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Paleoseismology, geomorphology, and fault interactions of the Raymond fault, Los Angeles County, California

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Paleoseismology, geomorphology, and fault interactions of the Raymond fault, Los Angeles County, California

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Content INFORMATION TO USERS This manuscript has been reproduced from the microfilm master. U M I films the text directly from the original or copy submitted. Thus, some thesis and dissertation copies are in typewriter face, while others may be from any type of computer printer. The quality of this reproduction is dependent upon the quality of the copy submitted. Broken or indistinct print, colored or poor quality illustrations and photographs, print bleedthrough, substandard margins, and improper alignment can adversely affect reproduction. In the unlikely event that the author did not send UM I a complete manuscript and there are missing pages, these w ill be noted. Also, if unauthorized copyright material had to be removed, a note w ill indicate the deletion. Oversize materials (e.g., maps, drawings, charts) are reproduced by sectioning the original, beginning at the upper left-hand comer and continuing from left to right in equal sections with small overlaps. Photographs included in the original manuscript have been reproduced xerographically in this copy. Higher quality 6" x 9’ black and white photographic prints are available for any photographs or illustrations appearing in this copy for an additional charge. Contact UMI directly to order. ProQuest Information and Learning 300 North Zeeb Road. Ann Arbor, M l 48106-1346 USA 800-521-0600 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. NOTE TO USER This reproduction is the best copy available. UMI” Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. PALEOSEIMOLOGY, GEOMORPHOLOGY AND FAULT INTERACTIONS OF THE RAYMOND FAULT, LOS ANGELES COUNTY, CALIFORNIA Kristin D. Weaver A Thesis Presented to the FACULTY OF THE GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA In Partial Fulfillment of the Requirements for the Degree Master of Science (Earth Sciences) August 1999 Copyright 1999 Kristin Weaver Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. UMI Number: 1409663 ___ __® UMI U M I Microform 1409663 Copyright 2002 by ProQuest Information and Learning Company. A ll rights reserved. This microform edition is protected against unauthorized copying under Title 17, United States Code. ProQuest Information and Learning Company 300 North Zeeb Road P.O. Box 1346 Ann Arbor, M l 48106-1346 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. UNIVERSITY O F SO U T H E R N CALIFORNIA THE GRAOUATE SCHOOL UNIVERSITY RARK LOS ANGELES. CALIFORNIA § 0 0 0 7 This thesis, written by J & L fU & O m x J fiB £ £ S £ ._________________________ under the direction of hex.— Thesis Committee, and approved by all its members, has been pre sented to and accepted by the Dean of The Graduate School, in partial fulfillment of the requirements for the degree of M aster o f Science______________________ TSntm January 14» 2000 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table of Contents Acknowledgments............................................................................................................. iv List of Figures................................................................................................................. viii List of Tables.....................................................................................................................ix Abstract..............................................................................................................................x Introduction........................................................................................................................ 1 Geomorphology................................................................................................................. 3 Previous Paleoseismic Work on the Raymond Fault........................................................ 12 This Study........................................................................................................................ 13 Los Angeles County Arboretum...........................................................................13 Stratigraphy.............................................................................................. 15 Age Control..............................................................................................20 Evidence for Faulting................................................................................ 23 Interpretation of Trench Results.............................................................. 24 An Additional Fault Along the Southern Side of the Pressure Ridge.........................................................24 Central Fault Zone....................................................................... 25 Southern Fault Zone..................................................................... 28 Sierra Madre Boulevard........................................................................................29 Bucket-auger Hole Transect..................................................................... 29 Ground-Penetrating RADAR Survey.......................................................33 Trench Results.........................................................................................34 Stratigraphy.................................................................................. 34 Age Control...................................................................... 39 Evidence for Faulting.........................................................43 Interpretation of Trench Results...................................................44 Discussion........................................................................................................................53 Conclusion........................................................................................................................62 Bibliography.....................................................................................................................64 Appendix 1 . Photomosaics of the Sierra Madre Boulevard Trench Walls.......................67 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Appendix 2. Field Logs from the Los Angeles County Arboretum Trench.................. 70 Appendix 3. Bucket-auger Hole Logs from the Sierra Madre Boulevard Site................ 109 Appendix 4. Ground-Penetrating RADAR Data from the Sierra Madre Boulevard Site...................................................................................................................... 114 Appendix 5. Field Logs from the Sierra Madre Boulevard Trench................................ 129 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Acknowledgments First and foremost I would like to acknowledge Dr. James F. Dolan for introducing me to the faults in and around Los Angeles and for suggesting this project on the Raymond fault in particular. James has shown me the difference between being a geologist and being a scientist, and his paleoseismological experience and comm unity contacts have been invaluable to me as I completed this work. I have enjoyed working with him as we conducted our research, taught classes, and socialized together. I hope that he has gained even a quarter as much knowledge, experience and fun as I have in our three years working together. I am grateful to the other members of m y thesis committee for putting up with long-distance thesis reviews and corrections. Dr. Greg Davis provided me with valuable critiques and useful suggestions for improving my work. Not only has Greg's influence helped me write this thesis, but it has also made my time here at USC rich and rewarding. He has taught me an amazing am ount about the geology of the western U.S. and the world. My other committee member, Dr. Charlie Sammis has also been a big help to me while writting this thesis. Not only is he a great resource, but Charlie is one of the funniest professors I know, and it was a pleasure being a teaching assistant in his Introduction to Earthquakes class. I w ould also like to thank John Provine and the Los Angeles County Arboretum Staff, and Debbie Bell and the residents and city officials of the City of San Marino for paving the way for us to excavate on their respective properties. Special thanks to Beta Analytic for processing the many carbon samples I collected and to Don Schwarzkopf of Terra Geosciences for conducting the Ground-Penetrating RADAR (GPR) study at the Sierra Madre iv Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Boulevard site. This investigation could also not have been completed without the expertise and equipment provided by TriValley Drilling and HazMat Backhoe. Steve Richardson of HazMat and Ron Cueller, our friendly backhoe operators, provided our daily requirement of interesting conversation and jokes during the opening and closing of the trenches. Dr. Richard Crook, Dr. Richard Proctor, and Dr. Robert Hill were incredibly helpful and provided me with data from their earlier investigations of the Raymond fault. I would also like to thank Steven Lipshie at the Los Angeles County Public Works and Jerry Treiman at the California Division of Mines and Geology for allowing me access to their departm ent's collections of aerial photos and maps. My deepest appreciation goes out to David Bowman and Cindy Waite for assisting me in various ways while I completed my thesis while working in Houston, TX. Both of these kind and tolerant individuals received somewhat frantic phone calls and emails from me over the course of the summer and were always willing to track down whatever it was that I needed. They went above and beyond the call of duty, and I owe them both a nice dinner at Killer Shrim p. I would also like to thank the current and previous USC Earth Science Administrative Staffers for helping me get through my masters program in various ways: Cindy Waite, Vardui Tersimmon, Brad "the business guy and cool office dude" Zagnoev, Barbara Grubb, Eric Hovanitz and Rene Kirby. These six people have made my time here much easier, and a lot of fun. Extra special thanks must go out to everyone who gave their time to help pound fence posts, erect chain-link fencing, place shores in the trenches, log the trenches and excavate the bucket-auger holes: Joe "Big Mama" Barr, Jeff Beard, Proto-doctor David Bowman, Steve "Dr. Funkinstein" Colbert, Arie v Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Margaret "Doo bee, doo bee doo" Glasscoe, Ross Hartleb, Soparkh "monkey boy" Khalsa, Ilene Cooper, Wayne Marko, Wende "I want to play in the trench" Owen, Mathew "breakfast burrito" Ragan, Brooks Ramsdell, Meredith "crash" Robertson, Allan Tucker, and Joel Wedberg. I could not have done it without all of you! (My sincerest apologies to anyone who was left out - your time in the trenches was still greatly appreciated). My family and the friends I have had while at USC deserve recognition, as well. These people have all sprinkled my life generously with hugs, kind words, and cheering jokes, even when they were experiencing their own stressful times. My parents have helped me, financially and otherwise, to get through the rough and tumble years of college in order to get to graduate school. I know they didn't want me to go as far away for my advanced degree as California, but they kept their mouths closed and continued to encourage me despite the distance. As for my friends, David "Proto-doctor" Bowman is the best (and only) TA I have ever had, as well as one of my best friends. I am indebted to him for making me see my strengths, but also for helping me grow stronger still. Along with Dave, Margaret "puppy toes" Fraiser, Steve "grotesquely tall" Dombos, and Dr. Adam " Dr. Wooooods" Woods have provided support and comic relief, and have been my requisite bowling buddies during my time at USC. Wende and Steve Owen also supplied wittisms, Mario Party and various comfort foods during those times when I just couldn't stay in front of a computer or in a trench any longer. Thanks, too, to Morgan for being cute. In addition, Lori Schacht, my best friend from high school, college and beyond, and Sue Bilek, whom I have known since our internship at the USGS, have always been there to urge me on when times got tough these past few years. vi Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. I could not have done any of this without the support of the Department of Earth Sciences, which provided me with several semesters of teaching assistantships and money for conference travel, the USC Graduate and Professional Student Senate, which gave me several conference travel grants, and the Amoco Corporation, which awarded me with three months of summer support. This research project was funded by the Southern California Earthquake Center. Finally, I would like to thank Mother Nature and Plate Tectonics, for without whom none of this would have been possible. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. List of Figures Figure 1. Map of Faults in Southern California................................................................2 Figure 2. Map of the Raymond Fault............................................................................... 5 Figure 3. 1988 Pasadena Earthquake............................................................................... 1 1 Figure 4. Topographic Map of the Los Angeles County Arboretum and Surrounding Area...................................................................................................................... 14 Figure 5. Trench 1 logs with C1 4 Dates........................................................................... 16 Figure 6. Central Fault Strand Detail...............................................................................18 Figure 7. Vertical T1 Strand Detail with C1 4 Ages.........................................................19 Figure 8. Re-exposed Central Fault Strand.....................................................................26 Figure 9. Topographic Map of Sierra Madre Boulevard and Surrounding Area.................................................................................................................... 30 Figure 10. Borehole and Sierra Madre Boulevard Trench Locations.............................. 31 Figure 11. Bore Hole Cross Section............................................................................... 32 Figure 12. Ground-Penetrating RADAR Survey........................................................... 35 Figure 13. Sierra Madre Trench Logs............................................................................. 36 Figure 14. Sierra Madre Trench Cl4Ages...................................................................... 38 Figure 15. Detail of Sierra Madre Site Surface-rupturing Events....................................46 Figure 16. Idealized Chronology of Sierra Madre Site Surface-rupturing Events........... 