison of at least the latter part of the 0.5-2 ka Garlock cluster. Interestingly, as shown in figure 9, the two most recent earthquakes in the Garlock cluster correlate with periods of either rapid slip during temporal clusters of SAF earthquakes (~1000 AD Garlock fault earthquake) or very large-displacement earthquakes on the SAF (~1500 AD Garlock fault earthquake). Coulomb failure function modeling (Rollins et al., 2011; McAuliffe et al., 2013) indicate that earthquakes on the Mojave section of the San Andreas fault will encourage failure of the western part of the Garlock fault, and vice versa. Thus, these data suggest a nested set of controls on the occurrence of supercycles along the Garlock fault, with the overall behavior of the fault over millennial time scales governed by one set of controls, and the detailed occurrence of Garlock fault events modulated by the incremental slip of nearby faults (especially the San Andreas fault). It is worth noting that all of the potential mechanisms described above could potentially be occurring simultaneously. 3.7.5 Implications for probabilistic seismic hazard assessment (PSHA) This inference of large variations in elastic strain accumulation rate suggests extreme temporal variations in fault strength. Specifically, inasmuch as the geodetic velocity field is thought to record motions below the brittle-ductile transition, the strength of the ductile roots of the Garlock fault would have to vary widely through time at the multi-millenial scale of our observations. For example, if the absence of earthquakes between 2 ka and 5 ka records a period of slower-than-average to zero elastic strain accumulation, the rate of elastic strain accumulation must have sped up immediately before and during the cluster. Such extreme variations in elastic 80 strain accumulation rates would be readily detectable with geodetic data and could indicate the impeding initiation of a cluster of major earthquakes. Recent geologic studies have indicated that the slip rate averaged over 10 5 timescales are consistent with those over 10 6 timescales, indicating that periods of elevated strain accumulation and release occur on much shorter timescales. Our 2 ka slip rate of ~12 mm/yr confirms that these periods exist at least within the Holocene. Nevertheless, our current understanding of the geodetic data does suggest that the Garlock fault, however much an outlier, does appear to be storing energy at a rate that is slower than its long-term average, and regardless of the actual values for the Garlock fault, this methodology provides a clear path forward for next-generation PSHA to estimate the likelihood of future earthquake occurrence. The behavior of the Garlock fault throughout these strain supercycles has a significant impact on the advancement of next generation probabilistic seismic hazard assessment. If the strain accumulation rate is constant at our preferred slip rate of 5-6 mm/yr, the similarity in elastic strain energy that would have been stored in the 4.5 ky encompassing the 4 event cluster and preceding lull with the 24 meters of strain that was accommodated in those earthquakes would suggest that the 0.5-5 ka strain supercycle is complete. If so, and if this supercycle behavior is typical of the Garlock fault, then this would indicate that the probability of future Garlock fault earthquakes is very low. It is worth noting however that if the average strain accumulation rate, as reflected in the long-term fault slip rate is faster than our preferred 5.3 mm/yr slip rate (McGill et al., 2009), then these strain supercycles may not be complete and there may be one or more remaining earthquakes required to complete the cycle. A comprehensive probabilistic seismic hazard assessment should include the range of possible earthquakes, and misrepresenting the range of earthquake scenarios by relying on a short 81 instrumental or even short paleoseismoloical record can introduce bias into earthquake prediction models by either underestimating or overestimating the size of earthquakes on that fault system. 3.8 Conclusions New advances in single grain feldspar IRSL (Infra-Red Stimulated Luminescence) dating have allowed us to accurately date 21 new luminescence samples from 2 alluvial fans (5 pits) at Christmas Canyon West along the Central Garlock fault. Together with LiDAR- and field-based mapping of small scale geomorphic offsets, we calculate a late Holocene minimum slip rate of ~12 mm/yr. This 2 ka age rate is significantly faster than the longer-term latest Pleistocene to early Holocene rate of ~7 mm/yr determined at both Clark Wash (McGill et al., 2009) and the Summit Range study area (Ganev et al., 2012), illustrating the temporal inconsistency of slip along this part of the Garlock fault. The 2 ka averaged rate calculated at Christmas Canyon West is an order of magnitude faster than the current rate of elastic strain accumulation determined from geodesy. The current strain transient experienced by the Garlock fault, together with periods of elevated strain accumulation and release, indicates that the Garlock fault must either experience two contrasting phases of strain accumulation and release, or our understanding of the geodetic data is incorrect and strain is stored at a constant rate across the Garlock fault. If the strain accumulation rate is constant at our preferred slip rate of 5-6 mm/yr, the similarity in elastic strain energy that would have been stored in the 4.5ky encompassing the 4 event cluster and preceding lull with the 24 meters of strain that was accommodated in those earthquakes would suggest that the 0.5-5ka strain supercycle is complete. If so, and if this “supercycle” behavior is typical of the Garlock fault, then this would indicate that the probability of future Garlock fault earthquakes is very low. It is worth noting however that if the average 82 strain accumulation rate, as reflected in the long-term fault slip rate is faster than our preferred 5.3 mm/yr slip rate (McGill et al., 2009), then these strain supercycles may not be complete and there may be one or more remaining earthquakes required to complete the cycle. The key to distinguishing these possibilities lies in refining the full Holocene-latest Pleistocene slip rate of the central Garlock fault and extending the record of paleo-earthquake ages and displacements back until the 7 ka event. Complicating this picture are the geodetic data that suggest that current rates of elastic strain accumulation are much slower than the long-term average slip rate. If these data have been interpreted correctly, and not everyone agrees that they have been (e.g. Chuang and Johnson, 2011; Platt and Becker, 2013), this would imply that the Garlock fault experiences two different modes of strain accumulation with the fault currently in a “slow mode”. Such periods of slow strain accumulation could possibly explain the long duration of the 2-5 ka lull in activity. But any such “slow modes” would have to be balanced by periods of faster-than-average strain accumulation, and consequently faster slip rates, that would have to initiate prior to and possibly during the earthquake clusters such as that observed between 0.5-2 ka. Any such changes would be readily detectable with geodetic data and could indicate the impeding initiation of a cluster of major earthquakes. Recent geologic studies have indicated that the slip rate averaged over 10 5 timescales are consistent with those over 10 6 timescales, indicating that periods of elevated strain accumulation and release occur on much shorter timescales. Our 2 ka slip rate of ~12 mm/yr confirms that these periods exist at least within the Holocene. Our results, together with the paleoseismic record at El Paso Peaks (Dawson et al., 2003) and the small-scale displacement estimates by McGill and Sieh (1991) can be used to infer slip estimates during the past 4 83 earthquakes, and validate the relationship between earthquake clustering and periods of elevated slip rate on the Garlock fault. These issues raise basic questions about the evolution of fault strength over multiple earthquake cycles and the potential for complete stress drop during earthquakes. As a basic point, in either scenario the fault strength must vary significantly during these supercycles. These observations call into question the well-established concept of the earthquake cycle, and downplay the notion of faults being “overdue”. The short instrumental (geodetic) and historical records are inadequate to characterize the complex and multi-scale seismic behavior of major fault systems, including the Garlock fault. A comprehensive probabilistic seismic hazard assessment should include the range of possible earthquakes, and misrepresenting the range of earthquake scenarios by relying on a short instrumental or even short paleoseismological record can introduce bias into earthquake prediction models by either underestimating or overestimating the size of earthquakes on that fault system. 3.9 Figure Captions Figure 1. Index map of the Mojave section of the eastern California shear zone showing active Quaternary faults (grey) and the Garlock fault (white). Star indicates location of our Christmas Canyon West study area. White circle indicates location of the El Paso Peaks trench site (Dawson et al., 2003) approximately 30 km west of our study site. B – Barstow; CCW – Christmas Canyon West; EPP – El Paso Peaks; M – Mojave; R – Ridgecrest; T – Trona. Figure 2. Christmas Canyon West study area showing four main study sites. Sites 1 and 2 are used in this paper to calculate our late Holocene slip rate. Colored regions show interpreted 84 surfaces of different age. Hash marked polygon at Site 4 indicates a highly eroded and slumped terrace riser. Only minor left-lateral displacement is observed on the secondary faults to the north of the main strand. The small inset map shows trace of San Andreas fault (SAF) and Garlock fault (GF). Figure 3. (a) Hillshade geologic and (c) slope aspect maps of Site 1 at Christmas Canyon West from LiDAR data (white star in Figure 1). (b and d) The geomorphic landscape is retro-deformed a preferred 23 m based on these data. Colored polygons on (a) and (b) indicate interpreted fan surfaces of different age. White squares in (a) indicate OSL sample locations. Figure 4. (a) Hillshaded geologic and (c) slope aspect maps of Site 2 at Christmas Canyon West from LiDAR data (white star in Figure 1). (b and d) The geomorphic landscape is retro-deformed a preferred ~24 m based on these data. Colored polygons on (a) and (b) indicate interpreted fan surfaces of different age. White squares in (a) indicate OSL sample locations. Dashed box indicates location of small trench (figure S5) Table 1. Results of single grain IRSL dating from the Christmas Canyon West site. Figure 5. (A) Available slip rate and paleo-earthquake age data from the central Garlock fault plotted versus time. Small circles represent preferred rates and horizontal and vertical black bars show uncertainties in slip rate based on uncertainties in age and displacement. Orange boxes outline possible slip rate range for the two preferred slip rate studies (McGill et al., 2009 and Ganev et al., 2012). Yellow horizontal band encompasses range of preferred average latest 85 Pleistocene-Holocene rates measured by Clark and Lajoie (1974); McGill and Sieh (1993); McGill et al. (2009); and Ganev et al. (2012). Red line shows variable short-term slip rate calculated using events documented at El Paso Peaks and displacements observed along central Garlock fault. Note correspondence of rapid latest Holocene rate with cluster of four earthquakes observed by Dawson et al. (2003) at their El Paso Peaks site 30 km west of Christmas Canyon West. Orange boxes between dashed red lines constrain interpreted maximum and minimum slip rates. 13-21.3 mm/yr slip rate between ~0.5-2 ka calculated from 4 most recent events documented at El Paso Peaks (Dawson et al., 2003) and 23.5 ± 2.5 meters of displacement observed at Christmas Canyon West. Dawson et al. (2003) documented an absence of earthquakes between 2 ka and ~5 ka, yielding a 0 mm/yr rate for this interval. For the two earthquakes they documented at ~5 ka and ~7 ka we assume a range of possible displacements for each event of 2 m (the smallest geomorphic displacement measured along the central Garlock fault by McGill and Sieh, 1991) to 6 m (average slip in four earthquakes totaling 24 m as measured at our Christmas Canyon West study site), yielding 4-12 m of slip within this ~2 ky period. Combined with the 24 m of fault slip measured by us over the past 2 ky, this yields a total of 28 to 36 m of slip on the Garlock fault since 7 ka. Green, Gray and pink boxes show possible incremental rates prior to 7 ka that are consistent with the 70 m of total displacement measured by McGill et al. (2009) and Ganev et al. (2012) that has accrued since latest Pleistocene-early Holocene time. Three possibilities are shown, given the uncertainty in the exact time at which the 70 m of displacement began to accumulate: (1) a 19-24 mm/yr incremental rate from 7.5 ka to 9.3 ka (preferred age of incision for ~70 m offset at McGill et al., 2009, study site); or (2) a 5.5-7.5 mm/yr incremental rate from 7.5 ka to 11.5 ka (following Ganev et al.’s [2012] suggestion that incision began during return to wetter conditions at the end of the Younger Dryas 86 climatic regime, 58 m offset); or (3) 5.8-7.2 mm/yr incremental rate from 7.5 ka to 13.3 ka (age of incised surface at Ganev et al., 2012, 70 m offset). Note that these three speculative “rates” assume constant steady slip during these time periods, a behavior we consider unlikely, especially for the 4-6 ky-long periods represented by the latter two possibilities. This method of presentation thus artificially reduces the average rates shown during these intervals. (B) Same data plotted as cumulative displacement through time. Triangular region between 7.5 ka and 13.3 ka denotes range of possible cumulative displacements. (C) Available paleoseismologic data for the central and western Garlock fault. The well-constrained site of Dawson et al. (2003) lies closest to our study site. Figure 6. Long term strain periodicity along the Garlock fault based on 6 earthquakes since 7ka. The sawtooth pattern illustrates the accumulation of strain represented by the red line at the constant long term late-Holocene/latest Pleistocene slip rate of (A and B) 5.3 mm/yr (Ganev et al., 2013) and (C and D) 7.6 mm/yr (McGill et al., 2009), and the strain release through earthquakes (black arrows) in meters of slip. Because the absolute strain level is unknown, we have arbitrarily determined the level of zero net strain. If the strain rate is 5.3 mm/yr and the slip during the two oldest events that we have record of is 2 meters (minimum slip value based on smallest offsets observed along Garlock fault [McGill and Sieh, 1991]), it appears as though we have yet to fully complete the strain supercycle and we may expect another earthquake in the near future. If we apply a strain rate of 7.6 mm/yr, we are still far from returning to a zero net strain level, regardless of the slip during events 5 and 6. (A) and (C) show scenarios where 6 meters of slip occurred in events 5 and 6. (B) and (D) illustrate scenarios where 2 meters of slip occurred in events 5 and 6. 87 Figure 7. (A) Slip-predictable behavior is determined if the earthquake displacement and the length of the previous interseismic period have a linear relationship that falls along the average slip rate trend (blue and red dashed line). (B) Time-predictable behavior is classified when the earthquake displacement and the length of the subsequent interseismic period follow a linear relationship that falls along the average slip rate trend (blue and red dashed line). These limited data indicate that the Garlock fault is neither slip nor time predictable. (C) Long-term strain periodicity along the Garlock fault based on 6 earthquakes since 7 ka. The sawtooth pattern illustrates the accumulation of strain represented by the red line at the constant long-term late- Holocene/latest Pleistocene slip rate of 5.3 mm/yr (Ganev et al., 2013) and 7.6 mm/yr (McGill et al., 2009), and the strain release through earthquakes (black arrows) in meters of slip. Because the absolute strain level is unknown, we have arbitrarily determined the level of zero net strain. If the strain rate is 5.3 mm/yr and the slip during the two oldest events that we have record of is 2 meters (minimum slip value based on smallest offsets observed along Garlock fault [McGill and Sieh, 1991]), it appears as though we have yet to fully complete the strain supercycle and we may expect another earthquake in the near future. If we apply a strain rate of 7.6 mm/yr, we are still far from returning to a zero net strain level, regardless of the slip during events 5 and 6. Figure 8. Strain predictable behavior is determined if the likelihood of a future earthquake on the fault depends on the relative level of elastic strain that has accumulated during the duration of the available record. As with the time- and slip-predictable models, strain-predictable behavior is based on the assumption of constant rate of strain accumulation. The brief contral Garlock fault record suggests that earthquake occurrence is weakly strain predictable. When calculating the 88 relative strain accumulation leading up to each event for the strain predictable model, we used a constant strain accumulation rate of 6.45 mm/yr. This is the average between the McGill et al., (2009) rate of 7.6 mm/yr and the Ganev et al., (2012) rate of 5.3 mm/yr. Figure 9. Plot showing cumulative displacement of the San Andreas fault and Garlock fault over the past 1.6 ka. Orange boxes show age uncertainties for the most recent event and the penultimate event on the Garlock fault. Between 600 and 900 A.D., the San Andreas fault accumulated slip at three times the mean rate. Interestingly, this period of rapid strain release coincides with the penultimate event on the central Garlock fault. Between ~1500 and 1550 A.D., the San Andreas fault experienced a large displacement event, and this appears to have occurred just prior to the most recent event on the central Garlock fault. Figure modified from Weldon et al., 2004. Data Repository Figure Captions Supplementary figure S1. Map showing Garlock fault and faults of the northern Mojave section of the eastern California Shear zone. The three values in grey boxes above the Garlock fault, separated by the dashed white lines are the slip rates and formal uncertainties for the three sections of the Garlock fault from Meade and Hager’s (2005) best-fitting elastic block model of available geodetic data. White circles indicate locations of Holocene and Late Quaternary geologic slip rates (CC – Christmas Canyon, Smith, 1975; CW – Clark Wash, McGill et al., 2009; KL – Koehn Lake, Clark and Lajoie, 1974; MC – Mesquite Canyon, Carter et al., 1994; OLF – Owl Lake fault, McGill, 1998; OC – Oak Creek, La Violette et al., 1980; SL – Searles 89 Lake, McGill and Sieh, 1993; 449100 – Summit Range, Ganev et al., 2012. All slip rates are in mm/yr. Red box indicates Christmas Canyon West study region (supplementary figure S2). Supplementary figure S2. Supplementary figure S4. (a) LiDAR image of Christmas Canyon West study site showing sample pits (white boxes) and different offset alluvial fans. (b) Sites 1 and 2 retro-deformed a preferred 23.5 ± 2.5 m. Older fan at west edge of image (red shading) appears to be much older than green and yellow-shaded fans used to generate >12 mm/yr late Holocene slip rate described in this paper. Yet older fan restores well at same 23.5 ± 2.5 m offset as younger dated fans. Supplementary figure S3. Digital trench log at site 2 showing lateral continuity of stratigraphy across the alluvial fan (dashed box in figure 4). Depth and width are taken from an arbitrary datum. By verifying the stratigraphy we have demonstrated that erosion of the channels precedes deposition of the dated units. 90 CHAPTER 4: Characterizing the recent behavior and earthquake potential of the blind Ventura fault system 4.1 Abstract Detailed analysis of high-resolution seismic reflection data, continuously cored boreholes and cone penetrometer tests (CPT), and 14 C and luminescence dates from Holocene strata folded above the tip of the Ventura fault constrain the ages and displacements in the two most recent earthquakes on this major blind thrust fault. These two earthquakes, which are recorded by the prominent surface fold scarp and a stratigraphic sequence that thickens across a buried fold scarp, occurred at <1.4 ka (most recent event) and between 3-5 ka (penultimate event). Minimum uplift in these two scarp-forming events was ~6 meters for the MRE and ~4.5 meters for the penultimate event. Individual uplift events of this magnitude would require very-large earthquakes, likely in excess of M w 7, likely involving rupture of the Ventura fault together with other Transverse Ranges faults to the east and west. The proximity of this large reverse-fault system to major population centers, including the greater Los Angeles metropolitan region, and the potential for tsunami generation during ruptures extending offshore along the western parts of the system, highlight the importance of understanding the complex behavior of these faults for future probabilistic seismic hazard assessment. 91 4.2 Introduction 4.2.1 Recognition of emerging thrust fault hazards The recognition of the hazards posed by thrust fault earthquakes to urban centers around the world has been highlighted by several recent events (e.g., 1994 M w 6.7 Northridge, 1999 M w 7.6 Chi-Chi, 2005 M w 7.5 Kashmir, 2008 M w 7.9 Wenchuan). These events emphasize the need to better understand the behavior of these faults and their associated folds, particularly when these faults are “blind”, that is, faults that do not reach (or “see”) the surface. The 2008 M w 7.9 Wenchuan earthquake, in particular, illustrated that ruptures may link together various thrust faults to generate extremely large-magnitude earthquakes. In southern California, such multiple- fault ruptures could potentially produce events similar in size to the largest earthquakes expected from the San Andreas fault, indicating they represent a serious hazard to property and life in the densely populated southern California region. Most current seismic hazard assessments and models of thrust fault earthquakes in southern California involve the rupture of individual reverse faults in the Transverse Ranges (e.g., Sierra Madre fault, San Cayetano fault) in moderately large magnitude (M w 7.2-7.4) events (WGCEP, 1995). While the seismic threat posed by these individual faults is significant, as for example the 1994 M w 6.7 Northridge earthquake, which was the costliest earthquake in US history prior to Hurricane Katrina [Scientists of the U.S. Geological Survey and the Southern California Earthquake Center, 1994], the biggest threat presents itself if several of these faults rupture together. The various thrust faults of the Transverse Ranges form an interconnected, >200-km-long network of faults that could potentially rupture together during extremely large- magnitude events similar in scale to the 2008 M w 7.9 Wenchuan earthquake. While the potential for these faults to link and rupture together has recently been recognized (e.g., Dolan et al., 1995; 92 Hubbard et al., 2014), relatively little is known about the ages, repeat times, and magnitudes of paleo-earthquakes generated by faults within the Transverse Ranges. In this paper we apply a multi-disciplinary approach utilizing continuously cored borehole and cone penetrometer test (CPT) data, in conjunction with high-resolution and deeper- penetration petroleum industry seismic reflection data, to document the characteristics and structural evolution of very young folds that have developed above the Ventura fault, a major reverse fault in the western Transverse Ranges. Together, these new data allow us to assess the geometry of buried fold scarps and identify periods of stratigraphic growth that record discrete uplift events along the Ventura fault. We use these data to validate previous paleoearthquake magnitude estimates for the Ventura blind thrust fault and discuss the results in light of their implications for seismic hazard in southern California. 