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Determination of paleoearthquake age and slip per event data, and Late Pleistocene-Holocene slip rates on a blind-thrust fault: Application of a new methodology to the Puente Hills thrust fault,...

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Determination of paleoearthquake age and slip per event data, and Late Pleistocene-Holocene slip rates on a blind-thrust fault: Application of a new methodology to the Puente Hills thrust fault,...

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Content DETERMINATION OF PALEOEARTHQUAKE AGE AND SLIP PER EVENT
DATA, AND LATE PLEISTOCENE - HOLOCENE SLIP RATES ON A BLIND-
THRUST FAULT: APPLICATION OF A NEW METHODOLOGY TO THE
PUENTE HILLS THRUST FAULT, LOS ANGELES COUNTY, CALIFORNIA
by
Shari Ann Christofferson
A Thesis Presented to the
FACULTY OF THE GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fufillment of the
Requirements of the Degree
MASTER OF SCIENCE
(EARTH SCIENCES)
AUGUST 2002
Copyright 2002 Shari Ann Christofferson
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UNIVERSITY O F SOUTHERN CALIFORNIA
T H E G R AD U A TE S C H O O L
U N IV E R SIT Y PA R K
LOS A N G E L E S. C A L IF O R N IA 9 0 0 0 7
This thesis, written by
Shari Ann Christoffejrson
under the direction of hfS 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
Master of Science
D ias
D ate Becenlb e r 1 8 , 2002
T
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Acknowledgments
There were many people who lent me helping hand (mind, ear, shoulder, cash, etc.)
during various stages of this research. I would like to first thank my mother and father for all
of their support over the past 3 years and for deferring my payments on their much deserved
“Parent Royalty Fee.” Dr. James Dolan made this research possible by lending his vision,
expertise and time. I thank him most for his open-mindedness and heart-felt enthusiasm.
Needless to say, I could not have done it without him. Ross Hartleb, my senior officemate,
gracefully acted as an advisor and field grunt at the same time. I am very lucky to have had
him around. I thank Dr. John Shaw for hosting me at Harvard University, where I was able
to get a broader perspective on the Puente Hills Thrust Fault, and for his constant scientific
advisement on this project. John McRaney was very patient and understanding as I tried my
hand and project management, which involved a trial-and-error approach to extracting
money from our NEHRP grant. Thomas Pratt, Robert Williams, and Jack Odum ran the
high-resolution seismic lines for Phase 1 of this project but they get 1st prize for best
practical joke, which reminded me of the importance of not taking yourself too seriously.
Thanks to Dr. George Burr at the AMS radiocarbon lab at the University of Arizona for
teaching me how to prepare my samples for radiocarbon dating, and for being so kind and
hospitable to me during my stays at the UofA. Dr. Lewis Owen took a lot of time to teach
me how to prepare samples for Optically Stimulated Luminescence Dating at University of
California, Riverside. Marcos Marin was an irreplaceable field assistant as he provided both
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physical assistance and mostly entertainment. I would also like to thank Scott Marsic, Allan
Tucker, Ken Austin, Francisca Staines-Urias, Gerald Grellet-Tinner, and everyone else who
offered their help in the “field” that was east Los Angeles County. Ron Hester and his crew
at Tri-Valley Drilling for rolling with the changes during this project and for their dedication
to safety. I thank Joanna McKeehan and the guys at Gregg Drilling for their expertise.
Shriley Bishop and Jerrry Stock at the City of Bellflower who helped me acquire permits. A
big thank you to all the residents on Carfax Avenue who put up with all the noise of the drill
rig and the temporary parking restrictions on their street. Finally, I thank Murat Alpaslan for
being by my side for the past three years, and for listening to all my exclamations of pain,
frustration, relief, and joy as I made my way through graduate school.
This research was funded by the United States Geological Survey National
Earthquake Reduction Hazards Program (NEHRP), with additional funding provided by the
Southern California Earthquake Center (SCEC). SCEC is jointly funded by the National
Science Foundation and the United States Geological Survey. Funding for the acquisition of
the high-resolution Mini-Sosie seismic reflection data was provided by the University of
Southern California and Harvard University.
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Table of Contents
Acknowledgements................................................................................................................ii-iii
List of Tables..............................................................................................................................vi
List of Figures.................................................................................................................... vii-viii
Abstract......................................................................................................................................ix
CHAPTER 1..................................................................................................... 1
I n t r o d u c t io n .............................................................................................................................................1
R e g io n a l Se t t in g .................................................................................................... 3
P u e n t e H il l s B l in d -Th r u s t F a u l t .................................................................................................6
P r e v io u s In v e s t ig a t io n s......................................................................................................................8
M e t h o d o l o g y ......................................................................................................................................... 13
CHAPTER 2......................................................................... 19
P h a s e 1: M in i - S o sie a n d H ig h R e s o l u t io n Se is m ic D a t a ...............................................19
CHAPTER 3................................................................................ 26
P h a se 2: B o r e h o l e T r a n s e c t s........................................................................................................ 26
Carfax Avenue....................................................................................................................26
Drilling Techniques and Potential Errors.............. 26
Stratigraphy........................................................................................................................29
SCE boreholes.....................................................................................................................38
Age Control......................................................................................................................... 40
Evidence for folding.......................................................................................................... 46
In t e r p r e t a t io n .................................................................................................................... 50
Evidence for paleoearthquakes..........................................................................................50
Kinematics........................................................................................................................... 69
Slip per Event and Magnitude o f Paleoearthquakes........................................................75
Slip Rates.............................................................................................................................78
CHAPTER 4................. 80
T r o ja n W a y ..........................................................................................................................80
Stratigraphy........................................................................................................................81
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Age Control................................................................................................................................ 86
Evidence for folding...........................................................................................................87
In t e r p r e t a t io n ...................................................................................................................................... 89
Late Pleistocene Slip Rates................................................................................................89
CHAPTER 5............. 91
D is c u s s io n ................................................................................................................................................. 91
C o n c l u s io n .............................................................................................................................................100
B ibliography................................................................................................................................................... 105
Appendix A................................................................................................................................ 110
Appendix B ................................................................................................................................ 125
Appendix C................................................................................................................................ 132
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vi
List of Tables
Table 1. Radiocarbon Analysis at the Carfax Avenue site........................................................41
Table 2. Estimated Slip and Magnitude Values for Paleoearthquake Events
at Carfax Avenue...........................................................................................................77
Table 3. Slip Rate Information....................................................................................................97
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vii
List of Figures
Figure 1. Regional Map of the Los Angeles basin..................................................................... 4
Figure 2. Structural Contour Map of the Puente Hills Thrust Fault.......................................... 7
Figure 3. Annotated Seismic Reflection Profile of the Santa Fe Springs Anticline............... 10
Figure 4. Annotated Seismic Reflection Profile of the West Coyote Hills Anticline............12
Figure 5. Models of Expected Stratigraphic Relationships...................................................... 15
Figure 6. Growth Strata Thickness and Scarp Height.............................................................. 17
Figure 7. Carfax Avenue Site Map and Surrounding Area.......................................................20
Figure 8. Trojan Way Site Map and Surrounding Area............................................................21
Figure 9. Industry, Mini-Sosie, and Hammer-Source Seismic Reflection Profiles
at Carfax Avenue........................................................................................................23
Figure 10. Industry and Mini-Sosie Seismic Reflection Profiles at Trojan Way....................24
Figure 11. Detailed Map of Carfax Avenue and SCE Borehole Locations............................ 28
Figure 12. Stratigraphic Correlation at Carfax Avenue............................................................30
Figure 13. Correlation of Carfax Avenue Stratigraphy with Borehole 1589S......................33
Figure 14. SCE Borehole Transect.............................................................................................39
Figure 15. Sedimentation Accumulation Rate Curve...............................................................45
Figure 16. Stratigraphic Growth Versus Distance along Carfax Avenue Borehole
Transect......................................................................................................................53
Figure 17. Reconstruction of Units 10 to 50........................................................................ ....62
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viii
List of Figures (continued)
Figure 18. Sedimentation Accumulation Rate Curve and Earthquake Events.......................67
Figure 19. Schematic Representation of Incremental Fold Development
at Carfax Avenue......................................................................................................68
Figure 20. Axial Surfaces and Growth Triangle at Carfax Avenue.........................................71
Figure 21: Annotated Hammer-Source Reflection Profile and Stratigraphic
Correlation at Carfax Avenue..................................................................................74
Figure 22. Borehole Transect at Trojan Way............................................................................85
Figure 23. Stratigraphic Correlation and Mini-Sosie Seismic Reflection Profile at
Trojan Way............................................................................................................... 88
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Abstract
The Puente Hills thrust fault (PHT) is an active, blind-thrust fault that generated the 1987 M
6.0 Whittier-Narrows earthquake. In this study, we developed and tested a multi
disciplinary, collaborative methodology for assessing the seismic hazards of blind-thrust
faults, focusing on the forelimb growth triangles of the Santa Fe Springs and West Coyote
Hills anticlines associated with slip on the PHT. High-resolution seismic reflection images
revealed dipping reflectors in a narrowing upwards zone that continues to with 10 m of the
surface. Borehole transects across this zone along the Santa Fe Springs segment revealed a
discrete zone of south-dipping strata measuring ~ 300 m wide at 25 meters depth and ~ 100-
160 m wide at 2 meters depth. Stratigraphic and structural relationships revealed evidence
for four magnitude > 7.0 events in the past 11,000 years on the Santa Fe Springs segment of
the PHT. The earliest of these four earthquakes, Event V, occurred between 10.2 ka and 11.5
ka, with -2.6 m of displacement, suggesting a magnitude of - 7.2. Event W occurred
between 6.6 ka and 8.2 ka, (7.2 - 7.8 ka preferred) with - 4 m of displacement, suggesting a
magnitude of -7.3. Event X occurred between 0.5 ka and 6.3 ka, (1.3 - 4.6 ka preferred)
with - 1.7 m of displacement, suggesting a magnitude of -7.1 The most recent event, Event
Y, occurred between 0.3 ka and 3.3 ka (0.3 - 1.3 ka preferred), with - 1.7 m of
displacement, suggesting a magnitude of - 7.1. Slip rates generated along the Santa Fe
Springs segment range from 0.95 to 1.1 mm/yr and slip rates along Coyote Hills segment
range from 0.8 to 2.1 mm/yr.
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Chapter 1
Introduction
Paleoseismological studies have become a peerless source of information on
earthquake occurrence in the pre-historic era. Knowledge of past earthquakes on a single
fault or a regional fault network permits further analyses of the space and time distribution of
seismic strain release. Such descriptions become increasingly important as we strive to
model and predict large seismic events. The methodologies typically utilized in
paleoseismological studies require that the fault either be exposed at the surface or have a
morphological expression that can be identified as a locus of deformation. The surficial
expression of a fault can then be excavated and deformation observed directly from the
deformation of stratigraphic horizons. These prerequisites limit the investigator to faults that
breech the surface, and thus blind faults are not suited to traditional paleoseismological
investigations. This thesis presents the results of a collaborative, multi-phase investigation
designed to develop methodologies for extracting paleoearthquake and latest Pleistocene -
Holocene slip-rate data from blind thrust faults, using the Puente Hills Blind Thrust fault
beneath Los Angeles as an example.
Blind thrust faults within metropolitan southern California have been recognized as
a major source of seismic hazard for more than 15 years (Davis et al., 1989; Scientists of the
USGS and SCEC, 1994; Shaw and Suppe, 1994; 1996; Dolan et al., 1995; Shaw and
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Shearer, 1999). Such faults generated both the 1987 Mw 6.0 Whittier Narrows and
1994 Mw 6.7 Northridge earthquakes. The 1994 earthquake caused over $40 billion dollars
in damages and directly caused more than 30 deaths, making it the most expensive natural
disaster in U.S. history (Scientists of the USGS and SCEC, 1994; Eugchi et al. 1998). Now
that the seismic risk posed by blind-thrust faults been made painfully obvious, it becomes
critical to understand the seismic hazard posed by these faults.
An accurate assessment of the seismic hazards posed by blind thrusts depends on
knowledge of Holocene slip rates and records of past earthquake activity, including both the
ages and slip per event in past earthquakes. The goal of this research is to develop and test a
multidisciplinary methodology to acquire these data from blind-thrust faults. My efforts have
focussed on the Santa Fe Springs and Coyote Hills segments of the PHT. To the best of my
knowledge, this study represents the first multi-phase effort to obtain a direct record of
paleoearthquakes from a blind thrust fault. The specific goals of this study are to: (1)
delineate the near-surface characteristics of syn-tectonic growth sediments associated with
an active blind-thrust fault; (2) test the feasibility of utilizing high-resolution seismic
reflection data, coupled with borehole excavations, to characterize near-surface deformation
of growth strata; (3) obtain late Quaternary slip rate and paleoearthquake ages and slip per
event data for the PHT.
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3
Regional Setting
Metropolitan Los Angeles is situated above a structurally complex region between
the Peninsular and Transverse Ranges (Figure 1). Active right-lateral strike slip, left-lateral
strike-slip, and reverse faults exist within the basin and surrounding areas, accommodating
the stresses related to the compressional bend in the San Andreas fault north of the Los
Angeles basin (e.g.Wright, 1991). Both northwest-southeast and east-west fold trends have
been recognized in the Los Angeles basin associated with three compressional uplift trends:
(1) the Santa Monica Mountains Anticlinorium, the Palos Verdes Anticlinorium;, (2) the
Verdugo Mountains together with the San Gabriel Mountains; and (3) San Rafael Hills
(Crook et al., 1987; Davis et al., 1989; Wright, 1991; Dolan et al., 1995; Shaw and Suppe
1996; Schneider et al., 1996; Dolan 1998; Shaw and Shearer, 1999; Oskin et al., 2000).
These trends represent the fold and thrust belt which began shortening across the basin
approximately 4-5 Ma, after Miocene extension of the region and the accumulation of up to
10 km of sediment in the basin (Davis et al., 1989; Wright 1991).
Dolan et. al. (1995) distinguished between the Santa Monica Mountains
Anticlinorium and the Elysian Park Thrust (EPT), which is a northwest-southeast fold trend
that shares a central decollement in the central basin with the Compton-Los Alamitos trend
(Shaw and Suppe, 1996). The EPT ramp underlies the Puente Hills thrust fault system,
which is the southernmost and deepest of a series of east-west blind-thrust faults and
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associated anticlines (Davis et al., 1989; Wright, 1991; Dolan et al., 1995; Shaw and Suppe
1996; Schneider et al., 1996; Dolan 1998; Shaw and Shearer, 1999; Oskin et al., 2000).