47 Figure 17. Summary of Events at Sierra Madre Boulevard............................................59 viii Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. List of Tables Table 1. C1 4 Samples from Raymond Fault Trench 1, L.A. County Arboretum, Arcadia, C A ................................................................................................................................................21 Table 2. C1 4 Samples from Raymond Fault - Sierra Madre Trench, San Marino, CA......40 Table 3. Calandric Ages for C ^ Samples from Raymond fault - Sierra Madre Trench, San Marino, CA................................................................................................... 58 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Abstract The geomorphically well-defined Raymond fault extends for 20 kilometers in a broad convex-to-the-south arc across the San Gabriel Valley northeast of downtown Los Angeles. Much of the fault may be characterized by consistently south-facing scarps, but not by the 60 to 425-meter-long stream offsets, the focal mechanism of the 1988 Pasadena earthquake, the very steep dip of the fault, and offset of a crystalline basement ridge at the eastern end of the fault. These all indicate a predom inant component of left-lateral motion. An apparent 3.4 kilometer offset of the basement ridge may represent total slip on the fault. Our paleoseismologic trench data, in combination with published data, indicate that the most recent Raymond fault surface rupture occurred -1,000 to 2,000 years ago. Data from another of our trenches yielded evidence for at least five latest Pleistocene earthquakes, including a group of four surface ruptures that occurred during a short-lived cluster of 3,500 to 7.000 years duration. The average recurrence interval for this cluster is much shorter than the published average recurrence interval for the fault. Rupture of the Raymond fault in its entirety could produce a Mw -6.7 event. However, the Raymond fault may interact with several other major faults, including the Sierra Madre, Verdugo, and Hollywood-Santa Monica fault systems. Paleoseismologic data from the central Sierra Madre and Verdugo faults are as-yet too poorly constrained to compare with Raymond fault events, but the -6,000 to 9,000 year-old most recent surface rupture on the Hollywood fault (Dolan and others, in prep.) conflicts with the 1,000 to 2.000 year-old age for the most recent Raymond fault event, indicating that the Raymond fault did not rupture together with the Hollywood fault during its most recent event. x Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Introduction The Los Angeles metropolitan area is built atop a major transition between two tectonic provinces. In the southern part of the metropolitan region northwest-trending right-lateral strike-slip faults accommodate most plate-boundary strain. In contrast, in the northern part of the urbanized area deformation is dominated by generally west-trending reverse faults and left- lateral strike-slip and oblique-slip faults (Figure 1). Over the past several decades, this complicated network of faults beneath the northern half of the urban area has generated a number of moderate to moderately large earthquakes, including most recently the 1994 Mw 6.7 Northridge event. This earthquake was the most expensive disaster in United States history and (Scientists of the USGS and SCEC, 1995) clearly demonstrates the hazards associated with active urban faults. Despite a heightened awareness of the potential hazards for destructive earthquakes generated by faults beneath metropolitan Los Angeles, the earthquake histories, kinematics, locations, and geometries of many of these faults remain poorly known. This greatly complicates our understanding of the seismic hazards facing the metropolitan Los Angeles region. Over the past several years, geologists of the Southern California Earthquake Center (SCEC) have been studying the paleoseismology and geomorphology of a number of the major faults in the northern part of the Los Angeles metropolitan region. In addition to better documenting the seismic hazards posed by these faults, a parallel focus of our research has been to begin the construction of a space-time history of earthquake occurrence across the fault systems underneath Los Angeles. Such data from regional fault networks will be a critical resource in testing various models of earthquake occurrence. Two of 1 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission o f th e copyright owner. Further reproduction prohibited without permission. Gabriel Cucamonga >«)■• fault k / \ Fernando fit Raymond Gabriel Figure 1. Map of the Los Angeles basin, including the Raymond fault and surrounding area. The black stars labeled 1 and 2 refer to our trench sites at the Los Angeles County Arboretum and Sierra Madre Boulevard in the City of San Marino, respectivly. Abbreviations: SJcf, San Jacinto fault; C-Sf, Clamshell-Sawpit fault; SMf, Santa Monica fault; Hoi fit, Hollywood fault; SAf, San Andreas fault; WN, Whittier Narrows; PV, Paleos Verdes; LB, Long Beach; LA, Los Angeles. the more fundamental questions we are trying to answer are: (1) can we rule out simultaneous rupture of mechanically linked faults within the system, or are the data permissive of the simultaneous rupture of adjacent faults?; and (2) do we see regional temporal clustering of earthquakes that may indicate some sort of intermittent criticality (e.g. Heimpel, 1997; Bowman and others, 1998; Huang and others, 1998; Jaume and Sykes, in press; Sammis and Smith, in press), or has earthquake occurrence been more of a random process (e.g. Ito and Matsuzaki, 1990; Main, 1997; Somette and Somette, 1989; Bak and Teng, 1989; and Geller and others, 1997)? In this paper we describe results from a trenching study of the Raymond fault, a northwest-trending left-lateral strike-slip fault located northeast of downtown Los Angeles. Figure 1 illustrates the relationship between the Raymond fault and neighboring structures, including the Hollywood, Sierra Madre, and Verdugo faults. In one possible scenario the Raymond fault may act as a tear fault that acts to transfer strain from the eastern Sierra Madre thrust fault system to the Verdugo thrust fault. Alternatively, the Raymond fault may project westward beyond the Los Angeles River valley and connect directly with the Hollywood-Santa Monica fault system along strike to form a through-going network of oblique reverse-left-lateral faults in excess of 80 km long. I will first describe our paleoseismologic and geomorphologic studies of the fault and then discus the implications of these data for seismic hazard assessment in the metropolitan Los Angeles region. I will then compare these data with available paleoseismologic data from adjacent faults. GEOMORPHOLOGY The Raymond fault is well-defined geomorphically, with common left- lateral stream offsets and pronounced south-facing scarps along much of its 3 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. length. The scarps generally increase in height from about 12 m to 45 m from east to west, with the exception of a high, crystalline ridge at the fault's eastern end in the City of Monrovia (discussed below). The 20-km-long fault trace is gently arcuate, convex-to-the-south, and extends across the San Gabriel Valley south of the San Gabriel Mountains (Fig. 1). Pronounced lateral changes in geomorphic character and fault orientation define four distinct zones, which we refer to as Zones 1 through 5, from east to west Fig. 2). The eastern end of Zone 1 marks the Raymond fault s confluence with the Sierra Madre fault, a major north-dipping reverse fault that extends along the southern edge of the San Gabriel Mountains for >90 km (Dolan and others, 1995). This portion of the Raymond fault is 3.4 km long and is characterized by south-facing scarps that extend along the southern edge of a prominent, 155-m-high, crystalline ridge made up of Cretaceous diorite and pre- Cretaceous gneiss. The fault forms a boundary between the crystalline rocks to the north and younger Quaternary alluvium to the south. This east- northeast-trending ridge of crystalline rock appears to have been transported westward, outward from the San Gabriel Mountain front along the Raymond fault. I infer that the 3.4 km along-strike length of the basement ridge records the total amounts of left-lateral strike-slip on the Raymond fault (Fig. 2). The western end of Zone 1 is located at the western end of the basement ridge, at Santa Anita Wash. To the west, the Raymond fault trace between Zones I and II is obscured by Quaternary alluvium in Santa Anita Wash (Fig. 2). Zone II stretches for 2.4 km westward from Santa Anita Wash (Fig. 2) to the western edge of the second pressure ridge (number 45 in Fig. 2). This zone exhibits predominantly south-facing scarps, except at two relatively sharp left bends, where the scarps face southeast (numbers 48 and 51 Fig. 2). Two pressure ridges also lie in this zone (numbers 45 and 50 in Fig. 2; Buwalda, 4 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Trench 2, Sierra Madre Boulevard Trench 1, Los Angeles County Arboretum Figure 2. Strip map showing strands of the Raymond fault, fault-related features, and study sites. The heavy dark lines are faults. The thin, pale gray lines and thin black lines represent drainage patterns and selected streets, respectivly. Fault locations have been modified from Weber (1980), Crook and others (1987), Dibblee (1989a and b) and Dolan (1997). The boxes indicate the locations of detailed topographic m aps containing the two study sites discussed. Reproduced with permission o f th e copyright owner. Further reproduction prohibited without permission. Evidence for Quaternary Faulting on the Raymond fault: Key to Figure 2 1) Hollywood fault trace, Dolan and others (1997) 2) Suggested trace of the Raymond fault, mapped by Dibblee (1989) 3) Inferred north-side-down fault, Dibblee (1989) 4) Eagle-Rock fault, Dibblee (1989) 5) Sharply defined break in slope that may mark the active trace of the Eagle-Rock fault (based on 1928 USGS 6’ topographic maps) 6) Two fault strands identified by north-facing scarps observed on aerial photos (Fairchild aerial photos C300 K230 - K 231) 7) Bedrock strands of Eagle-Rock fault, Dibblee (1989) 8) Boreholes 1 did not show evidence of faulting, Crook and others (1987). 9) 2.5 kilometer-long linear depression, Buwalda (1940 pg. 42) 10) North-facing scarps observed on aerial photos (Fairchild aerial photos C300 K230 - K 231) 11) Trenches excavated at the Meridian Tank Site revealed several fault strands, Sakado (1991). 12) South-facing scarp observed on aerial photos (Fairchild aerial photos C300 K230 - K 231) 13) Lineament parallel to bedding, possible stratigraphic control (Fairchild aerial photos C300 K265-267; 1928 USGS 6’ Altadena Quadrangle topographic map; Dibblee, 1989) 14) Bedrock fault exposure in Arroyo bottom interpreted as the Eagle Rock fault, Crook and others (1987); Dibblee (1989). Fault separates Cretaceous Wilson diorite from Tertiary Topanga formation. 15) Two springs in Arroyo Seco wall suggest two strands of the fault, Crook and others (1987) 16) Older edge of Arroyo Seco offset about 120 meters (Fairchild aerial photos C300 K266 - K 267) (R. Hill, unpublished data) 17) Linear, northeast side of Raymond Hill trends parallel with the presumed trace of the Eagle Rock fault, Dibblee (1987) 18) Two parallel, south-facing scarps observed on aerial photos (Fairchild aerial photos C300 K266 - K 267) indicate two strands of the Raymond fault on the south side of Raymond Hill. Dibblee also noted north-dipping terraces in these locations (1940, pg. 43) 19) South-facing scarp on the southern side of Oak Knoll (Fairchild aerial photos C300 K294 - K295). Back-tilted surfaces on top of Oak Knoll (Dibble, 1949, pg. 45) 20) Left-laterally deflected Alhambra Wash along base of steep, south-facing scarp, Buwalda (1940 pg. 46). Jones and o v others noted 330 - 500 meters of left-lateral deflection of the channel formed by uniting the ravines occupied by South Los Robles and South El Molino Avenues (1990). Reproduced w ith permission o f th e copyright owner. Further reproduction prohibited without permission. 21) Drainages offset left-lateraily 180 to 300 meters (based on Fairchild aerial photos C300 K293 - K294) 22) Spring in Kewen Canyon, Crook and others (1987) 23) Southeast-facing scarp (noted by Buwalda, 1940 pg. 48) bends north- and east-ward to connect with a south-facing scarp (based on Fairchild aerial photos C300 K294 - K295) 24) Lacy Park sag pond (depression), Crook and others (1987) (Fairchild aerial photos C300 K294 - K295) 25) Two scarps at Huntington Estate, Buwalda (1940 pg. 51) (also based on Fairchild aerial photos C300 K294 - K295) 26) Gentle north-facing scarp is on trend with south-facing scarps observed to the west (based on Fairchild aerial photos C300 K293 - K294) 27) Depression and pond, Buwalda (1940 pg. 51) (also based on Fairchild aerial photos C300 K317 - K318) 28) South-facing scarp and groundwater lineament (based on Fairchild aerial photos C300 K294 - K295 and K317- 318) 29) One west-trending strand of the Raymond fault was observed in our Sierra Madre Boulevard trench, this study. 30) Pressure ridge, Crook and others (1987) (also based on Fairchild aerial photos C300 K317 - K318) 31) Vegetation and groundwater lineaments suggest two fault strands north of the pressure ridge (based on Fairchild aerial photos C300 K317 - K318) 32) San Marino High School, the site of two trenches excavated by Crook and others (1987) 33) Spring in Rubio Wash, Crook and others (1987) 34) Drainage deflected -300 meters (based on 1928 Sierra Madre 6' Quadrangle Topographic Map) 35) Depression, Buwalda (1940 pg.53) 36) Sunny Slope Resovior, the site of one trench excavated by Crook and others (1987) 37) Drainage deflected 100 meters, Jones and others (1990) (also based on Fairchild aerial photos C300 K317 - K318) 38) Spring in gully, Crook and others (1987) 39) Depression, Buwalda (1940 pg. 54) 40) Drainage deflected -30 meters; drainage to the south was diverted by the construction of Huntington Drive and the Pacific Electric Railroad grade (based on 1928 Sierra Madre 6’ Quadrangle Topographic Map) 41) Vegetation lineaments, Crook and others (1987) (also based on Fairchild aerial photos C300 K337 - K339 and K362 - 363) 42) Deflected drainage and shutter ridge indicate 120 meters of probable offset (based on 1928 Sierra Madre 6’ Quadrangle Topographic Map) Reproduced with permission o f th e copyright owner. Further reproduction prohibited without permission. 43) Left-lateral deflection of gully, Crook and others (1987), and deflection of drainage along south-facing scarp (based on 1928 Sierra Madre 6’ Quadrangle Topographic Map) 44) Deflected drainages along north-facing scarp (based on Fairchild aerial photos C300 K337 - 338 and K362 - 363) 45) Pressure ridge, Buwalda (1940 pg. 55) 46) Drainage into Baldwin Lake offset left-laterally about 60 meters (based on 1928 USGS 6’ Sierra Madre Quadrangle Topographic Map) 47) Baldwin Lake, water-filled depression, Buwalda (1940 pg. 55). Groundwater has ponded against the fault. Baldwin Lake marks the boundary between Raymond fault zones II and III. 48) Southeast-facing scarp of a probable oblique-normal fault accommodating a local component of east-west extension (based on Fairchild aerial photos C300 K362 - 363) 49) Depression, Buwalda (1940 pg. 56) 50) Pressure ridge, Buwalda (1940 pg. 56) 51) Southeast-facing scarp of a probable oblique-normal fault accommodating a local component of east-west extension (based on Fairchild aerial photos C300 K362 - 363) 52) Reverse-displacement of Cretaceous Wilson Diorite over itself and Quaternary alluvium along the Sierra Madre fault, Crook and others (1987) 53) South-facing scarp on 1928 Sierra Madre 6' Quadrangle Topographic Map 54) Trench excavated across possible scarp observed by Crook and others (1987) failed to reveal evidence of faulting. 55) South-facing scarps and vegetation lineaments (based on Fairchild Photos C300 K386 - 388 and L9 and L10) 56) Bedrock spur made up of Cretaceous Wilson Diorite and pre-Cretaceous gneiss, Crook and others (1987) 57) Low scarps observed in drainages (based on Fairchild Photos C300 L9 and L10) 58) Pre-Cretaceous gneiss thrust over Cretaceous Wilson Diorite along the Sawpit Canyon fault, Crook and others (1987) 59) Deflected drainage (based on Fairchild Photos C300 L9 and L10) 60) Quartz monzonite thrust over the Plio-Pleistocene Saugus formation, Crook and others (1987) 61) Groundwater and vegetation lineament (based on Fairchild Photos C300 L9 and L10) 62) East-trending scarp (based on Fairchild Photos C300 L9 and L10) 63) Strands of the Sierra Madre fault, Crook and others (1987) 64) Duarte fault, a strand of the Sierra Madre fault system, Crook and others (1987) OO 1940). Zone III is distinguished from Zone II by the fact that it exhibits less pronounced topography along the fault. Specifically, Zone III does not exhibit any large-scale south-facing scarps or pressure ridges. Zone III does locally exhibit south-facing scarps, but they are much less pronounced than to the east and west. Zone III begins at the western tip of the western pressure ridge in Zone II (number 45 in Fig. 2) and extends westward to the eastern end of the pressure ridge at Sierra Madre Boulevard, (number 30 in Fig. 2). This zone is ~ 3.5 km long and is dominated by strike-slip fault features, including drainages that have been deflected by 30 to 300 m, beheaded streams, shutter ridges, closed depressions and ground w ater and vegetation lineaments (Buwalda, 1940; Crook and others, 1989; Jones and others, 1990; Fairchild aerial photographs C300 K317 - K318, C300 K337 - K339, and K362 - 363; and 1928 series USGS 6' Sierra Madre Quadrangle Topographic Map). These features suggest the presence of at least three fault strands exhibiting predominantly strike-slip displacement. Buwalda (1940) also noted that this portion of the Raymond fault has many north-dipping alluvial surfaces. While Zone III is dominated by strike-slip features, the ~ 5.5 kilometer- long Zone IV to the west is characterized by highly-dissected hills with south- facing scarps. These hills, including Raymond Hill and those upon which the Huntington Estate (number 25 in Fig. 2), increase in elevation to the west towards a 25° releasing bend in the fault at Raymond Hill. The hills rise up to 45 m above the alluvial fans at their bases and are incised by alluvial drainages. Lacy Park, formally a sag pond called Wilson Lake, is the product of a small left step in the fault that is superposed on the larger, right bend in the fault (number 24 in Fig. 2). At least one fault strand in Zone IV exhibits several offset stream channels (number 21 in Fig. 2). Zone IV extends from 9 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. the pressure ridge at Sierra Madre Boulevard (number 27 in Fig. 2) to Arroyo Seco (number 16 in Fig. 2). The Ml = 4.9 1988 Pasadena earthquake occurred within Zone IV at 16 km depth, and produced a nearly pure left-lateral strike-slip focal mechanism that, when plotted with its aftershocks, projected up to the surface trace of the Raymond fault with a dip of about 80° north (Jones and others, 1990; Fig. 3). The earthquake data, coupled with the offset-stream channels demonstrate the predominance of strike-slip motion even in the most contractional portion of the Raymond fault. Buwalda described back-tilted (i.e. north- dipping) terraces on the south side of Raymond Hill and on top of Oak Knoll, which he attributed to deformation along several strands of the Raymond fault (1940; numbers 18 and 19 in Fig. 2). He also noted north-dipping fan deposits in the hills north of Lacy Park and south of the pressure ridge east of Sierra Madre Boulevard (Buwalda, 1940). Zone V and stretches for 7.6 km from Arroyo Seco (number 16 in Fig. 2) to the Los Angeles River. On the western side of Arroyo Seco the Raymond fault