4.2.2 Regional Geology Located 50-100 km northwest of Los Angeles, the Ventura basin is a narrow, ~50-km- long basin bounded on both the north and south by a complex network of E-W reverse and oblique-left-lateral reverse faults (Figure 1). Bounded by Oak Ridge to the south, and the Ventura Avenue Anticline (VAA) and Topa Topa mountains to the north, the Ventura basin is ~4 km across at its widest near the city of Ventura, and narrows towards its eastern end where the northern basin-bounding San Cayetano fault overrides the south-dipping Oak Ridge fault (Huftile and Yeats, 1995). The stratigraphy of the Ventura basin is dominated by a competent sandstone sequence from the Cretaceous through the Paleocene, overlain by Miocene mudstones and Plio-Pleistocene coarse-grained clastics. With a maximum thickness of 15-17 km, the Ventura basin is one of the deepest Plio-Pleistocene sedimentary basins in the world (Yeats, 1977; Yeats, 1983; Huftile and Yeats, 1995). Extremely high sediment accumulation and 93 tectonic loading rates produce anomalously low heat flow values (approximately 30% lower than are typical for southern California), accounting for the anomalously deep background seismicity observed within the area (Bryant and Jones, 1992) – though these anomalously deep earthquakes may be related to tectonic loading of the basin and unrelated to slip on the major faults (Donnellan et al., 1993). The western Transverse Ranges are dominated by several major east-west trending faults and fold structures that interrupt the general northwesterly structural grain of coastal California. These structures are evidence of the north-south compressive forces that have been responsible for the observed deformation since early Pliocene time (Luyendyk et al., 1985). The deformation of Pleistocene and younger deposits along a similar structural grain, together with current geodetic data, illustrate the ongoing style of deformation within this region. Within the Ventura basin, shortening is accommodated by slip on opposing thrust systems (Figure 1). Along the basin’s southern edge, the Oak Ridge fault, a high-angle, reactivated and structurally inverted Miocene normal fault (Crowell, 1976; Yeats, 1988; Yeats et al., 1988; Sorlien et al., 2000), is responsible for the uplift of basin-bounding Oak Ridge. Geologic studies of the Oak Ridge fault estimate a mid-Pleistocene slip rate of 5.9-12.5 mm/yr (Yeats, 1988; Peterson and Wesnousky, 1994) and late Pleistocene rate of 3.7-4.5 mm/yr (Huftile and Yeats, 1996) – a rate at least twice as fast as that of the blind thrust fault responsible for the 1994 Northridge earthquake (Huftile and Yeats, 1996). Additionally, Yeats and Huftile (1995) interpret the south-dipping blind thrust fault responsible for the 1994 Northridge earthquake as the eastern continuation of the Oak ridge fault, which reaches the surface along the southern boundary of the Ventura basin. Along the Ventura basin’s northern margin, several faults are responsible for the uplift of the VAA and Topa Topa Mountains. The Ventura fault, and its offshore extension the Pitas Point 94 fault (Campbell et al., 1975; Sarna-Wojcicki et al., 1976; Yerkes and Lee, 1987; Kamerling and Nicholson, 1995; Hubbard et al., 2014) form the middle section of a series of range-front thrust faults that link the rapid north-south shortening accommodated by the San Cayetano fault, and ultimately the Sierra Madre and Cucamonga faults to the east, to the Red Mountain fault and related faults offshore to the west. Following the clockwise rotation of the western Transverse block during the mid- Miocene to its current east-west trend (Hornafius et al., 1986; Jackson and Molnar, 1990; Luyendyk, 1991), the Ventura basin has been the locus of very rapid convergence rates, with maximum N-S horizontal geodetic shortening rates of between 7-10 mm/yr (Donnellan et al., 1993a; 1993b; Hager et al., 1999; Marshall et al., 2008), and geologic fault slip rates between 7 and 15 mm/yr since 500ka (Huftile and Yeats, 1995a). In the western half of the Ventura basin, growth of the VAA accommodates much of this north-south shortening (e.g., Rockwell et al., 1988; Stein and Yeats, 1989; Hubbard et al., 2014), rising at a very rapid rate of ~5mm/yr (Rockwell et al., 1988). Some early studies suggested that the VAA is rooted in a shallow detachment within the Miocene Monterey Formation (e.g., Huftile and Yeats, 1995), and thus did not pose a major earthquake threat despite this being one of the fastest-deforming structures in California. However, others have suggested that is it a seismogenic structure in its own right (Sarna-Wojcicki et al., 1976: Sarna-Wojcicki and Yerkes, 1982). Based on a comprehensive set of geologic maps, well data, industry seismic reflection profiles, and high-resolution seismic reflection profiles described in a companion paper (Hubbard et al., 2014), we interpret the Ventura fault to be the dominant structure accommodating shortening and uplift of the VAA by fault-propagation folding. A decrease in the uplift rate of the anticline at 30 ka, as measured from terrace uplift rates (Rockwell et al., 1988), is consistent with a breakthrough of the Ventura fault 95 at that time, although the fault remains buried by a thin sedimentary cover and thus is blind (Hubbard et al., 2014). The blind Ventura thrust fault is a ~12 km long, east-west-striking, north-dipping reverse fault that is expressed at the surface by a monoclinal fold scarp through the city of Ventura (Ogle and Hacker, 1969; Sarna-Wojcicki et al., 1976; Yeats, 1982; Perry and Bryant, 2002). Though several different models have been proposed for the geometry and structure of the Ventura fault at depth (e.g. Yeats, 1982; Huftile and Yeats, 1995; Sarna-Wojcicki and Yerkes, 1982) results from a companion study (Hubbard et al., 2014) have been used by Hubbard et al. (2014) to interpret the Ventura fault as extending to seismogenic depths, indicating that it is a potential major seismic source. The Ventura fault is interpreted as a single planar surface that dips 50°±5° that has propagated through the VAA to the near-surface since 29.7 ka. Regionally, the Ventura fault acts as a transfer structure, accommodating significant north-south shortening as slip is transferred between the San Cayetano and Red Mountain/offshore fault system to the east and west, respectively. Whereas these distinct faults have separate surface traces, Hubbard et al. (2014) suggest that they are all merge below 7.5 km onto a regional detachment to form a nearly continuous fault surface. Previous slip rates on the Ventura fault have been calculated at 0.2-2.4 mm/yr (Peterson and Wesnousky, 1994; Perry and Bryant, 2002), however, based on our updated interpretation of the fault kinematics, Hubbard et al. (2014) have determined that the fault must be slipping at a rate of ~4.1-8.1 mm/yr for the last 30 ka. 96 4.3 Study area Located at the western end of the Ventura basin, the City of Ventura extends east-west along the base of the steep, south facing mountain front that bounds the city no the north. This mountain front is coincident with the forelimb of the VAA. The city itself is built on the intersection of Holocene floodplain deposits from the Santa Clara River and low-relief latest Pleistocene-Holocene alluvial fans deposited from over half a dozen rivers and creeks draining southward from the VAA. Most of these alluvial fans exhibit drainages that have been incised by ~1-6 meters, with the exception of the alluvial fan emanating from the north end of Day Road (Figure 5 and supplementary figure S1). We refer to this fan as the Arroyo Verde Fan after the name of the city park located within the source drainage. The topographic expression of the Ventura fault through the city is marked by a prominent, continuous south-facing scarp that extends roughly eastward for ~12 km from the eastern edge of the active channel of the Ventura River to where the mountain front takes a 1.5 km step to the north at the eastern end of the City of Ventura. At the eastern end of the scarp it appears that slip is transferred onto an en echelon segment of the greater fault system. To the east, this eastward continuation of the fault system, which is known as the southern San Cayetano fault, is interpreted as a major north-dipping blind thrust, with a south-dipping backthrust in the uppermost few kilometers that serves to transfer slip between the Ventura-Pitas Point fault to the west and the rapidly slipping eastern San Cayetano fault to the east (Hubbard et al., 2014). The scarp morphology of the Ventura fault varies markedly along strike. Within the city limits, the fault scarp is approximately 5 meters high and has a surface slope of up to 12°. Across 97 the active Arroyo Verde Fan, the scarp is ~6-6.5 meters high and has a gentler south-facing slope (~5°). Our efforts to document the earthquake history of this large, multi-segment reverse fault system focus on three sites within the city of Ventura (Figure 2) – from east to west, these are Evergreen/Hall Canyon, Day Road, and Brookshire Avenue. Each site comprises a fault- perpendicular study transect across the locus of active folding related to slip on the underlying blind Ventura fault. In addition to the high-resolution seismic reflection data acquired at each site, hollow-stem continuously cored borehole data and cone penetrometer tests (CPT) were collected at our primary transect (Day Road). Each of the three transects were taken across deposits of varying age – The Evergreen/Hall Canyon Road transect was acquired across the low-relief Pleistocene fan deposits and northward into the foothills of the VAA, which are underlain there by Pleistocene marine sandstone of the San Pedro Formation (Sarna-Wojcicki et al., 1976); the Brookshire Avenue transect was collected across the Holocene Harmon Canyon alluvial fan (5.7-15 ka; Sarna-Wojcicki et al., 1976; Clark et al., 1984); and the Day Road transect was taken across the active, unincised Holocene Arroyo Verde fan. The Day Road transect was selected as the preferred site for our borehole and CPT study due to its location on an active alluvial fan with the potential for continuous deposition; elsewhere along the fault, south-flowing drainages have incised into the alluvial fan surfaces, isolating them from active deposition (Figure 5 and Supplementary figure S1). The absence of incision into the Arroyo Verde Fan suggests that the surface was deposited more recently than those fans that are incised. This young fan deposition presents an ideal target for resolving the most recent slip history of the Ventura fault. 98 4.4 Data 4.4.1 High resolution seismic reflection During the summer of 2010, we collected high-resolution seismic reflection profiles across the locus of active folding above the blind Ventura fault, as inferred from analysis of deeper-penetration petroleum industry seismic reflection data, at three sites in Ventura - specifically, across the prominent fold scarp that has developed in response to recent slip on the underlying thrust ramp (Figure 2). These high-resolution data overlap with the uppermost parts of petroleum-industry seismic reflection data, and provide a near-continuous image of recent folding from several km depth to within 50-100 m of the surface (see Hubbard et al., 2014, for description of industry data). The three N-S transects, each between 1 and 2.5 km long, followed Brookshire Avenue, Day Road, and Evergreen/Hall Canyon across the Ventura fault (Figure 3). We used a trailer Minivibe borrowed from the University of Nevada, Las Vegas, as our seismic source, with geophone station spacing every four meters. Using a vibration sweep from 20 to 120 hz, we were able to image the upper 800 meters of sediment – deep enough to image the fault tip of the Ventura fault. The Evergreen/Hall Canyon high-resolution seismic reflection profile is 2.02-km-long, extending northward along Evergreen Drive from its intersection with Thompson Boulevard. At the north end of Evergreen Drive (shotpoint 270), the profile takes a 90° right bend towards the east and continues along Hall Canyon Road for an additional 1300 m into the VAA (Figures 2 and 3a). The meandering nature of this transect necessitated by access constraints led to clarity issues with the seismic profile. Reflections from the hills along the northern half of the profile also created imaging problems. Moreover, the deep water table in this area likely led to high signal attenuation and poor reflection returns. 99 The southern end of the 2.24-km-long Day Road high-resolution seismic reflection profile is 1.75 km south of the southern break in slope at the base of the hills formed by the VAA at the intersection of Lafayette Street and Bucknell Avenue. The southern section of the profile follows Bucknell Avenue northward to its intersection with Aurora Drive. At this point the profile traverses the incised drainage on the north side of Aurora Drive and continues northward along Day Road from its southernmost intersection with Telegraph Road. From the intersection of Day Road and Foothill Road (shotpoint 260), the profile extends an additional 600 m north into the VAA along the paved road through Arroyo Verde Park (Figures 2 and 3a). The Brookshire Avenue seismic reflection profile extends northward along Brookshire Avenue for 1.06 km from its intersection with Woodland Street and terminates at the north end where Brookshire Avenue intersects Kearny Street (Figures 2 and 3a). Due to the linearity of this transect, and by avoiding the hills to the north, this profile created the best seismic reflection profile with limited noise. At each of these three transects, our high-resolution seismic profiles revealed a section of south-dipping beds between two sections of sub-horizontal strata. Acoustic interference off of the mountains to the north of the break in slope created imaging issues for the two transects within the city of Ventura that continued north into the VAA (Evergreen/Hall Canyon and Day Road). The Brookshire Avenue profile provided us with the clearest seismic image to a depth of ~800 m. A well-defined, north-dipping active synclinal axial surface can be traced from the tipline of the fault at a depth of approximately 230 meters below sea level to the surface (supplementary figure S5). The south-dipping strata between the synclinal and anticlinal axial surfaces extends to the surface at a prominent, south-facing fold scarp lying approximately 500 100 meters south of the topographic range front. This scarp defines the surface expression of deformation associated with the most recent folding events on the underlying thrust ramp. 4.4.2 Borehole excavations As noted above, the absence of incision on the Arroyo Verde fan led us to choose the Day Road transect for more detailed analysis of the geometry of recent folding above the Ventura fault tipline. Accordingly, we acquired six, 15-cm diameter, 15- to 21-m-deep, continuously cored hollow-stem auger boreholes along the central section of the Day Road transect across the prominent fold scarp (Figure 4). Excavation of the boreholes along the same transect as the high- resolution seismic data acquisition provided overlap between the datasets. Moreover, the cores facilitated detailed observation of the subsurface structure and stratigraphy through correlations of the upper 25 meters. The continuously cored boreholes also allow us to document the ages of these youngest sediments (where dateable material is present) folded above the tipline of the Ventura fault. The core samples provided us with charcoal fragments for radiocarbon dating and sediment samples for luminescence dating. The continuous sampling method also allowed us to observe basic sediment characteristics, including grain size, sediment color, and degree of soil development. These sediment characteristics were used to identify and correlate the subsurface stratigraphy between the six boreholes. Supplementing the continuously cored boreholes, we acquired 13 Cone Penetration Tests (CPTs), which provided detailed measurements of grain size variations with depth. While the CPTs provide a faster method of observing the subsurface lithology, we lose the ability to date the material at any particular depth or determine the presence of any buried paleosols as no cuttings are brought to the surface. Nevertheless, the CPTs provide key data that allowed much more robust correlations of strata between boreholes. 101 In addition to the continuously cored boreholes and CPTs, we excavated two, 1.8-m- deep, 1 m x 1 m sampling pits on the hangingwall and footwall blocks of the fold scarp to verify that no post-MRE erosion or deposition had taken place. At each pit we collected sediment samples for luminescence dating, and logged the upper 1.8 m of sediment. 4.4.3 Stratigraphic observations The stratigraphic cross section for the Day Road profile extends a total of 368 meters, with the northernmost borehole located ~210 m north of the fold scarp (34.281901° N, 119.227480° E), and the southernmost borehole located ~150 m south of the fold scarp (34.278844° N, 119.227021° E; Figure 3a and 4). The fold scarp at the Day Road site lies at the north side of the intersection between Day Road and Loma Vista Road. The stratigraphy at Day Road is dominated by alternating silts and fine- to coarse-grained sands with several pronounced gravel layers. Several prominent sedimentary packages can be traced along the entire 368 m length of the borehole/CPT section. Specifically, the results from our borehole and CPT analysis can be generalized to show seven major stratigraphic packages. The uppermost 4 m of the section consists predominantly of fine-grained sands and silts (Units A and B). This is underlain by a sequence of sandy- to coarse-grained gravelly units (Unit C, Unit D and Unit E), which in turn overlie a prominent fine-grained silty interval (Unit G). Within the downthrown growth section of Unit C, Unit D is separated into three distinct layers labelled D1, D2 and D3. These three units appear to fan downslope and may represent onlapping of material onto a paleo-event scarp. The presence of gypsum in the upper 0.5 m of Unit C between boreholes DY-2B and DY- 3 together with prominent 0.5-3 cm detrital chips of what appear to be fire-baked clay, (likely from a brush or forest fire in the mountains north of Ventura) found between the depths of 6-10 m in boreholes DY-2, DY-2B, DY-2C and DY-3 (shown by letter B in cross section: Figure 7) 102 aided in the unit correlations. Unit G is well-defined deposited on top of a second sandy- to coarse-grained gravel unit (Unit H), which overlies Unit I, a fine-grained silt unit. 4.5 Age Control Age control for the Day Road transect is provided by 17 Infra-red luminescence (IR) samples and eight radiocarbon ages from small detrital charcoal fragments collected from the six boreholes and the two sampling pits. The single grain Post-IR 50 IRSL 225 method for feldspar dating recently developed by our colleague Ed Rhodes at UCLA was used to date our luminescence samples (Brown et al., in review). The Post-IR 50 IRSL 225 method is a two-step infra-red luminescence dating technique used to date feldspar grains (Buylaert et al., 2009; 2012; Thiel et al., 2011). After an initial preheat at 250°C for 60 seconds, the grains are stimulated with IR light for three seconds at 50°C, which is done to mimic the natural fading that occurs in feldspar over geologic timescales. Following the first IR stimulation, the grains are once again stimulated at 225°C for three seconds additional seconds to release the electrons in the deeper, stable traps (Buylaert et al., 2009; 2012; Thiel et al., 2011). The luminescence samples were collected in 15-cm brass cores placed inside the hollow-stem drill sampler as the cores were collected. Due to the light-sensitive nature of the brass cores, we were unable to determine the sediment characteristics (i.e. grain size and color) for the 15 cm that were collected for our luminescence samples. The sample cores were prepared for dating and processed at the UCLA luminescence laboratory. The luminescence ages revealed that the borehole transect spans the entire Holocene, with the youngest samples collected from the middle of Unit A at a depth of 1.08 m providing an age of 770 ± 90 years before present and the oldest sample from a depth of 103 18.21 m in borehole DY-1 yielding an age of 11720 ± 770 years before present. The complete geochronology age data are provided in Table 1. Of the 28 radiocarbon samples that were sent to the Keck Carbon Cycle AMS facility at the University of California, Irvine, only 11 samples yielded allowable ages. The remaining samples did not provide suitable ages because either the sample sizes were too small and/or no organic material were left after the standard acid-base-acid pre-treatment. Several of those samples that did provide ages have extremely large uncertainties due to the very small sample size (e.g.; DY-C12, DY-2C:CL-1, CL-2-BR-3, CL-4-BR-3, CL-5-BR-4). All the results in Table 1 have been corrected for isotopic fractionation according to the conventions of Stuiver and Polach (1977), with  13 C values measured on prepared graphite using the AMS spectrometer. The absence of any well-developed soils within the upper 20 m suggests a rapid rate of uplift consistent with the rapid slip rate inferred by Hubbard et al., (2014) for the Ventura fault. Although it has been determined that the soils in the Ventura basin generally show minimal lateral variations – a common problem with soil stratigraphic correlations (Rockwell, 1983; Rockwell et al., 1985) – the two prominent deep red/brown colored soils that were identified at the Briggs Road site were not seen at the Day Road transect. In coastal environments along the Ventura basin, soil development has been recorded as occurring at much faster rates when compared to other soils of equivalent age in California’s inland areas (Rockwell et al., 1985). The influence of sea fog, which contributes higher sodium ions (a clay deflocculant) to the soil in these coastal areas, promotes favorable soil development conditions (Rockwell, 1983). Despite the coastal setting of the Day Road site, there is little evidence for the development of significant soils within the generally pale-colored sediments, suggesting that sediment accumulation during 104 alluvial fan aggradation has been relatively continuous at this site, an inference that is consistent with our geochronologic results showing a relatively young section. These age data reveal that the Arroyo Verde alluvial fan is Holocene in age and has been actively depositing sediment within the past few hundred years. The luminescence and radiocarbon ages provide evidence for relatively steady sediment accumulation rates throughout the Holocene. The similarity in ages between the youngest samples collected from sampling pits DR-13 (770 ± 90 Cal. Yr. BP) and DR-14 (1335–1415 Cal. Yr. BP) reveals that the shallowest unit that is correlative across the fold at Day Road is younger than ~1.5 ka. The deepest identifiable laterally continuous sedimentary unit is Unit I, which is dated at ~9ka. 4.6 Discussion At Day Road, our stratigraphic correlations based on the borehole and CPT data reveal two episodes of uplift with the possibility of a third event. Evidence for these events are described below. 4.6.1 Most recent event (Event 1) Several observations from the two shallowest units (Units A and B) in our cross-section detail evidence for recent folding of sediments due to the most recent event on the Ventura fault at Day Road. First, the stratigraphy of the uppermost 4 m (Units A and B in Figure 7) tracks the ground surface of the fold scarp, indicating that: (1) this stratigraphic interval, which was deposited at the gently south-dipping slope of the Day Road alluvial fan (based on its laterally continuous thickness), has been folded subsequent to deposition; (2) the fold scarp has not yet been buried by young Day Road alluvial strata following the most recent event(s); and (3) the ~6.0-6.5 m height of the fold scarp (measured vertically from the northward and southward 105 projections of the average far-field ground surface slope) records at least 6.0-6.5 m of uplift, which occurred during the most recent large-magnitude earthquake (or earthquakes) on the Ventura fault. The similar ages of the youngest samples collected from the two sample pits above and below the scarp (770 ± 90 Cal. Yr. BP from DR-13 and 1335–1415 Cal. Yr. BP from DR- 14) suggest minimal post-MRE erosion of the hangingwall or post-MRE deposition on the footwall. Thus, the height of the current topographic scarp records uplift during the most recent events on the Ventura fault. These ages from the single charcoal and luminescence ages from the two sample pits indicate that the MRE occurred within the past 1400 years (Table 1). Specifically, charcoal sample DY-14:CL-01 [calibrated age (95.4% confidence limits) of 437 to 615 AD, with almost all of the possible ages (92.5%) between 535-615 AD] was collected from a depth of ~1.26 m from sample pit DR-14 and luminescence sample DR13-02 from ~1.08 m depth in sample pit DR-13. Both samples were collected from the same stratigraphic interval, Unit A. Charcoal sample DY-14:CL-01 was collected from a well-bedded silt layer with sharply defined boundaries and no evidence of bioturbation (Supplementary Figure S3). Thus, the youngest folded strata are less than ~1300-1400 years old, and possibly much younger, since the approximately one meter of sediment atop the radiocarbon and luminescence sample depths in the two sample pits required some unknown amount of time to be deposited. These observation indicate that the ~6-6.5-m-high fold scarp has developed since that time. Because no strata have been deposited across the scarp since the MRE, we cannot determine growth stratal geometries that might constrain whether this large uplift occurred in a single earthquake or more than one event. 106 4.6.2 Event 2 Evidence for an older event (or events) at Day Road comes from a second episode of uplift and folding that is marked by an interval of stratigraphic growth deeper in the section where Unit C thickens southward across the fold scarp by ~4.5 m (Figure 7). We interpret this sedimentary growth as evidence of deposition against a now-buried paleo-fold scarp that developed during the penultimate folding event. The event horizon for the this period of fold growth is located in the lower part of the growth interval at ~ 8.2 m depth in borehole DY-4, above Unit D3, which is folded parallel to underlying strata at the scarp and has been truncated on the upthrown side by erosion of the hangingwall (Figure 7). Thickening of this unit by ~4.5 m on the downthrown side of the scarp indicates that at least this much uplift occurred during fold growth; this is a minimum measurement of the vertical component of fold growth in this event because we cannot completely quantify the erosion of Unit C that may have occurred on the upthrown side of the fold scarp. Significant (up to 1.3 m) erosion is suggested by the buttressing of Unit D2 (suggesting that it had to be deposited onlapping the paleo-scarp) and the consistent thickness of Units D3 and the lowest parts of Unit C (suggesting that these strata were deposited at the gently sloping pre-earthquake gradient). In addition to the absence of sedimentary growth in the upper 4 m of the Day Road section, the ~ 7-m-thick sequence of strata observed below the prominent stratigraphic growth interval does not change thickness across the fold, indicating that those units were deposited during a period of structural quiescence. 