Seismic reflection and refraction data acquired from the basin to the San Andreas fault
suggest that an upper crustal decollment extends northward from the hypocentral region of
the 1987 Whittier Narrows event beneath the San Gabriel Mountains (Fuis and Ryberg,
2001).
Global Positioning System (GPS) slip rate vectors reveal present-day contraction
across the Los Angeles basin at rates ranging of ~ 4 to 6 mm/yr in a north-northeast
direction, (Walls et al., 1998; Argus et al., 1999; Bawden et al., 2001). Gradients in GPS
data collected from sites in and around the Los Angeles basin suggest that the majority of the
horizontal contraction in the basin is accommodated within a contractional zone that has a
southern margin coincident with the forelimb of the Santa Fe Springs and Coyote Hills
segments of the PHT (Argus et al., 1999). InSAR interferograms reveal an anomalous area
of uplift at the location of the Santa Fe Springs oil field, with a short-term uplift rate of 5 - 9
mm/yr, despite the fact that oil extraction exceeded fluid injection over the 5 year period of
the interferogram (Bawden et al. 2001). The geodetic data suggest that there is a strong
component of contraction and uplift in the Los Angeles basin, and that much of this
contraction may be localized on the PHT and associated faults.
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Puente Hills Blind-Thrust Fault
Located in the central to eastern Los Angeles Basin, the Puente Hills fault is an
active blind-thrust fault that extends southeastward for ~ 40 km from downtown Los
Angeles to northern Orange County (Shaw and Shearer, 1999; Shaw et al. in press). The
PHT comprises three en-echelon, right-stepping blind-thrust ramps. These fault segments
are, from west to east, the Los Angeles segment, the Santa Fe Springs segment, and the
Coyote Hills segment (Figure 2). Rupture of a small part of the central PHT generated the
1987 Mw 6.0 Whittier Narrows earthquake, providing direct evidence that this fault is active
(Shaw and Shearer, 1999). Hypocentral relocations for this event and its aftershocks reveal
that the mainshock fault plane was coincident with the down-dip projection of the Santa Fe
Springs segment of the Puente Hills thrust fault plane at approximately 13 km depth (Shaw
and Shearer, 1999).
The PHT strikes approximately east-west and dips 25-30° to the north. The ramp of
this blind-thrust fault terminates at ~ 3 km depth. Above the fault tip, distinct hangingwall
folds have developed above each of the three fault segments (Shaw and Suppe, 1996; Shaw
et al., in press). Neither the Los Angeles nor Santa Fe Springs segments show topographic
evidence of the hangingwall anticline observed on seismic reflection profiles.
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7
380000 390000 400000 410000 420000
5 km Los Angeles j,
segment
Santa Fe Springs
segment
Coyote Hills
segment^
Los
A ngeles
1987 M6.0
1 0 km •1 0 k m -
= upper tip line of blind thrust
Figure 2: Structure contour map of the Puente Hills blind thrust showing the Los Angeles,
Santa Fe Springs, and Coyote Hills segments and the location of the 1987 Whittier
Narrows (M 6) earthquake sequence from Shaw et al. (in review). Locations of petroleum
industry seismic reflection profiles A® - A A B - B^, and C® - C-. Lines T and C correspond to
high-resolution seismic reflection profiles shown in Figures 9 and 10.
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3790000 3780000 3770000 3760000 3750000
The forelimb of the Coyote Hills anticline, however, is marked by a pronounced south-
facing fold scarp along much of its length. Relief on this scarp is greatest near the City of
Fullerton, with a height of ~ 30 m. This scarp was originally thought to be the locus of slip
on the postulated Norwalk fault (e.g. Richter, 1958).
Previous Investigations
Much of what is now known about the Puente Hills thrust fault comes from
petroleum-industry seismic reflection data. Shaw and Shearer’s (1999) original identification
of the fault was expanded by Shaw et al. (in press), who used 25 industry seismic profiles to
describe the subsurface geometry of the PHT and to interpret its interaction with other Los
Angeles basin faults at depth. All three en-echelon anticlines associated with the Santa Fe
Springs, Coyote Hills, and Los Angeles segments of the PHT are imaged in the seismic
reflection profiles. Each anticline has a narrow southern forelimb composed of south-dipping
Pliocene strata. These forelimb kink bands extend upward into Quaternary strata as upward-
narrowing growth triangles, which are indicative of fault-bend and fault-tip propagation
folding (Shaw et al., in press).
The Santa Fe Springs segment, due to the exceptional quality of the reflection data
across this segment, has been described in the most detail (Shaw et al., in press). The fault
ramp dips 25-29° to the north and fault-plane reflections extend from 7 km to 3 1cm depth,
where the ramp terminates upward at the base of the forelimb kink band. An older fold
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structure underlies the Santa Fe Springs thrust ramp and does not deform upper Pliocene and
Quaternary strata (Shaw et al., in press). These observations indicate that this deeper fold
formed during an earlier stage of development of the greater PHT system. The currently
active period of folding is associated with reactivation of the southernmost fold following a
period of quiescence in the late Pliocene (Shaw et al., in press).
The forelimb of the Santa Fe Springs anticline has an upward narrowing growth
triangle composed of folded Quaternary strata bounded by an active anticlinal axial surface
and an inactive synclinal axial surface (Shaw and Suppe, 1996; Shaw and Shearer, 1999;
Pratt et al., 2002; Shaw et. al., in press) (Figure 3). The axial surfaces are evident in the
industry seismic reflection data up to the upper limit of these data, ~ 200-300 m below the
surface. The continuity of the axial surfaces to the upward limit of the industry reflection
data suggests the possibility that discrete zones of deformation extend into the very shallow
sub-surface, providing a potential target for paleoseismologic investigations.
The Coyote Hills segment is divided into the Western and Eastern Coyote Hills
anticlines at the surface. Analysis of fault dips and locations by indicates that the two thrust
ramps responsible for the Western and Eastern Coyote Hills anticlines most likely intersect
at a depth of ~ 5 km (Shaw et al., in press). Above that depth, the faults vary in strike. The
Western Coyote Hills segment has an east-west strike, whereas the Eastern Coyote Hills
segment strikes approximately N 60° E, parallel to the trend of the East Coyote Hills. The
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10
Santa Fe Springs Anticline
1 km
Figure 3: Migrated seismic reflection profile image of the Santa Fe Springs section of the PHT
after Shaw and Shearer (1999). The folded section of Q(t) Quaternary, (Tfu) Pliocene upper
Fernando Formation, (Tfl) Pliocene lower Fernando Formation, and (Tp) Miocene Puente
Formation has forelimb and backlimb growth triangles bounded by axial surfaces. The forelimb
has an active anticlinal axial surface (a) and inactive synclinal axial surface (a’) and the backlimb
kink band is marked by the active synclinal axial surface (b) and the inactive anticlinal axial
surface (b’).
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forelimb structure of the Coyote Hills is similar to the Santa Fe Springs forelimb in that a
narrow kink band and upward-narrowing growth triangle can be identified by northward-
thinning Quaternary strata above the kink band. These relationships suggest that fold growth
began at the same time as growth of the Santa Fe Springs anticline. Unlike the Santa Fe
Springs segment, however, the forelimb has an active synclinal axial surface that projects
upward to a south-facing fold scarp (Figure 4).
The Los Angeles segment, which underlies downtown Los Angeles and may extend
northwestward beneath Hollywood and Beverly Hills (Dolan, 1998), is located to the north
and west of the Santa Fe Springs segment (Shaw and Shearer, 1999) (Figure 2). This
segment of the PHT is poorly imaged on industry seismic reflection data above the base of
the Quaternary sequence (Shaw et al., in press). Although the age of the forelimb kink band
is not as well constrained, upper Pliocene strata do not change thickness across the fold,
whereas Quaternary strata do change thickness across the fold. This observation suggests the
kink band of the Los Angeles segment developed during Quaternary time, similar to both the
Santa Fe Springs and Coyote Hills segments of the PHT.
There have been no previous attempts to collect paleoearthquake age and slip per
event data from the PHT. Similarly, Holocene rates have previously not been obtained for
the PHT, although Sundermann and Mueller (2001) have used regional aquifer data to infer
Pleistocene slip rates for the PHT. Long-term (1.6 Ma to present) average slip rates
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12
Western Coyote Anticline
1 km W estestates
Santa Fe Springs #1
Chevron Rivera
Community #1
Tfu
Tfl
PHT
Figure 4: Migrated and depth converted seismic reflection profile across the
forelimb of the Coyote Hills segment o f the PHT after Shaw et al. (in press).
Kink band inactive anticlinal axial surface (a’) and active synclinal axial surface
(a) are shown for the forelimb growth triangle. Stratigraphic nomenclature is
described in Figure 3.
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determined for the each segment of the PHT average between 0.44 -1 .7 mm/yr (Shaw et al.,
in press).
A collaborative, high-resolution seismic reflection study conducted during Spring of
2000 by geologists from the University of Southern California, the United States Geological
Survey, and Harvard University, imaged the kink band in the forelimb of the Santa Fe
Springs and Coyote Hills anticlines (Pratt et al, 2002). Our results, which span the depth
range of 10 to 600 m, provide an upward continuation of the industry seismic reflection
profiles to the near surface, thus bridging the gap between excavations conducted at the
surface and interpretations of the deeper structure. Results of the high resolution seismic data
collection are discussed below (see Phase 1: Mini-Sosie and High-Resolution Seismic
Reflection Profiles).
Methodology
Knowledge of the subsurface geometry is critical to identifying the locus of active
deformation for a fault-related fold. Specifically, it is necessary to define the width of the
kink band as well as the location and geometries of the active and inactive axial surfaces.
After identifying the locus of active deformation in the near surface stratigraphy using high-
resolution seismic reflection data, we excavated a transect of boreholes perpendicular to the
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14
active axial surface. These boreholes facilitated direct sampling of the sediments for datable
material, allowed us to make accurate stratal correlations, and provided a detailed record of
thicknesses of various marker horizons, both vertically and along the length of the profile.
The methodology for extracting paleoearthquake data is based upon geometrical
relationships expected at the tip of a kink-band on the forelimb of a fault-related fold.
Geometrical relationships established by Suppe et. al. (1992) and Navoa et al. (2000) show
that growth strata are folded as the kink-band migrates during fold growth, incorporating
sedimentary layers into the kink band. The most recent major slip event on the blind-thrust
fault will be recorded by the youngest sedimentary layer incorporated into the kink band.
The geometry of the growth strata is dependent upon two variables: (1) the location of the
active axial surface (i.e. whether it is the anticlinal or synclinal axial surface); and (2) the
relationship between the sedimentation rate and the the uplift rate.
Figure 5 shows possible end-member models. If the anticlinal axial surface is active
and the sedimentation rate is greater than the uplift rate, the result is that each sedimentary
layer deposited across the active axial surface is folded in an uplift event (Figure 5a). If the
sedimentation rate is less than the uplift rate in the same configuration of axial surfaces, then
the uplift is characterized by changes in thickness of the growth strata by formation of a
colluvial wedge, or no detectable change at all (Figure 5b). In the case of an active synclinal
axial surface and high sedimentation rate, the geometry does not differ greatly from the
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(a)
Growth
Strata
I
T
Pre-growth
Strata
Active Anticlinal Axial Surface
(b)
colluvial wedge
Sedimentation rate > Uplift Rate Sedimentation rate < Uplift rate
Active Synclinal Axial Surface
(d)
(c)
Figure 5: Expected growth stratal geometries for a fault-bend fold, (a) Active axial surface is anticlinal and sedimentation rate is greater
than uplift rate, (b) Same active axial surface as in (a) but sedimentation rate is less than uplift rate, (c) Active synclinal axial surface and
sedimentation rate is greater than uplift rate, (d) Same as (c) but with a faster uplift rate than sedimentation rate.
16
predicted geometry of an anticlinal axial surface with a high sedimentation rate (Figure 5c).
An active synclinal axial surface and greater uplift rate than sedimentation rate results in
progressive folding of the on-lapping strata (Figure 5d).
Another feature bom of fault-related folding theories, as well as observation, is a
change in bed thickness across the growth axial surface. Suppe et al. (1992) describe the
thickness of the sedimentary layers in the basin as equal to the thickness of the sedimentary
layers on the uplifted side of the active axial surface, plus the amount of tectonic uplift. The
difference in thickness of a section at the top of a scarp and the base of a scarp is simply the
scarp height. The amount of sediment that can accumulate between the base and top of the
scarp is the accommodation space, which is directly proportional to the scarp height. When
this accommodation space is filled, it is referred to as growth (Figure 6).
The methods employed for this study involve both geophysical and geological
techniques. Due to the lack of surface rupture along blind thrusts and high sedimentation
rates within the Los Angeles basin, a multidisciplinary effort was required to obtain slip rates
and paleoearthquake data for the PHT. This study was divided into two phases.
Phase 1 involved the acquisition of high-resolution seismic profiles at two sites, one
on the forelimb of the Santa Fe Springs anticline along Carfax Avenue in the City of
Bellflower, and a second site on the forelimb of the Coyote Hills anticline along Trojan Way
in the City of La Mirada. Details of the data acquisition and processing can be found in Pratt
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(b)
growth
growth
. ultimate
y earthquake
A . penultimate
* earthquake
A'
(a)
Thickness
at base o f scarp .
(Tb) growth *
accomodation
* space
growth = Tb-Tu - scarp height
Thickness
above scarp
(Tu)
Figure 6: Relationship of the growth strata and scarp height at the tip o f the growth triangle above the Santa Fe Springs segment o f the
PHT. (a) Growth of a sedimentary package that fills the accommodation space created by the fold scarp, (b) Identification o f a
sequence of uplift events based on the growth of strata.
et al. (2002). The high-resolution seismic profiles acquired during Phase 1 provide a link
between the structural and stratigraphic relationships found in the industry seismic reflection
profiles, which image strata between 7 km and 300 m depth, and the near-surface
excavations of Phase 2. The results of Phase 1 provided information on the locations of the
axial surfaces in the uppermost several hundred meters, and were used to guide site selection
for the borehole transects drilled during Phase 2.
Phase 2 involved the excavation of boreholes along the high resolution seismic
profiles at both sites. Two parallel borehole transects were excavated at the Carfax Avenue
site (21 holes), and one transect (3 holes) was excavated at the Trojan Way site. In addition
to the borehole data obtained during this study, water-well logs on file with the Los Angeles
Department of Public Works provide additional information near our Phase 2 transects.