is characterized by both north- and south-facing fault scarps bounding a linear depression (Buwalda, 1940). In this zone the Raymond fault bifurcates into two geomorphically expressed splays that trend approximately east (Fig. 2). Farther to the west, in the Los Angeles River valley, Dibblee (1989) noted a possible fault with north-side-down displacement that may represent the portion of the Raymond fault that connects with the Hollywood fault to the west of the Los Angles River (number 3 in Fig. 2). The Raymond and Hollywood faults are approximately on trend with each other, and both show evidence for oblique-reverse, left-lateral displacement, but it is difficult to confidently connect these two faults through the Highland Park area and across the Los Angeles River floodplain on the basis of 10 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced w ith permission o f th e copyright owner. Further reproduction prohibited without permission. Ml = 4.9 1988 P asad en a Earthquake 13'- 12’ - 11'- 1 0 - 8 - 3 K m a Raymond Fault A ' I -8 £ -9 S - io -11 -12 -13 -14 -15 -16 -17 0 1 2 3 4 5 Distance (km) Figure 3. a) Map showing the locations of the December 3, 1988 ML= 4.9 Pasadena earthquake (solid star) and its aftershocks together with focal mechanisms for the mainshock and the four largest aftershocks are shown with dark compressional quadrents (From Jones and others, 1990). R is Raymond Hill, b) A cross section showing the hypocenters of the Pasadena earthquake and its aftershocks projected onto the plane A-A', shown in 4a (From Jones and others, 1990). Note that the mainshock and aftershocks lie on a plane that projects to the surface trace of the Raymond fault. geomorphic expression (Figs. 1 and 2). Several structural trends exist in this area, but they do not appear to define a simple, single throughgoing fault between Arroyo Seco and the Hollywood fault (Fig. 2). Instead it appears that the Raymond fault bifurcates westward into several east- to southeast- trending, oblique-reverse(?) faults that connect with the Hollywood fault across the Los Angeles River Valley (Weber and others, 1980; Crook and others, 1987; Dolan, 1997). Similarly, several workers have attempted to connect the Raymond fault and the Verdugo-Eagle Rock fault system (Fig. 1 and numbers 4, 5 and 7 in Fig. 2). Weber and others (1980) mapped the Eagle Rock fault along the north eastern side of Raymond Hill and connected it to the Raymond fault. They based their location of the Eagle Rock fault on several outcrops in Arroyo Seco where they observed bedrock faults separating Cretaceous Wilson Diorite from the Miocene Topanga Formation. However, the absence of any other well-defined scarps or geomorphic features along its trace suggest the possibility that the fault they mapped may not have had any major Quaternary slip. Other workers have suggested that the Eagle Rock fault bends more easterly to the east of Raymond Hill and forms a diffuse scarp roughly parallel to the trace of the Raymond fault (Crook and others, 1987; Dibblee, 1989a, 1989b). PREVIOUS PALEOSEISMIC WORK ON THE RAYMOND FAULT Crook and others (1987) conducted a study of the seismic hazard potential of the Sierra Madre and Raymond faults that included the excavation of a large number of trenches along both faults. They reported results from several trenches across the Raymond fault, including sites at the Sunny Slope Reservoir and San Marino High School in the city of San Marino (numbers 12 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 32 and 36 in Fig. 2), and described evidence for at least five, and possibly as many as eight, surface rupturing events since 36,000 yrs B.P. They were able to approximately constrain the possible age ranges of five of those earthquakes with bulk soil radiocarbon ages, but the other three ruptures could only be assigned a minimum age of 29,000 yrs B.P. Their data suggest a recurrence interval of 5,000 to 9,000 yrs. Crook and others (1987) however, recognized that some events may have remained undetected in their study, and speculated that the actual recurrence interval was shorter than the 5,000 - 9,000 year measured interval. They suggested 3,000 years as a plausible recurrence interval for the surface ruptures on the Raymond fault. THIS STUDY The Raymond fault lies beneath a highly developed region covered with homes, streets, schools and commercial buildings. Consequently, few undeveloped trench sites still exist along the fault. Using published geologic maps (Buwalda, 1940; Crook and others, 1987; and Dibblee 1989a and 1989b) topographic maps, aerial photographs, and field reconnaissance, James Dolan and I identified the main trace(s) of the Raymond fault and located the best paleoseismic study sites along those traces (Fig. 2). We identified several good trench locations, despite the urban development, and executed our primary field work during the summers of 1997 and 1998. Los Angeles County Arboretum Our Trench T1 was excavated at the Los Angeles County Arboretum in Arcadia, California along the east-central part of the fault (near the east end of Zone III; Fig. 4). The southwestern portion of the Arboretum extends around and to the east of a 500-meter-long, 22-meter-high, west-trending pressure ridge that lies between two strands of the Raymond fault. During their earlier 1 3 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission o f th e copyright owner. Further reproduction prohibited without permission. A bandoned Railroad G rade ^ /- /-/ y . 575 Baldwin Lake P ressure Ridge P ressure Ridge. TRENCH SITE 300 A bandoned Railroad G rade meters Figure 4. Detail of the 1928 USGS Sierra Madre Quadrangle topographic map showing our Los Angeles County Arboretum trench site and the geomorphic expression of the Raymond fault in this area. Bold lines indicate splays of the Raymond fault and light lines indicate drainages. Elevations are in feet above mean sea level. Based on Crook and others (1989) and our own geomorphologic mapping. C.l. = 5 feet study, Crook and others (1987) excavated a short trench on the north side of the pressure ridge. They found weak evidence for a south-dipping splay of the fault that deformed Pleistocene and Holocene alluvium, but recovered no paleoseismic information. Early topographic maps and aerial photographs indicated that active stream channels flowed parallel to the southern slope of the pressure ridge. This suggested to us that the base of the slope at the southern edge of the pressure ridge may delineate strata that onlap deformed strata deposited since the most recent surface rupture. During July and August of 1997 we excavated a trench totaling 42 m in length across the southern pressure ridge scarp (Figs. 4 and 5; App. 2). The north end of the trench was located about 2 m south of the access road that winds around the pressure ridge, and continued down the scarp. That trench bent once to avoid an irrigation line, and terminated about one meter north of the Arboretum property boundary (Fig. 5). We encountered evidence for five fault strands, referred to from north to south as strands a through d. Stratigraphic units described in the text are referred to by capital letters. Stratigraphy The trench exposed well-bedded, predominantly sandy and silty strata that can be divided into three blocks, which are separated by two strands of the fault (Fig. 5). The northernmost stratigraphic block consists of friable sands overlain by a silty sand which is itself covered by a two meter thick section of sandy gravels. The gravels grade upwards over about 40 cm into silty, clayey sandy gravels (unit B) about two-m-thick. Based on the orange-brown color of the unit and the abundance of clay that has been leached from the upper part of the soil profile and translocated downward into the top of the gravels by meteoric waters, we interpret this unit as an argillic (Bt) soil horizon. The Bt horizon is capped by 40-60 cm of dark brown, organic-rich silty sand (unit 15 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission o f th e copyright owner. Further reproduction prohibited without permission. meters Arboretum Property Boundary Road Raymond Fault Trench 1 LA County Arboretum, Arcadia, CA unit A (A Hor). A D . 1215 a .D. 1450 -45/+35 unjtK unit I -15/+35 unitJ unit E m £ m eters unit A (A Hor.) unit H unit G 34,300 ± 470 yr B.P. unit K a .D. 1030 unit unitU Hr t -20/+120 A D- 1670 j r1 n i * 4 c \ a c l uii't B i B . H^r unit E unit A (A Hor > •". • a 'S * .'3 . ' ?• o '.1 ? ' . 5 ’ fa * 5 1 * £ > • 1 ^ e unit G A.D. 1,020 ± 20 8,045 ± 25 B.C. 26,610 ± 2 0 0 yr B.P. 27,240 ± 220 yr B.P. Sands ( Bt Soil Horizon Gravels Figure 5. Logs of the west and east walls of our trench at the Los Angeles County Arboretum in Arcadia, CA. Faults are shown by heavy black lines. Stars show locations of detrital charcoal samples. Note that 5 the east wall (bottom diagram) has been mirror imaged in order to compare it to the west wall. Inset map at the top shows the layout of this trench site. < 1 A) which pinches out to the south at the surface projection of fault a. We interpret this top unit as the soil A horizon. The middle block is bounded by fault a to the north and fault c to the south (Fig. 5). The stratigraphy in the lower three m of the middle block consists of gently south-dipping interfingering sands and silts, overlain by gently south-dipping gravel channels. These channels are succeeded by two bioturbated, wedge-shaped silty sands. The lower wedge-shaped deposit consists of massive silt (unit C), while the overlying unit is made up of several deposits that may be broken into separate, smaller wedge-shaped deposits (units D through F). These are capped by a one-meter-thick dark brown, organic-rich A horizon that pinches out to the north at the surface projection of fault a (Fig. 6). The southern structural block is made up of cobble to pebble gravels overlain by interfingering sand and silt layers that include unit G (Figs. 5 and 7). These deposits dip moderately to steeply to the south and are covered by several pebble to cobble channels. One of the younger channels (unit H) appears to have preferentially eroded into fault zone e (Fig. 7). Several of these channels grade into moderately south-dipping, well-stratified sands and pebble gravels that are overlain by a package of horizontal, finely bedded medium to coarse sands and granules (units I). These sands and granules overlie the upward terminations of faults c and d. The friable sands and gravels are covered by a gray, well-stratified, silty sand (unit J), which is itself overlain by a darker gray, stratified, organic-rich silty sand (unit K). On the basis of their color, organic content, and stratification, we interpret the stratigraphically highest two units as Aj and A2 soil horizons. It appears that this A horizon material above the southern stratigraphic block was not developed in situ, but instead represents reworked A horizon material 17 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission o f th e copyright owner. Further reproduction prohibited without permission. Central Fault Zone - West Wall meters unit A (A Hor.) Figure 6. Detail illustration of the upper portion of the south-dipping fault zone on the west wall of the trench. Bold black lines indicate faults. Patterns are the sam e as in Figure 6. 0 0 Vertical Fault Zone - West W all L A Co. Arboretum A.D. 1215 ±45 unit J B.C. 8045 ± 2 5 A .D 1670 -1945 A.D. 1450 -15/+35 unjtKN A .D 1020 + 20 34,300 ± 470 yr B.P. A .D 1030 - 20/+120 meters Figure 7. Diagram of the southern end of the west wall of the Arboretum trench showing the relationship of undeformed units to deformed units. Faults are indicated by bold lines and are lettered. The hatchured region indicates slough on the benched portion of the trench. White and grey stars show locations of detrital charcoal samples collected for age analyses from the east and west walls of the trench. Patterns are same as in figure 6. 19 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. washed down the slope and deposited at the base of the pressure ridge. Soil A horizons are generally thoroughly bioturbated, with little to no original stratigraphy discernible. The surface deposits south of fault strand a, however, contain thin beds and lenses of pale yellow-brown sand and beds of darker gray and paler gray silty sand that do not appear bioturbated. Farther to the south the A horizon material itself is stratified, indicating that it has probably been reworked. We interpret these units as deposits of A horizon material that has been washed down the slope and deposited nearer the base of the pressure ridge. Age Control We collected 10 pieces of detrital charcoal for radiocarbon analysis from the Arboretum trench (Table 1). Three samples came from the middle stratigraphic block and were collected from the fine-grained silty sands at the base of the trench (Fig. 5). Two of these three samples were combined into one sample. The combined sample and the third sam ple yielded radiocarbon ages of 26,610 ± 200 yrs B.P. and 27, 240 ± 220 yrs B.P., respectively. These ages are consistent with our field estimates of the degree of soil development at the north end of the trench based on the thickness and color of the argillic horizon. The remaining charcoal samples came from the southern stratigraphic block (Fig. 7). One of these, the oldest sample in this trench, yielded a radiocarbon age of 34,000 ± 470 yrs B.P., however, this sample was collected from unit G which also yielded a calandric date of B.C. 8045 ± 25, showing that the 34 ka sample is reworked. One sample collected from a lower section of unit I yielded a calandric date of A.D. 1020 ± 20. We collected three other pieces of detrital charcoal from units I which yielded A.D. 1030, A.D. 1215 ± 45 and A.D. 1670-1945, in stratigraphic succession. The latter sample is substantially younger than the 20 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission o f th e copyright owner. Further reproduction prohibited without permission. Table 1. C1 4 Samples from Raymond fault Trench 1, L.A. County Arboretum, Sample Location* Dimensions (mm) Description Conventional 14 C Age** r f r east wall m 21(H), -3.84 one 6 X 4 X 5 ; six -1 X 1 X I organic sediment; moderately rounded very small pieces all from silty fg ss layer @ bottom of trench; same as RF 5 and 8 27,240 ± 220 B.P. RF 3 west wall RF 5e east wall and RF 8E RF 1(H ) wall m 32.60, -1.40 -1X1X2 m 21.77, -4.20 m 22.52, -3.71 west wall m 37.60, -3.10 many (>14) one 3 X 3 X 2 four 2 X 2 X 1 many small pieces one 2 X 2 X 1 two or three 1X1X1 two 3 X 3 X 3 nine 2 X 2 X 2 many small pieces detrital charcoal; very small pieces all from 20 cm below A hor. organic sediment; angular piece; moderately rounded all from silty fg ss layer @ bottom of trench; same as RF I and 8 organic sediment; all from silty fg ss layer @ bottom o f trench, about mid-trench; same as RF I and 5 (RF 5 and 8 were combined) detrital charcoal; angular pieces; angular pieces all from micaceous fg ss in s. fault zone same as RF 102 430 ± 50 B.P. 26,610 ± 200 B.P. 9,030 ± 50 B.P. RF 101 m 4 0 .3 7 ,-l.5 1 one 3 X 4 X 2 detrital charcoal; moderately 1,000 ± 50 B.P. wall many small pieces rounded all from horizontal eg ss @ end o f extension Arcadia, CA Calibrated _______ Calandar Year*** A.D. 1450-15/+ 35 A.D. 8,045 ± 25 east A.D. 1020 ± 2()east Reproduced w ith permission o f th e copyright owner. Further reproduction prohibited without permission. Sample Location* Dimensions (mm) Description Conventional 14 C Age** Calibrated Calandar Year*** RF 102 RF 103 west wall RF 106 east wall RF 109 east wall m 37.43, -2.76 west wall m 40.76, -1.20 one 3 X 3 X 4 m 35.85, -0.85 m 35.50, -1.00 small pieces C-6^ west wall Trench 7, Sunny Slope Reservoir many small pieces one 1 X 2 X 1 three 3 X 3 X 3 angular two 1 X 2 X 3 three 2 X2 X1 one 7 X 2 X 4 -1X1X1 Bulk Soil detrital charcoal; angular pieces all from micaceous fg ss in s. fault zone same as RF 100 detrital charcoal; (wood grain is visible); all from horizontal eg ss @ end o f ext. detrital charcoal; angular pieces all from horizontal eg ss @ end o f extension detrital charcoal; well rounded large piece angular small pieces all from horizontal eg ss @ end of extension collected from within a fracture thought to be the result of the most recent surface rupture 34,300 ± 470 B.P. 850 ± 50 B.P. 130 ± 70 B.P. 980 ± 50 B.P. 2160 ± 105 B.P. A.D. 1215 -45/+35 A.D. 1670 - 1945 A.D. 1030 -20/+120 B.C. 189 -1 17/+212 °c All samples were pretreated with acid, alkali and acid washes before analyzing, except for those noted by e . e Indicates sample that the sample was acid washed only. * Please see Figures 5 and 9 for sample locations on the trench logs. ** Conventional Dates are reported as radiocarbon years before present (RCYBP), where “present” = 1950 A.D. and incude two sigma, or 95% probability, error bars. *** Calibrated Calandar Years were calculated by the lab using the Pretoria Calibration Procedure program, and include one sigma error bars. Some sample Conventional ages were too old to fit to the calibration curves and are designated by dashes, p Indicates a sample collected and dated by Crook and others (1987). others and was collected from only 2 cm below a lens of gray, silty fine- to medium-grained sands that we interpret as reworked A horizon material. Based on the date yielded by this sample and its proximity to the possible A horizon material, we suspect that it has been bioturbated dow n into unit I along a cryptic burrow. Our tenth sample was collected from the silty sand (Unit H in Figs. 5 and 7) just below the pale gray, organic-rich A 2 horizon, and yielded a calandric age of A.D. 1450 +35/-15. This sample was collected from a highly bioturbated. predominately massive section. Based on its proximity to numerous animal burrows mapped at the base of the A2 unit, and the anomalously young radiocarbon age it yielded, we believe this sample was collected from a cryptic animal burrow and was contaminated. Therefore, we feel this sample should also be discarded. Evidence for Faulting The trench revealed two major fault zones comprising five fault splays. The northern fault zone, which encompasses strands a and b on Figures 5 and 6, consists of a narrow, well-defined zone of faults at the base of the trench that anastomose and separate into four and five planes on the east and west walls, respectively. Several of these fault planes terminate about three m from the ground surface, but one fault continues upward and juxtaposes unit B, the Bt horizon, against wedge-shaped unit E to the south (Fig. 6). That splay also appears to reach the ground surface. The dark organic-rich A horizon (unit A) at the ground surface pinches out where the fault projects to the surface and thickens to both the north and the south. Faults a and b also separate gently south-dipping strata to the south from horizontal strata to the north. In addition, a 7 meter-wide fault zone consisting of three strands that cut upward through the south-dipping strata is exposed near the south end of the 23 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. trench (Fig. 7). The northernmost of these faults (strand c in Fig. 7) dips steeply to the south and juxtaposes cobble gravels in a sand matrix with interfingering sands and silts. The middle strand (strand d) also dips steeply to the south, but has not accommodated any vertical separation of the gravels, sands and silts. The southernmost fault (strand e) comprises several closely spaced, vertical, anastomozing faults that cut through the silts, sands and younger gravel channels. This fault zone is clearly truncated by a narrow, trough-shaped channel deposit, which appears to have eroded down through the fault zone. None of the faults cut the horizontal sands and gravels overlying the south-dipping sands and gravels at the south end of the trench. Interpretation of Arboretum Trench Results A n a d d i t io n a l f a u l t a lo n g th e s o u th e r n s id e o f th e p r e s s u r e rid g e In addition to the two major fault zones described above, stratigraphic and structural relationships observed in the trench lead us to infer a fault zone to the south of our trench, beyond the Arboretum property boundary. This inferred fault zone is required to explain the deformation of the moderately south-dipping layers. We could not excavate any farther south because the property on the other side of the Arboretum boundary fence is occupied by the backyards of several houses, most of which have below-ground swimming pools. Although we did not expose all of the strands along the southern boundary of the Arboretum pressure ridge, we can use the structural and stratigraphic relationships exposed in our trench to place some constraints on the earthquake history of the Raymond fault at this site. 24 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. C e n tr a l F a u lt Z o n e Although the south-dipping fault in the central part of the trench exhibits what initially appeared to be south-side-down normal displacements in the upper few m, the lack of any vertical separation of distinctive stratigraphic units below 2 m depth (Figs. 5 and 6) demonstrates that this is a pure strike- slip fault that has locally juxtaposed dissimilar stratigraphic sequences. In the upper two m the fault separates northern cobble gravels in a sand matrix capped by the 1.5 meter-thick argillic horizon from the massive, southern wedge-shaped deposits (Fig. 6). The origin of these wedge-shaped deposits remains unclear. If the stratigraphic relationships exposed lower in the trench had not precluded major south-side-down normal displacements, we would interpret these as colluvial wedges shed off a south-dipping oblique-normal fault scarp. But since strata exposed below the wedge do not exhibit any vertical separation, the wedges must represent either: (1) colluvial wedges shed off a south-facing scarp caused by strike-slip juxtaposition of irregular topography; or (2) channel deposits. We exposed the fault and associated wedge-shaped deposits in three faces: the two original trench walls, and a later exposure we cut 3 m east of the original east wall of the trench (Fig. 8). Geometrically, the wedges appear to extend along the strike of the fault, which is, of course, along the slope of the scarp. If these are channel deposits, then the channel must have flowed along the fault. While the channel that was active at the site before urban development did flow along the base of the scarp, the base of the wedge- shaped deposits within the scarp is more than one meter higher than the highest possible correlative units within the recent channel deposits at the south end of the site (discussed below). This relationship, could, of course, be 25 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Central Fault Zone Bench (mirror image of east wall) unit A (A Hor.) meters Figure 8. Re-exposed east wall (mirror imaged) of the south-dipping central fault zone. Patterns are the same as in Figure 6. 26 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. due to uplift of the wedge-shaped deposits during an earthquake along a deeper strand to the south. We suspect, however, that the wedge-shaped deposits are not channel deposits because of the exact coincidence of the northern edge of the wedge-shaped deposits with the upward-projection of faults a and b, and the very abrupt, 60° angle formed by the base of the wedge- shaped deposits and the steep northern edge of the deposit. We believe that the coincidence of the north edge of the deposit with the upw ard projection of faults a and b argues against this being a purely sedimentary contact, and that the contact is, in fact, a fault that extends all the way to the ground surface. Nevertheless, we approach this interpretation with caution because the three- dimensional topographic context of the deposit is not well established (i. e., we do not know if the paleo-surface topography was consistent with the existence of a south-facing paleo-scarp which could have generated colluvial wedges). One additional point that suggests that the wedges may be true colluvial wedges and not channel deposits is that the wedges are completely massive and composed of what appears to be, at least in part, old A horizon material. In contrast, all of the channel deposits that we observed elsewhere in the trench, including those at depths similar to the subsurface depth of the wedges, were pervasively and finely bedded on the millimeter to centimeter scale. These relationships, and the preservation of distinct contacts between the wedge-shaped deposits, argues against their massive nature being due to bioturbation. Thus, we think that these deposits probably are colluvial wedges, but we consider this interpretation to be tentative, and the origin of these features remains somewhat puzzling. If these are colluvial wedges formed during strike-slip of irregular topography, then the wedge-shaped deposits we observed record at least two distinct surface ruptures of the 27 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. central strand of the Raymond fault exposed in our trench. The three -27,000 yrs B.P. detrital charcoal ages recovered from the base of the central block provide a maximum age for the wedge-shaped deposits described above. If the wedge-shaped deposits are colluvial wedges that resulted from earthquakes, then at least two surface ruptures have occurred on the south- dipping strand of the Raymond fault since 27,000 yrs B.P. S o u th e r n F a u lt z o n e The 5 meter-wide southern fault zone comprises faults c, d, and e. This fault zone cuts upward through south-dipping strata that are similar in composition to those sands and silts in the hanging wall of strand c (Fig. 5) but does not cut the reworked A horizon material that lies at the ground surface. The west wall shows fault d cutting several strata without any apparent vertical separation, while fault c truncates the sands and silts to the south against the gravels to the north in such a way as to suggest south-side- down motion (Fig. 7). This geometry suggests that the units are laterally discontinuous and that fault splays c and d have experienced predominately horizontal motion. The southernmost fault strand e may also accommodate some of the horizontal motion along the Raymond fault, but the relationship between it and the silts and channels near it suggest at least some north-side-up motion, as well. Note that unit G in (colored white in Fig. 7) is gently south-dipping at its northern end, but becomes steeply south-dipping at fault e. The upper contact of this unit may have been modified during erosion by the overlying channel (unit H in Fig. 7), but the lower contact of unit G has been juxtaposed with the underlying units in a north-side-up relationship. The -8,050 B.C. date obtained from deformed and faulted unit G provides a maximum possible age for the most recent surface rupture. As discussed 28 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. above, these faulted, tilted strata are onlapped by horizontal, well-bedded friable sand, granule and pebble beds, from which we obtained three stratigraphically consistent calandric ages: A.D. 1000 -1040, A.D. 1010 - 1150, and A.D. 1170 -1260 that do not appear to have been contaminated by recent A horizon material. These age data constrain the most recent tilting event on the southern fault strand exposed in our Arboretum trench to between -B.C. 8050 and A.D. 1000 to 1040. Sierra Madre Boulevard Our second Raymond fault trench was excavated along the median strip of Sierra Madre Boulevard in the City of San Marino, California, about four km west-southwest of the Arboretum trench (Fig. 2). Sierra Madre Boulevard is a four-lane city street, with a 10-meter-wide median strip that follows a man- modified stream channel (Fig. 9). Aerial photograph analysis and geomorphic mapping show that Sierra Madre Boulevard winds through the eastern end of a west-trending line of hills, and that the fault zone at this location is probably at least 400 m wide and comprises several anastomozing strands (Fig. 9). Because the fault zone is so wide at this location, we excavated a 240 meter-long, northeast-trending transect of nine large- diameter (70 centimeter) bucket auger holes in order to correlate stratigraphy along Sierra Madre Boulevard and to locate the active trace(s) of the Raymond fault prior to opening the trench (Fig. 10; App. 3). The augerholes ranged in depth from seven to twelve m. Bucket-Auger Hole Transect Strata in boreholes 1, 2, 3 and 5 could be correlated and revealed very gently south-dipping, apparently undeformed alluvial, fine- to coarse-grained sands, silty sands and gravels (Figs. 10 and 11). Holes 4, 6 and 7 were excavated in much more friable pebble gravels and fine- to coarse-grained 29 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission o f th e copyright owner. Further reproduction prohibited without permission. Offset Drai lages Huntington Estate Pressure Ridge I I TRENCH SITE m eters Figure 9. Detail of the 1928 Altadena topographic map showing our Sierra Madre Boulevard site and the geomorphic expression of the Raymond fault in this region. Bold lines indicate splays of the Raymond fault and light lines indicate drainages. Elevations are in feet above mean sea level. C.l. = 5 feet. Loction of RFSM Trench along the borehole transect / Main Thrust Fault Euston R < ± Fault and dip direction m easured down-hole meters Figure 10. Map of the Sierra Madre Boulevard bucket auger hole transect and trench site. Faults observed in Holes 3 and 8 are indicated by thick lines. 31 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission o f th e copyright owner. Further reproduction prohibited without permission. Bucket-auger Holes and Trench Location Sierra Madre Boulevard, San Marino, CA N30E ««-------1 ------ ► 9/10 H1 ■ Cohesive Sand ■ Silt ■ Marsh deposits □ Cohesive Sand and Silt □ Friabable Sand and Gravel 19/20 j 17/18 H9 N45E + 7/8 *r 5/6 N58E 3/4 i* 1/2 H5H8H7 H4 Outline of the Trench Continuous Section 0 10 meters N18E N30E 17/18 t 15/16 T 15/16 -p 13/14 13/14 11/12 I H2 I 11/12 T 9/10 T f 9 Figure 11. Cross section looking northwest showing logs of the nine bucket-auger holes and the location of the second trench in this study. Because the bucket-auger hole transect followed Sierra Madre Boulevard, its orientation changed at three points, as indicated by the black arrows and labels. The light grey arrows refer to ground pentrating S radar lines measured parallel to the bucket-auger transect. sands that were distinctly different from the strata encountered in the more southerly boreholes. Because we could not correlate between these two portions of the bucket-auger transect we drilled Holes 7 and 8 between them where we suspected a structural discontinuity existed. Hole 9 was excavated at the southern end of the borehole transect to confirm that there was no discernible vertical separation of the alluvial strata indicative of faulting in that area. We directly observed faults in two of the nine holes. Hole 3 revealed small, steeply south-dipping fractures at 1 to 4 m depth. These fractures disrupt many layers of finely laminated, iron-oxide-stained sands and silty sands and show only one to several cm of vertical separation across any given bedding plane. Hole 8 penetrated a major fault. This fault dips steeply to the north and juxtaposes near black to dark blue-gray, organic-rich, silty fine-grained sands on the north with iron-oxide-stained, medium- to fine-grained sands and silty sands to the south. In the borehole this structure could only be traced from 4 m depth to about 30 cm depth. The top of the fault is truncated by a dark gray, organic rich, silty sand through which the A horizon of the active surface soil has developed. We observed a second, approximately vertical fault in this hole at ~8 m depth. Ground-Penetrating RADAR Survey In addition to the borehole transect, we conducted a ground-penetrating RADAR (GPR) survey of the fault zone in order to locate any structures that may have been missed by the widely spaced bucket-auger holes (App. 4). The majority of the GPR lines revealed flat-lying reflectors. However, we imaged several vertical discontinuities between very reflective and non-reflective strata several m north of Hole 7 and southward to near Hole 8. As discussed 33 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. below, this feature corresponds to the main traces of the Raymond fault exposed in our trench (Feature C in Fig. 12). No structures were observed with the GPR south of borehole 3. Several lines also imaged gently north- and south-dipping structures that are probably the bases of channel deposits. One of these features was located north of our borehole transect (Fig. 12). This gently north-dipping feature was easily identified on the GPR lines because of the dry, friable, and coarse grained nature of the cobble sands and gravels through which it cuts. However, those same sediments are too dry and friable to support a trench, which prevented us from directly observing the gently north-dipping structure in a trench or bucket-auger hole. Trench Results S tr a tig r a p h y We excavated our 28 meter-long, four meter-deep trench from just south of Hole 3 to just north of Hole 7 in order to observe the entire fault zone identified from the bucket-auger holes. The trench revealed two main fault zones consisting of several smaller faults and predominately flat-lying to gently south-dipping alluvium (Fig. 13; App. 5). These units were well exposed in the trench and were photographed with a 1 meter by 0.5 meter grid for construction of a photomosaic (App. 1). The strata tend to thin to the north, and many of them pinch out between the two main fault zones. Based on differences in lithology two distinct stratigraphic sections separated by a vertical fault (fault b) near the north end of the trench have been identified. The package north of fault b consists of predominately friable sands and gravels and will be discussed below. The strata south of fault b consist of more cohesive silty sands, sands and gravels. A major north-dipping fault (fault d) displaces this stratigraphic section about 1.5 m, but the same units can be correlated across the fault. The 34 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission o f th e copyright owner. Further reproduction prohibited without permission. Ground-penetrating RADAR at Sierra Madre Boulevard Northern extent of trench meters Figure 12. Ground penetrating RADAR data collected at the San Marino site. Vertical exageration is approximate 3:1. Feature A , a gently southwest-dipping feature, is probably a stream channel. Feature B is also gently southwest- dipping, but has a more irregular surface than A. This is probably an artifact of crossing a paved intersection that bisects the median strip. C denotes a series of vertical changes in reflection intensity and appears to be an ~ 10-meter- wide vertical fault zone. We observed the southern two meters of this zone in our trench. U > U \ Reproduced with permission o f th