4.6.3 Possible 3 rd Event Several lines of evidence suggest the possibility of a third event documented by the buried paleo-scarp attributed solely to Event 2 described above. Specifically, stratigraphic 107 correlations show a distinct change in bed dip between Unit D2 and Unit D3 within the growth stratigraphic section (7.62 m to 9.75 m in DY-3). This change in bed dip may be due to an additional event and the processes of limb rotation (Novoa et al., 2000) just prior to the deposition of Unit D2, causing the beds beneath the event horizon at 7.62 m to 9.75 m in DY-3 depth to have distinctly steeper dips than those above. Alternatively, fanning of material off the paleo-fold scarp may have produced the change in bed dip. These observations are not definitive, however, and folding and subsequent deposition of the growth section could all be due to a single earthquake on the Ventura fault. 4.6.5 Uplift measurements and fault displacement estimates In order to determine vertical uplift in each of the past two earthquakes, we measure the total minimum scarp height for each event. During the MRE, the present day topographic scarp developed sometime after deposition of Unit A. Very little, if not zero, sedimentary growth has occurred since the deposition of this shallowest unit as shown by the similar ages of samples collected at ~1 m depth from our sample pits above and below the scarp. We therefore use the top of Unit A at the current ground surface is used as the restoration horizon for the MRE. The interval of sedimentary growth following the penultimate event is separated from the MRE by units A and B, which do not change thickness across the transect, indicating that these units were deposited at the gently south-dipping pre-earthquake fan gradient. Measuring uplift in the penultimate event is slightly more complicated due the truncation of Unit D on the upthrown side of the scarp. With material having been eroded off the hangingwall, any uplift measurements recorded will be minima. Based on sedimentary growth occurring from the top of Unit D3 to the base of Unit B, we calculate total minimum uplift in the penultimate event at ~4.5 m. 108 To convert the scarp heights to thrust displacements we must divide by the dip of the fault. Along fault slip estimates for Events 1 and 2 can be calculated by dividing our uplift measurements by the sine of the 50°±5° north dipping Ventura fault (Hubbard et al., 2014). For the MRE, 6 meters of uplift yields a fault displacement of 7.32-8.49 m (the range in displacement is due to the uncertainty in fault dip angle). For the penultimate event, the minimum 4.5 m of uplift yields 5.49-6.36 m of thrust slip (Table 2), although we reiterate that this is a minimum because it does not account for possible erosion of the hangingwall. For example, increasing the height of the paleo-fold scarp by 1 m would result in an estimate of total thrust displacement in Event 2 of 6.71-7.78-m. We can estimate conservative paleo-magnitudes for the two most recent events on the Ventura fault by utilizing published empirical equations that relate earthquake magnitude, fault area, and average displacement. These equations incorporate the global regression data of Wells and Coppersmith (1994). Though the calculated displacement at the Day Road site is only a single measurement along the ~60-km-long fault Ventura-Pitas Point fault that likely exhibits some degree of lateral variability in displacements, similar uplift values at Pitas Point (Rockwell et al., 2011) confirm our 4.5-6 m uplift estimate as a suitable assumption for the average uplift during a system-wide rupture of the Ventura-Pitas points fault system. Results from three different empirical equations relating earthquake magnitude, and average displacement are recorded in Table 2. Using the simplifying assumption that the entire Ventura fault slipped with an average displacement of 7.32-8.49 m during the MRE (displacement range due to uncertainty in fault dip), we calculate a paleoearthquake magnitude of M w 7.64-7.69. Using the thrust-fault- only regression of Wells and Coppersmith (1994), the calculated paleoearthquake magnitude decreases to M w 6.75-6.76. Applying these same regressions for our penultimate event gives us a 109 paleoearthquake magnitude of M w 7.54-7.59 when using the all-slip-type displacement regression and M w 6.74 when using the thrust-fault-only regression. Using the slightly modified empirical equations of Biasi and Weldon (2006), we calculate estimated paleoearthquake magnitudes of M w 7.91-7.98 for the MRE and M w 7.76-7.84 for the penultimate event. These paleo-event magnitudes are similar to those estimated by Hubbard et al. (2014) using slip values based on the uplifted marine terraces measured at Pitas Point. Calculating the potential earthquake magnitudes based on rupture area to magnitude regressions (rather than slip to magnitude) allows us to speculate on the potential maximum earthquake magnitudes for earthquakes within the Ventura basin. Using the empirical relationships discussed in Wells and Coppersmith (1994) and Hanks and Bakun (2002; 2008), we have estimated rupture magnitudes for several multi-segment rupture scenarios. These paleoearthquake magnitudes are recorded in Table 3. Rupture of just the Ventura fault can produce an earthquake of M w 6.07-6.21. With the inclusion of the Pitas Point fault, fault rupture area increases significantly from 122 km 2 to 446.2 km 2 and the magnitude estimates increase to M w 6.63-6.71. Including the downdip blind thrust portion of the Ventura fault significantly increases the rupture area and thus the hypothetical earthquake magnitude. A system wide rupture involving the Ventura, Pitas Point, San Cayetano and the blind thrust ramp has the potential to produce a M w 7.28-7.45 earthquake. Though these magnitudes are significant when compared to the most recent earthquakes in southern California (e.g. M w 6.6, 1971 San Fernando earthquake, M w 5.9, 1987 Whittier Narrows earthquake; M w 6.7, 1994 Northridge earthquake), if displacements recorded by the uplifted terraces at Pitas Point (Rockwell et al., 2011) are used to calculate magnitude estimates, the 10 m uplift events suggest an earthquake magnitude up to M w 8.1 (Hubbard et al., 2014). 110 One additional complication in determining the paleomagnitude estimates for a rupture on the Ventura fault is the position of the Day Road transect along strike. The eastern end of the Ventura fault forms a “soft” boundary with the southern San Cayetano fault and thus may not record the average displacement of a rupture along the entire length of the fault. 4.6.6 Implications for seismic hazard in southern California From a seismic hazard standpoint, one of the most critical questions about the faults of the Ventura basin concerns the size of future earthquakes that they might produce. The seismogenic potential of the Ventura and southern San Cayetano fault has been debated for some time. The persistent disagreement on this matter stems from the uncertainty of the fault geometry at depth. The new structural model from our companion study (Hubbard et al., 2014) elucidates the fault geometry and validates the potential seismic hazard that these two faults pose. The Ventura fault forms the middle section of a >200-km-long, east-west belt of large, discrete yet interconnected reverse faults that extends across the western Transverse Ranges. Although each individual fault represents a major seismic source in its own right, a system-wide multi-segment rupture involving the Ventura fault together with other major faults of the western Transverse Ranges could cause catastrophic damage to a very large area encompassing the densely urbanized areas of the Ventura and Los Angeles basins. One of the largest of these potential multi-fault earthquakes involves rupture of the rapidly slipping eastern San Cayetano fault westward via the blind, southern San Cayetano fault, onto the blind Ventura thrust fault together with correlative faults to the west (e.g., Lower Pitas Point thrust, Red Mountain fault; Figure 1). This could produce a 75- to 100-km-long rupture plane on multiple gently dipping thrust faults. The resulting fault-plane area could be as much as several thousand square kilometers – on par with the rupture area of the great 1857 Fort Tejon and 1906 San Francisco earthquakes on the 111 San Andreas fault. Alternatively, the Ventura fault could rupture together with the faults to the east, however, the fault mechanics that connect the San Cayetano fault with the high slip-rate north dipping fault to the east, the Santa Susana fault, is complicated and such a scenario is less likely than a multi-segment rupture to the west. No damaging earthquakes have occurred on any of the faults surrounding the Ventura basin since records began over 200 years ago suggesting that recurrence intervals for these faults are quite long and that these faults tend to rupture in larger, multi-segment events. The December 21, 1812 earthquake however, has been speculated to have occurred on the San Cayetano fault (Dolan and Rockwell, 2001) and on one of the main offshore faults in the Santa Barbara channel (Toppozada et al., 1981). Furthermore, the threat posed by a rupture along the Ventura fault is compounded by the location of the city of Ventura, which sits on a deep sedimentary basin and has the potential for significant amplified ground shaking during large earthquakes (Field, 2000; 2001). These various potential rupture scenarios for the faults of the Ventura basin must be considered in all future seismic hazard assessments of the region. Assessing the short- and long- term seismic risk posed by the Ventura fault to a significant portion of southern California’s population hinges on our understanding of the slip behavior of the Ventura fault and the likelihood of a near-future rupture. Though additional slip-event data for the Ventura fault is limited, what data are available suggests that very large earthquakes may have indeed occurred along the faults bounding the northern Ventura basin. Specifically, Rockwell (2011) observed four paleo-shore faces along the Ventura coastline at Pitas Point each uplifted by 5-10 m, which he concluded as evidence for the 4 most recent events; ~800-1,000 years ago for the MRE; ~1,900 years ago for the penultimate event; ~3,500 years ago for Event 3; ~5,000 years ago for Event 4. Uplift of this magnitude 112 would require a very large (M w 7.7-8.1) earthquake, likely rupturing a fault area equivalent to the entire Ventura-Pitas Point fault combined with the Red Mountain and San Cayetano faults (Hubbard et al., 2014). The 3D fault model of Hubbard et al (2014) illustrates the connectivity of the Ventura, San Cayetano and Red Mountain faults, and demonstrates the permissibility of large-magnitude, multi-segment ruptures in the western Transverse Ranges. The similarity in age between the MRE and the penultimate event on the Ventura fault (this study), and the MRE and events 3 and 4 documented at Pitas Point based on uplifted marine terraces (Rockwell, 2011) suggests the possibility that the two (or three) events that we observe at Day Road may be the same events recorded at Pitas Point and suggests that sometimes the whole Ventura/Pitas Point fault (and potentially other faults) ruptures in its entirety. Interestingly, the penultimate event observed at Pitas Point (~1.9 ka) does not appear to have ruptured the Ventura fault at Day Road suggesting that in between these fault/system wide ruptures, the fault may rupture partial segments in smaller events. Though such bimodal rupture behavior is not unusual (e.g. Zielke and Arrowsmith, 2008; De Pascale et al., 2014), the similarity in uplift height (and presumably magnitude) during each of the past 4 events at Pitas Point would suggest that each event presumably ruptured a similar area and it is unusual for only Event 2 to not have made it as far east as Day Road. An unlikely yet possible alternative scenario may involve the rupture of both the Pitas Point fault and the San Cayetano fault whereby slip is transferred along the blind thrust and southern San Cayetano fault, bypassing the shallower Ventura fault. 113 4.7 Briggs Road 4.7.1 Regional geology of the central Ventura basin The San Cayetano fault proper is the major fault system along the north edge of the central-east end of the basin. The San Cayetano fault consists of two distinct sub-linear segments, separated by a 4-km-wide, right-stepping lateral ramp near the city of Fillmore (Figure 1). Whereas the eastern, Modelo lobe segment extends to the surface as a range front thrust fault east of the town of Fillmore, the western strand differs markedly in its geomorphic expression and extends to the surface on the side of the mountain to the north of the valley ~100-500 m north of the topographic mountain front. The location of this fault indicates that the western strand of the San Cayetano fault lies entirely within the hanging wall of another thrust fault that lies deeper in the section, beneath the northern edge of the Ventura basin (e.g., Huftile and Yeats, 1995a). This secondary thrust fault, which was first documented in Hubbard et al. (2014), is referred to as the southern San Cayetano fault. The blind, southern San Cayetano fault segment extends upward from the Sisar Decollement at depth of ~ 7 km and is interpreted as either 1) a gently north dipping blind thrust in the footwall of the San Cayetano fault, or 2) an eastward extension of the Lion backthrust into the footwall of the San Cayetano fault. This type of active wedge folding has been successfully imaged by high-resolution seismic data along the Coyote segment of the Puente Hills blind thrust in Los Angeles (Pratt et al., 2002; Shaw et al., 2002). The slip rate for the San Cayetano fault proper varies from east to west, as slip is gradually transferred onto the blind southern San Cayetano fault along the northern edge of the Ventura basin. The Modelo lobe segment of the San Cayetano fault exhibits an exceptionally rapid slip rate of at least 7.5 mm/yr, and possibly as fast as ~10 mm/yr – the fastest rate documented for any fault in the Transverse Ranges (Huftile and Yeats, 1995a; Dolan and 114 Rockwell, 2001; Nicholson et al., 2007) and probably any known reverse fault in California south of the Cascadia subduction zone (Dolan and Rockwell, 2001). This high slip rate indicates that the San Cayetano fault produces either very large or very frequent earthquakes, or both. This was confirmed in a trench study across the San Cayetano fault by Dolan and Rockwell (2001) who observed a very large (>4.3 m) surface rupturing event within the past ~350 years. The slip rate of the western part of the fault, which is slower than the eastern, Modelo lobe segment, diminishes westward to zero just east of Ojai (Rockwell et al., 1984; Rockwell, 1988; Dolan and Rockwell, 2001) as more slip is transferred onto the blind southern San Cayetano fault. 4.7.2 Briggs Road study area In addition to our primary study site within the city of Ventura, we also focused on the central section of the Ventura basin, where the fastest north-south convergence rates are recorded (Figure 2). Our study site at Briggs Road ~ 2 km west of Santa Paula was aimed at documenting the slip history of the blind southern San Cayetano. The central section of the Ventura basin is similar in geomorphology to the western end with the basin dominated by south-flowing Holocene alluvial fans and stream terrace deposits associated with the Santa Clara River. Here, north-south compression is accommodated on the north-dipping San Cayetano and the blind southern San Cayetano faults. The southern San Cayetano fault is expressed at the surface by a gently-dipping, ~230 m-tall, south-facing escarpment that trends sub-parallel to the northern range front. The Briggs Road transect extends N-S across an active, latest Holocene fan that has been deposited on top of the low-gradient south-dipping bajada (Tan et al., 2004). 4.7.3 High-resolution seismic reflection at Briggs Road Complementary to our three transects across the Ventura fault, we acquired an additional high-resolution seismic profile perpendicular to the blind southern San Cayetano fault along 115 Briggs Road. The 2.41-km-long Briggs Road profile extended along a linear trajectory, northward from the intersection of Briggs Road and Telegraph Road and terminated in an orchard on the south-facing range front 0.6 km north of the intersection of Briggs Road and Foothill Road. At the intersection of Briggs Road with Foothill Road, the profile takes a 90° bend and continues east along Foothill Road for 90 m (between shotpoints 933 and 950) before continuing north within the orchard for an additional 600 m. This profile, which was collected across the south-facing fold scarp that forms at the base of the topographic mountain front along the northern edge of the Ventura basin, provides an exceptionally clear image of recent folding above the blind, southern San Cayetano ramp. The data at this transect provide evidence for a well-defined, north-dipping synclinal axial surface that separates sub-horizontal reflectors beneath the basin from steeply south-dipping reflectors within the mountains to the north. This synclinal axial surface projects to the surface at the intersection of Briggs Road and Foothill Road (Figure 8). 4.7.4 Borehole excavations at Briggs Road At our Briggs Road site, we drilled seven six-inch diameter, 18- to 23-m-deep, continuously cored and sampled hollow stem auger boreholes. Three of the boreholes (BR-5, BR-6, BR-7) were drilled north of the projected synclinal axial surface determined from our high-resolution seismic reflection data (Figures 8 and 9). The varying borehole termination depths were based on refusal of the drill rig within very coarse-grained gravel layers. Both luminescence and radiocarbon samples were taken from two of the cores (BR-3 and BR-4) to date the subsurface stratigraphy. In addition to our boreholes, we acquired six CPTs to provide us with sediment characterization data between the borehole locations. As with the Day Road transect, these borehole and CPT data allow us to construct subsurface stratigraphic correlations 116 and identify areas of stratigraphic growth, which would indicate discrete uplift events on the underlying blind Southern San Cayetano fault. At the Briggs Road transect, we drilled a hollow stem auger borehole (BR-1) within 2 meters of a CPT site (CPT-3) to serve as a calibration borehole. This allowed us to compare the physical results of the cores that we were collecting with the interpreted CPT logs. Although there were minor differences between the CPT and borehole that can be attributed to small lateral changes in stream/alluvial fan morphology, both the borehole and adjacent CPT generally showed similarities in coarse- and fine-grained intervals. 4.7.5 The stratigraphic cross section at Briggs Road The Briggs Road site is a 2.41km long transect that trends north-north-west, located ~14 km east of the city of Ventura, and ~2 km west of Santa Paula, CA. Both the borehole and CPT data allow us to determine grain size variations with depth and thus correlate the subsurface stratigraphy along the entire profile. Analysis of our borehole data confirms our high-resolution seismic interpretation of a synclinal axial surface that projects to the surface at the base of slope between the hills to the north and the gently south-sloping alluvial apron to the south. The stratigraphy of the Briggs Road profile shows interfingering between coarser-grained alluvial fan deposits and fine grained silts and clay flood plain deposits from the Santa Clara River to the south (Figure 9). Specifically, each of the boreholes show alternating coarse-grained sands and gravels, with fine-grained silts. The only exception being the southernmost borehole, CPT-1, which exhibits only fine-grained silts and clays that we interpret as overbank flood deposits from the west-flowing Santa Clara River in the basin. Analysis of the subsurface stratigraphy to the south of the mountain front also indicates two prominent paleosol horizons (highlighted in grey in cross-section, Figure 9) as well as several distinct gravel units that can be tracked up the 117 south-facing hill slope north of Foothill Road. The seismic reflections and borehole-CPT data indicate that this south-facing slope represents the dip-slope of the folded strata in the forelimb of the fold. Stratigraphic growth at the Briggs Road transect is confined to two intervals – between the ground surface and the prominent soil horizon at a depth of 9 m in borehole BR-4 (15 m in borehole BR-3 and 18.3 m in borehole BR-2), and above a coarse-grained sandy layer at a depth of 6 m in borehole BR-2 (10.7 m in borehole BR-1). These two growth intervals likely correspond to two separate uplift events caused by slip on the southern San Cayetano fault. 4.7.6 Age control at Briggs Road Determining sediment ages at the Briggs Road transect proved more challenging than at Day Road. Analysis of the borehole cores revealed two prominent, well-developed soils within the upper 23 meters of the cross-section. The top 2 meters of boreholes BR-1 through BR-7 exposed a well-developed A and B horizon. Additionally, a ~9 meter thick buried paleosol that appears Holocene-latest Pleistocene in age was observed in boreholes BR-2 through BR-7. From four of the boreholes, we were also able to collect 14 charcoal samples. Of the 14 charcoal samples that were collected, only three were suitable for dating following the preparation of the samples; most of the samples were too small and lacked sufficient organic material after the standard acid-base-acid treatment. The three samples that did produce a measureable age provided ages between 28587–69704 Cal. Yr. BP (this upper age limit exceeds the datable range for 14 C), however they proved somewhat problematic as they show an inversion (Figure 9). To supplement the 14 C ages, we collected two luminescence samples from borehole BR-6 (18.05 m and 18.21 m depths). These samples were run with the conventional multiple grain single aliquot regenerative (SAR) method as opposed to the single grain method used for the Day Road 118 samples. Both samples appeared to be poorly zeroed and as a consequence their ages are extremely over-estimated (in the range of 80-100 ka). Both the 14 C and luminescence samples failed to provide well constrained age estimates for any of the units in the Briggs Road transect, but as noted above, the degree of soil development indicates a likely latest Pleistocene age fan. The extremely old ages of the three radiocarbon samples suggests that the charcoal samples have been removed from older strata upstream. 4.7.7 Event at Briggs Road The slight observation of stratigraphic growth above the prominent buried paleosol indicates the presence of an event horizon between Unit B and Unit C (Figure 9). However, the lack of correlative units between many of the boreholes and CPTs together with unconfident age control does not allow us to constrain the event(s). 4.8 Conclusions Results from newly acquired high-resolution seismic reflection data, borehole cores, CPTs and 14 C and luminescence geochronology, reveal evidence of two large magnitude earthquakes on the Ventura fault during the past 5 ka. Specifically, the borehole-CPT data reveal a discrete stratigraphic interval at 6.09 m to 10.36 m depth in borehole DY-3 exhibiting southward thickening strata that is bounded by intervals in which sediments do not change thickness across the transect. These zero-growth intervals are interpreted as periods of structural quiescence where deposition occurred at the regional, gently (~2.5°) south-dipping fan gradient. The growth interval occurred after the formation of the now buried paleo-scarp during the penultimate event, followed by subsequent restoration of the gently south-dipping stream gradient. This package of growth strata, together with the current topographic scarp, provide 119 evidence for the two most recent ruptures on the underlying Ventura fault. These new borehole data from Day Road, together with results from earlier paleoseismologic studies to the west along the same fault system, illuminate the potential and likely possibility of system wide ruptures along the several individual faults of the western Transverse Ranges. Rupture of the Ventura fault together with its offshore extension, the Pitas Point fault with average displacement similar to those observed at Day Road would produce an earthquake of M w 6.63-6.71. A system wide rupture involving the Ventura/Pitas Point fault together with the San Cayetano fault and the adjoining blind thrust ramp could produce an earthquake of M w 7.28-7.45. The likelihood of similar large magnitude earthquakes in the future poses a significant seismic hazard to both the Ventura and Los Angeles basins. Ground motion amplification and possible liquefaction due to the deep sedimentary basin would significantly augment damage to the area. In addition, the offshore Pitas Point fault poses a tsunami hazard that would be devastating to many of the coastal communities including Santa Barbara and Ventura. Though the recurrence interval for the events documented at our Day Road site are an order of magnitude less frequent than those on the San Andreas fault system, with inter-event times measurable in thousands of years rather than hundreds, it is critical that the activity and seismogenic potential of the Ventura fault together with other faults in the western Transverse Ranges be properly considered in future regional seismic hazards assessment. 