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19
Chapter 2
Phase 1: Mini - Sosie and High Resolution Seismic Data
Two study sites were selected on the basis of: (1) proximity to the location of
industry seismic reflection profiles; (2) location on the forelimb of the anticline, specifically
within the bounds of the active and inactive axial surfaces projected to the surface from
depth; (3) geological factors such as the expected relatively continuous sedimentation of the
river floodplain environment at Carfax Avenue and the obvious scarp at the Trojan Way site;
(4) ability to obtain permits, and; (5) instrumentation and excavation logistics. The Carfax
Avenue profile, which is located ~ 100 m west of and parallel to, the San Gabriel River, was
excavated along the east side of a residential street. This profile extends north-south for 672
m, beginning ~ 100 m north of Rosecrans Avenue and terminating at the cul-de-sac near
Muroc Street (Figure 7).
The Trojan Way profile was located ~ 10 km southeast of the Carfax site and 1.2 km
west of the industry seismic reflection profile (C2 - C0 ) shown in Figure 2. All three
boreholes were excavated along the western edge of Trojan Way. The profile extends
southward, across a 9 m high south-facing scarp for 792 meters, from a point 15 m south of
Alondra Boulevard (Figure 8).
Two high-resolution seismic reflection profiles were obtained using two different
energy sources at each location in order to create a continuous image of the growth triangle
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Reproduced w ith permission o f th e copyright owner. Further reproduction prohibited without permission.
(b) (a)
Mini-Sosie
seismic profile
@ 1 589S
9 0 -
Extent of Carfax
Ave. borehole
transect and
hammer source
profile
1 5 8 9 S '
rx
1 609EI
High Set
6 1 9 C"
Arthurda • Extent of SCE
t borehole transect
Area bound by
axial surfaces
as projected from
seismic profiles
(g\ Well log supplied
by the LADPW
SCALE 1 :2 4 0 0 0
Rosecrans
COUNTOUR INTERVAL 2 0 FEET
DOTTED LIN ES REPRESENT 5 - FOOT CONTOURS
100 300 METERS 0
Figure 7: (a) Site m ap showing the projection o f the axial surfaces from industry seismic profiles at the locations near Carfax
Avenue and N orw alk High School, (b) M ap detail o f the Carfax Avenue site. o
21
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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 8: Trojan W ay site map.
22
to within 10-50 m of the surface. In order to ensure overlap with the upper part of the depth
range imaged in the industry data, a 60-channel seismic reflection system was used with a
Mini-Sosie (Barbier, 1983; Stephenson et al., 1992) source to image the depth range of 50-
600 meters. The second set of profiles used a sledge hammer source to image the uppermost
50 m.
The Mini-Sosie profiles at both sites show that the growth triangles and active axial
surfaces extend upward into the strata less than 300 m from the surface (Pratt et. al, 2002).
At the Carfax site, three prominent reflectors located at ~ 280 m, ~ 100 m, and ~ 90 m depth
can be identified on the Mini-Sosie profile (Figure 9). These three reflectors are folded
within a discrete zone, interpreted as the upward continuation of the kink-band identified at
depth by Shaw and Shearer (1999). The hammer profile exhibits gently south-dipping
reflectors at 10- 20 m depth located at the upward projection of the growth triangle from the
Mini-Sosie profile. These observations suggest that the growth triangle extends to within 10
- 20 meters of the surface.
At the Trojan Way site, reflectors change in dip southward, from south-dipping
beneath the scarp to horizontal beneath the flay lying area south of the base of the scarp,
suggesting that the active synclinal axial surface extends to the surface (Figure 10). The
presence of dipping strata in the northern half of the Mini-Sosie profile indicates that the
kink-band imaged in the industry profile extends upward into the depth range of the Mini-
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23
distance (m) distance (m)
300
2x vertical exaggeration
Santa Fe Springs Anticline Chevroo C he1
zrowth N C : ! l H C i
hiion
V £H 2
\ fault-plane EE^t:
j reflections
Figure 9: M igrated seismic reflection profiles along Carfax A venue showing dis
crete anticlinal and synclinal axial surfaces (dashed lines) that define an upward-
narrowing zone o f deform ation on the forelim b o f the Santa Fe Springs anticline.
The M ini-Sosie profile shows a reflector (orange line in (b)) at a depth of 200-280
m that is folded in a zone ~ 270 m wide. T he ham m er profile shows a reflector
(green line in (c)) at a depth of 10-20 m dipping to the south within a zone ~ 90 m
wide.
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elevation (m)
distance (m)
-I • ,
H* _|
Mini-Sosie/
weight drop
industry
Anticline Western Coyote
growth
S ^ C triangle
Puente Hills fa u lt
Figure 10 M igiated seisnnc reilection prohlcs along the Coyote IIills segment of
the PH T show ing an upw ard narrow ing folded zone o f south dipping reflectors. The
synclinal axial surface corresponds with the base o f the south-facing fold scarp at
the surface. The M ini-Sosie profile shows a prom inent reflector that is interpreted
as an unconform ity. Figure after Pratt et al. (2002).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
depth (km)
25
Sosie profile. The anticlinal axial surface is not imaged on the Mini-Sosie profile and is
presumed to be north of the profile. A probable unconformity at approximately 30-80 m
depth separates more steeply dipping strata below from more shallowly dipping reflectors
above. Hammer-source profile data from this site did not clearly image the subsurface due to
interference from local utilities.
In summary, the high-resolution seismic reflection data provide a continuous image
of the growth strata from the industry data upward into the near-surface. These data show
that the deformation in the forelimb of the Santa Fe Springs and Coyote Hills anticlines is
localized in narrow zones on the southern margin of the fold forelimbs. The most steeply
dipping strata are within upward-narrowing bands and exhibit discrete axial surfaces, which
can be identified to within 30-80 m of the surface at the Coyote Hills anticline and to within
10-20 m of the surface at the Santa Fe Springs anticline. Thus, these complementary and
collaborative studies (Shaw and Shearer, 1999; Pratt et al., 2002; Shaw et al., in press; this
study) provide an image of the subsurface structure of the PHT from as deep as 13 km to as
shallow at 10 m. Most importantly, these results clearly delineate the locus of active
deformation to shallow depths that can be sampled directly with borehole excavations.
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26
Chapter 3
Phase 2: Borehole Transects
Carfax Avenue
The Carfax Avenue borehole transect is located along the east curb of Carfax
Avenue. This site is on the distal floodplain of the San Gabriel River, approximately 100
meters west of the active channel. The borehole transect is coincident with the hammer-
source and Mini-Sosie seismic reflection profiles. A shorter borehole transect was also
completed 25 meters east of and parallel to Carfax Ave. This is referred to as the “SCE site”
because it is located in a Southern California Edison power line right-of-way (Figure 11).
The Carfax Avenue transect, which comprises 14 boreholes, extends north-south for 311
meters and spans hammer-source shotpoint numbers (sp) 496 to sp 185 (Figure 11). The 80-
m-long SCE transect consists of seven boreholes that span sp 491 to sp 411, projected 25 m
due east from the Mini-Sosie profile.
Drilling Techniques and Potential Errors
Both bucket-auger and hollow-stem drilling techniques were used in this study.
There is error involved in the measurement of depth in both the bucket-auger and hollow-
stem auger sampling techniques. One possible source of error for the bucket-auger holes is
the estimation of penetration depth, which can easily be checked by sending a weighted
measuring tape down hole. The error is minimized by using short sampling drives. Drives of
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27
30 - 60 cm (1 to 2 ft) were typically taken by the bucket-auger driller. The uncertainty on a
depth measurement, short of missing a stratigraphic unit or contact entirely due to an empty
bucket, is considerably less than 50% of the sample drive. If the hole was stable enough to
be examined directly, then contacts inferred from the sample barrel could be checked
directly by a geologist being lowered into a hole, except for samples from below the water
table. In the 16 bucket auger holes that I logged down-hole, the estimated error on contacts is
no more than 15 cm (0.5 ft).
If the hole was not directly viewed by a geologist, another source of error occurs,
when recovery is less than 100%. If all drilled core does not stay within the core barrel or
bucket, then it is unclear which section was lost from the sample barrel and which depth
section has remained.
Hollow-stem auger drilling allows for continuous sampling of sediments retrieved in
the core. At the Carfax site, recovery on each 2 ft drive was on average 90%, but was locally
reduced to 50% or less in water saturated sands and gravel. The hollow-stem auger depth is
tracked by the amount of 5 ft auger has been drilled below the ground surface. Estimating
error for the hollow-stem auger is more difficult since the hole is never viewed directly, but
the primary source of error is, again, in the case of less than 100% recovery. On Carfax
Avenue, 46 cm and 60 cm (1.5 and 2 ft) sample barrels where used. Except in the lower 20
meters of saturated sands and gravels (-20-50 m), recovery was on average 90%. Estimated
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28
13754
13802
13914
BH 19,
sp 185
Figure 11: Detailed map of Carfax
Avenue and the Southern California
Edison right-of-way land west of the
San Gabriel River showing the
surveyed locations of boreholes and
residential property lines.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
29
error on contact depths is ± 15 cm (6 in). In sections of poor recovery, we estimate that
potential errors can be up to ± 30 cm (1 ft), however. Core and cuttings were logged in feet
because this is the system used by drill rig operators, however, depths are presented here as
meters with footage in parenthesis.
Eight of the boreholes along the Carfax transect (boreholes 10-17) were drilled with
a bucket-auger drilling technique, whereas the remaining seven were drilled with a hollow-
stem auger drilling technique. The SCE boreholes were all drilled with the bucket-auger
technique. The SCE boreholes penetrate to 11-11.5 m (36 to 38 ft). Maximum drilling depth
for the bucket-auger boreholes along the Carfax Avenue transect varied from 11 - 22 m (36
- 75 ft), depending on whether or not a steel casing was used. Without the steel casing the
borehole became unstable below an unconfined aquifer at 11 m (36 ft).
Typically, the 70-cm-diameter bucket sampled in 30-45 cm (1-1.5 ft) increments and
the sediment collected from the teeth of the bucket auger was logged at each depth interval.
These holes where then examined directly for the purpose of accurately defining contacts
and collecting datable material from the sediments.
Stratigraphy
The 14 Carfax Avenue boreholes reveal a stratigraphic section comprising laterally
continuous friable sands and gravels, and cohesive slits and clays (Figure 12). These units
range in thickness from tens of centimeters to several meters. In the uppermost 20 meters,
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— North 1
Friable san d s
U nit 47, m ark er u n it
0
Soil
1 1 C ohesive u nits o f silt an d ciay
© U nit n u m b e r 1 0
100 feet 50 0
4x Vertical Exaggeration
£ 20
a
c v
Q
CO h s i r j
( N ( N ro
^ 's f r
a q. o. q.
fN r s
f N V O
X X X X X X
CQ CD CO
o
Figure 12: Stratigraphic correlation across the Carfax Avenue borehole transect. Please refer to A ppendix A for key to
borehole logs and soil color logs.
o
31
sands ~ one to several meters thick are separated by centimeter- to meter-scale cohesive silt
and clay units. The lowermost part of the section (e.g. basal 30 m of boreholes 18 and 19)
consists primarily of finable, medium- to coarse-grained sands and pebble to cobble
conglomerates with the exception of three submeter to meter-scale cohesive (clay and silt)
sections at the south end of the transect. These cohesive intervals do not extend north of
borehole 18, and have most likely been eroded by channels. All other units, however, are
traceable continuously for the entire north-south length of the borehole transect. Four major
sand units (10, 20, 30, and 40), are continuous both the north-south and the east-west
directions as demonstrated by the correlation with the SCE borehole transect approximately
25 meters to the east of Carfax Avenue.
There is an unconfmed aquifer at ~11 m (36 ft), which is saturated to a depth of 15
m (49 ft) in the northern part of the transect. This same aquifer lies at a depth of 12-15.8 m
(42-56 ft) at the south end of the transect. Water flows through predominantly medium to
fine-grained sands. A clay to silt unit separates this saturated zone from a lower confined
aquifer at -19 m. The clay and silt in some cases grades downward into a silty, very fine
grained sand. This dry silt and clay section is - 4 m thick and acts as an aquitard that is
underlain by an ~ 18 m thick section of water-saturated, medium- to coarse-grained sands
and pebble to cobble gravels.
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32
The fine-grained strata of the uppermost 20 m of beneath Carfax Avenue is
interrupted by continuous sections of friable, well-sorted medium-grained sands. These
sediments were deposited in a river floodplain environment intermittently channelized by
river meanders. Below 21 m (69 ft) depth (as measured in the southern end of the transect)
the sediments are predominately coarse-grained sand with pebble gravel.
I have identified seven laterally continuous units of friable, fine-grained sand and
greater grain size (units 10, 20, 30, 40, 50, 60, and 70). Six laterally continuous units of
cohesive, silty-sand to clay (units 12, 15, 25, 35, 45,and 47) have been identified across the
311m extent of the borehole transect (Figure 12). In addition to the major depositional units,
several pedogenic and diagenetic sub-units (11, 46 and 61) have also been identified.
The contacts between units 40 and 45,47 and 50, and also 50 and 60, are correlated
with major contacts observed in the log of a water-well (1589S) located approximately 300
meters north of our borehole 22 (Figure 13). This indicates that these units are continuous
for a considerable distance in the direction of the transect. All stratigraphic units found in the
SCE boreholes (10 - 35) are present in correlative boreholes along Carfax Avenue,
suggesting units are also continuous perpendicular to the borehole transect.
Unit 10 is the shallowest laterally extensive sand below the < 30 cm of artificial fill
and pavement. Unit 10 is a friable, very-fine grained sand that ranges in thickness from 30
cm to 1 m. Below the friable sands of unit 10 lies a cohesive section comprised of silty-sands
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Reproduced w ith permission o f th e copyright owner. Further reproduction prohibited without permission.
Friable sands
l l l l l Unit 47, marker unit th a t
shows no change in thickness
Soii or m arker horizon
| | Cohesive units of silt and clay
Carfax Avenue Borehole Transect “ 5 s ** N o rth
4x Vertical Exaggeration
Unit num ber
-— 10
S
-2 0
- 3 0
Figure 13: Carfax Avenue borehole transect including the additional borehole log (1589S) supplied by the Los Angeles Department of Water and
Power which is located ~ 300 m to the north of our northernmost borehole. Three major contacts can be identified in 1589S. The shallowest
contact (orange line) separates the upper ~12 m of alternating sands and silt from the clay rich section at ~17 m. The pink line represents the
contact separating the clay-rich section from the coarse-grained section below. At ~ 20 m the dashed line represents a color change in these coarse
grained sands from greenish gray to olive.