e copyright owner. Further reproduction prohibited without permission. Raymond Fault - San Marino Trench a2 NE W est Wall Fill of Bucket Auger Hole 3 m eters E ast Wall (mirror im age) S i l t s ^ Marsh Deposits ■ I /r-° Sands( ■ Pebble, Cobble Gravels( ^ I U ) ON Figure 13. Logs of the faults and stratigraphy observed on the walls of the San Marino trench. Heavy black lines indicate faults. The stippled pattern represents gravels. The locations of detrital charcoal and bulk soil samples are indicated by white stars and triangles, respectivly. (See Figure 14 for carbon 14 age data.) base of the stratigraphic block bounded by faults b and d is made up of about ten layers of sand and silty sand units (below unit A, Fig. 14) that vary from 10 to 30 cm in thickness. The sands and silty sands are overlain by about one meter of crudely stratified pebbly, sandy gravels (unit A), which are both cut by fault group c. This section is overlain by a sandy silt (unit B) about 40 cm thick, which is not cut by fault group c and is covered by an unstratified red- brown sandy silt (unit C). A 1.5 m-thick section of dark blue-gray to black sandy silt covers the red-brown silts (unit D), but also appears to be bounded by the red brown silts to the north and south, where faults b and d cut through the stratigraphy. We suspect that the red-brown color of at least the silt bounding the southern edge of the black sandy silt is diagenetic, and is related to oxidizing fluids flowing along fault d. All of these units dip moderately to the south. At the top of the blue-gray silt is a thin (up to 10 cm) layer of very charcoal- rich black silt, which is overlain by a series of sandy silts (light gray in Figs. 13 and 14) interfingering with sand units (units F through N, Fig. 14) dipping moderately to gently south. Silt stringers were observed at several locations in the sand units, and the internal stratigraphy of the sand and silt units is disrupted in several locations. A pebble and cobble gravel channel visible on the sandy pebble to cobble gravel that varies in thickness from several to 70 cm (unit O, Fig. 14). A silty sand (unit P) about 30 cm thick and a thin (~ 8 cm) clayey silt (unit Q) overlie the gravel. The clayey silt is covered by a series of silty sands. These units, along with the interfingering silts, sands and gravels below it, can be followed southward, beyond the north-dipping fault, to the south end of the trench. The sediments between faults a2 and b are a series of gently- to steeply- south-dipping sands, granules and silty fine-grained sands. These sands and 37 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced w ith permission o f th e copyright owner. Further reproduction prohibited without permission. San Marino Site Age Control North End of Raymond Fault - San Marino Trench - west wall SW 29,900 +/- 160 20,350 unit L 32,820 32,240 + /-Q O 34,270 +/- 830 unit J unit O m eters >27,340 +/-130 Figure 14. Detail of the north end of the west wall of the San Marino trench with radiocarbon ages in yrs B.P. Stars and triangles refer to sample locations of detrital charcoal and bulk soil samples, respectively. Samples collectedfrom the east wall have been projected to the west wall for comparasion and are represented by gray stars. Capital and lower w case letters designate specific units and faults discussed in the text. Pattern scheme is the sam e as in Figure 13 0 0 silts overlie a pebbly sand which itself overlies several gravel deposits. These units gradually dip more southward with depth until they are parallel to the fault (Fig. 14). North of fault a2 the sediments consist of friable sand and gravel. Faint channel edges were demarcated by pebble lines within the gravel. The gravel is overlain by predominately coarse-grained sand and small pebble gravel (unit R, Fig. 14) that contains several vertically elongate lenses of silty, fine grained sand. The coarse-grained sands and silty sands are themselves cut by several vertical, approximately 70 cm-long, 1 to 10 cm-thick lenses of packed coarse-grained sands and very small pebbles (units S). A 30-60 cm-thick package of horizontal- to gently south-dipping, fine- to coarse-grained sands, granules and very small pebble layers (unit T) lies on top of the vertical sands- and granule-filled lenses. Material from one unit within the T strata, labeled unit U in Figure 14, has fallen into and filled a vertical fissure that cuts up through the R, S and T strata. The stratigraphy described above is compieteiv blanketed along the entire length of the trench by a 30 to 40 cm-thick unit of dark brownish-gray to black, organic-rich sand that extends to the surface. The base of this unit is nearly planar and horizontal, and contains abundant m odem tree and grass roots at the trench site and is therefore the A horizon of the active soil. A ge C o n tro l Alternate acid-base-acid washes were applied to the 1 4 C samples collected from our trench. These pretreatments are designed to remove any organic material that the sample has acquired since burial. However, on two very small detrital charcoal samples (RFSM 215 and 216; Table 2) and two bulk soil samples (RFGL 3 and 24a; Table 2), only the acid pretreatment was applied in order to retain some datable sample. Thus, it is possible that these samples 39 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission o f th e copyright owner. Further reproduction prohibited without permission. Table 2. Sampleoc RFSM 106 RFSM 108 RFSM 110 RFSM 162 RFSM 185 RFSM 215 E and RFSM 216 E o C1 4 Samples from Raymond fault - Sierra Madre Trench, San Marino, CA ________ D im ensions (m m t Description_________________ Location* Conventional .14 C Age ** west wall + 3 .5 5 , +3.31 west wall + 3 .7 1 , + 3.49 west wall + 8.75, + 2.56 east wall + 3 .1 2 , +3.75 east wall 3 .3 2 , +2.95 east wall + 0 .4 8 , + 0.49 + 0 .6 1 ,+ 0 .4 1 east wall several pieces several 5X 5X 5 several < 3X 3X 3 pieces one piece several 3X 3X 5 pieces several 10X5X5 several smaller pieces 3X 4X 4 + 2X 3X 4 (one each) one v. small piece -2 X 2 X 2 charcoal fragments from charcoal-rich layer above marsh contorted fine-grained sand; sam e level as RF SM 102 / Silt 2 gray-beige sandy silt south o f n-dipping fault stratigraphicaliy above unit M beige slty unit; should be datable grey-black charcoal layer on top o f marsh 3 cm below top o f sit; should be datable; com bined with 216 from 4 cm below top o f grey silt; com bined with 21 3 2 ,2 4 0 ± 250 3 2 ,8 2 0 ± 4 4 0 27 ,5 3 0 ± 150 2 9 ,9 0 0 ± 160 3 3 ,6 6 0 ± 530 > 2 7 ,3 4 0 ± 130 Reproduced with permission o f th e copyright owner. Further reproduction prohibited without permission. Sampleo Location Dim ensions (m m t Description Conventional C 14 Age** RFSM 233 e west wall +0.24 to 0.34 + 3 .7 9 RFSM 246 east wall - 0 .5 5 , + 3.63 RF GL3 £ 9-14 RF G L 24a 119-124 E 2 flakes -1 X 3 X 6 several 2X 3X 4 pieces Bulk soil sample Bulk soil sample can probably date individually; could be com bined with 234 silty fine-grained sand between fracture fills dark gray sandy silt top o f probable marsh deposit med. - dark gray sandy silt; bottom o f probable marsh deposit > 20,350 ± 90 3 4,250 ± 830 3 3,330 ± 840 > 37,300 « All sam ples were pretreated with acid, alkali and acid washes before analyzing, except for those noted by E. RFSM sam ples are detrital charcoal or plant material. RF GL are bulk soil sam ples collected from the dark gray-black sandy silt. E Indicates sample that the sample was acid washed only. * RFSM sample locations given in x,y coordinates o f a meter-spaced grid on the original trench logs. RF GL sam ple locations given in centim eters from the top o f the dark gray-black silt deposit. Please see Figures 5 and 9 for exact sample locations on the trench logs. ** Conventional Dates are reported as radiocarbon years before present (RCYBP), where “present” = 1950 A .D. and incude tw o sigma, or 95% probability, error bars. Sam ples collected from the Sierra Madre trench site were too old to calculate calendar ages. have been contaminated by younger carbon material, and are therefore slightly older than their conventional 1 4 C ages suggest. We note, however, that the two detrital charcoal samples are from the base of the trench at 4.3 m depth and the two bulk soil samples were collected from two and three m depth. Moreover, as discussed below, about 1.5 m of material was removed from above the trench within the last one hundred years. Due to their depths we therefore believe that any contamination of these samples by young carbon associated with soil development during the past century was negligible. The radiocarbon ages of the two bulk soil and nine detrital charcoal samples collected from our trench (triangles and stars, respectively, in Fig. 14; Table 2) indicate that the trench contains a latest Pleistocene geologic record. The bulk soil samples collected from the base and top of the dark blue-gray silt (unit D) yielded radiocarbon ages of > 37,300 yrs B.P. and 33,330 ± 840 yrs B.P., respectively. The two pieces of detrital charcoal that received only the acid pretreatment were collected from a very thin layer of silt at the base of the trench, several layers below unit A and were combined for analysis because of their very small sizes. This combined sample yielded a conventional carbon age of >27,340 ± 130 yrs B.P. This sample provides no new information, as the bulk-soil sample collected from a unit stratigraphically above the silt gave an age of >37,300 yrs B.P. The remaining detrital charcoal samples were rim through the acid-base- acid washes prior to being burned. Two pieces of detrital charcoal collected from the layer of charcoal-rich silt overlying unit D give ages of 33,660 ± 530 and 32,240 ± 250 yrs B.P. (Table 2). Three samples of charcoal collected from Silt 2 (unit I in Fig. 14), Gravel 4 (unit O), and a layer sandy silt south of the 42 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. north-dipping fault (younger than unit O) yielded ages of 32,820 ± 440, 29,900 ± 160 and 27,530 ± 150 yrs B.P., respectively. The eight carbon samples we analyzed are in stratigraphic order, suggesting that they have not been reworked. Although the preferred age of the two samples collected from the charcoal-rich layer is slightly younger than the age of the overlying Silt 2 (unit I) sample, these samples are all about the same age within error (95% confidence limits), and thus probably do not indicate any reworking. A detrital charcoal sample collected from within one of the brown silty- sand packages in the friable sands and gravels at the north end of the trench (unit T) yielded a conventional carbon age of 34,250 ± 830 yrs B.P. Another sample collected from the horizontal to gently south-dipping sands (above unit U) yielded 20,350 ± 90 yrs B.P. These two samples are also in stratigraphic succession, and thus have probably not been reworked. E v id e n c e f o r F a u ltin g Two main fault zones were visible in the trench (Figs. 13 and 14). The northern strand, fault b, is near-vertical and juxtaposes different lithologic sequences, suggesting that it has experienced significant strike-slip motion. In contrast, the southern strand d, which dips northward 45°, has experienced at least two m of apparent reverse displacement. Strata above and below fault d can readily be correlated across the fault. The northern 1.5 m-wide fault zone is made up of several vertical- to near-vertical, anastamozing faults overlain by the base of the modem A horizon. One group of faults in this zone could be traced across the lower -1.5 m from the base of the trench, but could not be traced into the higher levels. Another set of faults could be traced two m from the base of the A Horizon, 43 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. but were also lost in the gravels. This wide fault zone apparently accommodates predominantly strike-slip motion, indicated by the juxtaposition of brown sandy silt units to the south of the main fault splay in the zone with pale yellow-brown, fine- to coarse-grained sands and pebble- to cobble gravels to the north. South of the vertical fault zone lies a north-dipping fault plane with 1.6 m of apparent north-side-up reverse-slip, indicated by reconstructing a series of black- to blue-gray marsh deposits (black in Figs. 13 and 14). The fault plane strikes N86°W and dips 45°N and divides in to several anastomozing splays about two m below the ground surface. These splays cut up to the base of the modem A horizon at the ground surface. Two splays in this fault zone do reach the base of the horizontal silty sand and are discussed in the next section. A third zone of six approximately vertical fault splays was observed between the two main fault planes. These faults cut through various sands and silty sands in the lower two m of the trench, but could not be traced through the ~one-meter-thick gravel deposits (speckled in Fig. 14) on either trench wall. Interpretation of Trench Results The dark brownish-gray, A horizon that caps the upper 20 to 30 cm of the trench is an old railroad grade that was leveled during the early 1900's and removed and landscaped in 1951. Assuming that the original slope across the site of the trench was relatively planar, the geometry and slope of the hills on either side of the trench site suggest that -1.5 m of material was removed from above the north end of the subsequent trench site. In contrast, only -20 to 30 cm appears to have been removed from above its south end. Based on the latest Pleistocene radiocarbon ages of the charcoal and bulk-soil samples 44 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. that we recovered from the trench, it appears that any Holocene sediments at the trench site, if ever present, were removed during construction of the railroad. Nevertheless, the stratigraphic relationships and 1 4 C ages indicate that the trench contains an excellent record of latest Pleistocene earthquakes on the Raymond fault. Within this record we identified evidence for five Raymond fault surface ruptures, as well as three shaking related events that may or may not have been on the Raymond fault itself. The fault-bounded block between the two fault zones contains evidence for at least five events (Fig. 15). The oldest well-defined surface rupture, Event 1, is defined by at least six faults at the bottom of the trench that terminate at the base of a pale yellow-brown-orange, iron-oxide-stained sand and pebble gravel unit (unit A). This event tilted some of the sands and gravels cut by the fault strands, but the abrupt thickening, thinning and disappearance of different sedimentary layers across individual fault strands suggests that it was predominately a strike-slip event. We interpret the organic-rich, dark gray to blue gray sandy silt (unit D) that overlie the iron- oxide-stained sands and gravels as marsh deposits. Unit D is mostly massive with only a few faint pebble lines oriented parallel to the top and base of the unit. Bulk soil samples from the base and the top of the marsh unit yielded radiocarbon ages of >37,300 and 33,330 ± 840 years B.P. (Table 2), respectively. Therefore the oldest of the five surface ruptures in the fault-bounded block occurred > 37,300 years ago. Evidence for Events 2, 3, and 4 is found in a sequence of sand and silt beds that overlie the marsh unit. Figure 16 illustrates our interpretation of the evolution of the sediments observed in the trench, beginning with the deposition of Silt 1 atop the dark blue-gray to black marsh unit D. Event 2 followed deposition of the oldest portion of Silt 1, and was succeeded by 45 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced w ith permission o f th e copyright owner. Further reproduction prohibited without permission. Raymond Fault Events - San M arino Trench S W Event Horizon A unjt N £venf Horizon 4 N E unit U Event Horizon 5 Fracture Fill Event Horizon 3 Event Horizon 2 Event Horizon Event Horizon Fracture Fills a, faults Event Horizon B Event Horizon A unit S Event Horizon 5 Event Horizon C r . w -.. W est W all Event Horizon 4 Event Horizon 3 Event Horizon 1 d * units E+G ^ ^ lir.i»n unit C un,t D East W all - M irror Im age m eters O s Figure15. Detail of the north end of the trench walls illustrating the paleoseismic events we observed at the Sierra Madre site. Bold numbers refer to Raymond fault events, while shaking-only events are labeled with letters. See Figure 13 for pattern descriptions. d) Event 3 - propagation of fault strands into Sandstone 1 d c) Deposition of Gravel 1, Silt 1 b and Sandstone 1 b) Event 2 - tilting - 13 degrees unitQt jjnit F (Gravel • o " . O - ' a) Deposition of Marsh deposits and Silt 1 unit E /Silt 11 l . " . t ; ) M . H M ■ ■, I! ■, Figure 16. Idealized snapshots in time of the depostional and seismic events at the San Marino trench site that followed the depostion of the marsh unit. Figures a through h and figure j show the relationships observed on the west wall of the trench, while figure i represents Event 5 as it was observed on the east wall. Figures i and j represent the sam e period of time on the west and east trench walls. Note that Event 4 manifested itself as a fracture fill on the west wall and as several fault splays on the east wall. Event 1 occured prior to deposition of unit D and is not shown. Patterns are as in Figure 13. Bold, dark gray lines are faults. 