4.9 Figure Captions Figure 1. Location map showing the major faults and fold structures in the western Transverse Ranges. The darker shaded region outlines the extent of the Ventura basin. Selected cities are identified with green circles. The Pitas Point fault is the offshore continuation of the Ventura 120 fault. Seismic reflection profiles from left to right: industry line VB1, Evergreen/Hall Canyon, Day Road, Brookshire Ave. The southern San Cayetano fault has been interpreted to having two possible geometries (both shown with dashed lines). Hubbard et al., (2014) suggest that the southern San Cayetano fault is either a south dipping, eastward continuation of the Lion fault, or a north dipping, blind thrust ramp. Black inset box shows location of geologic map (Supplementary figure S1). Figure modified from Hubbard et al., 2014. Figure 2. Location map of central and western Ventura basin showing location of four high- resolution seismic reflection profiles and major roads in the Ventura area. GoogleEarth satellite image used as base image. Figure 3. (a) location map showing high-resolution seismic reflection transects through the city of Ventura. (b) location of Briggs Road high-resolution seismic reflection profile transect in the central Ventura basin. Figure 4. East facing perspective view of high-resolution seismic reflection profile (red line), and continuously cored borehole (yellow ovals) and CPT (green ovals) locations along the Day Road transect using a GoogleEarth base image with 3x vertical exaggeration. Orange swath highlights south-facing scarp of the Ventura fault. Red squares show locations of two sampling pits used to constrain the age of the most recent earthquake on the Ventura fault at this location. Figure 5. North facing oblique aerial view of high-resolution seismic reflection profile (red line), and continuously cored borehole (yellow ovals) and CPT (green ovals) locations along the Day 121 Road transect using a GoogleEarth base image with 3x vertical exaggeration. The prominent east-west trending fold scarp associated with the underlying Ventura fault is shown with the orange swath. Extent of the active Holocene Arroyo Verde alluvial fan is highlighted by the blue polygon. Figure 6. High-resolution seismic reflection profile at Day Road. Black dashed line shows projected synclinal axial surface associated with the underlying blind Ventura fault. Upper image shows local topography (5x vertical exaggeration) and locations of continuously cored boreholes (green) and CPTs (pink). Table 1. Radiocarbon and luminescence ages and calibrated, calendric dates of samples from the Briggs Road and Day Road transects. Figure 7. Cross section of the Day Road transect showing major stratigraphic units. Black vertical lines are continuously cored boreholes and red vertical lines are CPT data. Green horizontal line is the present day topography (3x vertical exaggeration). Colors denote different sedimentary units. Continuously cored boreholes include color-coded sediment grain size on left (blue [clay] to green [silt] to yellow [sand] to dark brown [gravel]) and soil color on the right. CPT logs show grain size with the same color scale as the boreholes. Red vertical arrows on the right side show regions of stratigraphic growth indicative of discrete uplift events. Green vertical arrows show no growth intervals. Black horizontal lines along the top of the profile show the far field topographic slope of the alluvial fan. Yellow stars indicate locations of charcoal samples and pink hexagons show locations of luminescence samples. 122 Table 2. Uplift, along fault displacement, age limits, and estimated moment magnitude (M w ) for the two paleoearthquakes on the Ventura fault from Day Road borehole and CPT results. Table 3. Earthquake magnitude estimates based on rupture area to magnitude regressions. Figure 8. High-resolution seismic profile at Briggs Road across the active fold limb and synclinal axial surface associated with the underlying blind southern San Cayetano fault. Upper image shows local topography (3x vertical exaggeration) and locations of continuously cored borehole (green) and CPTs (red). Yellow box in second image shows region of the cross-section (Figure 9). Figure 9. Cross section of the Briggs Road transect showing major stratigraphic units. Black vertical lines are continuously cored boreholes and red vertical lines are CPT data. Green horizontal line is the present day topography (5x vertical exaggeration). Colors denote different sedimentary units. Continuously cored boreholes include color-coded sediment grain size on left (blue [clay] to green [silt] to yellow [sand] to dark brown [gravel]) and soil color on the right. CPT logs show grain size with the same color scale as the boreholes. Yellow stars indicate locations of charcoal ages. Data Repository Figure Captions 123 Supplementary figure S1. Geologic map of the Ventura area showing fold scarp associated with the underlying Ventura fault (red polygon), locations of 2D high-resolution seismic reflection profiles (blue lines) and borehole/CPT locations (green and yellow circles). The Day Road transect is the only line that transcends a late Holocene active alluvial fan. The black dotted overlay shows the built up area of the city of Ventura. Map modified from Sarna-Wojcicki et al., (1976). Supplementary figure S2. East wall of sampling pit DR-13. This pit is located on the downthrown side of the Ventura fault along the Day Road transect. This sampling pit was excavated adjacent to CPT-10. Locations of five luminescence samples are shown with yellow circles. Red lines show contacts between discrete stratigraphic units. Upper 1.5 feet of material is non-native fill. Supplementary figure S3. (a) East wall of sampling pit DR-14. This pit is located on the upthrown side of the Ventura fault along the Day Road transect. This sampling pit was excavated 20 meters north of CPT-3. Locations of three luminescence samples are shown with yellow circles. Orange circle highlights location of charcoal sample DR14-CL01 from a depth of 126cm. Oblique black box shows projection of image b. Red line marks discrete contact between silty sand unit and the darker clay rich soil horizon below. (b) close up of sample locations. Orange circle shows location of charcoal sample DR14-CL01. The sediment surrounding the charcoal sample shows no signs of bioturbation. 124 Supplementary figure S4. High-resolution seismic reflection profile at Evergreen/Hall Canyon. Upper image shows local topography of the transect, middle image shows position of shotpoints with profile transect in map view. Supplementary figure S5. High-resolution seismic reflection profile at Brookshire Avenue. Upper image shows local topography. Supplementary figure S6. Cross section of the Day Road transect showing major stratigraphic units. Individual borehole and CPT logs are not shown. Black vertical lines are locations of continuously cored boreholes and red vertical lines are locations of CPT data. Green horizontal line is the present day topography (3x vertical exaggeration). Colors denote different sedimentary units. Red vertical arrows on the right side show regions of stratigraphic growth indicative of discrete uplift events. Green vertical arrows show no growth intervals. Black horizontal lines along the top of the profile show the far field topographic slope of the alluvial fan. 125 CHAPTER 5: Conclusions 5.1 Summary The three studies documented here employ a multi-disciplinary methodology, utilizing light detection and ranging (LiDAR) digital topographic data, luminescence (OSL and IRSL) and 14 C radiocarbon geochronology, and paleoseismic trenching, to calculate slip rates and determine the timing of paleo-earthquakes along the Garlock and Panamint Valley faults respectively. In addition, I combine both high-resolution seismic reflection data, together with borehole excavations and cone penetration testing (CPT) to constrain the timing of previous slip events on the blind Ventura and southern San Cayetano faults. The results from these three studies allow us to better understand the complex behavior of plate boundary faults and fault systems. Specifically, they give us a better understanding of patterns of strain accumulation and release, seismic clustering along the faults of the eastern California shear zone, and the increasing likelihood of multi-segment ruptures in the western Transverse Ranges over millennial timescales. Each of these individual results illustrates the temporal and spatial variability in earthquake behavior over millennial timescales. The following is a summary of the results discussed in each of the three previous chapters. 5.2 Patterns of seismicity along the eastern California shear zone north of the Garlock fault Two paleoseismologic trenches excavated across the southern Panamint Valley fault document three surface ruptures during the past 3600 years, with an additional event occurring at some time before 4.1 ka (Event 1: 328 to 485 cal. yr. BP; Event 2: 910 to 2553 cal. yr. BP; Event 126 3: 3271 to 3549 cal. yr. BP; Event 4: >4094 cal. yr. BP). The timing of these events reveal the complex patterns of earthquake occurrence along the major faults of the eastern California shear zone north and south of the Garlock fault. The timing of events along the Panamint Valley fault suggest the intriguing possibility that the major faults of the ECSZ north of the Garlock fault may rupture together during relatively brief clusters that occur sometimes independently of clusters on ECSZ Mojave faults to the south of the Garlock fault. Such observations have obvious implications for seismic hazard in southern California. Furthermore, coulomb failure function modeling suggests that an earthquake on the Panamint Valley fault can strongly influence future earthquakes on the central and western sections of the Garlock fault if that fault is close to failure. Similarly, an earthquake on the central and western Garlock fault increases ΔCFF stresses along the Mojave section of the San Andreas fault. These modeling results illuminate the seismic hazard a fault several hundred kilometers away can have on the major metropolitan regions of southern California and these considerations need to be taken into account in future seismic hazard assessments. 5.3 Temporal variations in slip rate along the Garlock fault The constancy of strain accumulation and release along the Garlock fault has been studied through the integration of LiDAR topographic data and field based mapping together with new advances in single grain luminescence geochronology. Small scale geomorphic offsets have allowed us to calculate a late Holocene slip rate for the central Garlock fault that is significantly faster than the full Holocene/latest Pleistocene rate, and an order of magnitude faster than the current rate of elastic strain accumulation determined from geodesy. This highly documented discrepancy illustrates an apparent extreme variability in slip rate during the past 127 ~10 ka. These results from our slip rate study at Christmas Canyon West illustrate the very complicated behavior of strain accumulation and release along the Garlock fault. The current strain transient indicates that the Garlock fault must either experience two contrasting phases of strain accumulation and release, or our current understanding of the geodetic data is incorrect and strain is stored at a constant rate across the Garlock fault. A scenario involving two phases of strain accumulation and release must involve the evolution of fault strength over multiple earthquake cycles, or supercycles. The documentation of such fault behavior will be critical for updating future seismic hazard assessments that currently rely on simpler ideas on the behavior of faults over these timescales. 5.4 Slip history and the potential for system-wide ruptures of the faults along the Ventura basin The blind thrust faults of the western Transverse Ranges present a hidden seismic hazard to many of the greater southern California metropolitan areas. Recent advances in detecting and documenting the activity of these blind thrust faults have illuminated the threat from these hidden faults. The 1994 Northridge and 1987 Whittier Narrows earthquakes illuminate the need to determine the activity and seismic threat of these particular faults in southern California. Compounding this threat is the recent recognition of thrust faults producing multi-segment ruptures. The most damaging continental earthquakes have been produced by multi-fault ruptures along previously unknown or poorly mapped fault systems (England and Jackson, 2011). Deformed Holocene strata above the tip of the Ventura fault imaged through a combination of high-resolution seismic reflection profiles and borehole correlations, provide a complete record of incremental fold growth during large earthquakes on this major blind thrust. Through this 128 multi-disciplinary approach, we document evidence for at least two large magnitude earthquakes during the past 5 ka. The timing of these two paleo-earthquakes (or paleo-earthquake clusters) have been constrained to <1.4 ka for the most recent event (MRE) and 3-5 ka for the penultimate event. Minimum uplift in each of the two scarp-forming events are ~6-6.5 meters for the MRE, and ~4.5 meters for the penultimate event. Combining the results from our paleoseismologic borehole study at Day Road, together with results from a paleoearthquake study on river terraces along the Ventura River (Rockwell et al., 2011) we demonstrate the increasing likelihood that the faults of the western Transverse Ranges experienced two multi-segment system wide ruptures over the past 5 ka. Individual uplift events of this magnitude would require large earthquakes (M w 6.7-7.9, and possibly as large as M w 8.1), likely involving multiple other major faults across the western Transverse Ranges. Such a rupture is comparable in size to the largest earthquakes expected from the San Andreas fault and represents a serious hazard to the densely populated southern California region. 5.5 Complex behavior of plate boundary fault systems over millennial timescales The three projects documented here as parts of this dissertation highlight the extreme spatial and temporal variability in fault behavior over 1000 year timescales. These observations call into question the well-established concept of the earthquake cycle, and downplay the notion of faults being “overdue”. The short instrumental (geodetic) and historical records are inadequate to characterize the complex and multiscale seismic behavior of major fault systems. The fundamental principles of fault behavior and the elastic rebound theory are demonstrated to be a highly simplified view of fault behavior and the work done at these three study sites demonstrates the importance of being cognizant of these complexities when discussing such fault 129 characteristics as slip rate, recurrence intervals and potential earthquake magnitudes. The complex nature of fault ruptures needs to be accounted for in all seismic hazard assessments including the possibility for earthquake clustering, periods of elevated strain accumulation and release during earthquake supercycles, the possibility of multi-segment ruptures, and the possibility of complex cascading ruptures. Our current geophysical and seismic hazard models heavily rely on the oversimplification of fault behavior both temporally and spatially, and the recognition of such complex fault behavior is critical to accurately assessing the hazards to the major metropolitan regions of southern California. A comprehensive probabilistic seismic hazard assessment should include the range of possible earthquakes, and misrepresenting the range of earthquake scenarios by relying on a short instrumental or even short paleoseismoloical record can introduce bias into earthquake prediction models by either underestimating or overestimating the size of earthquakes on that fault system. Several recent technological advances over the past ten years have allowed us to investigate the extent of this variability, including extremely high- resolution airborne LiDAR data, and single grain luminescence dating techniques. Recent advances in luminescence dating have allowed us to investigate the extent of this variability by allowing us to accurately date geomorphic features directly. The advancement of luminescence geochronology has given geomorphologists a means of dating alluvial and colluvial deposits directly without having to rely on the haphazard availability of datable material within those deposits. Furthermore, the single grain dating method has increased the accuracy of luminescence dating by allowing us to be selective with the grains that are used for the age estimate. 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Qa Qa4 Qa2 PLAYA Main alluvial input Subtle w-facing scarp T1 T2 Qa3 117°7’0”W 117°7’10”W 117°6’50”W 35°45’20”N 35°45’10”N 35°45’0”N 37°0’0’’N 36°0’0’’N 35°0’0’’N 118°0’0’’W 117°0’0’’W GF GF PVF PVF SVF SVF CF CF GLF GLF BF BF LoF LoF HF HF SAF SAF HLF HLF ALF ALF BMF BMF SNF SNF AHF AHF LF LF CRF CRF FIF FIF TF TF NDVF NDVF TMF TMF DSF DSF OVF OVF FLVF FLVF HMSVF HMSVF TPF TPF EVF EVF RVF RVF HCF HCF 38°0’0’’N 119°0’0’’W BLF BLF CLF CLF MF MF LuF LuF LLF LLF PF PF EPF EPF QVF QVF RF RF WF WF BSF BSF PSF PSF LMF LMF SDVF SDVF WMF WMF R T San Gabriel Mtns. San Gabriel Mtns. Mina Deflection Mina Deflection BrF BrF a c b CA NV image area 0 50 km N N 170 Main alluvial input P L A Y A Trench site 171 E W 1 2 3 4 5 6 7 8 14 15 16 19 18 17 22 21 20 9 10 13 11 12 E W 0.0 0.5 1.5 2.0 2.5 3.0 1.0 3.5 ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? PVT-200 831 - 963 PVT-208 518 - 626 PVT-207 795 - 920 PVT-47 1519 - 1613 PVT-206 697 - 790 PVT-204 2857- 2958 PVT-46 2184 - 2350 PVT-302 504 - 535 PVT-97c 550 - 655 PVT-44 796 - 957 PVT-205 1065 - 1230 PVT-36 3169 - 3337 PVT-7 796 - 930 PVT-17 477 - 521 PVT-63 3009 - 3215 PVT-70 3982 - 4145 Depth below surface (m) Horizontal distance (m) PVT-OSL-4 140 - 260 PVT-OSL-3 470 - 1050 North Wall 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 1 2 3 4 5 6 7 8 14 15 16 19 18 17 22 21 20 9 10 13 11 12 Fault 3 Fault 4 Fault 5 Fault 2 Fault 1 EH 1 EH 2 EH 3 EH 4 1 2 3 4 5 6 7 8 14 15 16 19 18 17 22 21 20 23 9 10 13 11 12 0.0 0.5 1.0 1.5 2.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 2.5 3.0 3.5 3.5 E W 1 2 3 4 5 6 7 8 14 15 16 19 18 17 22 21 20 23 9 10 13 11 12 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? PVT-402 304 - 454 PVT-52 335 - 511 PVT-53 477 - 521 PVT-14 3991 - 4153 PVT-11 4087 - 4240 PVT-49 3845 - 4237 PVT-26 2960 - 3142 PVT-76 3986 - 4148 PVT-75 3470 - 3582 PVT-86 3892 - 4162 PVT-88 3907 - 4084 PVT-50 2894 - 3070 PVT-211 3987 - 4150 PVT-30 325 - 502 W E PVT-300 564 - 674 PVT-28 2489 - 2719 PVT-23 3210 - 3354 PVT-214 3982 - 4145 Depth below surface (m) Horizontal distance (m) PVT-OSL-1 2940 - 4500 PVT-OSL-2 2300 - 3040 South Wall Fault 1 Fault 2 Fault 3 Fault 4 Fault 5 EH 1 EH 2 EH 3 EH 4 EH 1 EH 3 Unit X - coarse sands and gravels. Fluvial deposit Unit B - alternating bands of dark to light silts. Layers are bounded by darker silts at the bottom that gradually lighten upwards Paleosol - moderately well-developed incipient argillic horizon. Some laminations are still visible in this layer indicating areas where the soil had not fully developed Unit Z - coarse cobble bed Unit Y - granular/pebble gravel. Not as coarse as unit Z Unit L - friable massive very pale brown (10YR 7/3) silt layer bounded by gravel bed below Unit A - silty top soil with four to five prominent pale olive (5Y 6/3) medium silty clay layers which can be traced for most of the length of the trench. Moderate soil development is observed along parts of this top layer Unit G - light olive brown (2.5Y 5/4) silt layer bounded at the top by a 1 cm brown (10YR 5/3) silt layer Unit D - friable very pale brown (10YR 7/3-8/3) silt layer with some areas showing alternating dark and pale bands. The base of Unit U - pale brown (10YR 6/3) and milky laminated silt Unit H - vesicular pale yellow (2.5Y 7/4) silts with no clear laminations visible Unit P - massive pale yellow (2.5Y 8/4) silt layer Unit R - soft grayish brown (2.5Y 5/2) clay layer Unit S - laminated very pale brown (10YR 7/3) silty sand layer 477 - 521 1020 -1154 3210 -3354 2960 -3142 3470 -3582 3991 - 4153 2894 -3070 304 - 454 2489 -2719 3009 -3215 2857 -2958 3892 - 4162 EH 2 Unit Age of unit (cal. yr. BP) EH 4 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 Approximate depth (m) 1030 -1155 3169 -3337 3987 - 4150 4087 - 4240 3982 - 4145 a b c Unidentifiable areas of the trench given diamond pattern the Paleosol above is discrete in places with a brown (10YR 5/3) very fine silt layer Unit F - alternating light olive brown (2.5Y 5/3) sand, very pale brown (10YR 7/3) silt, grayish brown ((2.5Y 5/2) silt and pale yellow (2.5Y 8/3) clay layer Unit I - friable fine grain silts and clays with alternating light olive brown (2.5Y 5/3) and light yellowish brown (2.5Y 6/3) bands Unit E - very pale brown (10YR 8/3) fine grained silt layer unbroken by fault 2 Unit O - very fine white (10YR 8/1) silt layer 172 Sample number UCI lab number Altitude (m asl) Wall Horizontal (m) Depth (m) Fraction modern ± Δ 14 C (‰) ± 14 C age (BP) ± OxCal calibrated age (yr. BP) OxCal trimmed (Cal. yr. BP) 4 3 4 - 2 0 3 4 5 4 - 4 0 3 0 2 0 1 3 1 . 2 8 . 7 3 - 1 2 0 0 . 0 2 2 6 9 . 0 2 2 . 0 5 8 . 7 1 h t u o S 4 0 5 2 4 7 6 7 2 0 4 - T V P 1 0 5 - 2 3 4 2 0 5 - 5 2 3 0 2 5 7 3 9 . 1 5 . 5 4 - 9 1 0 0 . 0 5 4 5 9 . 0 1 5 . 0 9 2 . 5 1 h t u o S 4 0 5 7 3 7 6 7 0 3 - T V P A / N 1 1 5 - 5 3 3 0 2 5 0 4 9 . 1 3 . 9 4 - 9 1 0 0 . 0 7 0 5 9 . 0 3 0 . 1 3 0 . 2 1 h t u o S 4 0 5 4 4 7 6 7 2 5 - T V P 1 2 5 - 0 8 4 1 2 5 - 7 7 4 0 2 5 3 4 9 . 1 5 . 2 5 - 9 1 0 0 . 0 5 7 4 9 . 0 0 3 . 0 0 1 . 3 h t r o N 4 0 5 6 3 7 6 7 7 1 - T V P A / N 1 2 5 - 7 7 4 0 2 5 3 4 9 . 1 5 . 2 5 - 9 1 0 0 . 0 5 7 4 9 . 0 1 2 . 0 6 7 . 3 1 h t u o S 4 0 5 1 4 7 6 7 3 5 - T V P A / N 5 3 5 - 4 0 5 0 2 0 8 4 3 . 2 2 . 8 5 - 3 2 0 0 . 0 8 1 4 9 . 0 9 1 . 2 2 8 . 1 1 h t r o N 4 0 5 3 7 3 5 8 2 0 3 - T V P A / N 6 2 6 - 8 1 5 0 2 0 4 5 9 . 1 2 . 5 6 - 9 1 0 0 . 0 8 4 3 9 . 0 2 6 . 0 2 7 . 6 1 h t r o N 4 0 5 0 4 7 6 7 8 0 2 - T V P A / N 5 5 6 - 0 5 5 5 2 5 1 6 3 . 2 7 . 3 7 - 3 2 0 0 . 0 3 6 2 9 . 0 0 2 . 2 7 7 . 1 1 h t r o N 4 0 5 4 7 3 5 8 c 7 9 - T V P A / N 4 7 6 - 4 6 5 0 2 0 8 6 2 . 2 0 . 1 8 - 2 2 0 0 . 0 0 9 1 9 . 0 2 3 . 2 7 8 . 1 1 h t u o S 4 0 5 2 7 3 5 8 0 0 3 - T V P Sample age Trench location A / N 0 9 7 - 7 9 6 0 2 5 4 8 8 . 1 1 . 0 0 1 - 8 1 0 0 . 0 9 9 9 8 . 0 2 6 . 0 5 7 . 6 1 h t r o N 4 0 5 8 3 7 6 7 6 0 2 - T V P 0 3 9 - 6 9 7 0 3 9 - 6 9 7 0 2 5 6 9 8 . 1 9 . 2 1 1 - 8 1 0 0 . 0 1 7 8 8 . 0 1 5 . 0 5 8 . 7 h t r o N 4 0 5 5 3 7 6 7 7 - T V P 9 1 9 - 5 9 7 0 2 9 - 5 9 7 0 2 0 4 9 8 . 1 3 . 0 1 1 - 8 1 0 0 . 0 7 9 8 8 . 0 0 8 . 0 3 8 . 6 1 h t r o N 4 0 5 9 3 7 6 7 7 0 2 - T V P A / N 7 5 9 - 6 9 7 5 2 5 8 9 4 . 2 4 . 5 1 1 - 4 2 0 0 . 0 6 4 8 8 . 0 6 1 . 2 1 1 . 1 1 h t r o N 4 0 5 1 7 3 5 8 4 4 - T V P A / N 3 6 9 - 1 3 8 0 2 5 0 0 1 8 . 1 5 . 7 1 1 - 8 1 0 0 . 0 5 2 8 8 . 0 4 3 . 1 7 9 . 1 1 h t r o N 4 0 5 5 4 7 6 7 0 0 2 - T V P A / N 0 3 2 1 - 5 6 0 1 0 2 5 1 2 1 1 . 2 1 . 0 4 1 - 1 2 0 0 . 0 9 9 5 8 . 0 2 1 . 2 1 7 . 6 1 h t r o N 4 0 5 0 7 3 5 8 5 0 2 - T V P A / N 3 1 6 1 - 9 1 5 1 0 2 5 5 6 1 7 . 1 1 . 6 8 1 - 7 1 0 0 . 0 9 3 1 8 . 0 9 5 . 1 9 9 . 9 h t r o N 4 0 5 7 5 7 6 7 7 4 - T V P A / N 0 5 3 2 - 4 8 1 2 0 2 5 8 2 2 5 . 1 6 . 7 4 2 - 5 1 0 0 . 0 4 2 5 7 . 0 4 7 . 1 1 2 . 0 1 h t r o N 4 0 5 6 5 7 6 7 6 4 - T V P 9 1 7 2 - 9 8 4 2 9 1 7 2 - 9 8 4 2 0 2 5 9 4 2 8 . 1 2 . 7 6 2 - 8 1 0 0 . 0 8 2 3 7 . 0 5 3 . 1 1 5 . 8 h t u o S 4 0 5 9 6 3 5 8 8 2 - T V P 7 5 9 2 - 7 5 8 2 8 5 9 2 - 7 5 8 2 0 2 5 0 8 2 5 . 1 8 . 4 9 2 - 5 1 0 0 . 0 2 5 0 7 . 0 0 3 . 2 6 0 . 6 1 h t r o N 4 0 5 0 6 7 6 7 4 0 2 - T V P 6 5 0 3 - 5 2 9 2 0 7 0 3 - 4 9 8 2 0 2 5 6 8 2 6 . 1 2 . 0 0 3 - 6 1 0 0 . 0 8 9 9 6 . 0 5 5 . 1 2 6 . 9 h t u o S 4 0 5 8 5 7 6 7 0 5 - T V P PVT-26 76755 504 South 800 158 0 6968 0 0014 -303 2 14 2900 20 2960-3142 2968-3138 PVT-26 76755 504 South 8.00 1.58 0.6968 0.0014 -303.2 1.4 2900 20 2960-3142 2968-3138 1 1 2 3 - 1 7 0 3 5 1 2 3 - 9 0 0 3 0 2 5 5 9 2 4 . 1 6 . 7 0 3 - 4 1 0 0 . 0 4 2 9 6 . 0 3 6 . 1 1 8 . 6 h t r o N 4 0 5 9 5 7 6 7 3 6 - T V P 1 2 3 3 - 7 6 1 3 4 5 3 3 - 0 1 2 3 5 2 0 5 0 3 8 . 1 8 . 5 1 3 - 8 1 0 0 . 0 2 4 8 6 . 0 2 8 . 1 8 1 . 9 h t u o S 4 0 5 6 7 3 5 8 3 2 - T V P 9 3 3 3 - 5 2 2 3 7 3 3 3 - 9 6 1 3 0 2 5 3 0 3 7 . 1 7 . 4 1 3 - 7 1 0 0 . 0 3 5 8 6 . 0 8 7 . 1 9 1 . 7 h t r o N 4 0 5 5 7 3 5 8 6 3 - T V P 6 8 5 3 - 1 7 4 3 2 8 5 3 - 0 7 4 3 0 2 5 0 3 3 4 . 1 2 . 7 3 3 - 4 1 0 0 . 0 8 2 6 6 . 0 8 8 . 2 7 1 . 5 1 h t u o S 4 0 5 0 5 7 6 7 5 7 - T V P 3 4 0 4 - 7 8 9 3 3 5 1 4 - 1 9 9 3 0 2 5 3 7 3 3 . 1 0 . 2 7 3 - 3 1 0 0 . 0 0 8 2 6 . 0 4 0 . 2 0 7 . 5 h t u o S 4 0 5 7 4 7 6 7 4 1 - T V P 4 5 0 4 - 0 9 8 3 2 7 0 4 - 2 9 8 3 0 2 0 4 6 3 4 . 1 4 . 4 6 3 - 4 1 0 0 . 0 6 5 3 6 . 0 8 3 . 3 4 9 . 1 2 h t u o S 4 0 5 2 5 7 6 7 6 8 - T V P 7 4 0 4 - 4 8 9 3 8 4 1 4 - 6 8 9 3 0 2 0 2 7 3 4 . 1 7 . 0 7 3 - 4 1 0 0 . 0 3 9 2 6 . 0 0 3 . 3 2 9 . 4 1 h t u o S 4 0 5 1 5 7 6 7 6 7 - T V P A / N 7 3 2 4 - 5 4 8 3 0 7 0 9 6 3 4 . 5 5 . 8 6 3 - 4 5 0 0 . 0 5 1 3 6 . 0 0 9 . 1 5 0 . 6 h t u o S 4 0 5 8 4 7 6 7 9 4 - T V P 0 5 1 4 - 1 1 0 4 0 5 1 4 - 7 8 9 3 0 2 5 2 7 3 5 . 1 9 . 0 7 3 - 5 1 0 0 . 0 1 9 2 6 . 0 8 9 . 1 5 2 . 5 h t u o S 4 0 5 4 5 7 6 7 1 1 2 - T V P 4 3 2 4 - 7 8 0 4 0 4 2 4 - 7 8 0 4 5 2 0 8 7 3 7 . 1 3 . 5 7 3 - 7 1 0 0 . 0 7 4 2 6 . 0 6 0 . 2 1 7 . 5 h t u o S 4 0 5 6 4 7 6 7 1 1 - T V P 6 4 1 4 - 0 1 0 4 5 4 1 4 - 2 8 9 3 0 2 0 1 7 3 3 . 1 0 . 0 7 3 - 3 1 0 0 . 0 0 0 3 6 . 0 4 4 . 3 1 0 . 7 1 h t r o N 4 0 5 9 4 7 6 7 0 7 - T V P 21 83 0 S 21 3 36 0 6302 0 001 369 8 1 310 20 3982 1 /A / N 5 4 1 4 - 2 8 9 3 0 2 0 1 7 3 5 . 1 8 . 9 6 3 - 5 1 0 0 . 0 2 0 3 6 . 0 7 6 . 3 3 4 . 1 2 h t u o S 4 0 5 7 7 3 5 8 4 1 2 - T V P 6 8 0 4 - 0 3 0 4 4 8 0 4 - 7 0 9 3 0 2 0 6 6 3 3 . 1 9 . 5 6 3 - 3 1 0 0 . 0 1 4 3 6 . 0 1 5 . 3 6 6 . 0 2 h t u o S 4 0 5 3 5 7 6 7 8 8 - T V P A / N A / N 0 2 5 1 8 1 - 6 . 2 9 . 3 5 2 6 2 0 0 . 0 9 3 5 2 . 1 7 1 . 0 4 2 . 