U J
o j
and clay, unit 15. It has variable thickness between 1 and 1.5 m (3 - 5 ft). Two buried A-
horizons exist within the upper 2.5 meters and are characterized by a dark olive-gray to dark
brown-gray color and iron staining. The upper A-horizon, unit 11, is recognized at ~ 150-
160 cm depth and is characterized by its dark gray to olive color (2.5 Y 3/2), which may be
due to an abundance of organic material. This unit is a poorly-sorted mix of medium to fine
grained sand and clay. This soil grades into a predominately fine to very-fine grained sand of
a paler color (2.5Y 3/3) which represents the eluviated section in the A/C transition.
Approximately 60 cm below the top of unit 11 another A-horizon, unit 12, has been
identified on the basis of its color, a gray-olive brown (2.5-5Y 3/2-3).
Unit 12 is not as well developed as unit 11, which is robust in the majority of
boreholes and can be tracked across the transect by its dark color and mix of sands and clay.
The maximum change in depth of unit 11 is at most 1 m and at least 60 cm. Units 10-15
thicken southward across the transect by ~ 90 cm.
Below unit 15 is a 2.5m to 3m thick section of friable, light yellowish brown (2.5Y
6/4), fine- to medium-grained sand with fine-scale cross-bed structure, unit 20 (Figure 12).
The top of unit 20 is at a depth of ~ 3m (10ft) in boreholes north of borehole 15 and is ~ 1
meter deeper in borehole 19. The base of these sands has an irregular base with rip-up clasts
present in the lower 5 cm of sands, which is evidence of scoured clay and/or silt. This
observation coupled with the relative thickness of the section and presence of cross-beds
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35
suggests that these sands represent a channel or system of multiple channels that cut through
the floodplain locally eroding the underlying cohesive section. Unit 20 sands are
distinguishable in all boreholes, with the exception of borehole 14, including all 7 of the
SCE boreholes.
Unit 25, below the unit 20 channel sands, consists of cohesive sandy silt to clay.
This cohesive section, where present, ranges in thickness from ~ 50 cm to 2 meters. In
boreholes 14 and 20, this cohesive section has been cut out by the channel erosion associated
with the unit 20 sands. The combined thickness of units 20-25 increases to the south by a
maximum of 80 cm between holes 23 and 18. A very sharp contact is observed between the
base of this cohesive section and the top of the underlying sands. Unit 25 is a laterally
extensive deposit as it is also present in all of the SCE boreholes. The olive to olive-brown
color (5Y - 2.5Y 4/3) suggests little to no soil development within this unit. Some horizons
have iron-staining and roots, but generally evidence for prolonged soil development is weak.
In boreholes 18 and 23 this cohesive section exhibits a greenish-gray (Gley 4-5/10y-5gy)
coloring.
Unit 30 is a friable, pale-colored (olive - olive yellow) small pebble gravel to fine
grained sand that ranges from 10 cm to 2 meters in thickness. Its contact with unit 25 above
is not found in boreholes 14, 15, 20, but is found in boreholes along both the north and south
ends of the transect. Total change in depth of the top of this unit is ~ lm (3-3.5 ft) deepening
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
36
to the south. Unit 30 underlies the cohesive section of unit 25 and is underlain by the
cohesive sediments of unit 35 (Figure 12).
Unit 35 is a 3 m thick, cohesive, fine-grained clay-rich section that grades downward
to a sandy silt to a very-fine-grained to fine grained sand. Together units 30 and 35 thicken
by 50 cm - 1 m across the Carfax Avenue transect. The base of unit 35 is ~ 1.8 m deeper in
the southernmost borehole. At the SCE transect, however, units 30 and 35 do not change
depth and do not thicken as they are north of the point at which the contact deepens.
The upper part of unit 40 consists of very fine to fine-grained sands that grades over
~ 45-60 cm to a medium to fine-grained sand. A color change is found at a depth of ~ 12
meters from olive - olive brown (2.5Y-5Y 4/4) to dark gray (5Y,2.5Y, 4/1) or dark greenish-
gray (Gley 4-3/5gy). Unit 40 sands are saturated at ~11 m in the northern boreholes and is at
a depth of ~ 12 m in the southern most borehole. The top contact is 1.8 meters deeper at the
south end of the transect.
Unit 45 consists of gray (5y-2.5Y 6-5/1) to greenish gray (Gley 5/5g) silty sand to
silty clay and is separated by sub-meter scale sands in boreholes south of borehole 13. It has a
variable thickness of 20 cm to 2.5 m due to the downcutting of the unit 40 channel sands.
Units 40-45 show a thickness change of ~ 2 m, thickening towards the south.
Cohesive units 45 and 47 are separated by unit 46, a distinctive, dry, gastropod-
bearing very dark gray to black clay that is observed in all boreholes along the Carfax
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37
Avenue transect. This continuous unit provides a distinctive marker bed across the study
area. The thickness of the gastropod bearing clay is ~ 5 — 30 cm. Unit 47, which underlies
the gastropod-bearing clay, consists of ~ 2 m of cohesive dry silty sand to clay strata that
does not change thickness, but is ~ 3.5 - 4 m deeper at the southern end of the borehole
transect.
The unit 47/50 contact separates the dry, cohesive silts and clays of unit 47 from the
underlying water-saturated sands and pebble to cobble gravels below. The base of unit 50 is
a sharp color change from the greenish gray (Gley 3-4/ 5g-10gy) colors of the overlying 8-10
meters to olive (5Y 3/2-5). This color change is interpreted to be the base of a saturated zone
in the sands that has since migrated to greater depths. This is indicated by the observation
that the units below are now saturated. A laterally discontinuous cohesive section exists
within the unit 50 sands in holes 19, 20, and 15. This section, unit 55, was likely a
continuous silty section that was eroded by subsequent channelization. Unit 55 is between 30
cm and 2 m thick where exposed in boreholes 15, 19, 20. Its lower contact lies at the
greenish-gray to olive color change. Although unit 55 is not observed continuously across
the section we assume the base of this cohesive section was coincident with the sharp color
change that marks the top of the underlying sands. Only boreholes 19, 18, and 24 continue to
depths below 27.4 m (90 ft) and therefore the remaining stratigraphy is limited to these
boreholes.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
38
Unit 60 is a saturated, olive-colored, medium-grained sand to small pebble gravel. A
40 cm thick silt interrupts the sand at 25.9 m (85 ft) in borehole 24. In borehole 19, unit 60
sands are punctuated by centimeter scale dark gray to black clay and silty sands with clay at
depths of 35 - 35.5 m (115-116.5 ft). A sharp color change from primarily olive and olive
brown (5 Y -2.5 Y 5-3/ 2-5) downward to yellow brown or brown (10Y -7.5 Y 3-S/3-6) is
indicative of a moderately well-developed soil that can be traced from boreholes 24
southward to borehole 19. This soil, unit 61, separates the unit 60 sands from the unit 62
sands below and deepens southward by 8.5 m (28 ft) between boreholes 24 and 19.
Unit 62 sands are identical to the unit 60 sands. In boreholes 18 and 19 these coarse
grained sands and gravel are underlain by a silty sand, unit 65. The silty sand section is
cohesive and is 2 times thicker in borehole 19,(4.8 m / 16 ft), than in borehole 18 (2.4 m / 8
ft). This cohesive section is not present in borehole 24 suggesting that the silty sands where
eroded by channels. Thus the distinction between unit 65, the silty sand section in boreholes
18 and 19 and the underlying sands does not exist north of borehole 18. Where unit 65 is
present we refer to the underlying sands and pebble gravels as unit 70.
SCE boreholes
The SCE borehole transect lies ~ 50 m east and parallel to the Carfax Avenue
transect. Seven boreholes were excavated along an 80-m-long transect extending from sp
411 to 491 (Figure 14). These boreholes are parallel to boreholes 15 through 21 on the
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Reproduced w ith permission o f th e copyright owner. Further reproduction prohibited without permission.
10 0 m eters
4x Vertical Exaggeration
The SCE Borehole Transect
o
a.
_ co r^
fN fN m
a a. a. Cl
-X-
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a a.
©
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18
r
O N
a.
iA
o
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N orth
0
10
15
Figure 14: Borehole transect at the Southern California Edison (SCE). Location o f Carfax Avenue boreholes -
the w est shown in gray. Shaded areas correspond to correlative units in the Carfax Avenue transect.
50 m to
40
Carfax Avenue transect, as projected due east from Carfax Avenue, parallel to the axial
surface trends mapped by Shaw et al., (in press). The bucket-auger drilling technique was
used for all seven boreholes. Maximum penetration depth was between 11 and 11.5 m (36-
38 ft), at which depth the water table was encountered. The use of a steel casing was not
implemented at this site to drill below the water table due to site logistics.
Four of the sand units (10, 20, 30, and 40), are correlative between the Carfax and
SCE transects. The two A-horizons identified along Carfax Avenue (units 11 and 12), as
well as cohesive units 15, 25, and 35, are also observed on the SCE transect. The upper
contacts of units 10, 20, and 30 are at the same depth in both transects indicating that the
tops of these units are horizontal in an east-west direction, parallel to the strike of the PHT
identified by Shaw et. al (in press). Unit 40’s upper contact is irregular and is 0 .6 - 1.8 m
(2-6 ft) shallower at this site.
Age Control
More than 100 samples of detrital charcoal, bulk-soil and wood fragments were
collected for radiocarbon dating. These samples spanned the depth range of 1.5 - 40 m (5 -
130 ft). Seventeen detrital charcoal samples and three bulk-soil samples have been
radiocarbon dated thus far by accelerator mass spectrometer (AMS) 1 4 C analyses at the
University of Arizona AMS lab (Table 1). All samples underwent an acid-alkaline-acid
(AAA) pretreatment prior to combustion. Pending dates on an additional 30 samples will
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Table 1: Radiocarbon Analysis at the Carfax Avenue Site
depth at depth at Thousands
Sample
number U nit M aterial Tvoe
Collection
D etrthm Iff)
south end
(m)
north end
(m)
AM S 14C
A ge (BP)
2 sigm a calibrated,
calendric age range
o f years
ago (ka)
PHT 1800 11 charcoal fragm ent 2.06 (6.75) 2.7 2.1 3 7 2 + /-3 1 A.D. 1440-1640 0.5 0.3
PHT 1803 20/25 charcoal fragm ent 6.0 (19.8) 6.0 5.5 3,078 +/- 43 B.C. 1430- 1140 3.4 3.0
PHT 1101 30 charcoal fragm ent 7.53 (24.7) 9.4 7.6 5430 +/- 40 B.C. 4350-4230 6.3 6.2
PHT 1700 30 charcoal fragm ent 7.6 (24.9) 9.4 7.6 5375 +/- 52 B.C. 4340-4040 6.3 6.0
PHT 1804 35 charcoal fragm ent 9.6 (31.5) 10.7 8.8 5922 +/- 51 B.C. 4910-4690 6.8 6.6
PHT 1704 35 charcoal fragm ent 8.3 (27.5) 10.7 8.2 6 1 3 8 + /- 62 B.C. 5280-4850 7.2 6.8
PHT 2301 40 organic rich layer 14.5 (47.6) 16.2 13.4 7284 +/- 60 B.C. 6240-6010 8.2 8.0
PHT 2303 45 charcoal fragm ent 15.5 (51) 17.4 14.3 7265 +/- 69 B.C. 6240-5930 8.2 7.9
PHT 2007 45 charcoal fragm ent 16.7 (55.8) 17.5 14.3 7165 +/- 59 B.C. 6200-5910 8.1 7.9
PHT 1806 45 charcoal fragment 16.4 (53.8) 17.7 14.3 6 8 1 9 + /-6 1 B.C. 5840-5620 7.8 7.6
PHT 2103 45 charcoal fragm ent 14.5 (47.5) 17.7 17.7 7240 +/- 280 B.C. 6640-5560 8.6 7.5
PHT 1301B 46 bulk soil 15.3 (50.2) 18.1 14.6 7 1 6 0 + /-8 1 B.C. 6220-5840 8.2 7.8
PHT 1302B 46 bulk soil 15.3 (50.2) 18.1 14.6 7390 +/- 79 B.C. 6420-6030 8.4 8.0
PHT 1400B 46 bulk soil 15.54 (51) 18.0 14.9 7350 +/- 60 B.C. 6380-6060 8.3 8.0
PHT 1807 47 charcoal fragment 17.1 (56.3) 18.7 15.2 8009 +/- 63 B.C. 7523-7081 9.5 9.0
PHT 1808 47 charcoal fragment 17.3 (56.8) 19.1 15.5 8287 +/- 68 B.C. 7523-7081 9.5 9.0
PHT 1813 47 charcoal fragm ent 19.5 (64) 20.7 17.1 8754 +/- 72 B.C. 8200-7597 10.2 9.9
PHT 2108 47 organic rich layer 16.6 (54.5) 21.0 21.0 8291 +/- 80 B.C. 7539-7079 9.5 9.0
PHT 2010 55 charcoal fragment 23.5 (77) 24.1 18.9 9274 +/- 71 B.C. 8722-8289 10.7 10.2
PHT 1910 60 wood chip w/ block shape 31.8 (104.4) 31.8 30490 +/- 440 B.C 38630-37750 36.7 35.8
PHT 1809 65 charcoal fragment 38 (124.9) 40.5 23230 +/- 460 B.C 29240-28320 27.3 26.4
42
allow is to refine the age constraints discussed here. All sample ages, except 1809 and 1910,
were converted to calibrated, calendric ages using CALIB 4.3 (Stuiver and Reimer, 1993)
and are reported as the 2 Sigma (95% confidence limit) age range. Samples that yielded
radiocarbon ages older than 20,000 were approximately calibrated using the U C production
rate curves of Voelker et al. (1998).
Radiocarbon-dated samples, including charcoal, bulk-soil, and wood collected from
units 11, 20, 30, 35, 45, 46, 47, 55, 60 and 65, provide age control for the stratigraphic units
in our section. Units 45, 46, 47 contained an abundance of datable material and therefore
have the greatest number of dated samples and the best-constrained ages. Sample ages
throughout the section are generally in correct stratigraphic order, indicating minimal
reworking of datable material. Only one sample (1910) is clearly reworked, as discussed
below. All depths in the following discussion refer to the depth at which the sample was
retrieved. Table 1 shows the depth at which the sample was collected, as well as the depth of
the sample projected to its correlative stratigraphic level at the southern and northern ends of
the transect.