47 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. g) Deposition of Sandstone 2b and Siit 3 iriMKinr* f) Event 4 - tilt and shaking unit F (Gravel 1) e) Deposition of Silt 2, Sandstone 2a and Silt 2.5 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission unit Q (Sandy Silt 6) j) Deposition of Sandy Silt 6 (unit P) ' » X a • v - ' o w a • O 9 o . O a - O o • . a ■ ®. * .C J . o ’ ® • O. o * o O. o '® » O. ?o ° . »'«■• tb 'i V .1 ?faV. ?; (o ° » unit 0 (Gravel 4)^ IT nit F (Gravel (Gravel 4) unitKSijtg uniacs*'6' i) Event 5 (as observed on the east wall) O' • .O' • • • o ' * ••*• c : o « ' • « • : o « « * . ' ® : o # - o » : o « • * .o ' - ........... o unit 0 (Gravel 4) h) Event 5 fracture filled with Silty Sand 5 nit F (Gravel 1 49 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. deposition of Gravel 1 (Figs. 16b and c). The top of this gravel dips to the south at a shallower angle than its base, which indicates that there is a buttressed unconformity at the base of this unit. In other words, Gravel 1 is wedge-shaped and was deposited against a slope that was presumably created by tilting during Event 2. Likewise, the top of Silt lb on the east wall dips more shallowly than its base, which indicates that it, in the absence of Gravel 1, was deposited against a scarp. The Gravel 1 wedge was not observed on the east wall, so it must pinch out laterally between the two trench walls. After Gravel 1 was deposited against the scarp, the younger part of Silt 1 and all of Sand 1 were deposited over it. Because we observed several faults on both walls of the trench that cut through Sand 1, but do not disturb Silt 2, Event 3 m ust have occurred between the deposition of these two units (Figs. 16c, d and e). In addition, liquefaction features observed on the west wall that involve Sand 1 and Silt 1 could have been produced during this earthquake. A piece of detrital charcoal collected from within Silt 2 yielded a age of 32,820 ± 440 years B.P., statistically the same as a 32,240 ± 250 yrs B.P. detrital charcoal age collected from the black silt atop unit D. Thus, if the sample from Silt 2 is not reworked, Events 2 and 3 must have occurred during a very short period of time about 32,500 yrs B.P. If, however, the charcoal sample from Silt 2 is reworked, then Events 2 and 3 are constrained between the 32,240 ± 250 yrs B.P. age and a 29,990 ± 160 yrs B.P. age from a detrital charcoal sample in unit O (Table 2). Silt 2 is overlain by the older portion of Sand 2 and all of Silt 2.5 (Fig. 16f). We observed on both walls that a package made up of Silt 2.5 had slumped down onto the top of the existing Sand 2. In addition, the sand unit that blankets the Silt 2.5 slump is a northward-thinning wedge and was therefore 50 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. deposited against a gently south-facing scarp, unlike the underlying units (Fig. 16g). The same earthquake that produced the Silt 2.5 slump must have tilted the stratigraphic column a few degrees to the south in order to form the scarp. This is Event 4 and can be constrained by dates to between 32,240 ± 250 and 29,900 ± 160 years B.P., the age of a detrital charcoal fragment from unit O which was deposited after Event 4 (Table 2). Event 5 was observed on both walls of the trench. On the west wall a fracture fill containing Silty Sand 5 extends down to the main north-dipping fault (Fig. 16i). At about the same location on the east wall, several fault strands terminate within Silty Sand 5 (Fig. 16j). These features were probably produced by the same earthquake, which we term Event 5. Sandy Silt 6 was deposited over Silty Sand 5 after to Event 5. This event can be constrained to between 29,900±160, the age of the sample from unit O, below silty sand 5, and 27,530±150 year B.P., the age of a detrital charcoal sample from a unit stratigraphically above silty sand 5 in the footwall of fault d (Fig. 14; Table 2). Besides these five Raymond fault surface ruptures, we observed the two sets of vertical, finger-like stringers of fine- to coarse-grained sands and granules that cut across the near-horizontal, medium- to coarse-grained sands and pebbles north of the main vertical fault zone (Fig. 16). We interpret these features as fractures that were produced during two surface ruptures on the Raymond fault. These fractures are filled with the material that was at the ground surface at the time of the rupture. The older of the two fracture fill sets contains at least three individual fractures that are truncated at their tops by thin, gently south-dipping medium- to coarse-grained sand lenses (Fig. 15). The younger fracture cuts through the older group and is filled with packed, coarse-grained sand and granules derived from unit U overlying it. These sets of fracture fills represent two separate surface ruptures on the Raymond 51 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. fault. AMS dates of detrital charcoal collected from above the youngest fracture fill and from units that were cut by it indicate that this earthquake occurred between 20,350 ± 90 and 34,250 ± 830 years B.P. The older fracture fill earthquake can only be constrained by a minimum age of 34,250 ± 830 years B.P. (Fig. 14). Our age constraints on the fracture fills permit us to consider these two surface rupturing events as two of the five events observed in the fault-bounded block to the south. In addition, we observed three slumped silts that we interpret as shaking- induced features that could not be attributed to any of the five Raymond fault earthquakes described above. We refer to these as Events A through C in order to avoid confusion with surface rupture designations. These shaking events may or may not have resulted from earthquakes that ruptured the Raymond fault, but nonetheless correspond to strong ground shaking that affected this site. The first shaking event, Event A, followed Event 4 but occurred before deposition of Sand 3. This shaking event is identified by slumping of Silt 3 on both walls (Fig. 15). On the west wall this slump surface is manifested as a nearly planar rootless sedimentary "fault" that terminates downward in Sandstone 2 and upward in Gravel 4. On the east wall Silt 3 occurs as two relatively thick layers (~20-cm-thick) and one thin (1- 2 cm-thick) neck between them. We believe that Silt 3 has been thinned and extended along a listric slump surface to produce this geometry. Both the planar slump on the west wall and the concave-up slump on the east wall are stratigraphically younger than Event 4, and older than Sand 3. Neither slump is associated with tilting of the units below it. Therefore, we believe these two slumps are the result of shaking induced during a single earthquake on a fault other than the Raymond fault. We observed evidence for Event B on 52 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. the east wall of our trench in the form a ribbon-shaped silt slump, called Silt 4 (Fig. 15). This silt cuts Sandstone 3 and Silt 3 and is in turn overlain by Gravel 4. Because it cuts Silt 3 it must be younger than Event A. Likewise, because it is overlain by Gravel 4 this event is older than Event 4. As with Event A, Event B was not accompanied by tilting, so it is presumably also a shaking event that was not accompanied by surface rupture of the Raymond fault. We have been able to constrain shaking Events A and B to between 32,240 ± 250 and 29,900±160 years B.P. (Table 2). Shaking Event C was identified by thrust movement of several sand and gravel layers over two other gravel layers (Fig. 16). We were not able to trace the plane on which this occurred down into Gravel 4, nor could we follow it all the way through Sandy Silt 6. Event C must be younger than Event 5 because it involves Sandy Silt 6. Event C can be constrained to between 29,900±160 and 27,530±150 years B.P. (Table 2). The absence of events between 34,000 and 37,000 yrs B.P. in the interval between Events 1 and 2 may simply reflect inadequate stratigraphic control to resolve events within the predominately massive organic silt marsh deposits. Thus we could be missing events in this span of time. DISCUSSION The well-defined fault zone geomorphology of the Raymond fault confirms the earlier interpretation of Jones and others (1990) that the Raymond fault is a predominately left -lateral strike-slip fault, in contrast to earlier suggestions by Buwalda (1940) and Crook and others (1987) that it accommodates predominately reverse motion with some unknown component of strike slip. Specifically, the consistent left deflection of numerous drainages by as much as 400 m and the apparent 3.4 km left-lateral 53 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. displacement of the basement ridge at the east end of the fault confirm major left-lateral strike-slip on the Raymond fault. O ur geomorphological Zone III differs from zones to the east and west in that it does not exhibit a consistently well-developed south-facing scarp. We interpret this as an indication that Zone III has predominately pure strike-slip displacement, rather than the transpressional behavior of the fault to the east and west. The westward- increasing height of the south-facing scarp west of Zone III is a result of motion through the Raymond Hill - H untington Estate restraining bend at which the fault undergoes an abrupt 25° change in strike (Fig. 2). Although our geomorphic studies have failed to reveal a connection between the Raymond and the Hollywood faults, the similar orientation and kinematics - both are predominately left-lateral strike-slip with subordinate reverse components - of the two faults suggests that some connection may exist. Weber (1980) described north-facing scarps approximately along strike of the Hollywood fault in the town of Atwater on the eastern edge of the Los Angeles River floodplain. Although these scarps may be fluvial in origin (Dolan and others, 1997), they are superposed on an overall south-facing slope in the alluvial-fluvial deposits that is directly along strike with, and to the east of, the easternmost well established location of the Hollywood fault. This east-trending slope also overlies a major, very sharp gravity gradient similar to that seen to the west across the Hollywood fault (Chapman and Chase, 1979). These relationships suggest that there may be a through-going connection between the Raymond and Hollywood faults approximately where it has been mapped by T. Dibblee (1989) and Weber (1980). If such a connection exists, then the Raymond fault, together with the Hollywood and Santa Monica faults to the west, form part of a continuous, >80-km-long system of oblique-reverse left-lateral strike-slip faults. 54 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Previous authors (Weber, 1980; Crook and others, 1989; Dibblee 1989) have suggested that the Eagle Rock fault connects with the Raymond fault somewhere near Raymond Hill, but these interpretations are based on very sparse and equivocal geomorphic indicators and the presence of several bedrock faults which may not necessarily be active. The absence of straightforward geomorphic indications for a direct Eagle Rock - Raymond fault intersection raises the possibility that the Eagle Rock fault may not be active. This remains as open question and awaits further study. In contrast to the understood Raymond-Hollywood and Raymond-Eagle Rock fault connections, the connection between the Raymond fault and the Sierra Madre fault is a structurally complex, but well-expressed zone at the east end of the offset crystalline basement ridge (Crook and others, 1989). Our paleoseismologic data from the Arboretum trench indicate that the most recent surface rupture along one well-exposed strand of the fault occurred before ~A. D. 1,050. These trench data also show that this event on this strand has occurred after B. C. 8,050 years, the age of the youngest sample recovered from faulted strata. We can narrow these broad age constraints on the most recent event by calling upon published data from an earlier study of the fault by Crook and others (1987). Their trenches near Sunny Slope reservoir, 2.5 km west of our Arboretum trench site (Fig. 2), revealed a series of sharply defined, -50 centimeter-long (top to bottom), 10- to 20-centimeter- wide fissures filled with black, peaty marsh deposits. These near-vertical, finger-like fissure fills extend down to -2 m depth, and project downward over 30-50 cm into several well-defined, steeply north-dipping faults. The fissure fills were interpreted by Crook and others (1987) as direct evidence of the most recent surface rupture on the Raymond fault. The organic-rich, clayey mudstone fill of one of these fissures yielded a calibrated bulk-sample 55 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 14C age of B.C. 189 +212/-261 (their sample C-6; Table 1). This date provides a maximum age for the most recent Raymond fault surface rupture. However, given the very sharply defined edges of the fissure fills, it appears that the most recent surface rupture extended up into pre-existing m arsh deposits, which then fell into the fissures. Thus, the carbon in Crook and others' (1987) bulk-soil sample had probably been accumulating for some period of time before it became incorporated into the fissure during the most recent surface rupture. This means that the age of the sample from the fissure fill may be older than the age of the most recent event by some unknown amount of time. While the trench at Sunny Slope Reservoir and our trench at the Los Angeles County Arboretum were not excavated across the same fault strand, we can assume that these two trenches exposed geologic evidence for the same event because an earthquake that is large enough to produce surface ruptures along the Raymond fault is probably large enough to rupture multiple strands of the fault. In summary, the most recent Raymond fault surface rupture occurred sometime after deposition of the B. C. 189 +212/-261 sample from the Crook and others (1987) trench, but before deposition of the ~A. D. 1050 sample from the unfaulted deposits in the southern part of the Arboretum trench. Our Sierra Madre Boulevard trench revealed a latest Pleistocene paleoseismic record of five surface ruptures on the Raymond fault and three shaking events that may have occurred on other area faults. Although we did not recover a Holocene record at this trench site, we can use the ages of the latest Pleistocene events that we did observe to comment on the recurrence characteristics of the Raymond fault. Previous work (Crook and others, 1987) revealed at least 5 to 8 surface ruptures during the past 36,000 yrs 56 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. B.P., yielding an average recurrence interval of -5,000 - 9,000 years. The oldest event they recognized displaces a layer that yielded an -35,000 yrs B.P. bulk- soil radiocarbon age. This may correspond to our Event 1 (Fig. 17). Our event 5 could be the same event as their Event 1A or 2A, based on the very long possible age range for event 2A. The Crook and others (1987) results imply an average recurrence interval of -5,000 to 9,000 years (based on five to eight large events in the last 35,000 years). However, they recognized the possibility that some events may have remained undetected in their study, and speculated that the actual average recurrence interval was much shorter than their measured interval. They suggested 3,000 years as a plausible recurrence interval for large events on the Raymond fault. Current research indicates that 14C production rates between 32,000 and 34,000 yrs ago were much higher than previously thought, making 14C calandric ages up to 8,000 yrs. older than their conventional ages (Voelker and others, 1998). Based on these results, and geomagnetic models of 1 4 C production rates (Laj et al., 1996), the corrected calendric ages of our Sierra Madre trench 14C samples are 4,000 to 8,000 yrs. higher than their radiocarbon ages (Table 3). One of the major outstanding questions facing seismic hazard planners in Los Angeles is whether or not faults such as the Raymond can rupture simultaneously with adjacent faults, producing very large earthquakes similar to the 1992 Mw 7.3 Landers multi-fault rupture (Sieh and others, 1993). Our data show that the recurrence interval on the Raymond fault appears to be somewhat shorter than the published average. For example, our Sierra Madre Boulevard trench reveals a temporal cluster of at least five events during a < 9,000-year-long period between - 31,500 and 40,000 yrs B.P. (Fig. 17). Thus, the average recurrence interval for this period was no longer 57 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 3. Calandric Ages for C1 4 Samples from Raymond fault Sierra Madre Trench, San Marino, CA Sample°c Conventional C1 4 Age fvears) Calandric Ages in years (based on Voelker and others. 