0 2 h t u o S 4 0 5 3 4 7 6 7 3 0 4 - T V P The charcoal samples received a standard acid-alkali-acid (AAA) pre-treatment. All results were corrected for isotopic fractionation according to the conventions of Stuiver and Polach [1977], with δ13C values measured on prepared graphite using the AMS. Of the 39 samples sent to the AMS, four samples did not survive the pre-treatment and could not be measured. 173 2 3 4 5 6 7 8 ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? PVT-63 3009 - 3215 PVT-7 796 - 930 PVT-17 477 - 521 North Wall Horizontal distance (m) 0.0 0.5 1.0 1.5 2.0 2.5 EH 1 EH 3 PVT-36 3169 - 3337 PVT-OSL-4 140 - 260 PVT-OSL-3 470 - 1050 EH 2 Fault 1 Fault 2 Paleosol Unit A Unit B Unit E Unit G Unit X Depth (m) 174 Depth below ground surface (m) Calibrated age (yr. BP) Present 1000 2000 3000 4000 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 11 15 1 2 4 5 7 6 8 9 10 13 14 12 17 19 29 16 18 21 28 24 26 20 23 3 27 25 22 18 PVT 23 3210 3354 PDF Sample # 14C cal. yr. BP 1PVT Ͳ402 304 Ͳ454 2PVT Ͳ53 477 Ͳ521 3PVT Ͳ52 335 Ͳ511 4PVT Ͳ30 325 Ͳ502 5PVT Ͳ208 518 Ͳ626 6PVT Ͳ17 477 Ͳ521 7PVT Ͳ206 697 Ͳ790 8PVT Ͳ7 796 Ͳ930 9PVT Ͳ207 795 Ͳ920 10 PVT Ͳ205 1065 Ͳ1230 11 PVT Ͳ28 2489 Ͳ2719 12 PVT Ͳ47 1519 Ͳ1613 13 PVT Ͳ204 2857 Ͳ2958 14 PVT Ͳ46 2184 Ͳ2350 15 PVT Ͳ50 2894 Ͳ3070 16 PVT Ͳ26 2960 Ͳ3142 17 PVT Ͳ63 3009 Ͳ3215 18 PVT 23 Ͳ 3210 3354 Ͳ 19 PVT Ͳ36 3169 Ͳ3337 20 PVT Ͳ75 3470 Ͳ3582 21 PVT Ͳ49 3845 Ͳ4237 22 PVT Ͳ214 3982 Ͳ4145 23 PVT Ͳ14 3991 Ͳ4153 24 PVT Ͳ86 3892 Ͳ4072 25 PVT Ͳ76 3986 Ͳ4148 26 PVT Ͳ211 3987 Ͳ4150 27 PVT Ͳ88 3907 Ͳ4084 28 PVT Ͳ11 4087 Ͳ4240 29 PVT Ͳ70 3982 Ͳ4145 Event 1 Event 2 Event 3 Event 4 Paleosol - No reliable radiocarbon samples from these depths 20 23 1 20 3 2 1 175 14 9 10 13 11 12 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 ? ? ? ? ? ? ? Depth below surface (m) Horizontal distance (m) (south wall) A B C D E Event 3 ? ? Event 1 Event 3 ~50cm Unit G deposited flat throughout the length of the trench Event 3 Event 2 ~20cm Soil development continues on newly deposited silts until units B and G are deposited Unit G vertically separated by ~20cm between the east and west side of the “pit” 70cm of vertical separation of units below the Paleosol reflects fault slip in Events 1 and 2 Event 2 Event 3 Event 2 Event 3 Event 1 Event 2 Event 3 Event 2 Event 1 Event 3 Event 2 Event 1 F G H Event 2 Event 3 Event 4 Event 4 Event 4 Event 4 Event 4 Event 4 Event 4 Event 4 Paleosol Unit B Unit G Unit E Unit L Unit F Paleosol Unit E Unit L Unit F Unit B Unit G 176 PVT_402 PVT-17 PVT-7 PVT-207 PVT-28 PVT-204 PVT-50 PVT-26 PVT-63 PVT-23 PVT-36 Event 1 Event 2 Event 3 PVT-75 PVT-86 PVT-14 PVT-76 PVT-211 PVT-11 PVT-70 PVT-88 Phase Phase Phase Modeled cal. yr. BP 5000 4000 3000 2000 1000 0 OxCal v4.1.7 Bronk Ramsey (2010); r:5 Atmospheric data from Reimer et al (2009); 328-485 cal. yr. BP 3271-3549 cal. yr. BP 910-2553 cal. yr. BP Event 4 >4094 cal. yr. BP PVT-30 177 Event 1 (MRE) Event 2 910 - 2553 cal. yr. BP Event 3 3271 - 3549 cal. yr. BP Event 4 4094 - 4647 cal. yr. BP Modeled date (cal. yr. BP) Modeled date (cal. yr. BP) Modeled date (cal. yr. BP) Modeled date (cal. yr. BP) 350 300 400 450 500 950 1450 1950 2450 3150 3250 3350 3450 3550 3650 3950 4450 4950 5450 5950 6450 6950 Event 1 (MRE) 328 - 485 cal. yr. BP 178 19 22 21 20 23 0.0 0.5 1.0 1.5 2.0 2.5 3.0 EH 3 PVT-88 3907 - 4084 PVT-214 3982 - 4145 PVT-86 3892 - 4162 EH 2 pre-E3 Depth (m) South Wall Horizontal distance (m) 179 1999 Mw 7.1 Hector Mine earthquake surface rupture 1992 Mw 7.3 Landers earthquake surface rupture 1872 Mw ~7.6? Owens Valley earthquake surface rupture CA NV image area This paper Ganev et al., 2010 McGill, 1992 Dawson et al., 2003 Rymer et al., 2002 Rubin & Sieh, 1997 Melville Gap site (Rockwell et al., 2000) Hondo site (Rockwell et al., 2000) Bodick Road site (Rockwell et al., 2000) Batdorf site (Rockwell et al., 2000) Thrust site (Rockwell et al., 2000) Houser & Rockwell, 1996 Padget & Rockwell, 1994 Bryan & Rockwell, 1994 Bryan & Rockwell, 1995 Madden et al., 2006 Foster, 1992 Camp Rock site (Rubin et al., unpublished) Lee et al., 2001 Madugo et al., 2012 Hecker et al., 1993 Lee et al., 2001b McGill et al., 2009 0 25 50 75 100 Kilometers 116°0'0"W 116°0'0"W 117°0'0"W 117°0'0"W 118°0'0"W 118°0'0"W 37°0'0"N 37°0'0"N 36°0'0"N 36°0'0"N 35°0'0"N 35°0'0"N 180 Johnson Valley Melville Gap Rockwell et al., 2000 Johnson Valley/Landers/ Kickapoo Batdorf House Rockwell et al., 2000 Johnson Valley Hondo Road Rockwell et al., 2000 Johnson Valley/Landers/ Kickapoo Bodick Road Rockwell et al., 2000 Homestead Valley Thrust Rockwell et al., 2000 Homestead Valley Playa Hecker et al., 1993 Camp Rock Camp Rock Graben Rubin et al., unpublished Emerson Playa Rubin and Sieh, 1997 Old Woman Springs Old Woman Springs Ranch Houser and Rockwell, 1996 Helendale Rabbit Springs & Waverly Road Bryan and Rockwell, 1995 Lenwood Soggy Lake Padgett and Rockwell, 1994 Calico Ganev et al., 2010 Mesquite Lake Foster, 1992 Mesquite Lake Madden et al., 2006 Lavic Lake Rymer et al., 2002 Panamint Valley Playa Verde This paper Owens Valley Lee et al., 2001 Garlock McGill, 1992 Garlock Dawson et al., 2003 Garlock Madugo et al., 2012 Garlock ECSZ north of Garlock Mojave region of ECSZ Newberry Springs Twin Lakes Owens Valley Bacon and Pezzopane, 2007 El Paso Peaks McGill et al., 2009 Clark Wash Garlock Fault Trench Site Reference Age of Events (in thousands of years) 0 5 10 15 181 Panamint Springs Death Valley Trona Ridgecrest -117.5˚ 36.5˚ 37.5˚ -117.0˚ 37.0˚ Panamint Springs C. Garlock C. Garlock C. Garlock C. Garlock C. Garlock C. Garlock E. Garlock Panamint Springs Death Valley Trona Ridgecrest -117.5˚ 36.5˚ 37.5˚ -117.0˚ 37.0˚ E. Garlock Death Valley Trona Ridgecrest -117.5˚ 36.5˚ 37.5˚ -117.0˚ 37.0˚ E. Garlock Panamint Springs Death Valley Trona Ridgecrest -117.5˚ 36.5˚ 37.5˚ -117.0˚ 37.0˚ Panamint Springs Death Valley Trona Ridgecrest -117.5˚ 36.5˚ 37.5˚ -117.0˚ 37.0˚ Panamint Springs Death Valley Trona Ridgecrest -117.5˚ 36.5˚ 37.5˚ -117.0˚ 37.0˚ a) M=6.38, southern Panamint Valley fault ΔCFF resolved on central Garlock; μ’ = 0.2 f) M=7.33, Panamint + Brown Mtn. Fault ΔCFF on central Garlock; μ’ = 0.6 d) M=7.27, Panamint Valley Fault system ΔCFF on central Garlock; μ’ = 0.6 b) M=6.38, southern Panamint Valley fault ΔCFF on central Garlock; μ’ = 0.6 e) M=7.33, Panamint + Brown Mtn. Fault ΔCFF on central Garlock; μ’ = 0.2 c) M=7.27, Panamint Valley Fault system ΔCFF on central Garlock; μ’ = 0.2 source source source 25 km 25 km 25 km 25 km 25 km 25 km source source source Coulomb stress change (bars) -2.5 -2.0 -1.5 -1.0 -0.5 0.0 +0.5 +1.0 +1.5 +2.0 +2.5 Slip (m) on source fault 3.5-3.9 3.0-3.4 2.5-2.9 2.0-2.4 1.5-1.9 1.0-1.4 0.5-0.9 0.0 182 Panamint Springs Death Valley Trona Ridgecrest -117.5˚ 36.5˚ 37.5˚ -117.0˚ 37.0˚ source source source source source source E. Garlock Panamint Springs Death Valley Trona Ridgecrest -117.5˚ 36.5˚ 37.5˚ -117.0˚ 37.0˚ E. Garlock Death Valley Trona Ridgecrest -117.5˚ 36.5˚ 37.5˚ -117.0˚ 37.0˚ E. Garlock Panamint Springs Death Valley Trona Ridgecrest -117.5˚ 36.5˚ 37.5˚ -117.0˚ 37.0˚ Panamint Springs Death Valley Trona Ridgecrest -117.5˚ 36.5˚ 37.5˚ -117.0˚ 37.0˚ Panamint Springs Death Valley Trona Ridgecrest -117.5˚ 36.5˚ 37.5˚ -117.0˚ 37.0˚ Slip (m) on source fault Panamint Springs a) M=7.39, central Garlock fault ΔCFF resolved on s. Panamint Vy. Fault; μ’ = 0.2 f) M=7.77, entire Garlock fault ΔCFF on s. Panamint Vy. Fault, μ’ = 0.4 d) M=7.72, W and C Garlock fault ΔCFF on s. Panamint Vy. Fault; μ’ = 0.4 b) M=7.39, central Garlock fault ΔCFF on s. Panamint Vy. Fault; μ’ = 0.4 e) M=7.23, eastern Garlock fault ΔCFF on s. Panamint Vy. Fault; μ’ = 0.4 c) M=7.39, central Garlock fault ΔCFF on s. Panamint Vy. Fault; μ’ = 0.6 S. Panamint N. Panamint S. Panamint N. Panamint S. Panamint N. Panamint 25 km 25 km 25 km 25 km 25 km 25 km S. Panamint S. Panamint S. Panamint Coulomb stress change (bars) -2.5 -2.0 -1.5 -1.0 -0.5 0.0 +0.5 +1.0 +1.5 +2.0 +2.5 7.0-7.9 6.0-6.9 5.0-5.9 4.0-4.9 3.0-3.9 2.0-2.9 1.0-1.9 0.0 183 ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ??? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ??? ? b 2. Mojave section of SAF 1. M>7.5, western and central Garlock 2. PVF 50 km promoted promoted ? ? ? ? ? ? ? 37° 36° 117° 118° 119° 116° 35° 37° 36.5° 117° 118° positive ΔCFF feedback? negative ΔCFF feedback positive ΔCFF feedback eastern Garlock d a central Garlock PVF 1. Mojave section of SAF 2. M>7.5, western and central Garlock 3. PVF 50 km 25 km -2.5 +2.5 0.0 Coulomb stress change (bars) MRE >1450 AD [Madugo et al., 2012] MRE 1450-1640 AD [Dawson et al., 2003] MRE 1465-1622 AD [this study] Event at 1457-1568 AD [Scharer et al., 2011] Garlock (end-to-end) promoted promoted ? ? ? ? ? ? ? 37° 36° 117° 118° 119° 116° 35° Source: M=7.6, Mojave section of SAF CFF increased on western Garlock Source: M=7.8, CFF increased on Mojave section of SAF ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ??? ? c 3. Mojave section of SAF 2. M>7.5, western and central Garlock 1. PVF 50 km promoted promoted ? ? ? ? ? ? ? 37° 36° 117° 118° 119° 116° 35° 184 East West 0 0.5 1.0 1.5 2.0 2.5 3.0 0 0.5 1.0 1.5 2.0 3.0 2.5 1 2 3 4 5 6 7 8 9 10 11 12 0 Horizontal distance (m) Depth below surface (m) Trench T-1 North wall 185 East West 0 0.5 1.0 1.5 2.0 3.0 2.5 0 0.5 1.0 1.5 2.0 2.5 3.0 Depth below surface (m) Trench T-2 South wall 1 2 3 4 5 6 7 8 9 10 11 12 0 Horizontal distance (m) 186 14 15 16 9 10 13 11 12 0.0 0.5 1.0 1.5 2.0 2.5 3.0 ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? PVT-50 2984 - 3070 PVT-23 3210 - 3354 PVT-300 564 - 674 PVT-76 3986 - 4148 PVT-75 3470 - 3582 PVT-52 335 - 511 PVT-53 477 - 521 PVT-30 325 - 502 EH 1 EH 3 EH 4 EH 2 Depth (m) 187 2 3 4 5 6 7 8 0.0 0.5 1.0 1.5 2.0 2.5 ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? PVT-49 3845 - 4237 PVT-26 2960 - 3142 PVT-11 4087 - 4240 PVT-14 3991 - 4153 PVT-211 3987 - 4150 EH 1 EH 3 EH 2 Depth (m) 188 19 18 21 20 0.0 0.5 1.5 2.0 2.5 3.0 1.0 3.5 EH 1 EH 3 EH 2 Depth (m) 189 Event 1 (including PVT-52) 364 - 498 cal. yr. BP Modeled age (cal. yr. BP) 300 350 400 450 500 190 3524 - 3960 cal. yr. BP pre-Event 3 Modeled age (cal. yr. BP) 3550 3750 3950 4150 191 Sample ID Lab. Number 1 Depth (m) K 2 O (%) 2 Th (ppm) 2 U (ppm) 2 Cosmic Dose (Gy/ka) 3 Num. of aliquots 4 Dose Rate (Gy/ka) 5 Equivalent Dose (Gy) 6 OSL Age (ka, BP 1950 ) 7 PVT-OSL-1 USU-947 2.2 2.28±0.06 14.3±1.3 3.5±0.3 0.17±0.02 18 (42) 4.24 ±0.19 16.03±2.94 3.72±0.78 PVT-OSL-2 USU-948 1.7 2.42±0.06 13.7±1.2 3.1±0.2 0.18±0.02 19 (40) 4.25±0.19 11.62±1.09 2.67±0.37 PVT-OSL-3 USU-949 0.43 2.57±0.06 13.4±1.2 3.1±0.2 0.22±0.02 37 (70) 4.42±0.20 3.63±1.24 0.76±0.29 PVT-OSL-4 USU-950 0.02 2.56±0.06 17.2±1.6 4.4±0.3 0.22±0.02 33 (71) 4.91±0.23 1.29±0.26 0.20±0.06 1 All samples were analyzed at the USU Luminescence Lab, Logan Utah, USA. 2 Elemental concentraons determined by ICP-MS and ICP-A ES techniques from ALS Chemex. 3 Contribuon of cosmic radia on to the dose rate was calculated by using sample depth, elevaon, and longitude/la tude following Presco and Huon (1994). 4 Number of aliquots used for age calculaon, number of aliquots measured in parentheses. Rejecon of aliquots follows standard rejec on criteria (see Ri enour 2005 for example). 5 Dose rate is derived from concentra ons of radionuclides using conversion factors from Guerin et al. (2011) and includes cosmic contribuon and a enua ng e ffects of intersal water. 3±3% H 2 O content was assumed to be representave of burial history. 6 Analysis using the single-aliquot regenerave-dose procedure of Murray and Wintle (2000) on 5-mm small-aliquots of quartz sand for all samples except for USU-948 where 2-mm aliquots were used. Equivalent dose (De) calculated using the early background subtracon method (Cunningham and Wallinga, 2010). The central age model was used for samples USU-947 and USU-948 and the minimum age model was used for USU-949 and USU-950 (both models from Galbraith et al. 1999). Error on equivalent dose (De) is 2-sigma standard error. 7 Age reported in reference to the radiocarbon datum of AD 1950, 60 years were subtracted from the original OSL calendar age (reported in ka before AD 2010). Error on age is 2-sigma standard error. 192 Chapter 3 Figures 193 35° N 36° N 119° W 118° W 117° W 36° N 35° N 118° W 117° W 119° W western section central section eastern section CA NV image area San Andreas Fault San Andreas Fault SIERRA NEVADA SIERRA NEVADA Garlock Fault Garlock Fault B M T R Christmas Canyon West study area El Paso Peaks (Dawson et al., 2003) Death Valley Panamint Valley Wrightwood (Weldon et al., 2004) EPP CCW 025 50 75 100 Km N 194 ± 0 50 100 200 300 400 Meters Site 1 Site 2 Site 3 Site 4 117°23’0”W 117°22’30”W 35°31’30”N 35°31’0”N 117°23’0”W 117°23’30”W 117°22’30”W CA NV GF SAF 195 0 25 50 75 100 Meters ± ± 0 25 50 75 100 Meters ~23m 12C 11A 0 30 60 90 120 15 Meters 0 30 60 90 120 15 Meters ± ± ~23m 0 - 10 10 - 20 20 - 30 30 - 40 40 - 50 50 - 60 60 - 70 70 - 80 80 - 96 96 - 114 114 - 141 141 - 175 175 - 220 220 - 239 239 - 268 268 - 290 290 - 308 308 - 325 325 - 346 346 - 360 Aspect (°) Site 1 (Target 1A) A B C D 196 Site 2 (Target 1B) ± 0 25 50 75 100 Meters 0 25 50 75 100 Meters ± ~24m 12B 12B 12D 12D 12A 12A ± 030 60 90 120 15 Meters 0 - 10 10 - 20 20 - 30 30 - 40 40 - 50 50 - 60 60 - 70 70 - 80 80 - 96 96 - 114 114 - 141 141 - 175 175 - 220 220 - 239 239 - 268 268 - 290 290 - 308 308 - 325 325 - 346 346 - 360 Aspect (°) ± ~24m 030 60 90 120 15 Meters A B C D Trench Figure S5 * * 197 Site Pit number Sample number Strat unit Depth (m) Age (years ± 1σ error) Site 1 11A CCW 11A05 Unit 1 0.08 30 ± 20 Site 1 11A CCW 11A01 Unit 2 0.14 790 ± 70 Site 1 11A CCW 11A02 Unit 3 0.28 3770 ± 340 Site 1 11A CCW 11A03 Unit 3 0.49 3810 ± 270 Site 1 11A CCW 11A04 Unit 3 0.72 3760 ± 230 Site 1 12C CCW 12C01 Unit 1 0.13 520 ± 60 Site 1 12C CCW 12C02 Unit 2 0.29 2280 ± 140 Site 1 12C CCW 12C03 Unit3 0.62 5360 ± 330 Site 1 12C CCW 12C04 Unit 3 0.84 4980 ± 340 Site 2 12A CCW 12A01 Unit 1 0.10 2620 ± 190 Site 2 12A CCW 12A02 Unit 2 0.37 4550 ± 370 Site 2 12A CCW 12A03 Unit 3a 0.61 4740 ± 290 Site 2 12A CCW 12A04 Unit 3b 0.84 4490 ± 360 Site 2 12B CCW 12B01 Unit 1 0.21 1860 ± 150 Site 2 12B CCW 12B02 Unit 1 0.43 1910 ± 150 Site 2 12B CCW 12B03 Unit 2 0.58 4700 ± 290 Site 2 12B CCW 12B04 Unit 2 0.84 5620 ± 360 Site 2 12D CCW 12D01 Unit 1 0.18 1230 ± 160 Site 2 12D CCW 12D02 Unit 2 0.33 1570 ± 120 Site 2 12D CCW 12D03 Unit 3 0.58 5390 ± 340 Site 2 12D CCW 12D04 Unit 3 0.78 6990 ± 580 198 0 2 4 6 8 10 12 14 02 4 6 8 10 12 14 16 16 18 20 0 10 20 30 40 50 60 70 02 4 6 8 10 12 14 16 80 22 24 Variable short term slip rate (dashed where interpretted) Interpretted short term slip rate assuming steady slip over three time periods: 7.5ka to 9.3ka; 7.5ka to 11.5ka; 7.5ka to13.3ka Time (ka) Slip rate (mm/yr) Dawson et al., 2003 Madugo et al., 2012 McGill et al., 2009 This study Meade & Hager, 2005; McClusky et al., 2001; Miller et al., 2001 Clark & Lajoie, 1974 Ganev et al., 2012 option C 70m/13.3ka McGill & Sieh, 1993 McGill et al., 2009 Age of events (ka) N o p a l e o s e i s m i c d a t a ? ? ? ? ? ? ? 0 2 4 68 10 12 16 14 Paleoearthquake ages on the central and western Garlock fault No earthquakes Average Holocene-latest Pleistocene slip rate Ganev et al., 2012 option A 58m/8-10ka Ganev et al., 2012 option B 58m/11.5ka central Garlock fault western-central Garlock fault western Garlock fault Short term slip rate calculated using 4 events documented at El Paso Peaks (Dawson et al., 2003) and 23.5±2.5 meters observed at Christmas Canyon West (this study) Short term slip rate calculated using 2 events documented at El Paso Peaks (Dawson et al., 2003) and 4 -12 meters of displacement (assuming min = 2 meters, max = 6 meters per event) Time (ka) Total displacement (m) McGill et al., 2009 preferred displacement and surface age Possible incision at the end of the Younger Dryas period at ~11.5 ka (Ganev et al., 2012) possible end-member displacement values for 5 ka & 7 ka events minimum age maximum age Average Holocene slip rate (McGill et al., 2009) Short term slip rate Fault slip history 7m 7m 4m 6m Slip rate: 7.6mm/yr A B C Slip rate: 5.1mm/yr Average Holocene slip rate (Ganev et al., 2012) 199 4000 5000 3000 2000 BC/AD 1000 1000 2000 -6 -8 -10 2 0 -2 -4 8 6 4 10 12 14 16 18 20 Cumulative strain (m) Calendar years 5.3 mm/yr Zero net strain 4000 5000 3000 2000 BC/AD 1000 1000 2000 -6 -8 -10 2 0 -2 -4 8 6 4 10 12 14 16 18 20 22 24 26 28 Cumulative strain (m) Calendar years 7.6 mm/yr Zero net strain 4000 5000 3000 2000 BC/AD 1000 1000 2000 -6 -8 -10 2 0 -2 -4 8 6 4 10 12 14 16 18 20 22 24 26 28 Cumulative strain (m) Calendar years 7.6 mm/yr Zero net strain 4000 5000 3000 2000 BC/AD 1000 1000 2000 -6 -8 -10 2 0 -2 -4 8 6 4 10 12 14 16 18 Cumulative strain (m) Calendar years 5.3 mm/yr Zero net strain Elastic strain accumulation Fault displacement A B CD 200 4000 5000 3000 2000 BC/AD 1000 1000 2000 -6 -8 -10 2 0 -2 -4 8 6 4 10 12 14 16 18 Cumulative strain (m) Calendar years 5.3 mm/yr Zero net strain Potential Energy Gain (Strain accumulation) Kinetic Energy Loss (Earthquakes) 0 1 2 3 4 5 6 7 8 0 500 1000 1500 2000 2500 3000 3500 4000 Earthquake displacement (m) Slip Predictable? 5.3 mm/yr 7.6 mm/yr Length of previous interseismic period (yr) 0 500 1000 1500 2000 2500 3000 3500 4000 0 1 2 3 4 5 6 7 8 Length of subsequent interseismic period (yr) Earthquake displacement Time Predictable? 5.3 mm/yr 7.6 mm/yr C B A 201 R² = 0.4646 0 500 1000 1500 2000 2500 3000 3500 4000 -15 -10 -5 0 5 10 15 20 Length of subsequent interseismic period yr) Relative strain accumulation (m) Strain predictable? 6.45 mm/yr (2m) Event 6 Event 5 Event 4 Event 3 Event 2 MRE 202 2000 1800 1600 1400 1200 1000 800 600 400 50 45 40 35 30 25 20 15 10 5 0 Calendar years AD cumulative displacement, SAF (m) cumulative displacement, Garlock fault (m) 16 14 12 10 8 6 4 2 0 Slip rate: 31 mm/yr San Andreas fault slip history Garlock fault slip history Average slip rate (Weldon et al., 2004) Short term variable rate 203 1.8 ± 1.5 1.8 ± 1.5 3.2 ± 1.5 3.2 ± 1.5 1.1 ± 1.9 1.1 ± 1.9 SL: 4 - 9 OLF: ~ 2.5 CC: > 0.8 MC: 5.5 - 8 KL: 4.5 - 6.1 CW: 5.3 - 10.7 OC: 1.6 - 3.3 449100: 2.8 - 7.8 Eastern California Eastern California Shear Zone Shear Zone Sierra Nevada Sierra Nevada Christmas Canyon West study area 0 5 10 20 30 40 Kilometers 118° W 117° W 35° N 35° N 117° W 118° W N 204 0 50 100 150 200 25 Meters ± ± N 050 100 150 200 25 Meters N 12C 12C 11A 11A 11A 11A 11A 11A 12A 12A 12A 12A 12D 12D 12B 12B 12D 12D 12B 12B 23.5±2.5 m b a 205 Fold Line (90°) OSL sample OSL sample OSL sample Chistmas Canyon West Trench (Site 2) Sample pit 12A 0 20 20 40 60 80 100 40 60 -20 0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 420 440 460 480 500 520 540 560 580 600 620 640 0 20 20 40 60 80 100 40 60 Edge of active wash Depth (cm) Length (cm) 206 Chapter 4 Figures 207 km Oak Ridge fault Oak Ridge fault Ventura fault Ventura fault Red Mountain fault Red Mountain fault Southern Southern San Cayetano fault San Cayetano fault San Cayetano fault San Cayetano fault Lion fault Lion fault Pitas Point fault Pitas Point fault Ventura Avenue Ventura Avenue anticline anticline Fault Thrust fault Anticline Seismic reflection profile 34°30’ 34°20’ 34°10’ 34° -119°50’ -119°40’ -119°30’ -119°20’ -119°10’ -118°50’ -119° Western Transverse Ranges Western Transverse Ranges Ventura Ventura Oxnard Oxnard Fillmore Fillmore Santa Paula Santa Paula Santa Barbara Santa Barbara Ojai Ojai 208 Loma Vista Rd Foothill Rd Telegraph Rd Telegraph Rd Main St 126 1 Victoria Ave Foothill Rd Poli St 126 A B C D Santa Paula Ventura Ventura Avenue Anticline Santa Clara River N A B C D Hall Canyon Road/Evergreen Drive Day Road Briggs Road Brookshire Avenue 0 5 km Ventura Fault scarp High-resolution seismic reflection profiles Southern San Cayetano Fault scarp 209 126 Santa Paula St Telegraph Rd Foothill Rd Briggs Rd Cummings Rd 126 Telegraph Rd Loma Vista Rd Foothill Rd Thompson Blvd Polk St Day Rd Brookshire Ave Victoria Ave Main St Bucknell Ave Hall Canyon Rd Evergreen Dr Foothill Rd Ventura Avenue Anticline 1 Ventura Fault San Cayetano Fault 0 3 km N N 0 3 km (b) (a) 210 Day Road high-resolution seismic reflection profile Borehole (yellow)/CPT (green) locations Sample pit Fold scarp Loma Vista Rd Loma Vista Rd Day Rd Day Rd 3x vertical exaggeration 211 Telegraph Rd Telegraph Rd Day Road high-resolution seismic reflection profile Ventura Fault scarp 3x vertical exaggeration Day Road borehole/CPT area Arroyo Verde Fan 212 50 100 150 200 250 Elevation (meters) 0 400 800 approximate depth (m) 0 1000 2000 3000 4000 <<North distance (m) South>> 5x vertical exaggeration borehole (green)/CPT(pink) locations 213 Sample Sample type transect borehole actual depth, m projected depth to borehole DY-1, m Unit Age (before AD 2013) 1 sigma uncertainty Dy-OSL-1/1 Luminescence Day Road Dy - 0 7 1 3 B 8 5 . 3 8 5 . 3 1 ± 230 Dy-OSL-1/2 Luminescence Day Road Dy - 0 6 0 3 C 5 9 . 4 5 9 . 4 1 ± 230 Dy-OSL-1/3 Luminescence Day Road Dy - 0 6 4 5 E 8 7 . 6 8 7 . 6 1 ±330 Dy-OSL-1/4 Luminescence Day Road Dy - 0 2 0 5 G 0 0 . 8 0 0 . 8 1 ± 310 Dy-OSL-1/5 Luminescence Day Road Dy - 0 3 9 7 H 4 4 . 0 1 4 4 . 0 1 1 ± 530 Dy-OSL-1/6 Luminescence Day Road Dy - 0 6 0 9 I 7 2 . 2 1 7 2 . 2 1 1 ± 630 Dy-OSL-1/7 Luminescence Day Road Dy - 0 2 8 9 Z 0 4 . 4 1 0 4 . 4 1 1 ± 670 Dy-OSL-1/8 Luminescence Day Road Dy - 0 2 7 1 1 Z 1 2 . 8 1 1 2 . 8 1 1 ± 770 Dy-OSL-2/1 Luminescence Day Road Dy - 0 0 7 4 C 9 4 . 5 0 7 . 7 2 ± 350 Dy-OSL-2/2 Luminescence Day Road Dy - 0 7 0 5 E 8 7 . 9 1 8 . 1 1 2 ± 350 Dy-OSL-4/1 Luminescence Day Road Dy - 0 9 8 2 C 4 4 . 5 1 4 . 5 4 ± 210 Dy-OSL-4/2 Luminescence Day Road Dy - 0 8 4 5 C 9 7 . 5 3 5 . 9 4 ± 370 Dy-OSL-4/3 Luminescence Day Road Dy - 0 9 7 4 G 3 8 . 8 1 8 . 1 1 4 ± 350 Dy-OSL-4/4 Luminescence Day Road Dy - 0 3 3 7 Z 0 8 . 2 1 8 3 . 6 1 4 ± 500 Southern Pit DR13-02 Luminescence Day A 8 0 . 1 8 0 . 1 3 1 R D d a o R 770 ± 90 Southern Pit DR13-04 Luminescence Day A 8 7 . 1 8 7 . 1 3 1 R D d a o R 1020 ± 120 Northern Pit DR14-02 Luminescence Day A 2 1 . 1 2 1 . 1 4 1 R D d a o R final dates pending as of the wri Ɵng of this disserta Ɵon Northern Pit DR14-04 Luminescence Day A 8 5 . 1 8 5 . 1 4 1 R D d a o R final dates pending as of the wri Ɵng of this disserta Ɵon DY-OSL-2B/3 Luminescence Day Road Dy- C 9 4 . 5 4 2 . 7 B 2 final dates pending as of the wri Ɵng of this disserta Ɵon DY-OSL-2B/4 Luminescence Day Road Dy- C 9 4 . 5 7 3 . 9 B 2 final dates pending as of the wri Ɵng of this disserta Ɵon DY-OSL-2B/5 Luminescence Day Road Dy- E 1 0 . 7 1 8 . 1 1 B 2 final dates pending as of the wriƟng of this dissertaƟon DY-OSL-2B/8 Luminescence Day Road Dy- G 6 0 . 0 1 7 4 . 5 1 B 2 final dates pending as of the wri Ɵng of this disserta Ɵon DY-OSL-2B/10 Luminescence Day Road Dy- Z 1 1 . 3 1 7 6 . 8 1 B 2 final dates pending as of the wri Ɵng of this disserta Ɵon DY-OSL-2C/5 Luminescence Day Road Dy- C 9 4 . 5 9 0 . 7 C 2 final dates pending as of the wriƟng of this dissertaƟon DY-OSL-2C/6 Luminescence Day Road Dy- C 9 4 . 5 6 4 . 8 C 2 final dates pending as of the wri Ɵng of this dissertaƟon DY-OSL-2C/7 Luminescence Day Road Dy- C 9 4 . 5 9 2 . 0 1 C 2 final dates pending as of the wri Ɵng of this disserta Ɵon Sample Sample type transect borehole actual depth projected depth to borehole DY-1, . l a C t i n U m Yr. BP DY-C14 14 C Day Road DY- 8 2 1 4 4 G 7 2 . 9 7 2 . 9 1 -48526 DY-C12 14 C Day Road DY- 2 9 7 2 5 > C 8 1 . 5 4 9 . 5 3 DY-C5 14 C Day Road DY- 9 2 0 8 C 9 4 . 5 9 9 . 5 3 -8380 DY-C1 14 C Day Road DY- 1 5 0 8 G 0 6 . 9 6 4 . 3 1 4 -8409 DY-C7 14 C Day Road DY- 9 9 8 6 G 0 9 . 9 1 5 . 3 1 4 -7158 DY-C3 14 C Day Road DY- 2 0 8 3 2 I 9 8 . 1 1 9 6 . 6 1 4 -24446 Northern Pit DY-14:CL-01 14 C Day 5 3 3 1 A 6 2 . 1 6 2 . 1 4 1 R D d a o R -1415 DY-2C:CL-1 14 C Day Road Dy- 2 0 7 4 5 > E 2 3 . 7 6 0 . 2 1 C 2 CL-2-BR-3 14 C Briggs Road BR- A / N 5 7 . 7 3 - 45624-69704 CL-4-BR-3 14 C Briggs Road BR- A / N 6 8 . 2 2 3 - 28587-45940 CL-5-BR-4 14 C Briggs Road BR- A / N 9 5 . 8 4 - 31386-41111 214 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 DY - 4 DY-2C DY - 3 DY-2B DY - 2 DY - 1 CPT - 10 CPT - 8 CPT - 7 CPT - 6 CPT - 6B CPT - 5 CPT - 4 CPT - 3 CPT - 2 CPT - 1 CPT - 3B CPT - 11 CPT - 9 0 2 4 6 0 2 4 6 8 0 2 4 6 8 0 2 4 6 0 2 4 6 0 2 4 0 8 10 12 14 16 8 10 12 14 16 10 12 14 16 18 8 10 12 8 10 12 14 16 6 8 10 12 14 8 48 48 50 46 48 50 44 46 48 44 44 38 40 42 44 6 38 40 28 30 32 36 32 34 26 28 30 28 32 34 28 30 32 22 24 26 30 36 34 18 20 22 24 18 20 22 24 26 20 22 24 14 16 18 20 2 24 26 28 2 18 16 18 20 1 46 46 42 40 2 4 6 42 40 26 26 20 2 26 28 22 28 2 32 28 58 58 56 38 38 40 42 38 40 42 44 36 38 40 42 30 32 34 36 8 22 24 26 34 36 8 50 52 54 56 52 54 56 8 52 54 56 46 48 50 52 54 34 36 50 52 54 46 58 8 34 24 ? ? 58 ? ? 8 4 30 ? ? DR13-01 DR13-02 DR13-03 DR13-04 DR13-05 DR13 - pit DR14 - pit DR13-01 DR13-02 DR13-03 DR13-04 >52792 8029 - 8380 6899 - 7158 8051 - 8409 23802 - 24446 44128 - 48526 3170 +/- 230 3060 +/- 230 5460 +/- 330 5020 +/- 310 7930 +/- 530 9060 +/- 630 9820 +/- 670 11720 +/- 770 4700 +/- 350 5070 +/- 350 2890 +/- 210 5480 +/- 370 4790 +/- 350 7330 +/- 500 DY - C15 (19’ 5”) DY - C11 (19’ 4.5”) DY - C18 (20’) DY - C16 (19’ 10”) DY - C5 (19’ 8”) DY - C12 (19’ 6”) DY - C10 (39’ 5”) DY - C13 (39’ 7”) DY - C20 (35’ 5”) Dy-OSL-2/1 Dy-OSL-2/2 Dy-OSL-4/3 Dy-OSL-4/2 Dy-OSL-4/1 DY - C4 (43’ 6”) DY - C1 (44’ 2”) DY - C3 (54’ 9”) DY - C6 (47’ 6”) DY - C2 (55’ 3”) Dy-OSL-4/4 DY - C7 (44’ 4”) DY - C9 (44’ 8”) DY - C8 (35’) DY - C14 (30’ 5”) DY - C17 (58’ 2”) Dy-OSL-1/2 Dy-OSL-1/3 Dy-OSL-1/4 Dy-OSL-1/7 Dy-OSL-1/8 Dy-OSL-1/1 Dy-OSL-1/5 Dy-OSL-1/6 DY - C19 (36’ 1”) Dy-OSL-2B/1 Dy-OSL-2B/2 Dy-OSL-2B/3 Dy-OSL-2B/4 Dy-OSL-2B/5 Dy-OSL-2B/6 Dy-OSL-2B/7 Dy-OSL-2B/8 Dy-OSL-2B/9 Dy-OSL-2B/10 Dy-OSL-2C/1 Dy-OSL-2C/2 Dy-OSL-2C/3 Dy-OSL-2C/4 Dy-OSL-2C/5 Dy-OSL-2C/6 Dy-OSL-2C/7 Dy-OSL-2C/8 Dy-OSL-2C/9 DY - 2C -1 (39’ 7”) 770 +/- 90 1020 +/- 120 1335 - 1415 >54702 Loma Vista N S 3x vertical exaggeration N No growth Event(s) (~4.5m growth) No growth S Most Recent Event(s) (~6m uplift) Day Road, Ventura - CPT/Borehole A B C E G H I F D1 D2 D3 B B B B B B B B B B B B B B B 0 100m 200m Distance (ft) 180 120 60 0 Elevation (ft) 60 40 20 0 20m 0 Unit A - Fine silt Unit B - Sandy silt Unit C - Silt with sandy interbeds Unit D - Coarse gravel Unit E - Medium sand to gravel Unit F - Silty sand Unit G - Fine silt Unit H - Medium sand to gravel Unit I - Fine silt 215 Event Age (ka) Uplift (m) Min Max Min Max Min Max Min Max 1 <1.4 6 7.328.49 7.64 7.69 6.75 6.76 7.91 7.98 2 3-5 4.5 5.49 6.36 7.54 7.59 6.74 6.74 7.76 7.84 Mw Biasi and Weldon (2006) Wells and Coppersmith (1994) Slip (m) All-slip-type displacement Thrust fault-only displacement - Based on the simplifying assumption that our measured displacements represent the average along fault slip in each earthquake - Uplift is based on measured values from Day Road transect - Slip is based on a fault dipping 50° ± 5° providing minimum and maximum slip values - Biasi and Weldon (2006) incorporate the data of Wells and Coppersmith (1994) 216 Ventura fault Ventura + Pitas Point Ventura + Pitas Point + blind ramp Ventura + Pitas Point + blind ramp + San Cayetano Hanks and Bakun (2002; 5 4 . 