Unit 11 is the youngest unit from which datable material was retrieved and analyzed.
Sample 1800 yielded an age range of A.D. 1442-1635. The degree of soil development
within unit 11 indicates the soil was at the surface for at least several decades, and possibly
hundreds to as much as a few thousand years (Rockwell, 2000). Thus, the detrital charcoal
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sample could have been incorporated into the soil well before burial of unit 11. Unit 10 may,
therefore, be much younger than unit 11. The next deepest sample (1803) was collected in
borehole 18 from the contact between units 20 and 25. This sample yielded a calendric age
range of B.C 1432-1135. These samples indicate a latest Holocene age for unit 20.
Two sample ages (1101 and 1700) acquired from the same stratigraphic horizon in
two boreholes at the northern section of the borehole transect provide an age range of B.C
4043 - 4350 for unit 30. Unit 35 has an age range of B.C 4691-5282, based on two samples
(1804 and 1704) collected from approximately the same horizon in boreholes 17 and 18.
Charcoal fragments are not usually transported in bed load with medium-to-coarse-grained
sand such as that of unit 40. It is therefore not surprising that only one charcoal sample was
collected from unit 40. This detrital charcoal fragment (2301) was collected from the very
fine-grained sand in the lower section of unit 40. This sample yielded and age range of B.C
6235-5934. Four detrital charcoal fragments collected from unit 45 yield ages within the
range of B.C 5564 to B.C 6641.
Three bulk-soil samples (1301B, 1302B and 1400B) have been dated thus far from
unit 46, the black gastropod-bearing clay. Bulk-soil ages record a mixture of different-aged
carbon that has accumulated in the soil from the initial stabilization of the soil surface to the
time the soil was buried and pedogenesis ceased. Therefore, the age range given by the bulk-
soil samples is older than the burial age of the soil. For unit 46, the bulk-soil ages range from
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44
B.C 5842-6421. Sample 1806, a detrital charcoal fragment collected from just above unit 46,
has a calendric age of B.C. 5837-5622. This age is barely younger than the age range given
by the bulk-soil samples, suggesting that the clay was not at the surface for an extended
period of time, but rather was buried soon after stabilization of the soil surface. In other
words, the mean-residence time of the carbon in the soil was relatively brief, on the order of
no more than a few hundred years.
Three detrital charcoal fragments and one organic-rich layer were collected within
unit 47. These samples yielded an age range of B.C 8200-7079. The oldest charcoal
fragment (B.C 8200-7597) may be reworked, as another charcoal sample from the organic-
rich layer ~ 15 cm below (2108) yielded an age range of B.C 7539-7079. Sample 2010,
collected from unit 55 approximately 3 m below 2108, yielded an age of B.C 8722-8289.
Sample 1809 was collected in borehole 18 within unit 65, from 38 m (124.9 ft)
depth. This sample yielded an adjusted radiocarbon age of 25,230 ± 460 yBP. A wood
fragment (1910) collected from unit 60, when approximately calibrated using 14C
production rate of Voelker et al. (1998), yielded a crude calendric age of ~ 34,000. Although
this sample is ~ 9 m above sample 1809, it yielded an age that is ~ 10,000 years older. Thus,
sample 1910 is reworked and does not provide an accurate age range for unit 60.
The age ranges of all radiocarbon-dated samples plotted versus depth are shown on
Figure 15. This age-versus-depth plot provides a means of estimating an average
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Carfax Avenue Sedim ent A ccum m ulation Curve
45
40
✓ —4
E
35
+ - »
u
Q >
V)
c
T O
< * - *
o
_n
5 25
XI
2010 '
4 -
o
■ D
C
0 )
x:
4 - *
3
O
U 5
20 1400B 1806
1302B 2103
1 301 B 2303
■2301
+ - •
re
4— *
Q.
< U
-D
,1 70 4
1101
1803^
40000 35000 20000 25000 30000 0 5000 15000 10000
calibrated calendric age (years)
Figure 15: Sedim entation accum ulation rate curve estim ated w ith available calibrated calendric ages from the Carfax Avenue
borehole transect. Sam ple numbers indicated on graph.
46
sedimentation rate between samples. The average sedimentation rate for all samples in the
Carfax Avenue borehole transect is approximately 1.5 mm/yr. Sedimentation in a distal river
floodplain environment, such as along Carfax Avenue, is not continuous and steady, but is
more likely to be episodic as large flood events deposit a large amount of sediment in a very
short time. In overbank settings, sands may also be deposited in a pulse-like fashion
dependent upon the fluctuations in river discharge. Thus, it is appropriate to calculate the
sedimentation rates between units using 1 4 C ages from those units.
In the upper 25 m, the averaged sedimentation accumulation rate is 2.5 mm/yr. The
rate ranges from ~. 1.2 mm/yr for the stratigraphic interval between unit 11 and unit 35 to ~
6.5 mm/yr between units 35 and 46. The sediment accumulation rate decreases between units
47 and 55 to a rate of ~ 2 mm/yr. Unit 55 and unit 65, where the oldest 1 4 C age was obtained,
are separated by ~ 16 meters of section as measured in borehole 19. The resulting sediment
accumulation rate of 1.0 mm/yr is a rough estimate, because it is likely that the deposition of
the thick, coarse-grained sands and gravels that characterize the majority of sediments within
this interval was also episodic, with varying sedimentation rates throughout this section.
Evidence for folding
The stratigraphic observations described in the preceding section are indicative of a
depositional environment that consists of broad, shallow channels interspersed with silt and
clay overbank deposits. Locally, the sand- and gravel-filled channels have eroded down into,
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47
and cut out parts of underlying units. One of the most pronounced examples of this is the
thinning of unit 45 southward from borehole 13 in response to erosion by the unit 40
channel. These relationships are consistent with the distal floodplain environment of the
Carfax Avenue and SCE transects.
One of the most striking features of the Carfax transect is the pronounced coarsening
of the sediments below unit 47 (Figure 12). The available 1 4 C data suggest that this fine
grained over coarse-grained transition is earliest Holocene in age. This suggests that the
transition is related to the change from wetter climates in the late Pleistocene to the drier
Holocene climate.
Unit 46, the black, gastropod-bearing clay, is found in all boreholes and therefore is
a clear marker horizon. In addition to unit 46, many of the thick sand units are correlative
across both the entire lengths of the Carfax Avenue and SCE transects. Such laterally
continuous sand sheets must have been deposited at approximately the stream gradient at the
time of deposition. At the Carfax site, the stream gradient is very gentle, on the order of
0.001° (essentially horizontal). Moreover, given the fine-grained nature and thin bedding of
the clay and silt overbank deposits, we consider it unlikely that significant erosion of the
tops of the major sand units has occurred, except in a few isolated cases (e.g. unit 30 in
boreholes 14, 15 and 20). We can, therefore, use the top of these laterally continuous sands
as an indicator of the paleo-stream gradients at the time of their deposition. Since the
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48
channel sands do not drape, but instead onlap, escarpments (i.e. fold scarps) any change in
the depth of the top of sand units that is greater than the stream gradient would indicate that
the sand was deformed after its initial, near-horizontal to horizontal deposition. Changes in
the thickness of units and the depth changes of contacts between units thus provide a basis
for interpreting past folding events associated with slip on the underlying Santa Fe Springs
segment of the PHT.
The most general and important observation from the Carfax borehole transect is
that the upper contact of all major sand units (i.e. units 60, 50, 40, 30, 20, and 10) increases
in depth southward along the transect (Figure 12). Our survey of the ground surface, using a
laser electronic distancing monitor, indicates that the ground surface at the southernmost
borehole is < 30 cm shallower than the northernmost borehole. On the figures of the Carfax
Avenue transect all boreholes are shown projected to a horizontal line, eliminating the
effects of the negligible surface gradients in our interpretations.
The top of the unit 50 sand is the deepest depositional contact that can be traced
continuously across the transect. This geometry of the contact indicates that ~ 4 m more
sediment has accumulated at the southern edge of the transect than at the northern edge since
deposition of unit 50 during earliest Holocene time. Both the top and base of unit 47 increase
in depth southward by the same amount, ~ 3.7 m. This clay and silt unit does not change
thickness, although it is tilted to the south. The top contacts of the remaining major sands,
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49
units 40, 30, and 20, increase in depth southward by 1.5 m, 0.3 m, and 0.6 m, respectively. In
general, all of the contacts are found at greater depths in the south end of our borehole
transect.
The borehole log of a water well (1589S) located ~ 300 m north of our northernmost
borehole on Carfax Avenue, kindly supplied by the Los Angeles Department of Public
works, shows that there is no change in depth northward from our borehole 21 of two major
stratigraphic contacts that we can confidently identify in the water-well log (Figure 13). The
geologist’s logs for water-well 1589S show two major stratigraphic contacts and one
diagenetic color change contact in the uppermost 25 meters. At a depth of 12.8 m (42 ft) in
borehole 1589S, there is a sharp contact between sands above and a blue-colored clay below.
This clay section persists to 16.7 m (55 ft) depth, below which there is a thick section of
coarse-grained sands and gravels. Both of these contacts are at the same depths in our Carfax
boreholes 22 and 21, the northernmost, holes in our transect. These observations indicate that
there is no deformation of the flat-lying sediments north of our Carfax Avenue borehole
transect, at least down to the 21 m depth of the deepest correlation between Carfax Avenue
borehole 21 and borehole 1589S.
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50
Interpretation
Evidence for paleoearthquakes
The borehole results from the Carfax Avenue transect reveal the stratigraphic and
structural response of shallow sediments to folding above the PHT. When a fold scarp is
created in response to slip on a blind-thrust at depth, the difference in cross-sectional height
between the top of the scarp and the base of the scarp represents the accommodation space
(Figure 6). This space must be equilibrated either by erosion of the scarp and or deposition
across the scarp. As discussed above (Methodology), the geometry of the deformed
stratigraphy depends on: kinematics of the fold, i.e. anticlinal or synclinal axial surface, and
the relationship between sedimentation rate and tectonic uplift rate. At the Carfax Avenue
site, which is located above the Santa Fe Springs segment of the PFIT, the active axial
surface is anticlinal (Shaw and Shearer, 1999; Shaw et al., in press).
Figure 15 shows a sediment accumulation rate curve constructed from calibrated
radiocarbon ages obtained at various depths in the section and projected to correlative depths
in the south end of the transect. Accumulation rate varies from 1 - 6.5 mm/yr with a long
term average of 1.6 mm/yr in upper 38 meters. Long-term average Quaternary slip rate
estimates for the Santa Fe Springs segment, based on the base Quaternary surface, range
from 0.44 - 0.82 mm/yr with a preferred rate of 0.62 mm/yr, (Shaw et al, in press). These
slip rates translate to uplift rates of 0.2-0.4 mm/yr, assuming a 27° fault plane dip. Thus, the
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sediment accumulation rate at Carfax Avenue is faster than the long-term average tectonic
uplift rates, and the expected stratal geometries resemble those shown in Figure 5a.
In order to reconstruct past stratal geometries in an area that is actively folding two
assumptions must be made. First, it is assumed that scarp development occurs rapidly, i.e.
during an earthquake, and not slowly and progressively over time. This assumption is
supported by the seismogenic nature of the PHT and the episodic changes in thickness of
stratigraphic units. For example, unit 47 does not change thickness across the section,
although underlying and overlying units do thicken to the south. This indicates that little to
no tectonic uplift occurred while unit 47 was being deposited. Similarly, unit 30 exhibits
minimal (< 50 cm) change in thickness across the transect, suggesting that there was no
uplift during deposition of this unit. Second, it is assumed that sands, that is fine-grained to
coarse-grained sands that do no contain silt, will onlap rather than drape any existing
topographic slopes (e.g. older fold scarps). The tops of thick, laterally continuous sands can,
therefore, provide a paleo-stream gradient. It is probable that finer-grained, cohesive strata,
such as a silt or clay, can drape a slope and therefore, sections in our stratigraphy containing
silt are not used as a paleo-stream gradient. The scoured base of sand units is also not an
appropriate indicator of paleo-stream gradient because of the channelized, erosional nature
of these contacts. Thus, it is assumed that the top of a continuous sand section overlain by
cohesive sediments is a robust paleo-stream gradient at the time of deposition.
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We have identified one other paleo-stream gradient along the Carfax Avenue
transect, which we interpret as the base of a paleo-acquifer. The top of the unit 60 sands is
not separated from the overlying sands by a continuous cohesive section, however, the silt
and clay of unit 55 is almost 2 m thick in boreholes 20 and 19 and only 30 cm in borehole 15
suggesting that this cohesive section was possibly continuous prior to erosional scouring
during deposition of the unit 50 sands. The presence of a sharp color change from dark
greenish-gray to olive at the top of unit 60 also supports the presence of a cohesive section
that acted as an aquitard between the unit 50 and 60 sands. The greenish gray color change is
coincident with the base of unit 50 and is a contact that is at approximately the same depth in
the well logs of hole 1589S, 300 meters to the north as it is in borehole 22 (Figure 13). Thus,
it is assumed that this contact was a near horizontal surface at one time in the past and has
since been deformed by uplift of the fold scarp.
To constrain the location of the inactive and active axial surfaces, graphs of unit
thickness, and the depths from the tops of each sand unit to the top of the sand unit below
have been constructed for each package of sand (Figure 16 a-g). For each uplift event, the
anticlinal axial surface is identified as the point where the thickness of each section shifts
from no change in thickness to significant increase in unit thickness. The top of the fold
scarp does not create an increase in accommodation space and therefore does not require the
overlying unit to thicken. In contrast, dip of the sediments below the top of the scarp results
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D epth Range and Thickness
Top of Unit 10 - Top of Unit 20
3.
3.6
3.4
3.Z
3
2.
2.6
2.4
2.2
2
350 300 0 50 100 150 200 2S0
M eters s o u th along tra n s e c t
x D epth Range and Thickness
C) Top of Unit 30 - Top of Unit 40
2.0 H ---------------------1 -------------------- 1 -------------------- '-------------------- 1 ------------------------■ ' " 1
0.0 50.0 100.0 150.0 200.0 250.0 300.0 350.0
M eters so u th along tra n s e c t
Depth Range and Thickness
Top of Unit 20 - Top of Unit 30
0 0 50.0 100.0 150.0 200.0 250.0 300.0 350.0
M eters south along tra n se c t
Key to sym bols
— * - D epth o f unit to p
-... “ D epth of unit base
- A - Unit thickness
Figure 16 a-c: Plots of the depth range and thickness o f units from north to south along the Carfax Avenue borehole transect.