1998) RFSM 106 32,240 ± 250 -38,000 RFSM 108 32,820 ± 440 -40,000 RFSM 110 27,530 ± 150 -31,500 RFSM 162 29,900 ± 160 -34,000 RFSM 185 33,660 ± 530 -40,800 RFSM 215 and RFSM 216 >27,340 ± 130 -31,000 RFSM 233e >20,350 ± 90 -24,000 RFSM 246 34,250 ± 830 RFGL3 33,330 ± 840 -39,400 RF GL24a >37,300 -41,300 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced w ith permission o f th e copyright owner. Further reproduction prohibited without permission. Late Pleistocene - Late Holocene Events RFT1 RFSM A RFSM B Local RFSM Events from Crook and others (1989 | 3 | rand 4 l L A and B , * C I 1A X 2A 3 and 4 X X X 40,000 35,000 30,000 25,000 20,000 15,000 10,000 5,000 -6 h years B.P. Figure 17. Diagram showing age relationships between the events constrained by this study and by that of Crook and others (1987). The bars represent the maximum age limits of the specified surface ruptures and shaking events. RF T1 refers to the results of our first trench, at the Los Angeles County Arboretum. RFSM A and RFSM B refer to earthquakes that are interpreted from stratigraphy in the southern and northern portions, respectivly, of the Raymond fault zone at the Sierra Madre Boulevard site. Local RFSM refers to events infered from slumping, liqufaction and other shaking-induced features observed in this study. U \ SO than -2,000 years, and it could be much shorter; the minimum possible average recurrence interval is unconstrained because seismic events could be missing from the geologic record. Although the previously measured (-5,000 to 9,000 year) and suggested (-3,000 year) recurrence intervals published by Crook and others (1987) are much longer than the elapsed time since the most recent Raymond fault surface rupture, the variability in Raymond fault recurrence intervals that we have demonstrated indicates that the published recurrence interval is considerably longer than the minimum m easured recurrence interval on the fault. Jones and others (1990) calculated locations for the 1988 ML 4.9 Pasadena earthquake mainshock and many of its aftershocks. They found that these earthquakes fell on a plane dipping -80° north that projected to the surface trace of the Raymond fault. The 20 km length of the fault, coupled with an 80° fault dip and a seismogenic thickness of -16 km (Jones and others, 1990) yields a total fault plane area of -325 km2. Regressions of moment- magnitude versus fault-plane area and average slip (Wells and Coppersmith, 1994; Dolan and others, 1995) indicate that rupture of the Raymond fault in its entirety could produce a Mw -6.7 event with -50 to 100 cm of average slip. No reliable geological estimates of the slip rate on the Raymond fault are available. Walls and others (1998), however, using best-fit models of GPS geodetic results from the northern Los Angeles metropolitan region, have suggested a slip rate of 1.5 ± 0.5 m m /yrs on the Raymond fault. Combining this slip-rate estimate with the expected 50 to 100 cm of average slip suggests that if all strain on the Raymond fault was released during Mw 6.7 events, then such events would recur every - 250 to 1000 years. This is not in keeping with the 5,000-9,000 year average recurrence interval measured for the Raymond fault by Crook and others (1987). Nor is it consistent with the 60 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3,000 year average recurrence interval estimate suggested by Crook and others (1987) based on their assumption that there were events missing from the paleoseismologic record. The calculated average recurrence interval for Mw 6.7 events may be even slightly shorter than the average interval we measured during the latest Pleistocene "cluster" of events we observed in Sierra Madre Boulevard trench. There are three possible explanations for this apparent conflict of recurrence intervals: (1) The slip rate based on geodetic modeling may be too fast. This possibility has in fact been suggested by Argus and others (in review) who argue that much less east-west motion is occurring along faults in the northern metropolitan region (including the Raymond fault) than proposed by Wall and others (1998). (2) There are numerous missing events in the paleoseismologic record for the Raymond fault. This is certainly a possibility, as suggested by Crook and others (1987). (3) The Raymond fault may rupture less frequently than the calculations based on an assumed Mw 6.7, Raymond-fault-only rupture. If true, this latter possibility would suggest that the Raymond fault may sometimes rupture together with adjacent faults (e. g., Hollywood fault, Sierra Madre fault, Verdugo fault) in larger, less- frequent earthquakes. Paleoseismologic data from the central Sierra Madre and Verdugo faults are as yet too poorly constrained to compare with Raymond fault events, but the -6,000 to 9,000 year age of the most-recent surface rupture on the Hollywood fault (Dolan and others, in prep.) conflicts with the 1,000 - 2,000 yr age for the most recent Raymond fault event on the fault strands we have studied. We have trenched what appear to be the most These data indicate that the Raymond fault did not rupture together with the Hollywood fault during its most recent surface rupture. These data do not, of course, preclude 61 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. the possibility that earlier ruptures did involve both of these faults. Thus, development of a space-time history of strain release in large earthquakes along the faults of the northern metropolitan region awaits further paleoseismologic investigations. CONCLUSIONS The geomorphically well-expressed Raymond fault extends for 20 km in a gentle convex-to-the-south arc through the San Gabriel Valley northeast of Los Angeles. Geomorphologic data, including left-lateral stream deflections, shutter ridges, sag ponds, pressure ridges (Buwalda, 1940; Weber, 1980; Crook and others, 1987; Jones and others, 1990), and an apparent 3.4 km-long left- lateral offset of a basement ridge, coupled with the focal mechanism and very steep northward dip (-80°) of the fault revealed by seismological data from the 1988 Pasadena earthquake (Jones et al., 1990), all indicate that the Raymond fault is a left-lateral strike-slip fault. Common and consistently south-facing scarps along much of the fault are a result of a subordinate component of reverse slip as well as a major restraining bend along the central part of the fault in southwestern Pasadena. Whereas the confluence of the Raymond fault with the Sierra Madre fault at its eastern end is well-defined structurally and geomorphically, the interactions between the Raymond fault and the Hollywood and Eagle Rock faults are less well-defined. On the basis of published mapping (Weber, 1980 and Dibblee, 1989), gravity data (Chapman and Chase, 1979) and our geomorphologic analysis we believe there may be a complicated, but through- going fault trace between the Raymond fault and the Hollywood fault, along strike to the west. If true, the Ravmond-Hollywood-Santa Monica fault zone 62 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. would constitute a >80-km-long, thorough-going system of oblique reverse left-lateral faults. Our paleoseismologic trench data indicate that the most recent surface rupture along one well-exposed strand of the Raymond fault occurred before -A.D. 1050. Coupled with published data from an earlier study of the fault, this datum indicates that the most recent surface rupture on the Raymond fault occurred between -1,000 and 2,000 years ago. Another of our trenches revealed evidence for at least five latest Pleistocene surface ruptures, including four events closely spaced in time between 31,000 and 41,000. The <2,000-year-long average recurrence interval for these events is shorter than the published average latest Pleistocene-Holocene average recurrence interval (-5,000-9,000 years measured; 3,000 years suggested) (Crook and others, 1987). The quiescent period since the most recent Raymond fault surface rupture is within the shortest measured recurrence interval for the fault. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 63 BIBLIOGRAPHY Bak, P., and C. Tang, Earthquakes as a self-organized critical phenomenon ,J. Geophys. Res., 94,15635-15637 (1989). Bowman, D. D., G. Ouillon, C. G. Sammis, A. Somette, and D. Somette, An observational test of the critical earthquake concept, J. Geophys. Res., 103,24,359-24,372,1998. Chapman, R. H. and Chase, G. W., 1979, Geophysical investigations of the Santa Monica-Raymond fault zone, Los Angeles County, California, Calif. Div. Mines and Geology, Open-File Report 79-16, p. E-l to E-30. Crook, R., Jr., Allen, C. R., Kamb, B., Payne, C. M., and Proctor, R. J., 1987, Quaternary geology and seismic hazard of the Sierra Madre and associated faults, western San Gabriel Mountains, in eds., U.S. Geological Survey Professional Paper 1339: p. p. 27-63. Dibblee, Thomas W., 1989, Geologic Map of the Pasadena Quadrangle, Los Angeles County, California. 1:24,000 Dibblee Geological Foundation. Dibblee, Thomas W., 1991, Geologic Map of the Los Angeles Quadrangle, Los Angeles County, California. 1:24,000 Dibblee Geological Foundation. Dolan, J. F., Sieh, K., Rockwell, T. K., Guptill, P., and Miller, G., 1997, Active tectonics, paleoseismology and seismic hazards of the Hollywood fault, northern Los Angeles Basin, California: Bulletin of the Geological Society of America, v. 109,12, p. 1595-1616. Dolan, J. F., Sieh, K., Rockwell, T. K., Yeats, R. S., Shaw, J., Suppe, J., Huftile, G., and Gath, E., 1995, Prospects for larger or more frequent earthquakes in greater metropolitan Los Angeles, California: Science, v. 267, p. 199-205. Dolan, J. F., Stevens, D., and Rockwell, T. K., in prep., Paleoseismologic evidence for an early- to mid-Holocene age of the most recent surface rupture on the Hollywood fault, Los Angeles, California, to be submitted to the Bull, of the Seism. Soc. of Am. Geller, R. J., Jackson, D. D., Kagan, Y. Y. and F. Mulargia, Earthquakes cannot be predicted, Science, 275,1616-1617,1997. Heimpel, M., Critical behavior and the evolution of fault strength during earthquake cycles, Nature, 388, 865-868,1997. 64 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Huang, Y., H. Saleur, C. G. Sammis, and D. Somette, Precursors, aftershocks, criticality and self-organized criticality, Europhys. Lett., 41,43-48,1998. Jaume, S.C. and L.R. Sykes, Evolving towards a critical point: A review of decelerating m om ent/energy release prior to large and great earthquakes, Pure Appl. Geophys., (in press). Jones, L. M., Seih, K.E., Hauksson, E., and Hutton, L.K., 1990, The 3 December 1988 Pasadena earthquake: Evidence for strike-slip motion on the Raymond Fault: Bulletin of the Seismological Society of America, v. 80, p. 474-482. Ito K. and M. Matsuzaki, Earthquakes as self-organized critical phenomena, J. Geophys. Res., 95,6853-6860 (1990). Main, I., Long odds on predichon, Nature, 385, 19-20 (1997). Rubin, C. M., Lindvall, S. C., and Rockwell, T. K., 1998, Evidence for Large Earthquakes in Metropolitan Los Angeles: Science, v. 281, p. 398- 402. Sakado, M. M., 1991. Meridian Tank Site - Preliminary Geologic Report, December, 1991, AX 450-1" City of Los Angeles Department of Water and Power Sammis, C.G., and S. Smith, Seismic cycles and the evolution of stress correlation in cellular automaton models of finite fault networks, Pure Appl. Geophys, (in press). Scientists of USGS/SCEC, 1994, The m agnitude 6.7 Northridge, California, earthquake of 17 January 1994: Science, v. 266, p. 389-397. Seih, K., Jones, L., Hauksson, E., Hudnut, K., Eberhart-Phillips, D., Heaton, T., Hough, S., Hutton, K., Kanamori, H., Lilje, A., Lindvall, S., McGill, S. F., Mori, J., Rubin, C., Spotila, J. A., Stock, J., Thio, H. K., Treiman, J., Wernicke, B., and Zachariasen, J., 1993, Near-field investigations fo the Landers Earthquake Sequence, April to July 1992: Science, vol. 260, p. 171-176 Mori, J., Rubin, C., Spotila, J. A., Stock, J., Thio, H. K., Treiman, J., Wernicke, B., and Zachariasen, J., 1993, Near-field investigations of the Landers Earthquake Sequence, April to July 1992: Science, vol. 260, p. 171-176 65 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Shaw, J. H and Suppe P. M., 1998, A Blind-Thrust Fault beneath metropolitan Los Angeles identified from seismic reflection profiles and precise earthquake locations.: American Geophysical Union Annual Meeting, San Francisco, AGU, 79, p. F593. Somette, A. and D. Somette, Self-organized criticality and earthquakes, Europhys.Lett., 9,197 (1989). Voelker, A. FI., Samthein, M., Grootes, P. M., Erlenkeuser, H., Laj, C., Mazaud, A., Nadeau, M-J., Schleicher, M., 1998, Correlation of marine 14C ages from the nordic seas with the GISP2 isotope record: Implications for 14C calibration beyond 25 ka BP: Radiocarbon, v. 40, p. 517-534. Walls, C., Rockwell, T., Mueller, K., Bock, Y., Williams, S., Pfanner, J., Dolan, J. and Fang, P.. 1998. Escape tectonics in the Los Angeles metropolitan region and implications for seismic risk: Nature, v. 394, p. 356-360 Wells, D., and Coppersmith, K. , 1995, New empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement: Seismology Society of America Bulletin, v. 84, p. 974- 1002 Weber, F.H., Bennett, J.H., Chapman, R.H., and others, 1980, Earthquake hazards associated with the Verdugo-Eagle Rock and Benedict Canyon fault zones, Los Angeles County, California: California Division of Mines and Geology Open File Report 80-10 LA. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Appendix 1. Photomosaics of our two trench walls (east and west walls) at Sierra Madre Boulevard. Grid blocks are 1 meter by 0.5 meter. 67 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 69 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Appendix 2. Field Logs from the Los Angeles County Arboretum Trench. Each small grid box represents 10 centimeters. Note that dashed lines with numbers represent gradational contacts over the num ber of centimeters specified. Small circular to irregular outlines containing small triangles denote burrows. Key to abbreviations as follows: a/a abdnt A Hor (Ai, A2 ) bed blk brn ccl dy/dyey cob coh com dk fg flt/flted friab gty grn m gvl loc mg m n r mod m ott mtx or org peb pred rd s/sdy sim si slt/slty sm sort th an /a v w /. yel as above abundant soil horizon A bedding black brown charcoal coarse-grained day/dayey cobble cohesive comm on dark fine-grained fault/faulted friable gley (gray day) granuals gray gravel local m edium-grained m inor moderatly mottled matrix orange organic(s) pebble predominatly red sand/sandy sim ilar slightly silt/silty small sorted than/a very w ith yellow 70 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 72 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Wfc»T/*U- ' |.'T» Reproduced with permission of the copyright owner. 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Further reproduction prohibited without permission. t — ' f r ^ v v f- ' a r i «t»»i,f»iy#** 1 r»;i» U fijniv E 3 w T :,v S ? 5 ® - - ! - —- — - _ 1 ** — — • - • » » • • ^ 4 O -1 rv'j “ft i - r > - * s ; S r ^ ' p r v V / ? / • • • T » ^ * / A 4 105 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1 Stem* ENT) .4 r _ r — s»°ti s» * 7 « b 4 ) ■ » > n W M y V * 4 » , e»- lyt^ •rfm v w f ’ — ^ * * / - - - ’ 106 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. r K W d rn cM E «*i *i*» 107 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 9 108 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 73.07 meters to Hole 2 M o • H9 2 - 3 - 5 - 6 - 7 - 8 - 9 - 10 - -O o'* p« ♦ • K C ri». p r* * c ; V- < . o to 3 * . ••c > ; 3 * » •*c o * • • b * A H o t v . dk bm. abdnt org Si. coh. med. bm-yel-bm f-cg s and gm V . coh.. dk gry-bm, near-blk. v . (g sdy sit (no dy) Med. dk rdish-bm. v. fg sndy sit (no dy) Coh. dyey. slty. v. fg sod. M inr cd SI. - Mod. coh., v. poorly sort., pred. eg. f-cg s V . dk rd. blk and or-bm med.- eg s. Thin bed Mod. coh. v.-f to fg s. mod. well-sort a/a but si. dker to med. brown a/a but si. coarser, fg s. mod. well-sort Pred. eg s and gm. si. coh. med. bm., fgs-vsm peb. pred. fg s. loc. cob pred. srrvlg peb. gvl,sl-mod. coh mtx pred. med bg.-bm fg s sl-mod. coh. peb gvi. a/a. loc. cobb up to 0.13 m a/a ^ very sharp contact olive-yel-bm, to med or-bm.vfg coh. sdy sit. no dy mod. coh. fg-cg. pred. mgss. mott., loc. or-gry*ye(-bm slty intervals sl-mod coh . a/a. more bnght yel-or a/a. fnab.. solid or a/a. more Ig peb gv., fnab solid or loc cobb up to 0 15 m a/a but less or. pred. med si or-gry-yel-bm, fnab pred. fg s mtx. paler or-yel*bm. pebs common, si. coh. med yel-bm-bm fnab.. mod-well sort., pred. fgs coh., med bm. pred. fgs. vfgs-peb gvl interbedded coh. gry-yel-bm slty fgs. eg peb gvl. cgs - ^ or color increase (seen at 7.32 m in H2) pred vegs-gm, med or-bm. fnab a/a except more dk bnck rd-or. si. more fg a/a. or s med dk bm. coh.. v slty vfgs w./ ig peob. coh. vfg-egs. cobb. gvl a/a. but this is NOT as v dk bm as m other Holes a/a. but no or in this unit si coh., or mott. fgs-pebb gvl banded gry-yel-bm. fg-egs. fnab.. pred. or a/a. but pred. gry-yel-bm. pred. fgs a/a. but -1/2 or. 1/2 gry-bg, less gvl, fnab. a/a. but pred.gry-yel-bm. fnab. more eg than/a. otherwise fnab. a/a. mixed gry-yel-bm-or - more coh.. dker bm si. coh. pred fg-mgs. fgs-sm. pebb. med bm. damp a/a si coh a/a si coh K EY TO ABBREVIATIONS a/a abdnt AHor(As A]) bad blk bm c d e g cly/dyey cob coh com dk < g flt/ttted (nab gly gm gry gvl loc mg mnr mod mott mtx or org peb pred rd s/sdy sim s i slt/slty sm sort than/a v w/. yel as above abundant soil horizon A bedding black brown charcoal coarse-grained day/clayey cobble cohesive common dark fine-grained fault/faulted friable gley (gray day) granuals gray gravel local medium-grained minor moderatly mottled matrix orange organic(s) pebble predommatly red sand/sandy similar slightly silt/silty small sorted than/a very w ith yellow Appendix 3. Bucketauger hole data collected at the Sierra Madre site. Samples were collected and logged from every bucket of material pulled out of the hole. Scale to the left of each hole diagram is in meters. 