7 9 0 . 7 3 6 . 6 7 0 . 6 ) 8 0 0 2 Wells and Coppersmith 8 2 . 7 4 0 . 7 1 7 . 6 1 2 . 6 ) 4 9 9 1 ( Fault segment Area (km2) Ventura 122 Pitas point 324.2 Blind Ramp 583.5 San Cayetano 884 Fault area based on fault models produced by Hubbard et al. (2014) Mw 217 1100 1050 1000 950 900 850 800 750 700 650 600 550 approximate depth (m) <<North Station Number South>> 1100 1050 1000 950 900 850 800 750 700 650 600 550 <<North Station Number South>> Briggs Road profile, migrated Briggs Road profile, migrated approximate depth (m) 500 500 1100 1000 900 800 700 600 500 Elevation (m) 160 140 120 100 80 60 40 20 0 Projected synclinal axial surface Foothill road CPT (red)/borehole (green) locations 218 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 40 42 44 52 54 60 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 52 54 56 58 60 62 64 66 68 70 72 74 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 CPT- 1 CPT- 2 CPT- 3 CPT- 0 CPT- 4 CPT- 5 CPT- 6 Br - 1 Br - 2 Br - 3 Br - 4 Br - 5 Br - 6 Br - 7 14 42 44 46 48 50 52 54 56 58 22 24 26 28 16 18 20 24 26 28 30 32 34 36 38 40 12 14 16 18 20 22 20 22 22 20 0 60 62 64 66 68 70 72 42 44 46 48 50 52 54 38 28 30 32 34 36 38 4 38 Briggs Road, Ventura - CPT/Borehole 28587 - 45940 45624 - 69704 31386 - 41111 0 120m 60m 180m 20m 0m S N 5x vertical exaggeration Topography south of Foothill taken from GPS measurements taken from the field. Hill profile taken from Geocontext profiler Foothill Unit A - Fine/medium grained sand Unit B - Coarse grained gravel Unit C - Paleosol Unit D - Fine grained silt to clay 219 CA NV image area AZ San Andreas Fault Ventura Avenue Anticline Pleistocene bedrock (marine) Late Pleistocene terrace deposit (nonmarine) Late Pleistocene terrace deposit (marine) Late Pleistocene old alluvial fan deposits Holocene alluvium Holocene alluvial fan deposits Late Pleistocene/Early Holocene older alluvium Late Pleistocene/Early Holocene landslide deposit Holocene active young alluvial fan deposits City of Ventura Borehole (green) / CPT (yellow) location N Fold scarp 2000 Meters 1000 500 0 1 1 2 2 3 3 Evergreen/Hall Canyon Day Road Brookshire Ave. Arroyo Verde Fan Harmon Canyon Fan 220 0 6 feet DR13-02 DR13-01 DR13-03 DR13-04 DR13-05 Fill Ground surface Subsurface stratigraphy Weak paleosol 221 DR14-03 b a 0 12 cm Silty sand Silty clay DR14-CL01 222 Evergreen/Hall Canyon, Ventura, migrated 223 0 200 400 600 800 approximate depth (m) 200 300 <<South Station Number North>> Brookshire Avenue,Ventura,migrated 224 ? ? ? ? ? ? Loma Vista N S 3x vertical exaggeration N No growth Event(s) (~4.5m growth) No growth S Most Recent Event(s) (~6m uplift) Day Road, Ventura - CPT/Borehole A B C E G H I F D1 D2 D3 B B B B B B B B B B B B B B B 0 100m 200m Distance (ft) 180 120 60 0 Elevation (ft) 60 40 20 0 20m 0 Unit A - Fine silt Unit B - Sandy silt Unit C - Silt with sandy interbeds Unit D - Coarse gravel Unit E - Medium sand to gravel Unit F - Silty sand Unit G - Fine silt Unit H - Medium sand to gravel Unit I - Fine silt 225 APPENDIX A: Possible Evidence for a Pre-Event 1/Post-Event 2 Surface Rupture? On the north wall of the trench T-2 a small-displacement fault with 2-4 cm of vertical separation across several thin-bedded silty clay layers and a crack with no discernible offset are exposed near the base of the colluvial fissure fill at meters 11.5 and 11.0, respectively. The presence of this minor fault within the lower part of the fissure fill suggests either: (1) that minor fault slip has occurred since the MRE, possibly triggered by large aftershocks or by earthquakes on other, nearby faults; (2) that this small-displacement fault occurred during post-earthquake settling, or as a result of shrink and swell processes (desiccation cracks) that may have played a role in the early stages of post-event deposition; or (3) that the lowermost part of the large fissure actually formed during the penultimate event and was subsequently reactivated and enlarged in the MRE. Insofar as this one small-displacement fault is the only structural evidence suggestive of pre- Event 1 faulting in the trench, we consider this third scenario unlikely. Nevertheless, we cannot completely discount the possibility that the lowermost parts of the fissure developed during the penultimate event, and we discuss the possible evidence for a pre-Event 1 surface rupture in more detail below. Aside from the small-displacement fault exposed at m 11.5 on the north wall, the only other possible evidence for a possible pre-MRE, post-Event 2 surface rupture is: (1) the geometry of the fissure itself; (2) vertical changes in the sedimentary character of the fissure infill; and (3) a small-displacement strand of fault 1 that extends upward to approximately the top of the paleosol unit.. In terms of geometry, the fissure, especially in the south wall, consists of a narrow, fault- controlled lower domain, a somewhat wider, partially eroded middle part, and a much wider, 226 highly eroded upper part. This geometry may suggest that its shape is a function of multiple ruptures, with a pre-MRE event horizon located at or near the transition from a narrow to wide geometry. The lower part of the fissure is also filled primarily with colluvial blocks that fell off the fissure walls, whereas the upper fill consist of ponded playa sediments that gradually filled the remainder of the fissure. Based on these observations, the question arises: Do these features record the occurrence of a pre-MRE, post-Event 2 surface rupture? On balance, we consider this scenario unlikely, for the following reasons: (1) as noted above, the small-displacement fault near the base of the fissure fill at m 11.5 is the only structural evidence for a separate event; (2) the upward termination of a single individual strand, such as the strand of fault 1 noted above, does not provide robust evidence for a separate surface rupture; it is a common observation that some fault strands do not always extend all the way to the surface (e.g., Bonilla and Lienkaemper, 1990; Fumal et al., 1993); (3) the absence of structural evidence other than the one small- displacement fault at m 11.5 and the upward fault termination at fault 1 is inconsistent with the occurrece of a separate, pre-Event 1 earthquake – any significant surface rupture would likely have left much more structural evidence, as is the case for the other four surface ruptures documented in this paper; and (4) the shape and vertical changes in sedimentary infill of the fissure are entirely consistent with the fissure at fault 3 forming in a single surface rupture. Specifically, the initial opening of the fissure created sub-vertical walls that shed colluvial blocks that fell into the basal part of the fissure. Subsequent post-surface rupture evolution of the fissure likely involved gradual erosion of the free-standing sub-vertical walls of the fissure (i.e., those parts above the colluvium-filled lower part of the fissure) and consequent gradual erosion of the fissure walls, resulting in the upward-widening geometry we observed. 227 In summary, the absence of any structural evidence for a separate pre-MRE, post-Event 2 surface rupture other than the small fault at m 11.5 and a single upward fault termination at fault 1, coupled with the fact that the geometry and infilling of the fissure can be explained well in a single-event scenario, indicate to us that the fissure formed entirely during the Event 1. 228 APPENDIX B: Additional analysis and maximum slip rate on the central Garlock fault Slip rate data are often presented as minimum rates due of the nature of the landforms which are used in their calculations. Commonly alluvial fans or other depositional features are radiometrically dated, however the drainages that have been incised into those dated features are used to restore the offset feature and calculate a displacement. Because the incision event that formed the drainage occurred after the surface itself was deposited, the age derived from the deposit will provide a maximum age for the timing of the channel incision, and thus a minimum final slip rate. Calculating maximum slip rates on faults, however, is significantly more challenging and requires particular conditions within the tectonic landscape. An example of this would be a case where two fluvial terraces are preserved and can be dated. Using the terrace riser to calculate total displacement along the fault, the two ages calculated from both bounding terraces will provide both a minimum and maximum slip rate for that fault. Because of the availability of both a paleoseismologic record spanning the past ~7.5 ka, together with a full Holocene-latest Pleistocene slip rate, we can play some games to infer a maximum slip rate for the Garlock fault during the two seismic clusters – this would also give us an estimate for the late Holocene rate. McGill et al. (2009) determine a preferred slip rate of 7.6 mm/yr since a preferred date of 9.3 ka along the central/western Garlock fault. Combining the period of time during the two seismic clusters (total time between two blocks in part C of figure 5) we calculate between 3985 and 2505 years between the two seismic clusters going back 9.3 ka. Since the displacement required for the preferred 7.6 mm/yr since 9.3 ka is 70.68 meters, and assigning 0 mm/yr as the slip rate on the fault during the intervening seismic lulls, we can determine the necessary slip rate 229 during the two seismic clusters to achieve 7.6 mm/yr averaged over the full Holocene. This calculated slip rate is 17.7-28.2 mm/yr. This is a maximum slip rate because we are assuming that the paleoseismic record at El Paso Peaks is complete until 9.3 ka. This maximum slip rate is surprisingly high and upon further examination illustrates a possible problem with the slip rate of McGill et al., (2009). Given the assumption that the paleoseismologic record is complete going back 9.3 ka, the 70.68 meters of displacement would require a) an unreasonable amount of slip during events 4,5 and 6, or b) that the paleoseismologic record is in fact not complete and there are in fact multiple events between 9.3 ka and the oldest event recorded at the El Paso Peaks site. In addition, it is very possible that the incision event used to calculate the preferred slip rate is slightly older than predicted (likely following the end of the Younger Dryas event at 11.5 ka), which would both result in lowering the full Holocene slip rate, and provide more time for multiple events before event 6 (Event F in Dawson et al., 2003) at El Paso Peaks. It is however possible, and perhaps quite likely that we are in fact missing events between the 6 th (oldest) event recorded at El Paso Peaks and the preferred date used for calculating the 7.6 mm/yr slip rate (a time interval of ~2300 years). Additionally, the uncertainties on the slip rate by McGill et al. (2009) illustrate that the rate could in fact be averaged over a time interval nearly 4 ka older than the preferred age used, resulting in a slower total rate. It is possible and likely that if the age of the channel is much older, the 70.68 meters of displacement is accounting for events including those that are older than the six recorded at the El Paso Peaks trench site. Furthermore, the slip per event values do not seem realistic if we use only 6 events to record the 70.68 meters of displacement. Assuming even the maximum displacement of 20 meters during the last three events on the Garlock fault (McGill and Sieh, 1993), this would mean that we would need to account for over 50 meters of slip during the preceding three events. This seems 230 highly unlikely and leads to the conclusion that there are likely missing events that account for the ~71 meters of displacement. This detailed analysis of the slip rate and paleoseismologic data is puzzling because of the careful nature in which the preferred age of the incised drainage was determined. McGill et al. (2009) chose a preferred age of 9.3 ka as the timing of the incision event because of the climatic conditions favorable for incision after a period of likely fan deposition between 11.5 ka and 9.3 ka (Harvey and Wells, 2003). However, as noted in Ganev et al. (2013), the end of the Younger Dryas period at ~11.5 ka resulted in wetter conditions in the Mojave (Mott et al., 1986; Johnson et al., 1992; Edwards et al., 1993; Kenett & Ingram, 1995; Lowell et al., 1995; Osborn, 1995; Peteet, 1995; Benson et al., 1997; Mikolajewicz et al., 1997), suggesting that incision actually occurred slightly earlier than what has been suggested by McGill et al. (2009). Moving the preferred age of the incised channel back several thousand years would both decrease the preferred slip rate, as well as allow for the possibility of several more events along the Garlock fault – both of these would better fit the paleoseismologic and slip rate data. 231 APPENDIX C: Day Road CPT and borehole logs The following figures are the individual borehole and CPT logs from the Day Road transect. The CPTs were conducted by Gregg Drilling using an integrated electronic cone system. In each of the CPT logs the individual graphs show measurements of cone bearing resistance (q c ), total cone resistance (q t ), sleeve friction (f s ), friction ratio (R f ), penetration pore pressure (u) and soil behavior type (SBT). q t =q c +u(1-a), where ‘a ’ equals the net area ratio for the cone. The soil behavior type (SBT) is a basic interpretation based on current published empirical correlations based on Robertson, 1990 and Lunne et al., 1997. 232 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 DY - 1 Depth (m) 10YR 3/2 10YR 3/4 10YR 3/3 10YR 5/4 2.5Y 3/2 2.5Y 4/1 2.5Y 4/2 2.5Y 4/3 2.5Y 5/1 2.5Y 5/2 2.5Y 6/1 2.5Y 6/2 2.5Y 3/3 MUNSELL SOIL COLORS 10YR 5/2 10YR 4/4 2.5Y 6/3 No recovery/ cobbles 10YR 4/3 10YR 4/2 10YR 5/3 10YR 6/3 2.5Y 5/4 2.5Y 4/4 2.5Y 5/3 2.5Y 6/4 2.5Y 3/2 - 3/3 2.5Y 5/4 - 4/4 2.5Y 4/3 - 4/2 2.5Y 6/4 - 5/4 2.5Y 4/4 - 4/3 2.5Y 5/6 2.5Y 5/6 - 5/4 10YR 5/6 10YR 5/4 - 5/6 10YR 3/3 - 3/2 10YR 4/6 10YR 4/4 - 4/6 10YR 4/3 - 3/3 2.5Y 5/4 - 5/3 2.5Y 6/3 - 5/3 2.5Y 4/3 - 3/3 2.5Y 4/2 - 3/2 Grainsize Color Clay Silty clay Very coarse sand to gravel COLOR CODE FOR GRAIN SIZE Silty Sand (VFGS) - FGS Clayey silt Sandy silt - silt Med-coarse sand No recovery Dy-OSL-1/1 Dy-OSL-1/2 Dy-OSL-1/3 Dy-OSL-1/4 Dy-OSL-1/5 Dy-OSL-1/6 Dy-OSL-1/7 Dy-OSL-1/8 DY-C14 DY-C19 DY-C17 Luminescence sample Radiocarbon sample 233 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 DY - 2 Depth (m) 10YR 3/2 10YR 3/4 10YR 3/3 10YR 5/4 2.5Y 3/2 2.5Y 4/1 2.5Y 4/2 2.5Y 4/3 2.5Y 5/1 2.5Y 5/2 2.5Y 6/1 2.5Y 6/2 2.5Y 3/3 MUNSELL SOIL COLORS 10YR 5/2 10YR 4/4 2.5Y 6/3 No recovery/ cobbles 10YR 4/3 10YR 4/2 10YR 5/3 10YR 6/3 2.5Y 5/4 2.5Y 4/4 2.5Y 5/3 2.5Y 6/4 2.5Y 3/2 - 3/3 2.5Y 5/4 - 4/4 2.5Y 4/3 - 4/2 2.5Y 6/4 - 5/4 2.5Y 4/4 - 4/3 2.5Y 5/6 2.5Y 5/6 - 5/4 10YR 5/6 10YR 5/4 - 5/6 10YR 3/3 - 3/2 10YR 4/6 10YR 4/4 - 4/6 10YR 4/3 - 3/3 2.5Y 5/4 - 5/3 2.5Y 6/3 - 5/3 2.5Y 4/3 - 3/3 2.5Y 4/2 - 3/2 Grainsize Color Clay Silty clay Very coarse sand to gravel COLOR CODE FOR GRAIN SIZE Silty Sand (VFGS) - FGS Clayey silt Sandy silt - silt Med-coarse sand No recovery Dy-OSL-2/1 Dy-OSL-2/2 DY-C20 DY-C10 DY-C13 Luminescence sample Radiocarbon sample 234 DY-2B Depth (m) 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 Grainsize Color 10YR 3/2 10YR 3/4 10YR 3/3 10YR 5/4 2.5Y 3/2 2.5Y 4/1 2.5Y 4/2 2.5Y 4/3 2.5Y 5/1 2.5Y 5/2 2.5Y 6/1 2.5Y 6/2 2.5Y 3/3 MUNSELL SOIL COLORS 10YR 5/2 10YR 4/4 2.5Y 6/3 No recovery/ cobbles 10YR 4/3 10YR 4/2 10YR 5/3 10YR 6/3 2.5Y 5/4 2.5Y 4/4 2.5Y 5/3 2.5Y 6/4 2.5Y 3/2 - 3/3 2.5Y 5/4 - 4/4 2.5Y 4/3 - 4/2 2.5Y 6/4 - 5/4 2.5Y 4/4 - 4/3 2.5Y 5/6 2.5Y 5/6 - 5/4 10YR 5/6 10YR 5/4 - 5/6 10YR 3/3 - 3/2 10YR 4/6 10YR 4/4 - 4/6 10YR 4/3 - 3/3 2.5Y 5/4 - 5/3 2.5Y 6/3 - 5/3 2.5Y 4/3 - 3/3 2.5Y 4/2 - 3/2 Clay Silty clay Very coarse sand to gravel COLOR CODE FOR GRAIN SIZE Silty Sand (VFGS) - FGS Clayey silt Sandy silt - silt Med-coarse sand No recovery Dy-OSL-2B/5 Dy-OSL-2B/6 Dy-OSL-2B/7 Dy-OSL-2B/8 Dy-OSL-2B/9 Dy-OSL-2B/10 Dy-OSL-2B/4 Dy-OSL-2B/3 Dy-OSL-2B/2 Dy-OSL-2B/1 Luminescence sample Radiocarbon sample 235 DY-2C Depth (m) 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 10YR 3/2 10YR 3/4 10YR 3/3 10YR 5/4 2.5Y 3/2 2.5Y 4/1 2.5Y 4/2 2.5Y 4/3 2.5Y 5/1 2.5Y 5/2 2.5Y 6/1 2.5Y 6/2 2.5Y 3/3 MUNSELL SOIL COLORS 10YR 5/2 10YR 4/4 2.5Y 6/3 No recovery/ cobbles 10YR 4/3 10YR 4/2 10YR 5/3 10YR 6/3 2.5Y 5/4 2.5Y 4/4 2.5Y 5/3 2.5Y 6/4 2.5Y 3/2 - 3/3 2.5Y 5/4 - 4/4 2.5Y 4/3 - 4/2 2.5Y 6/4 - 5/4 2.5Y 4/4 - 4/3 2.5Y 5/6 2.5Y 5/6 - 5/4 10YR 5/6 10YR 5/4 - 5/6 10YR 3/3 - 3/2 10YR 4/6 10YR 4/4 - 4/6 10YR 4/3 - 3/3 2.5Y 5/4 - 5/3 2.5Y 6/3 - 5/3 2.5Y 4/3 - 3/3 2.5Y 4/2 - 3/2 Grainsize Color Clay Silty clay Very coarse sand to gravel COLOR CODE FOR GRAIN SIZE Silty Sand (VFGS) - FGS Clayey silt Sandy silt - silt Med-coarse sand No recovery Dy-OSL-2C/1 Dy-OSL-2C/2 Dy-OSL-2C/3 Dy-OSL-2C/4 Dy-OSL-2C/5 Dy-OSL-2C/6 Dy-OSL-2C/7 Dy-OSL-2C/8 Dy-OSL-2C/9 DY-2C-1 Luminescence sample Radiocarbon sample 236 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 DY - 3 Depth (m) 10YR 3/2 10YR 3/4 10YR 3/3 10YR 5/4 2.5Y 3/2 2.5Y 4/1 2.5Y 4/2 2.5Y 4/3 2.5Y 5/1 2.5Y 5/2 2.5Y 6/1 2.5Y 6/2 2.5Y 3/3 MUNSELL SOIL COLORS 10YR 5/2 10YR 4/4 2.5Y 6/3 No recovery/ cobbles 10YR 4/3 10YR 4/2 10YR 5/3 10YR 6/3 2.5Y 5/4 2.5Y 4/4 2.5Y 5/3 2.5Y 6/4 2.5Y 3/2 - 3/3 2.5Y 5/4 - 4/4 2.5Y 4/3 - 4/2 2.5Y 6/4 - 5/4 2.5Y 4/4 - 4/3 2.5Y 5/6 2.5Y 5/6 - 5/4 10YR 5/6 10YR 5/4 - 5/6 10YR 3/3 - 3/2 10YR 4/6 10YR 4/4 - 4/6 10YR 4/3 - 3/3 2.5Y 5/4 - 5/3 2.5Y 6/3 - 5/3 2.5Y 4/3 - 3/3 2.5Y 4/2 - 3/2 Grainsize Color Clay Silty clay Very coarse sand to gravel COLOR CODE FOR GRAIN SIZE Silty Sand (VFGS) - FGS Clayey silt Sandy silt - silt Med-coarse sand No recovery DY-C15 DY-C12 DY-C5 DY-C16 DY-C18 DY-C11 DY-C8 Luminescence sample Radiocarbon sample 237 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 DY - 4 Depth (m) 10YR 3/2 10YR 3/4 10YR 3/3 10YR 5/4 2.5Y 3/2 2.5Y 4/1 2.5Y 4/2 2.5Y 4/3 2.5Y 5/1 2.5Y 5/2 2.5Y 6/1 2.5Y 6/2 2.5Y 3/3 MUNSELL SOIL COLORS 10YR 5/2 10YR 4/4 2.5Y 6/3 No recovery/ cobbles 10YR 4/3 10YR 4/2 10YR 5/3 10YR 6/3 2.5Y 5/4 2.5Y 4/4 2.5Y 5/3 2.5Y 6/4 2.5Y 3/2 - 3/3 2.5Y 5/4 - 4/4 2.5Y 4/3 - 4/2 2.5Y 6/4 - 5/4 2.5Y 4/4 - 4/3 2.5Y 5/6 2.5Y 5/6 - 5/4 10YR 5/6 10YR 5/4 - 5/6 10YR 3/3 - 3/2 10YR 4/6 10YR 4/4 - 4/6 10YR 4/3 - 3/3 2.5Y 5/4 - 5/3 2.5Y 6/3 - 5/3 2.5Y 4/3 - 3/3 2.5Y 4/2 - 3/2 Grainsize Color Clay Silty clay Very coarse sand to gravel COLOR CODE FOR GRAIN SIZE Silty Sand (VFGS) - FGS Clayey silt Sandy silt - silt Med-coarse sand No recovery Dy-OSL-4/4 Dy-OSL-4/3 Dy-OSL-4/2 Dy-OSL-4/1 DY-C2 DY-C3 DY-C2 DY-C9 DY-C7 DY-C1 DY-C4 Luminescence sample Radiocarbon sample 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 APPENDIX D: Briggs Road CPT and borehole logs The following figures are the individual borehole and CPT logs from the Day Road transect. The CPTs were conducted by Gregg Drilling using an integrated electronic cone system. In each of the CPT logs the individual graphs show measurements of cone bearing resistance (q c ), total cone resistance (q t ), sleeve friction (f s ), friction ratio (R f ), penetration pore pressure (u) and soil behavior type (SBT). q t =q c +u(1-a), where ‘a ’ equals the net area ratio for the cone. The soil behavior type (SBT) is a basic interpretation based on current published empirical correlations based on Robertson, 1990 and Lunne et al., 1997. 265 Br - 1 Grainsize Color 10YR 3/2 10YR 3/4 10YR 3/3 10YR 5/4 2.5Y 3/2 2.5Y 4/1 2.5Y 4/2 2.5Y 4/3 2.5Y 5/1 2.5Y 5/2 2.5Y 6/1 2.5Y 6/2 2.5Y 3/3 MUNSELL SOIL COLORS 10YR 5/2 10YR 4/4 2.5Y 6/3 No recovery/ cobbles 10YR 4/3 10YR 4/2 10YR 5/3 10YR 6/3 2.5Y 5/4 2.5Y 4/4 2.5Y 5/3 2.5Y 6/4 2.5Y 3/2 - 3/3 2.5Y 5/4 - 4/4 2.5Y 4/3 - 4/2 2.5Y 6/4 - 5/4 2.5Y 4/4 - 4/3 2.5Y 5/6 2.5Y 5/6 - 5/4 10YR 5/6 10YR 5/4 - 5/6 10YR 3/3 - 3/2 10YR 4/6 10YR 4/4 - 4/6 10YR 4/3 - 3/3 2.5Y 5/4 - 5/3 2.5Y 6/3 - 5/3 2.5Y 4/3 - 3/3 2.5Y 4/2 - 3/2 Depth (m) 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 Clay Silty clay Very coarse sand to gravel COLOR CODE FOR GRAIN SIZE Silty Sand (VFGS) - FGS Clayey silt Sandy silt - silt Med-coarse sand No recovery Luminescence sample Radiocarbon sample 266 Br - 2 Grainsize Color 10YR 3/2 10YR 3/4 10YR 3/3 10YR 5/4 2.5Y 3/2 2.5Y 4/1 2.5Y 4/2 2.5Y 4/3 2.5Y 5/1 2.5Y 5/2 2.5Y 6/1 2.5Y 6/2 2.5Y 3/3 MUNSELL SOIL COLORS 10YR 5/2 10YR 4/4 2.5Y 6/3 No recovery/ cobbles 10YR 4/3 10YR 4/2 10YR 5/3 10YR 6/3 2.5Y 5/4 2.5Y 4/4 2.5Y 5/3 2.5Y 6/4 2.5Y 3/2 - 3/3 2.5Y 5/4 - 4/4 2.5Y 4/3 - 4/2 2.5Y 6/4 - 5/4 2.5Y 4/4 - 4/3 2.5Y 5/6 2.5Y 5/6 - 5/4 10YR 5/6 10YR 5/4 - 5/6 10YR 3/3 - 3/2 10YR 4/6 10YR 4/4 - 4/6 10YR 4/3 - 3/3 2.5Y 5/4 - 5/3 2.5Y 6/3 - 5/3 2.5Y 4/3 - 3/3 2.5Y 4/2 - 3/2 Depth (m) 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 75 Clay Silty clay Very coarse sand to gravel COLOR CODE FOR GRAIN SIZE Silty Sand (VFGS) - FGS Clayey silt Sandy silt - silt Med-coarse sand No recovery Luminescence sample Radiocarbon sample 267 Br - 3 Grainsize Color 10YR 3/2 10YR 3/4 10YR 3/3 10YR 5/4 2.5Y 3/2 2.5Y 4/1 2.5Y 4/2 2.5Y 4/3 2.5Y 5/1 2.5Y 5/2 2.5Y 6/1 2.5Y 6/2 2.5Y 3/3 MUNSELL SOIL COLORS 10YR 5/2 10YR 4/4 2.5Y 6/3 No recovery/ cobbles 10YR 4/3 10YR 4/2 10YR 5/3 10YR 6/3 2.5Y 5/4 2.5Y 4/4 2.5Y 5/3 2.5Y 6/4 2.5Y 3/2 - 3/3 2.5Y 5/4 - 4/4 2.5Y 4/3 - 4/2 2.5Y 6/4 - 5/4 2.5Y 4/4 - 4/3 2.5Y 5/6 2.5Y 5/6 - 5/4 10YR 5/6 10YR 5/4 - 5/6 10YR 3/3 - 3/2 10YR 4/6 10YR 4/4 - 4/6 10YR 4/3 - 3/3 2.5Y 5/4 - 5/3 2.5Y 6/3 - 5/3 2.5Y 4/3 - 3/3 2.5Y 4/2 - 3/2 Depth (m) 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 Clay Silty clay Very coarse sand to gravel COLOR CODE FOR GRAIN SIZE Silty Sand (VFGS) - FGS Clayey silt Sandy silt - silt Med-coarse sand No recovery CL-2-BR-3 CL-4-BR-3 Luminescence sample Radiocarbon sample 268 Br - 4 Grainsize Color 10YR 3/2 10YR 3/4 10YR 3/3 10YR 5/4 2.5Y 3/2 2.5Y 4/1 2.5Y 4/2 2.5Y 4/3 2.5Y 5/1 2.5Y 5/2 2.5Y 6/1 2.5Y 6/2 2.5Y 3/3 MUNSELL SOIL COLORS 10YR 5/2 10YR 4/4 2.5Y 6/3 No recovery/ cobbles 10YR 4/3 10YR 4/2 10YR 5/3 10YR 6/3 2.5Y 5/4 2.5Y 4/4 2.5Y 5/3 2.5Y 6/4 2.5Y 3/2 - 3/3 2.5Y 5/4 - 4/4 2.5Y 4/3 - 4/2 2.5Y 6/4 - 5/4 2.5Y 4/4 - 4/3 2.5Y 5/6 2.5Y 5/6 - 5/4 10YR 5/6 10YR 5/4 - 5/6 10YR 3/3 - 3/2 10YR 4/6 10YR 4/4 - 4/6 10YR 4/3 - 3/3 2.5Y 5/4 - 5/3 2.5Y 6/3 - 5/3 2.5Y 4/3 - 3/3 2.5Y 4/2 - 3/2 Depth (m) 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 Clay Silty clay Very coarse sand to gravel COLOR CODE FOR GRAIN SIZE Silty Sand (VFGS) - FGS Clayey silt Sandy silt - silt Med-coarse sand No recovery CL-5-BR-4 Luminescence sample Radiocarbon sample 269 Br - 5 Grainsize Color 10YR 3/2 10YR 3/4 10YR 3/3 10YR 5/4 2.5Y 3/2 2.5Y 4/1 2.5Y 4/2 2.5Y 4/3 2.5Y 5/1 2.5Y 5/2 2.5Y 6/1 2.5Y 6/2 2.5Y 3/3 MUNSELL SOIL COLORS 10YR 5/2 10YR 4/4 2.5Y 6/3 No recovery/ cobbles 10YR 4/3 10YR 4/2 10YR 5/3 10YR 6/3 2.5Y 5/4 2.5Y 4/4 2.5Y 5/3 2.5Y 6/4 2.5Y 3/2 - 3/3 2.5Y 5/4 - 4/4 2.5Y 4/3 - 4/2 2.5Y 6/4 - 5/4 2.5Y 4/4 - 4/3 2.5Y 5/6 2.5Y 5/6 - 5/4 10YR 5/6 10YR 5/4 - 5/6 10YR 3/3 - 3/2 10YR 4/6 10YR 4/4 - 4/6 10YR 4/3 - 3/3 2.5Y 5/4 - 5/3 2.5Y 6/3 - 5/3 2.5Y 4/3 - 3/3 2.5Y 4/2 - 3/2 Depth (m) 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 Clay Silty clay Very coarse sand to gravel COLOR CODE FOR GRAIN SIZE Silty Sand (VFGS) - FGS Clayey silt Sandy silt - silt Med-coarse sand No recovery Luminescence sample Radiocarbon sample 270 Br - 6 Grainsize Color 10YR 3/2 10YR 3/4 10YR 3/3 10YR 5/4 2.5Y 3/2 2.5Y 4/1 2.5Y 4/2 2.5Y 4/3 2.5Y 5/1 2.5Y 5/2 2.5Y 6/1 2.5Y 6/2 2.5Y 3/3 MUNSELL SOIL COLORS 10YR 5/2 10YR 4/4 2.5Y 6/3 No recovery/ cobbles 10YR 4/3 10YR 4/2 10YR 5/3 10YR 6/3 2.5Y 5/4 2.5Y 4/4 2.5Y 5/3 2.5Y 6/4 2.5Y 3/2 - 3/3 2.5Y 5/4 - 4/4 2.5Y 4/3 - 4/2 2.5Y 6/4 - 5/4 2.5Y 4/4 - 4/3 2.5Y 5/6 2.5Y 5/6 - 5/4 10YR 5/6 10YR 5/4 - 5/6 10YR 3/3 - 3/2 10YR 4/6 10YR 4/4 - 4/6 10YR 4/3 - 3/3 2.5Y 5/4 - 5/3 2.5Y 6/3 - 5/3 2.5Y 4/3 - 3/3 2.5Y 4/2 - 3/2 Depth (m) 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 Clay Silty clay Very coarse sand to gravel COLOR CODE FOR GRAIN SIZE Silty Sand (VFGS) - FGS Clayey silt Sandy silt - silt Med-coarse sand No recovery Luminescence sample Radiocarbon sample 271 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 Br - 7 Grainsize Color 10YR 3/2 10YR 3/4 10YR 3/3 10YR 5/4 2.5Y 3/2 2.5Y 4/1 2.5Y 4/2 2.5Y 4/3 2.5Y 5/1 2.5Y 5/2 2.5Y 6/1 2.5Y 6/2 2.5Y 3/3 MUNSELL SOIL COLORS 10YR 5/2 10YR 4/4 2.5Y 6/3 No recovery/ cobbles 10YR 4/3 10YR 4/2 10YR 5/3 10YR 6/3 2.5Y 5/4 2.5Y 4/4 2.5Y 5/3 2.5Y 6/4 2.5Y 3/2 - 3/3 2.5Y 5/4 - 4/4 2.5Y 4/3 - 4/2 2.5Y 6/4 - 5/4 2.5Y 4/4 - 4/3 2.5Y 5/6 2.5Y 5/6 - 5/4 10YR 5/6 10YR 5/4 - 5/6 10YR 3/3 - 3/2 10YR 4/6 10YR 4/4 - 4/6 10YR 4/3 - 3/3 2.5Y 5/4 - 5/3 2.5Y 6/3 - 5/3 2.5Y 4/3 - 3/3 2.5Y 4/2 - 3/2 Depth (m) Clay Silty clay Very coarse sand to gravel COLOR CODE FOR GRAIN SIZE Silty Sand (VFGS) - FGS Clayey silt Sandy silt - silt Med-coarse sand No recovery Luminescence sample Radiocarbon sample 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286