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d)
Depth Range and Thickness
Top of Unit 2 0 - Top of Unit 40
M eters s o u th along tra n s e c t
100 1S0 200 250 300 350
e)
D e p th R ange an d T hickness
T op o f Unit 4 7 - Top o f Unit 5 0
0.0 50.0 100.0 150.0 200.0 250.0 300.0 350.0
M eters so u th aiong tra n s e c t
f)
D e p th R ange and T hickness
T op o f Unit 4 0 - Top o f Unit 4 7
0.0 50.0 100.0 150.0 200.0 2S0.0 300.0 3S0.0
M eters so u th along tra n s e c t
g)
D epth R ange and Thickness
T op o f Unit 5 0 - T op o f Unit 6 0
0.0 50.0 100.0 150.0 200.0 250.0 300.0 350.0
M eters so u th along tra n s e c t
Figure 16 c-f: Plots o f the depth range and thickness o f units from north to south along the Carfax Avenue borehole transect.
Symbols used here are identical to those in Figure 16 a-c.
U \
55
in increased accommodation space. This, in turn, requires thickening of the overlying units
above the folded section. The synclinal axial surface for each uplift event was located by
identifying the point where thickness ceases to increase significantly south of the growth
section. The maximum sedimentary growth in the thickened section is equal to the paleo-
fold scarp height.
Restoring the top of sand units 60-20 to horizontal, each unit at a time, reveals the
shape of the deformation zone in the past, Figure 17 a-f. The oldest uplift event that we can
identify (Event V) is marked by folding of the unit 50 / unit 60 contact. The uplift event that
folded this stratum had an anticlinal axial surface located near ~ sp 493. The anticlinal axial
surface could not have been much farther north because the unit 50/ unit 60 contact is at the
same depth in borehole 1589S, 300 m to the north of sp 493. The synclinal axial surface for
Event V lies between sp 318 and sp 260. The kinkband width is therefore between 175 and
263 m. Growth within the unit 50 sands is -1.4 m suggesting that the paleo-fold scarp height
was also 1.4 m (Figure 17 a). Event V predates complete deposition of unit 50 and post-dates
deposition of the top of unit 60. Two charcoal fragments collected from borehole bracket the
age of sediments above and below Event V. At - 38 m depth a charcoal fragment (1809)
yielded an adjusted age of 25,230+/-460 y BP. This sample is - 15 m below the unit 50/ unit
60 contact. Sample 1813 was collected from 20.7 m depth, ~ 30 cm above the unit 50/unit 60
contact at the southern end of the borehole transect. This sample yielded a calibrated,
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56
calendric age range of B.C 8200-7597 (Table 1) and provides a minimum age for Event V.
Placing this event at the depth range of 21 to 25.6 m on the sediment accumulation rate
curve gives an age range of 10.2 ka - 11.5 ka. This approximation is limited, however, by
the assumption that the sediment accumulation rate was constant for the time interval
between two radiocarbon ages by which the rate is defined, (Figure 18).
The reconstruction of unit 40 sands to horizontal reveals the pronounced, post-
depositional folding of the unit 60 and 50 sands (Figure 17b). The stratigraphic interval
between the top of unit 40 to the top of unit 47 thickens by ~ 2 m between sp 493 and 185,
indicating that the underlying strata where uplifted by ~ 2 m. The anticlinal axial surface of
this folding event, Event W, is located between sp 445 and sp 493 and the synclinal axial
surface is located between sp 260 and 185, yielding a kink band width between 185 and 308
m. Event W occurred before the deposition of unit 40 sands was completed and after the
deposition of unit 46. Since unit 47 does not change thickness across the transect, the unit
was not deposited across a fold scarp, nor was it folded prior to completion of deposition.
The absolute age range for Event W is constrained by three bulk-soil ages (1301B,1302B
and 1400B) that give approximate ages for unit 46 between 8.4 ka and 7.7 ka, and two
detrital charcoal samples obtained from within unit 35 (1804 and 1704) that estimate the age
range for unit 35 between 6.6 ka and 7.2 ka. The absolute age range for this event is 6.6 - 8.4
ka. The event probably occurred after the burial of unit 46 and before the complete
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57
deposition of unit 40. If we assume constant average sediment accumulation rate between
sample locations, then we can use the sediment accumulation rate curve (Figure 18) to infer
more probable age ranges for each uplift event. Sample 1704, which was collected from ~
1.5 m above the top of unit 40, given a sedimentation rate of ~ 6.5 mm/yr, was deposited ~
230 years after the top of unit 40. This suggests a preferred youngest age of 7.4 ka. At the
maximum depth range for this event, the sedimentation rate curve provides an age of 7.9 ka.
This age assumes an average age for samples (2303, 2007, 1806, 2103) located centimeters
above the unit 46/unit 45 contact. The lack of thickening in units 46 and 47 leads us to
believe the oldest age for Event W is after the burial of unit 46 and therefore, we give a
preferred age range of 7.4 - 7.9 ka.
The present day configuration of stratigraphic units indicates a buried-fold scarp
height of the top of units 50 and 47, between 3.5 and 4 m. Approximately 2 meters of this
total uplift occurred prior to the deposition of the uppermost part of unit 40, indicating that
less than 2 meters of uplift occurred after deposition of unit 40 and before present day. The
correlation of unit 30 sands is less robust than those of units 50, 40, and 20, because of the
presence of younger sand channels that eroded the upper part of unit 30 in boreholes 14, 15,
20 and 24. For this reason, Unit 30 sands have been divided into two sections, the northern
section, north of shotpoint 421, and the southern section, south of shotpoint 354. Within
each of these 2 sections we are confident of our cross-borehole correlations. If an uplift
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58
event occurred during the deposition of either unit 30 or the underlying unit 35 at either the
north or south end of the transect, then an increase in the underlying buried fold scarps of
Events V and W, and growth of unit 30 would be expected. Figure 17 c depicts the
subsurface configuration for the top of units 47, 50 and 60 when the unit 40 sands were at
the surface. Figure 17 d shows these same relationships when unit 30 sands where at the
surface. After the deposition of unit 30 in the northern section (Figure 17 d) there appears to
be a 5 - 30 cm decrease in paleo fold scarp height whereas in the south, there is < 30 cm
paleo fold scarp at the top of unit 40 sands, a decrease of 10 cm at the top of unit 47, but an
increase of 60 cm at the base of unit 50. Given that a reduction of in height of buried scarp is
not plausible, there must be error in these estimations of 10 to 30 cm, reducing the 60 cm
increase in possible paleo-fold scarp height to only 30-50 cm, which is not entirely outside
of our sampling error of ~ 30 cm. Thickening of the southern section of unit 30 is ~ 50 cm
and the northern section shows thinning of ~ 30 cm. The small fluctuations in scarp height
and growth are probably within the noise and do not represent an uplift event. The
possibility exists, however, that a smaller uplift event occurred during the deposition of unit
30 that did not fold units 50, 47, or 40 sufficiently to generate a significant scarp, for
example a moderate or small sized uplift event that did not deform the near surface
sediments. This interpretation, however, is not favored. Given the discontinuous nature of
unit 30 it is not possible to unequivocally determine an event during its deposition.
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59
The penultimate uplift event on the SFS segment, Event X, occurred before
complete deposition of unit 20, and after deposition of unit 30, given the fact that a
significant amount of growth or paleo-fold scarp development cannot be clearly determined
to have occurred during the deposition of units 30 and 35. The 80 cm of southward
thickening within unit 20 is located between sp 318 and 185. The extent of growth in the
north-south direction of unit 20 is poorly constrained because both units 30 and 20 cannot be
accurately discerned in borehole 14, and the unit 30 sands north of boreholes 14 and 15 are
not continuous with the unit 30 sands to the south. For the same reason, the location of the
anticlinal axial surface cannot be precisely delineated, but is estimated to he south of sp 421
and north of sp 354 (Figure 17 e). The synclinal axial surface is located near the
southernmost borehole, however, the termination of southward thickening between units 20
and 30 cannot be resolved between the two southernmost boreholes (20 and 19). It is
possible that the synclinal axial surface is located between sp 260 and 185, or possibly
farther to the south of sp 185 since growth of unit 20 does not cease at borehole 19. The kink
band width for this event ranges from 169 to 236 m.
The absolute age range for Event X is constrained by the age range of unit 30 and 11
since the event occurred after the deposition of unit 30 and before deposition of unit 11. Two
detrital charcoal 1 4 C ages from within unit 30 (1101 and 1700) give an age range of 6.3- 6.0
ka. Thus, the oldest possible age of Event X is 6.3 ka. A sample obtained from unit 11, ~ 1 m
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60
above unit 20 and therefore younger than the age of Event X, yielded a calibrated age range
of 0.5-0.3 ka. The absolute age range of Event X is 6.3-0.5 ka. A detrital charcoal from the
base of unit 20 (1803), however, yielded an age of 3.4-3.0 ka. This sample provides a
constraint on the sediment accumulation rate curve around the approximate time of this
event. Based on the location of Event X on the sediment accumulation rate curve the
preferred age range of this event is -1.3 - 4.6 ka (Figure 18).
Event Y is the most recent uplift event found at this site. Unit 10 thickens - 76-90
cm within a zone ~ 106-161 m wide. Growth above unit 20 indicates that the anticlinal axial
surface lies near sp 421 and the synclinal axial surface lies between sp 354 and 260 (Figure
17 f). Unit 11, which is a distinctive soil in the upper 2.5m, deepens southward by 76-90 cm
between sp 404 and 185. The stratigraphic interval between the top of unit 11 and the top of
unit 20 does not increase, but actually decreases by 30 cm south of sp 318. This indicates
that no sedimentary growth occurred during the time between the end of unit 20’s deposition
and the burial of the unit 11 soil. Subsequent uplift and tilting of unit 11 occurred during
Event Y. The fold scarp marked by unit 11 has subsequently been buried by the onlapping
sands of unit 10. A fold scarp is not discemable at the ground surface along Carfax Avenue
or in the field along the SCE power line right of way 25 m to the east.
A detrital charcoal sample collected from within unit 11 (1800) yields a calibrated,
calendric age range of AD 1442-1635. Sample 1803, obtained from the base of unit 20
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61
sands, provides an absolute maximum age for Event Y of 3.4 ka. The youngest-possible age
for this event is unconstrained by our data. The most recent possible historical earthquake
that could correlate with event Y is also the first earthquake in the historic record. On July
28, 1769, while Gaspar de Portola and his men where camped along the Santa Ana River,
they experienced a moderate to moderately large earthquake (Toppozada et al., 1981;
Ellsworth, 1990). Members of Portola’s overland expedition from San Diego to Monterey
wrote about the strong quake and reported that numerous and some violent aftershocks
continued for the remaining 3 days the group was in the Los Angeles Basin. The absolute
age range for Event Y is, therefore, 230 years - 3.4 ka. Our preferred age range, based on the
sedimentation curve and the historic earthquake, is 230 years to 1.25 ka.
We have delineated a minimum of 4 events uplift events in the upper 25 m of the
Carfax Avenue transect. Uplift events in the lower 25 m, however, are difficult to identify
because of a lack of continuous, cohesive silt and clay units between sand sections.
Figure 19 is a schematic representation of the incremental development of paleo-fold
scarps due to Events V-Y. This simplified reconstruction illuminates the relationship
between growth, uplift, and axial surface locations and finally, relates the scale of our
borehole investigations to the larger Santa Fe Springs fold structure.
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Q .
a >
O
4x Vertical Exaggeration
5 ^ North 5 0 m e t e r s
1 0 0 f e e t
total accommodated
space
3.05 in 12 ft.
i2'>U' •’> '
> V
...IF
synclinal
axial su rfa c e
p a le o -
fold s c a rp
Event V
1.2 m (4ft) uplift
anticlinal
axial su rface
Figure 17 a: Reconstruction o f stratigraphic relationships w hen the top o f the unit 50 sands w ere the stream gradient. Interpreted locations o f
the anticlinal and synclinal axial surfaces are show n as w ell as the shape o f the paleo-fold scarp at the contact betw een unit 50 and 60. Total
accom m odated space refers to the am ount o f sedim ent that has been deposited since the reconstructed unit w as deposited. o x
to
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4x Vertical Exaggeration
50 m eters
North
to ta l a c c o m m o d a te d
/ s p a c e
E v en tW
2 m growth
anticlinal
axial surface
3.4 m (11ft) =
paleo-fold scarp
height
, I 'a n a l
axial su rface
Figure 17 b: Reconstruction of stratigraphic relationships when the top o f unit 40 was the paleo-stream gradient. Paleo-fold scarps
follow ing the top o f unit 60 and 47 w ith scarp height indicated to the right u>
64
90cm
p a le o fold scarps
S o u th e rn sec tio n j N orthern section
4xVertical Exaggeration
m 1 m (3-3.5 ft)
N o e v e n t
Figure 17: (c) Reconstruction of stratigraphic relationships when the top of unit 40 was the paleo- gradient
with the paleo-fold scarps shown (blue) and their respective heights indicated for both the northern and
southern sections of the borehole transect. The transect is divided into two sections based on the break in
continuity of unit 30 sands, (d) Configuration of paleo-fold scarps when unit 30 sands are at the surface.
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50 m eters 25
100 fe et 50
4x Vertical Exaggeration
North
CL
C U
O
75:S0,cnfL
80 cm of growth
Event X
76 cm
synclinal ^
an ticlin al
axial surface
4.5 m
w m
I g o
fold scarps~~
Figure 17 e: Reconstruction of stratigraphic and structural relationships o f units when unit 20 was the paleo-stream gradient. Paleo-fold ^
scarps are indicated in blue along with their respective heights to the right. u>
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40 meters
4X Vertical Exaggeration
Event Y
76 cm
0 SO cm Qrowth
Event
O
Event W
3.6 m
rrT p . i d - a a g g g g g g
0 growth
Event V
Figure 17 f: Present-day configuration o f stratigraphic relationships at Carfax Avenue. Paleo-fold scarps,
anticlinal axial surfaces, and earthquake events shown as in Figure 17 a-e.