109 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 61.60.98 meters to Hole 3 1 2 3 4 5 6 7 8 9 10 11 61.59 meters H1 • i— •o* .fc> .to *o* ■ C • o V dk gry-bm dy slty s; A hor. A 2hor7 paler dk gry-bm ooh. dy? slty s Con. org blk. sit; ponded water/marsh? Coti.bg slty s Less coh. than/a. bg slty s Friab or-gry-bg s w/ gvl Friab pale gty-bg s Friab or-bg s w/ gvl Friab. or cgs, w/. gvl SI coh bg-bm slty s a/a w/ peb-cob gvl Dker bg-bm slty sdy peb-cob gvl near-blk slty sdy org-rich. buried A? Coh. Aj’ Fnab bgs Coh., med bm-gry slty s Bright or-stalned stringers in med gty-bm s. lose ooh. Friab. to si. coh bm-or s SI. coh. Friab pred cgs org-bm a/a . but brighter or f More dk bm. si less or H2 73.07 meters to Hole 9 o -I 1 - 2 - 3 - 4 - 5 - 8 - 9 - fS . 3 .* . .* * ( t? 3 - '.' .* '< •d Med bm A? * roots Blk Peat!/ slty dy f paler Med bm dy Coh. med bm. fg-cgs Yel-bm coh. v slty vtgs ^incr. or Mixed gry-bm and or. still coh. slty s Med bm fg-v eg s w/ gvl. friab.. no tines nvegs. more gvl Med bm coh. slty tgs Bg vtgs. friab.. mixed coh sit * friab. ss (thin beds?) Med bm tgs Dk or-bm fg-cgs. v wet Bg. fgs. no gvl. si. coh?. v wet Flowing Water Dk. gty-bm slty fgs V coh. slty vfgs Coh. slty? fg-mgs Friab. or-bm fgs Coh. bm-or slty? fgs Abund well developed or stringers 110 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3.75 meters to Hole 8 15.09 meters H5 o I 2 - 3 - 65* S-dip on west wall 4 Fit drags bm up into gley___ w/ in fracture Cue 5.56 m 5 ■ Mod. well sort, si coh. cfg-fgs. no fines bg ‘granitic s', thinly bedded 7 - 8 - H3 60.98 meters to Hole 1 AHor.vdkbm Friab yel-bm-med bm fg-cgs Color change Or-bm-bright or, pred. fgs Coh. gry-bm to bhght or . dy sit fgs Coh. dy slty fg-cgs w/ mnr v sm peb Fgs-med peb. fnab.. gvl Coh or-bm slty vtgs Friab. or. pred. fgs, fg-cgs. mnr peb Thin coh. gry-yel-bm slty fgs 6 2.85 m Slty vfgs; si. coh.. gry-yel-bm to or mott Coh vfgs. thin beds Peb gvl somewehre betw 0.25 *3 56 m Coh. slty vfgs. thin beds Coh vfg sdy sit, dk gry-bm w/ or stains Blk v dy vfg sdy sit. plastic and coh upper part appears more bm than @ t Giey 401 m Gly. blu-gry slty dy. coh.. brecdated? SI. coh. fg-mgs. mnr sm peb. odd si purple-med gry-bm color Mod. coh fg-cg s. med gry-bg ■granitic s’ Mott or. gvl. friab.-sl coh. mott gry-yel -bm to or 'granitic s* cgs-cob Fnab. fg-mgs. or-bg. f more cgs-gm; fgs-gm. fnab . less or Friab fg-cg pale gry-yel-bm granite s Or-gr-yel-bm mott pred eg. fg-cgs. gm Dk gry-bm V dk blue-gry Or-gry-yel-bm vfg-fgs. coh. Color change, lose or. color Pred pale gry-yel-bm vfg-fgs. mod well sort., coh.. loc pale or-bm mott Slty vfgs Pred pale gry-yel-bm vfg-fgs. mod well sort., coh.. loc. pale or-bm mott Running water Oip^SSW Dip 11.5 SSI 8 - 9 - 10 - Dk bm A Nor SI. coh. (only when wet?) fg-cgs. pred eg Coh. pale gry-bg slty vfgs. more slty and coh than/a Color change, finer grained M ott, pale gry-bg to or vfgs. coh More or and sdyer than/a. loc slty mott gry. pred or s: fnab. Pale gry-bg s. coh. Gry-bg vfg-fgs. coh Some sit layers, coh Incredible cone, of cood/ccl @ -5.13 m V friab. pred. cgs w/. mnr pebb (bimodal w/cobb) No fines, mixed o r» gry-bg colors V . coh dy slt/slty dy, gry -bm Gley (blue) dy slty s Gry-bm fg-mg s Color change Bg fnab. fg-cgs SI. finer grained Some 'granitic's ' t 0 7.37 m Fg-cg 'granitic's Cly fg-sdy slty. med bry-bm. C Or slty vfgs. v coh unit Gry-bg fgs. friab Fg olive-bm fgs Fg-cgs Olive bm (no or) fgs. mod coh to coh. loc slty fgs 1 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 5.95 meters to Hole 4 2 3 4 5 6 H7 5.95 meters •; ° i ? ' • o O 3 ‘. • o > ’ ® *. o -. O ' Do O' D ‘o O ' D o A hor. v dk bm slt-cgs. pred. slty vtgs ‘granitic' vtgs-v sm pebb. friab df incr. cob vfgs. v (bin interbeds, overall mod cob More gvl f SI. dker fgs-med pebb. friab. SI. cob. pred. fgs. a/a tjf si paler ^ SI. more eg V mnr cob slty vfgs in bucket More gvl to sm cobb (-0 13 m). mnr coh slty vfgs a/a w/ mnr cob slty vfgs in bucket Pred. sm pebb gvl ^ Less gvl; daster finer-grained Pred cgs. still friab Pred. mgs. otherwise a/a, no gvl Fg-cgs - sm pebb gvl; gry-bg. friab.. more gvl SI more coh.. otherwise a/a SI. damp; incr damp below 5.00 m . a/a Pebb-sm cobb gvl. friab V . cob., slty? vfgs Pebb-cobb gvl Fnab. bm-bg, si damp. pred. fg-mgs to SI. coSr* ( f o e mod cob), fg-cgs w/ mnr SI ilrore eg. pred cgs Fnab fgegs-sm pebb gvl More gvl. pred sm-lg pebb SI coh a/a SI. more eg. pred cgs-gm. a/a H8 Fit. N89 W . , 53-63 SE ----- w /. loc. near vert strands 4 - 5 - 6 -6.86 m grad, mterfing. contact from vfgs down fi ■ into dy sit Fit @ 7 93 m - N67E. ------- 80-90 S 7 . on W wall 40 N on NE wall 3.75 meters to Hole 5 A Hor v . cob. v dk bm SI. cob. vfg-cgs (pred. fgs) ^ — mott gry-bg-or granite s contains dastgs of bm v. coh dy-slt w/ cd. contains flue-gley day M ult. CM Blk dy silt w/ abund. cd. v. coh. SI. blue, near blk dy sit. vfgs. org-ncb. gley? SI. paler bluish blk. dy? slty vfgs v cob. SI. dker. v. dk gry vtgsdy si! SI. less cob. (mod coh-coh), bluish dk gry sit I ” Mod. cob. damp, odd 'purplish' med bm cly slty fg-cgs; prod flted a/a. but dk or mott com a/a. but layerd w/. alt bm ♦ or layers9 a/a mnr sm pebb V coh. dk bm dy sit a/a paler bm Sim. to 3.35 - 3.96 m Mod. cob., med bg-bm pred. fg-mg s. fg-cgs a/a. but w/.o dker bm; mnr sm peb Color paler, lose cob. Fgs. fnab. odd pale yel-or color, mod a/a.'excepfmore med bm or or mott pred fgs, vfg-mgs. mod cob Color change V cob. med bm-bg dy? sit a/a Sdier • coh fg-fg pred bg-bm mott or Sltier. bg vslty vfgs or vfgsdy sit. cob. a/a. but mod or mott interbedded sit» vfgs. cob. Slty vfgs. cob. gry bg - or mott Fg-mgs. mod cob. abdnt or stain SI or-med bm mod coh vfg-fgs 112 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. H 6 S P 21.65 meters V . dusty pale-bm. v weak A? D iff man A in H1-5; maybe just drier man other holes? Pkd cgs-v sm peb gvl 1 - ’A - '1 Vi ’ r f .: o Mod. sm peb grvly < - fg-cgs. sev. clasts ^ .Ig peb-lg cobb gvl. pred. cgs-sm peb V The 0.23-0.31 m thick near-blk A hor. is missing in H6. only dry. dusty poss. RR ballast gvl Fg-cgs peb-cob gvl. pred cgs-sm peb Sm peb gvl. pred. cgs-sm peb. abdnt cob Fnab fg-cgs w/. sm peb-sm cob gvl to 6 40 m entire H6 is 'granite' fg-cgs w /. peb-cob gvl in various % except 6.71-7 01 m and 9.45-10.67 m. which are siltier 5 - 6 - 7 - 8 - 9 - 'd t V p 'a ' 1 'a * ' is .: \-9 SI. more fg. si coh . si slty? Slty vfg s. coh. damp SI. coh.. pred. fgs. fg-cgs. at 6 40-7 01 m . Fg-cgs. sm peb-sm cob congl.. friab Pred. cgs-sm peb gvl. more eg than above SI more fg * coh.. fg-cgs. few clasts Fnab fg-egs/peb gvl Mod slty fgs w/ mnr cgs Mod coh 1|f less rounded clasts Fnab fg-cgs w/. peb gvl Coh slty vfgs-peb gvl 0 - i 2 - 3 - 4 - 5 - 7 - H4 > • • 'k: l • i* ^ '• c j A > • • 'k : . ' c s : ’ d . * ► 5.95 meters to Hole 7 A Hor. V dk bm. org-rich slty ss w/ peb i ,V friab. bg-bm fg-cgs pred cgs. mgs-med peb granitic s. clasts also incl. meta y si. incr. in peb gvl v. friab.. lose, pred cgs-gm. fewer peb v fnab.. fg-cgs. gen more fg than above fgs-gm. v friab.s. gen a/a. but eg w/ mnr v sm. peb fgs-gm/sm peb gvly s Mod coh.. med bm. dker than above slty vfg-fgs. nothing more eg than fgs v. fnab fg-cgs w/ abdnt sm pebb-smcobb gvl y incr gvl ind. cob f iner, in size of dast; ind. loc sm boulder (1 83-3.05 m) pebb-boulder gvl • clasts >0.25 m pred. fg-cgs w/ mnr sm peb gvl y incr in size of dasts V less gvl than/a " A '' Fnab fg-cgs • peb gvl 113 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 25/26 27/28 Loction of RFSM Trench along the borehole/* transect X 3/4 1/2 5/6 Main Thrust Fault 7/8 9/10 11/12 13/14 15/16 Buried 6" Gas Line 17/18 19/20 Fault and dip direction measured down-hole 21/22 meters 23/24 Appendix 4. Ground Penetrating RADAR (GPR) data. The map above shows the locations of each line (1-28) collected for us by Terra Geosciences along the median strip of Sierra MadreBoulevard. The odd- and even-numbered GPR lines were collected using the 400 MHz and a 200 MHz antennas, respectivly. Numbers prefaced by H indicate bucketauger holes . Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission o f th e copyright owner. Further reproduction prohibited without permission. NE SW File 25: 400 MHz, 100 ns deep NE end starts 200' (70 m) north of the beginning of lines 1/2 and is 100' (30.5 m) long Collected parallel to the boulevard — File 26: 200 MHz, 200 ns deep Same location as File 25 io n o f th e copyright owner. Further reproduction prohibited without permission. < f) C / ) Fite 27: 400 MHz, 100 ns deep NE end starts at the 100' (30.5 m) mark of Lines 25/26 (100‘ (30.5 m) north of Lines 1/2] and is 100' (30.5 m) long Collected parallel to the boulevard and the bucketauger transect £ File 28: 200 MHz, 200 ns deep Same location as File 27 lission o f th e copyright owner. Further reproduction prohibited without permission. File 1: 400 MHz, 100 ns deep NE end of Line starts at 18'0“ (5.5 m) N of RFHole 6 and is 85' (25.9 m) long Collected parallel to the center of the median strip, N52E File 2: 200 MHz, 200 ns deep Same location as File 1 Reproduced with permission o f th e copyright owner. Further reproduction prohibited without permission. NE SW File 3: 400 MHz, 100 ns deep NE end starts at 17' (5.2 m) N of trench and is 100' (30.5 m) long Collected parallel to and west of the trench, inside our chainlink fence. o o File 4: 200 MHz, 200 ns deep Same location as File 3 Reproduced w ith permission o f th e copyright owner. Further reproduction prohibited without permission. NE SW File 5:400 MHz, 100 ns deep NE end starts at the 100' (30.5 m) mark of Lines 3/4 and is 55' (16.8 m) long The southern end of trench is at 12' 9" (3.9 m) on Lines 5/6 Collected parallel to and west of the trench, outside our chainlink fence 3 File 6: 200 MHz, 200 ns deep Same location as File 5 lission o f th e copyright owner. Further reproduction prohibited without permission. File 7:400 MHz, 100 ns deep NE end starts at the 55' (26.8 m) mark of Lines 5/6 and is 100' (30.5 m) long Collected along the bucketauger transect. g File 8: 200 MHz, 200 ns deep Same location as File 7 Reproduced with permission o f th e copyright owner. Further reproduction prohibited without permission. NE SW File 9: 400 MHz, 100 ns deep NE end starts at the 100' (30.5 m) mark of Lines 7/8 and is 100* (30.5 m) long Collected parallel to bucketauger transect 13 File 10: 200 MHz, 200 ns deep Same location as File 9 Reproduced w ith permission o f th e copyright owner. Further reproduction prohibited without permission. Fite 11: 400 MHz, 100 ns deep NE end starts at 100' (30.5 m) mark of Lines 9/10 and is 100' (30.5 m) long Collected parallel to bucketauger transect 13 File 12: 200 MHz, 200 ns deep Same location as File 1 1 lission o f th e copyright owner. Further reproduction prohibited without permission. File 13: 400 MHz, 100 ns deep NE end starts at the 100' (30.5 m) mark of Lines 11/12 and is 75' (22.9 m) long Collected along and parallel to the bucketauger transect File 14: 200 MHz, 200 ns deep Same location as File 13 lission o f th e copyright owner. Further reproduction prohibited without permission. File 15:400 MHz, 100 ns deep NE end starts at the 55' (22.9 m) mark of Lines 13/14 and is 100' (30.5 m) long Collected along and parallel to the bucketauger transect 13 File 16: 200 MHz, 200 ns deep ^ Same location as File 15 Reproduced with permission o f th e copyright owner. Further reproduction prohibited without permission. NE SW — I f f i B W File 17: 400 MHz, 100 ns deep NE end starts at 100' (30.5 m) mark of Lines 15/16 and is 100* (30.5 m) long Collected along the bucketauger transect, south of the intersection of Euston and Sierra Madre Blvd. 73 File 18: 200 MHz, 200 ns deep Same location as File 17 Reproduced with permission o f th e copyright owner. Further reproduction prohibited without permission. NE SW File 19: 400 MHz, 100 ns deep NE end starts at the 100' (30.5 m) mark of Lines 17/18 and is 100' (30.5 m) long Collected south of the bucketauger transect and parallel to the boulevard. ^ File 20: 200 MHz, 200 ns deep Same location as File 19 Reproduced w ith permission o f th e copyright owner. Further reproduction prohibited without permission. File 21: 400 MHz, 100 ns deep NE end starts at the 100' (30.5 m) mark of Lines 19/20 and is 100' (30.5 m) long Collected parallel to the boulevard £3 File 22: 200 MHz, 200 ns deep Same location as File 21 Reproduced with permission o f th e copyright owner. Further reproduction prohibited without permission. NE SW File 23: 400 MHz, 100 ns deep NE end starts at the 100' (30.5 m) mark of Lines 21/22 and is 100' (30.5 m) long Collected parallel to the boulevard i3 File 24: 200 MHz, 200 ns deep 0 0 Same location as File 23 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Appendix 5. Field Logs from the Los Angeles County Arboretum Trench. Each small grid box represents 10 centimeters. Note that dashed lines with numbers represent gradational contacts over the number of centimeters specified. Small circular to irregular outlines containing small triangles denote burrows. Key to abbreviations as follows: a/a as above abdnt abundant A Hor (Aj, A2 ) soil horizon A bed bedding blk black bm brown cd charcoal eg coarse-grained dy/dyey day/dayey cob cobble coh cohesive com dk fg flt/flted friab giy grn gry gvl loc mg mnr mod mott mtx or org peb pred rd s/sdy sim si slt/slty sm sort than/a v w /. yel com m on dark fine-grained fault/faulted friable gley (gray day) granuals gray gravel local medium-grained m inor moderatly mottled matrix orange organic(s) pebble predominatly red sand/sandy similar slightly silt/silty small sorted than/a very with yellow 130 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 131 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. U K * * ', * 1 (.9 7 * 0 132 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 133 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Co»- * O 134 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 135 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. * » -/-•** ti - m i i « C 3* W W & W - ■ ■ 11 1 1 1 I H f * T ‘T 6' t j n f r 'i * — V £r* « . * » « I m i ^ ' 1 * ' 'r i V t) i * ) ^ j.y - / « . . » % U t * l > 4 ^ 5 J b y * > « • » I • « « U # « i r ^ j i. • > ? > » - 136 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 137 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. **-V« Cot MU T w « fc.l ► w. ^ r n —f fcti i«k < • » < * < ■3^ " ‘VO V 138 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. r ~ . .. ’ — w r r j < - .:.c t.c , » A W • S« 6 it 44l /I ff / / 139 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 140 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 141 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. V ’SM sh i« M . ' • - ^ w * y* '»fr • * •* «+ — w- c«* <•«-*» 3* • * < * » ***^~ 1 — * ■ ) ■ * ««■**-* ^ u > mX C ' * > / » 8 C e * J , * f SH > 142 Reproduced with permission of the copyright owner. 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Further reproduction prohibited without permission. *PV1 f c r t t v \ V s * - ? * * “Imp c U a k « _ *»»0 'JUj IJ ) r* S y$ *-1 * - -• j v “ 1 m- * 7 * e W * ^ ^ - i f c n 149 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 150 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. •m* o» i o < > •.' » fit. <wo tfc-M tff*.. %»w • /• •' k.,. «tew fK. •»-! ^ .4 . ■J.5/ l •< * ■ < * - • f'*'*’ 151 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 152 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Co" - \W.' < o 2 — 153 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. f t 9* fa * ,a 154 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. • f 9* C«* 4 ) « • * fe7 5 * • * / *•' • * * J A t a r * * IM * I ii,ii h J P * » T'*3* y f t ' W ir. C _. .4. « • » U •* 3 * ^ • / • » * £ j 155 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. W - J r C m * t k i •j y V * * * • • ■ * * h - « • * c: ^7 /« y»«< ^ " ‘• n 156 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. **•1* Cat M b I I i t Ti« h w. ■rtn — / * * • / 157 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. | I f . t h M H k>~. Z f — r r . i •- <- ,:.c t.E ,a fc r f .£ . £**^x f % r / t / 158 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. > V / ■ * % ,-T -> T * ■ ; T / * ' *•»•“* • f r?* * - V " • * j r - j ' — - i; ' " ■ / ■ - . ( / ^ ■ ■ ■ i ' * > v u s * 5 “ ' 2 ’" (fS 'l *rt«Wj 3 5 HvJ ‘■ jra^r V l - < * T k /J.. 159 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 160 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 161 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. •r-«« «** «** 4 f c - i b » - « > T 3 * * < JB C 3 * 7 ’“• 162 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. v - w * « « • * u m » - y ( g . — w H « « S — v < ’ • » - « '■ " * • ) n < - '~ - * * '1 *H-ly - I - T ‘ f* ■ * ) - » “ >ki)*f Wf«.M»t<W ***■ £" • ”5'J* “* ■ j • tR < * — r,k5*,il — / * 1 163 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. r f j A w / •. k« 7 l i Ik.^ «•*. «> **W fe* ♦ 5K » <«M-y W < J» , ( > » k * " ' ! 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Asset Metadata

Creator Weaver, Kristin Derry (author)

Core Title Paleoseismology, geomorphology, and fault interactions of the Raymond fault, Los Angeles County, California

Degree Master of Science

Degree Program Earth Sciences

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

Tag Geology,OAI-PMH Harvest

Language English

Contributor Digitized by ProQuest (provenance)

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c16-290191

Unique identifier UC11341079

Identifier 1409663.pdf (filename),usctheses-c16-290191 (legacy record id)

Legacy Identifier 1409663-0.pdf

Dmrecord 290191

Document Type Thesis

Rights Weaver, Kristin Derry

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 au...

Repository Name University of Southern California Digital Library

Repository Location USC Digital Library, University of Southern California, University Park Campus, Los Angeles, California 90089, USA