Abstract (if available)

Abstract Understanding the complex behavior of plate boundary faults and fault systems has been an area of ongoing study in the geological sciences. This dissertation focuses on the spatial and temporal patterns of paleo‐earthquakes at three different study sites in southern California, which exhibit non‐steady slip histories over millennial timescales. Elucidating the long‐term patterns of seismicity along a fault or fault system by extending the paleoseismic record of that fault allows us to better understand fault mechanics, possible fault interactions and improve next generation seismic hazard analysis. ❧ Using Light Detection and Ranging (LiDAR) digital topographic data to map and measure offset geomorphic features, paleoseismic trenching to determine paleoearthquakes recorded in the rock record, and luminescence (OSL and IRSL) and ¹⁴C Radiocarbon dating methods, I constrain the timing of paleo‐earthquakes along the Panamint Valley Fault and calculate a late‐Holocene slip rate for the Garlock Fault. These new paleoseismic data from the Panamint Valley Fault support the notion that earthquake occurrence in the eastern California shear zone (ECSZ) may be spatially and temporally complex, with earthquake clusters occurring in different regions at different times. Our new slip rate for the Garlock Fault provides evidence for an elevated late‐Holocene slip rate that is significantly faster than the full‐Holocene/latest Pleistocene rate. This new slip rate provides insight into the behavior of the Garlock Fault on 10³ timescales and documents the temporal variability in slip behavior during the Holocene. The results presented here further validates the suggestion that the Garlock Fault experiences periods rapid fault slip that correlate with earthquake clusters interspersed with millennia‐long periods of no activity and presumably a 0 mm/yr “slip-rate”. These new data also provide a unique opportunity in which we can compare Holocene slip rates at multiple time intervals with a well‐constrained late Holocene paleoseismologic record. ❧ In addition, I use a multi‐disciplinary approach that combines high‐resolution seismic reflection profiles, borehole excavations, and cone penetration testing to analyze the near‐surface fault‐related folding related to the blind faulting of the Ventura and southern San Cayetano Faults at seismogenic depths. Our results provide evidence for at least two temporally discrete uplift events during the Holocene, which together with events recorded by marine terraces to the west, document the possibility of system wide ruptures in the western Transverse Ranges. ❧ The results of this dissertation provide a better understanding of the paleo‐earthquake behavior along the Panamint Valley, Garlock and Ventura‐southern San Cayetano Fault systems and illustrate the complexities of their spatial and temporally variable slip histories. Understanding the complex behavior of these faults has significant implications for regional seismic hazard in southern California.