O N
O s
Depth (m)
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Sedim ent Accum ulation Rate Curve and Paleoearthquake Events
4 0
E
3 0
Q )
o
JZ
4-Event V
■f-pref: 1 0 .2 - 1 1 .5 ka
o
_ Q
o
-a
c
a >
J C
3
O
1/ 5
-E v e n t W
'p r e f : 7 .2 -7 .8 ka
'a b s o lu te : 6 .6 - 8 .3 k;
- E v en t X
- pref: 1.3 - 4 .6 ka -
'a b s o lu te : .5 - 6 .3 ka'
—' 1 --------1 --------T “ f — I I -
•E v en tY
-p ref: .3 - 1 .2 5 k a
'a b s o lu te : .3 ,- 3 .4 ka;
5 0 0 0
4 0 0 0 0 3 5 0 0 0 20000 2 5 0 0 0 3 0 0 0 0 0 1 5 0 0 0 10000
calibrated calendric age (years)
Figure 18: A ge ranges for paleoearthquake events interpreted from stratigraphic relationships at Carfax Avenue. Yellow os
boxes indicate absolute age range and preferred age range in pink. "J
68
S tag e 2:Ti!ting o f u n it 60 a n d sy n te cto n ic d e p o sitio n o f unit la n d 60 S tag e 1: D ep o sitio n o f sa n d y se ctio n , u n it 60.
o f u n it 50. P a le o ea rth q u ak e e v en tV occu rs d u rin g d e p -
o sition o f unit 50.
5 ta g e 3: D ep o sitio n o f a co h esiv e se c tio n ca p p e d by a very
d istin ctiv e black clay co n tain in g g a s tro p o d shells, u n it 47.
N o d e te c ta b le u plift d u rin g d e p o sitio n o f u n it 47.
S ta g e 3: D ep o sitio n o f a n o n ta p p in g cohesive section,
u n it 45 (blue) follow ed b y sa n d d e p o sitio n , u n it 40
(m agenta). P a le o e a rth q u a k e e v e n t W tilts ail u n its below
th e sy n te cto n ic un its 45 a n d 40.
S tag e 4: D eposition o f co h esiv e se ctio n , u n it 35 (blue) a n d
u n it 30 (pink). N o d e te c ta b le uplift o ccu rs d u rin g d e p o sitio n
o f th e s e units.
S t a g e 5: D ep o sitio n o f on la p p in g sa n d s a n d silts, un its 25 a n d 20
(g re en a n d pink) a cro ss sc arp w ith th ic k en in g to th e so u th to fill
in c re a s e d a c c o m o d a tio n sp a ce d u e to uplift. P a le o ea rth q u ak e
e v e n t X folds all un its b e lo w th e g ro w th strata.
S ta g e 6: D ep o sitio n a n d p ro b a b le soil d e v e lo p m e n t, u n it 1 1,
n o u p lift e v e n t d u rin g this tim e,
;n tY J
S tag e 7: P a le o ea rth q u ak e e v e n t Y folds all u n its b e lo w th e : Y folds all b e lo w th e ev en t u n its
soil (vertical lines) d u rin g d e p o s ito n o f th e sa n d a n d silt
axial surface
N
Z one,of active.anticllnal
axial surface locations
s -
\ '
■iii*
growth strata
pre-growth strata
B alan c ed m o d e l o f in te rp r e te d kink b a n d g e o m e tr y a b o v e th e S a n ta Fe S p rin g s
s e g m e n t (b e lo w ) fro m P ra tt e t al 2 0 0 2 w ith o u r In te r p re ta tio n o f th e n e a r
s u rfa c e g e o m e trie s .
Figure 19: Schem atic representation o f the developm ent o f grow th stratal geom etries at
Carfax Avenue.
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69
Kinematics
The stratigraphic relationships in the upper 50 m at the Carfax Avenue study site
reveal a discrete zone of south-dipping strata that is bound on the north, and the south, by
more gently dipping to horizontal strata. (Figure 20). This near-surface zone of deformation
is the locus of active folding along the forelimb of a fault-bend fold. The south-dipping panel
beneath Carfax Avenue is the uppermost part of an upward-narrowing zone of south-dipping
strata. Such syn-tectonic upward-narrowing dip panels in growth strata are known as growth
triangles (Suppe et al, 1992). The growth triangle is a upward-narrowing fold panel created
by the migration of sediments through the active axial surface during slip on the buried
blind-thrust ramp (Suppe et al, 1992).
The northern extent of the growth triangle beneath Carfax Avenue is defined by the
trace of the anticlinal axial surfaces for each paleo-uplift event (Figure 19 and 20). The
southern extent of the folded zone for each uplift event, however, is not coincident with the
synclinal axial surface for all folding below the most recent event. For example, the top of
unit 60 deepens southward by ~3 m between boreholes 21 and 18 (sp 493 - 318), with an
additional ~1 m of deepening to the southernmost borehole 19 (sp 185). The location of the
synclinal axial surface for the uplift event that folded the top of unit 60 lies between sp 354
and 318, -130 m to the north of the southern extent of the growth triangle for this event. In
contrast, at the top of our uppermost buried sand, unit 20, the growth triangle extends from
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70
borehole 15 to borehole 20 (sp 404 - 260). The zone of total folding of unit 20 is within the
range of the locations of both axial surfaces. These observations show that the zone of
cumulative deformation, i.e. the growth triangle, narrows upwards from ~311 m wide at 15 -
18m depth and ~ 144 m wide at a depth of 3 - 4 m.
The kink band width has been determined for each folding (i.e. uplift) event. The
growth for each of these uplift events, as defined by the increase in thickness of major
stratigraphic units in our borehole transect provides a means of determining the location of
both the active anticlinal and inactive synclinal axial surfaces for individual uplift events that
have occurred prior to complete deposition of the growth stratum under consideration.
Therefore, we can reconstruct the incremental development of the kink band over numerous
discrete uplift events. For the oldest 3 of the 4 folding events found at Carfax Avenue, the
kink band width is 236 ± 72 m. For the youngest folding event (Event Y), the kink band
width is between 101 and 161 m, narrower than the previous 3 events. This would suggest
that the kink band width does not necessarily increase significantly with each folding event,
as predicted by theory, but instead widens cumulatively after a few or several uplift events.
Both axial surfaces are non-stationary and exhibit a general migration through the strata to
the south over time (Figure 20 and 19).
We can distinguish between the growth triangle, defined as the cumulative zone of
folded strata, and the kink band for individual uplift events, which is the width between the
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40 meters
4X Vertical Exaggeration
EventY
Event X
E ventW j I
?
XXX
CO £Q CO CO
Z&m&ngr&wffi
80 cm growth
* <50 cm
2 m growth
f (50) ' / 2 m growth '
Event V
l _
50
Figure 20: Carfax Avenue stratigraphic section showing the trace o f the anticlinal (red dashed line) and synclinal axial
surface (green dashed line) and the cum ulative width of the grow th triangle.
Depth (m)
anticlinal and synclinal axial surfaces. These interpretations are compatible with the
interpretations from both the industry and high-resolution seismic reflection images, which
show upwards-narrowing of the folded reflectors (Shaw and Shearer, 1999; Pratt et al., 2002;
Shaw et al., in press). Although the general upwards-narrowing growth triangle is observed
in all seismic reflection images and the stratigraphic relationships discovered in the borehole
transect, there is one major differences between what is observed in the stratigraphy versus
what is interpreted from the seismic reflection profiles. The width of the zone of deformation
estimated based on the extend of dip of prominent reflectors in the Mini-Sosie and hammer-
source seismic reflection profiles is narrower than the size of the dipping panel based on dip
of stratigraphic horizons.
Three prominent reflectors identified in the Carfax Avenue Mini-Sosie seismic line,
Figure 9, dip southward in a discrete zone which allowed for identification of the growth
triangle between 90 and 280 m below the surface. The width of this zone at the 90 m
reflector (purple reflector in Figure 9) is ~ 150 m wide, less than both the growth triangle
and kink band width determined stratigraphically at 25 m depth. Reflectors in the hammer-
source seismic reflection profile between 10 and 25 meters also reveal a localized zone of
folding. The width at this shallow reflector is ~ 90 m wide, narrower than kink band and
growth triangle at that depth as determined stratigraphically. These observations do not
imply that the growth triangle widens as it moves closer to the surface. Instead, there are two
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73
possible reasons for the difference in deformation zone width as determined by change in
reflector dip on the seismic images and the width of this zone determined stratigraphically.
First, growth of stratigraphic units cannot clearly be determined from the seismic reflection
images, individual events and their axial surface locations cannot be delineated. What can be
determined from the seismic images, however, is the zone of onlap and buttressing of sands
against buried-fold scarps indicated by the sand deposits. Second, the bottom of sand
channels are steeper than the dip of folded strata, which is subtle and may not be discemable
on the seismic reflection images. An example of the reflectors on the hammer-source seismic
image following a scarp buttressing sand channel, is seen at the 10 m depth reflector. This
reflector traces out the base of the unit 40 sand channel (Figure 21). The change in dip of this
reflector gives the approximate location of the fold scarp but underestimates the kink band
width by at least 85 meters and underestimates the zone of folding by ~ 160 meters.
Analysis of paleo-foldscarps based on reconstructions of paleo-stream gradient at
the upper contacts of continuous sand units reveals the incremental increase in paleo-fold
scarp height. Figure 17 a-f highlights paleo-fold scarps in blue. The scarp height for each
unit is indicated for the subsequent uplift events. A marked increase in height, or increase in
depth of underlying folded units, occurs with each subsequent uplift event. For example, in
Figure 17 a, when the top of unit 50 was the paleo-stream gradient, the fold scarp is only ~
1.4 m high. When the unit 40 sands were the paleo-stream gradient, however, (Figure 17 b)
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M 1
■ ■ ■
■
Figure 21: Migrated hammer-source seismic image with sands (brown) and silt and clay (transparent or white). The oran
and green reflectors are identical to those in Pratt et al. 2002. Red, blue and yellow are interpretations of this study. The
reflector mimicks the base of the unit 40 sand deposit. The red reflectors approximate the shape of unit 30 and 20 sand
deposits
• P -
75
the paleo-fold scarp marked out by the top of unit 50 is over 3 m high. Presently, the buried-
fold scarp height of unit 50 is ~ 5 m (Figure 17 f). Thus, the dip of strata are acquired
incrementally.
Slip per Event and Magnitude o f Paleoearthquakes
An estimated uplift based on the height of paleo-fold scarps for each
paleoearthquake event can be translated into slip on the fault plane by dividing the amount of
uplift by the sine of the fault dip. For the paleoearthquakes observed at Carfax Avenue,
which overlies the SFS segment, the range of fault dips based on the kink band width and
industry seismic image interpretation is 25° - 29°, with a preferred dip of 27° (Shaw and
Shearer, 1999; Shaw et. al, in press). Calculations of total cumulative slip on the Santa Fe
Springs segment based on kink band width and structural relief of the base Quaternary
reflector in industry seismic profiles indicate that slip at the Carfax site is 65% of the
maximum slip on the Santa Fe Springs segment, (Shaw et al., in press). These observations
suggest that, over the long-term, the maximum displacement in earthquakes is probably
significantly greater near the center of the Santa Fe Springs segment than at the Carfax
Avenue site. Magnitude estimates for each slip event are based on the displacement-versus-
moment magnitude (M) regressions of Wells and Coppersmith (1994). The M-versus-
average displacement regressions appear to overestimate the magnitude for reverse faults
compared to the estimates based on strike-slip, normal, and reverse faults together. We have
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therefore used both average displacement- versus-M and maximum displacement-versus-M-
regressions of Wells and Coppersmith (1994) to estimate the magnitude of paleoearthquakes
at Carfax Avenue. The displacement value determined from calculating slip based on uplift
as explained above was used for an average displacement in the average displacement-
versus-M regression (M = 6.93+0.82*log (average displacement)) (Wells and Coppersmith,
1994). The estimated maximum possible slip for each uplift event was determined by
increasing the calculated slip calculated by 35% for each event to account for the greater
cumulative displacement along the center of the Santa Fe Springs segment, east of the
Carfax site. The estimated maximum slip amounts were then used as the displacement
parameter in the maximum slip-versus-M regressions (M =, 6.69 +0.74* log (maximum
displacement)) (Wells and Coppersmith, 1994). Given the average slip estimated from a
preferred fault dip of 27°, M estimates are made for Events V, W, X, Y (Table 2).
Event V, the earliest event identified on the SFS segment, has an estimated moment
magnitude range of M = 7.1 to 7.3. Event W is the largest earthquake event found at Carfax
Avenue, with a possible range of M = 7.3 - 7.4. Events X and Y are comparable in size with
an estimated size of M = 7.0-7.1.
The estimated magnitude values calculated here represent a minimum value, as our
uplift measurements provide a minimum estimate of fault slip. These slip measurements are
minimum values because it assumes rigid hangingwall block translation, with no strain
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Table 2: Estimated Slip and Magnitude Values For
Paleoearthquake Events at Carfax Avenue
Event
Paleo-fold
scarp height
(m)
Slip on 27
degree
dipping
thrust ramp
(m)
Estimated
maximum
slip (m)
Magnitude
(Maximum
Displacement
Equation)
Magnitude
(Average
Displacement
Equation)
V 1.20 2.64 4.07 7.14 7.28
W 1.80 3.96 6.10 7.27 7.42
X 0.80 1.76 2.71 7.01 7.13
Y 0.80 1.76 2.71 7.01 7.13
78
accommodated by folding in the hangingwall. Thus, the true slip per event at seismogenic
depths on the Puente Hills thrust ramp is probably somewhat greater than our estimates.
Since only relatively small amounts of deep thrust slip is actually consumed by hangingwall
deformation on the PHT, however, our minimum estimate may be closer to the true value
(J.Shaw, pers. comm., 2002)
The M values determined in this study are somewhat larger than those predicted by
Shaw et. al.(in press). Based on a rupture area of 260 km2 for the SFS segment and the
empirical relations of Wells and Coppersmith (1994), Shaw et al. (in press) determined that
an earthquake rupturing the entire segment would generate a M 6.5 earthquake. One possible
explanation for this discrepancy is that slip on the SFS segment is rarely confined to this
segment alone, and in large slip events the rupture area of an event on the SFS segment may
include all of that segment and fractions of adjacent sections of either both or one of the
neighboring segments. Another explanation is that the magnitude vs. rupture area regressions
by Wells and Coppersmith underestimate the magnitude of southern California earthquakes,
(Dolan et. al. 1995).