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University of Southern California Dissertations and Theses

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Asset Metadata

Creator McAuliffe, Lee Joseph (author)

Core Title Paleoseismologic and slip rate studies of three major faults in southern California: understanding the complex behavior of plate boundary fault systems over millenial timescales

School College of Letters, Arts and Sciences

Degree Doctor of Philosophy

Degree Program Geological Sciences

Publication Date 07/31/2014

Defense Date 05/20/2014

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag eastern California shear zone,Garlock fault,neotectonics,OAI-PMH Harvest,paleoseismology,Panamint Valley fault,Ventura fault,western Transverse Ranges

Format application/pdf (imt)

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Dolan, James F. (committee chair), Davis, Gregory A. (committee member), Hogen-Esch, Thieo E. (committee member), Sammis, Charles G. (committee member)

Creator Email lee_mcauliffe@hotmail.com,lmcaulif@usc.edu

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c3-451310

Unique identifier UC11286949

Identifier etd-McAuliffeL-2755.pdf (filename),usctheses-c3-451310 (legacy record id)

Legacy Identifier etd-McAuliffeL-2755-0.pdf

Dmrecord 451310

Document Type Dissertation

Format application/pdf (imt)

Rights McAuliffe, Lee Joseph

Type texts

Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the a...

Repository Name University of Southern California Digital Library

Repository Location USC Digital Library, University of Southern California, University Park Campus MC 2810, 3434 South Grand Avenue, 2nd Floor, Los Angeles, California 90089-2810, USA