Slip Rates
Early Holocene slip rates for the Santa Fe Springs segment of the PHT based on the
interpretation of stratal geometry at the Carfax Avenue location range from 0.9 - 1.2 mm/yr.
Fault slip was determined from the ~5 m total height of the buried-fold scarp at the unit
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50/60 contact gives ~ 11 m of slip since folding of this horizon. The age range for the event
that deformed this stratum is between 10.2 and 11.5 ka, based on the average sediment
accumulation rate curve (Figure 18). These age estimates yield a slip rate of ~ 0.95 - 1.0
mm/yr assuming a 27° fault plane dip. For comparison slip rates have been determined for
the top of unit 50 and the base of unit 45, as well. Uplift since Event W is 3.6 m, which
translates into ~ 8 m of slip since the preferred age for this event of 7.2 - 7.8 ka. These data
yield a slip rate ranging from 1.0- 1.1 mm/yr.
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80
Chapter 4
Trojan Way
The Trojan Way borehole transect is located on Trojan Way between Alondra
Boulevard and Desman Avenue in The City of La Mirada, -1 .5 km west of the industry
seismic profile shown in figure 4, and directly above the Mini-Sosie seismic reflection
profile (Figure 8). Three boreholes (TW-1, TW-2, and TW-3) have been excavated thus far
at Mini-Sosie shotpoints 119, 184 and 256, respectively. The borehole transect spans 548
meters across a 9-m-high, south-facing scarp. Due to the relief of the scarp, the ground
surface at TW-1 is 3 meters higher in elevation than the surface at TW-2 and at TW-2 is - 5
meters higher than TW-3. The Trojan Way borehole transect is an ongoing project, and data
collection is incomplete at this point. Thus, all of the following conclusions should be
considered preliminary.
TW-1 and TW-2 were drilled using a hollow-stem auger technique with a
continuous coring system. Samples were taken in 5 ft (1.5 m) intervals. TW 3 was drilled
with a mud rotary system. This drilling system utilizes drilling mud to add strength to the
hole when drilling through water-saturated sediments. Samples where collected in
continuous 5 ft (1.5 m) increments.
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81
Stratigraphy
TW-1, the northernmost, borehole, is located ~ 20 meters south of the southwest comer
of Alondra Blvd and Trojan Way. Total penetration depth was 38.1 m (125 ft). The
stratigraphic section at this location consists of massive, alternating, meter-scale, friable
sands and cohesive sections composed of silty sand to clayey silt. There is one water-
saturated section from 22.5 m to 35 m depth.
Unit 1, the uppermost stratigraphic unit is a relatively well-developed soil with a
distinctive brown color (7.5 Y 4/2-4) and prismatic soil structure. Clay coatings cover large
grains within the soil. The soil is very porous and contains black, and reddish-brown
staining. The soil is ~3 m thick, extending from 1.2 m (4 ft) to 4.5 m (15 ft) depth. This unit
consists primarily of silt and clay down to 3.5 m depth, where it grades downward to a fine
grained sand to clay-rich sand. This change in grain size is coincident with a downward
change in soil color. The color gradually changes downwards to a light-yellowish brown (10
YR 6/4).
Stratigraphic unit 2 is located between 12 m (40 ft) and 13.7 m (45 ft) depth in TW-
1. This fine-grained section is pale olive-brown (2.5 Y 5-4/3-4) in color. Unit 2 is iron
stained, contains calcite viening, and pods of caliche. Roots and small charcoal fragments
also characterize this section.
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82
TW-2 had a total drill depth of 56.3 m (183 ft). The stratigraphy is predominately
massive sections of silty-sand separated by centimeter- to meter-scale sand- and clay-rich
sections. There is a water-saturated section between 21 and 33 m (69-110 ft) depth and
below that each sand section is completely saturated, whereas the cohesive interbeds are
damp or dry. At 4.5 m to 6.7 m depth is a soil with dark brown to yellowish brown color (10
Y 3-4/3-4). This soil exhibits clay coatings and high porosity similar to unit 1 of TW-1.
Below this section is a color change downward to light olive brown (2.5Y 5/3-4). As in TW-
1, this color change coincides with a downward change to a primarily sand section that
extends to 10 m depth.
TW-3 is located 288 meters south of TW-2 and ~ 200 m south of the base of the
topographic scarp on Trojan Way (Figure 8, 22). Total drill depth was 80 m (260 ft). The
upper 6 m (20 ft) is a cohesive section consisting of predominately gray to dark greenish-
gray (Gley 3-5/5g-10y) and black clay and silty sand. Except for a 1.5 m (5 ft) interval of no
recovery between 6 - 7.5 m depth, the dark grayish cohesive section appears to extend to 9
m (30 ft), at which depth there is a sharp color change downward to dark yellowish brown
(10 YR 4/4) concomitant with a downward increase in sand content. Below this depth are
intermittent sand sections tens of centimeters thick, with one 1.5 m section of no recovery at
15 m. This section of no recovery most likely consists of saturated sands. Between 19.8 and
39.6 m (65 - 130 ft) depth, the recovery was only 25%, probably indicating a large section
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of predominately saturated sands. From 48.8 to 54.8 m (160 to 180 ft) depth are ~ 2.5 m (8
ft) thick sections of alternating, massive silty sands and fine- to medium-grained sands.
Below this depth is another section of poor recovery (57%) that appears to consist of olive to
olive-gray, alternating sands and silty sands and clay. A massive section of clay and silty
sand exists between 64 and 75 m (220 - 245 ft) depth with a color change to greenish-gray at
69 m (228 ft). The remainder of the section to the base of the borehole consists of greenish-
gray to greenish-black, very-fine grained sand.
Correlations of specific horizons based on texture, color, and soil development have
been made between TW-1 and TW-2 and TW-3. The 3 m-thick soil at a depth of 3.6 m (12
ft) in TW-1 (unit 1) is correlative with to the soil horizon at a depth of 5.2- 5.4 m (17-18 ft)
in TW-2. In borehole TW-3, the only section with potentially correlative soil development,
based on color and soil characteristics, is the section at 9 - 18.3 m (30 - 60 ft) depth. We
tentatively correlate this buried soil in TW-3 with unit 1 in TW-1 and TW-2.
The location of TW-2 at the ground surface is 3 meters lower than TW-1, with the
actual change in depth of the top of unit 1 being 4.8 m (Figure 22). The dip of the top of unit
1 across the 256 m horizontal distance between TW-1 and TW-2 is 1°, slightly greater than
the slope of the present day ground surface. Between boreholes TW-2 and TW-3, unit 1 has
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a possible range of dips between ~2-4°. The actual geometry of the soil between TW-2 and
TW-3 most likely follows the general shape of the ground surface. This would suggest the
soil dips more steeply north of the base of the slope and then flattens south of the inflection
point (Figure 22). Exact resolution of the subsurface geometry of unit 1 awaits planned
borehole results.
The next-deepest, potentially correlative section consists of three horizons spaced <
3 m apart between 12.6 m and 13.4 m depth. The shallowest of the three, unit 2a, is a very
fine-grained to fine-grained sand at a depth of 12.6 - 12.8 m (41.5-42ft) in TW-1. This unit is
equivalent to a sand section at 14.9 - 15.2 m (49-50ft) in TW-2. Making corrections for the
change in ground surface elevation for between the two boreholes, the dip of the top of this
section is 1.2 °. The next-deepest stratigraphic correlation (unit 2b) is a silty-clay that occurs
at a depth of 12.9 m (42.5 ft) in TW-1 and 15.4 m (50.5 ft) at TW-2. This horizon has a dip
of 1.2°. Unit 2c is at a depth of 13.2 - 13.4 m (43.5 ft - 44 ft) in TW-1 and 16.4 - 16.7 , (54 -
55 ft) in TW-2. It is a light olive to olive brown silty sand and is parallel to unit 2b.
Another horizon, unit 3, that may be correlative between TW-1 and TW-2, is a
greenish-gray-colored clay, approximately one meter in thickness, that acts as an aquitard to
the overlying saturated sections. A sharp, downward color change from olive and brown
colors to dark greenish gray marks this transition in both boreholes. The contact is at a depth
of 37.2 m (122 ft) in TW-1 and ~ 44.5 m (146 ft) in TW-2, yielding a southward dip of 2.3°.
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SOUTH
D esm an A ve
'^TW-3
100 m
VE = 3:1
NORTH
A lo n d ra B lvd
N o rth am A ve
TW -1
T W -2
UNIT 1
UNIT
- 2 5
UNIT 3
- 50
— 75
100
Figure 22 : Borehole transect across Trojan Way showing stratigraphic correlations.
O O
Lh
Depth (m )
86
Age Control
Nine detrital charcoal samples and one bulk soil sample were collected from the
boreholes along Trojan Way. Three samples have been dated thus far. Two detrital charcoal
samples from 13 m (43 ft) depth in TW-1 yielded radiocarbon ages of 30,130 ± 1960 yBP
(sample TW-431) and 28890 ± 1920 yBP (sample TW 432). From a depth of 15.9 m (52.2
ft) in TW-2 a detrital charcoal yielded a radiocarbon age of 26,800 ± 2430 yBP. These dates
are older than the age range of samples that can be calibrated by CALIB 4.2.1 (Stuiver and
Reimer, 1993). Instead, we can use the 1 4 C production-rate curve of Voelker et. al. (1998) to
approximately calibrate these radiocarbon ages. These corrections add between -3600 and -
4000 years to each date to yield crudely calibrated ages of B.C 36,000, B.C 35,000, and B.C
33,000, respectively. These sample ages provide an age range for unit 2 of 30 - 37 ka. These
ages support the correlations of units 2 a-c, discussed above, as they are the same age within
error of the dating technique.
Unit 1 is stratigraphically above unit 2 and is therefore younger. Without more
specific age control from additional radiocarbon dates, it is difficult to the age of the soil.
The color, clay content, and soil structure of unit 1, however, suggest that the soil required at
least a few tens of thousands of years to develop (Rockwell, 2000).
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87
Evidence for folding
The stratigraphic correlations in our borehole transect are dipping to the south across
a zone at least 240 meters wide. The most obvious observation that suggests active folding at
this site is the 9 m change in elevation at the surface (Figure 8). The shape of the scarp at the
surface is consistent with the geometry of reflectors imaged on the Mini-Sosie profile during
Phase 1 of this study. The correlations of units 1, 2a, 2b, and 2c in boreholes TW-1 and TW-
2 suggest that the strata have a similar geometry to that of the general geometry of the
reflectors in the upper 50 m of the Mini-Sosie profile (Figure 23). It cannot be determined
from the available borehole data whether or not the contact of unit 1 follows the shape of the
scarp and the seismic reflections. Specifically, the current borehole spacing is too great to
identify the south-dipping to horizontal inflection point in the sub-surface. The reflections
imaged on the Mini-Sosie seismic reflection profile, which I believe represent the general
geometry of correlative units, suggest that it does. Overall, the strata identified in our
boreholes are inclined to the south, with a greater slope than the present ground surface
across the borehole transect.
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Trojan Way Boreholes and Mini-Sosie Reflection Profile
100m
No Vertical Exaggeration
SOUTH
TW-3
250
200
TW-2
150
NORTH
TW-1 !00
"■ m il . irm s u ttL tiL flitiiS
i iipifliiiiss
i M i i i i i S
H i . !
ill
&
9 0
lUliill, iiilllllllllfl*1
Figure 23: Trojan Way Mini-Sosie profile with borehole locations and unit correlations (red lines) and
topographic profile (blue line).
O O
00
D ep th (m)
^
58
89
Interpretation
Late Pleistocene Slip Rates
The continuous seismic imaging from almost 7 km depth to within 50 m of the
ground surface reveals an upward-narrowing kink band with an active synclinal axial surface
(Pratt et al., 2002; Shaw et al., in press). Our borehole results confirm that shallow strata
within the active growth triangle dip to the south. By making the assumption that these strata
have been deformed by slip on the underlying Coyote Hills segment of the PHT, slip-rates
may be calculated from the dated horizons in our transects.
The Coyote Hills section of the PHT has an estimated dip 25- 30° (Shaw et al., in
press). Slip is calculated translating the uplift into slip on the thrust ramp by dividing the
uplift by the sine of the fault dip. Unit 1 has been uplifted by 18 to 27 m, which yields a
range of thmst displacement values between 36 and 42.5 m. Assuming a very crude age
estimate for unit 1 of 20 ka, this yields a slip rate of 1.8 - 2.1 mm/yr, given a rough age
estimate of 20 ka. Considering an older age estimate of 25 ka, the slip rate would be 1.4 -
1.7 mm/yr. Unit 1 is a soil horizon, however, not a sedimentary unit, and it is possible that it
was developed on an existing slope (i.e. fold scarp). Therefore, the age ranges for the uplift
of unit 1 may be older than the age of the soil, yielding slightly slower slip rates than
estimated here. Unit 2c, however, is a depositional unit.
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90
The base of unit 2c in TW-1 and TW-2 gives a minimum uplift of 10.3 m in ~
31,000 years, resulting in a minimum slip rate of 0.8 - 1 mm/yr. Slip estimates based on
uplift at the Trojan Way site reveal a slip rate of 0.8 - 2.1 mm/yr.
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91
Chapter 5
Discussion
The borehole data at Carfax Avenue reveal the details of incremental folding and
associated sedimentary growth during latest Pleistocene - Holocene slip on the PHT. Large
earthquakes on the thrust ramp of the PHT result in the growth of folds (i.e. the Los Angeles,
Santa Fe Springs, and Coyote Hills anticlines) above the thrust ramp and flat. Analysis of
industry seismic reflection data across the PHT shows that Quaternary sediments deposited
across the axial surfaces of these anticlines have been folded within a localized zone referred
to as a growth triangle. A growth triangle is an upward-narrowing zone of dipping strata
bounded by flat-lying strata on both sides. Of particular interest is the width of the growth
triangle at its apex, where recent uplift has deformed the youngest sedimentary layers. This
study has shown that the growth triangle of the PHT extends upward into the near surface.
The depth of the dipping strata deformed by the most recent event is within two meters of
the present day ground surface at Carfax Avenue.
The structural and stratigraphic relationships observed in the borehole data at Carfax
Avenue prove that the sediments are folded within a discrete, upward-narrowing zone that is
bounded by near-horizontal to horizontal strata to the north and south of the folded zone. At
a depth of 25 m, the zone of south-dipping strata is ~ 300 m wide. At a depth of 2 meters, the
depth range of the most recent earthquake (Event Y) this zone is ~ 100 - 160 m wide.
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92
Although geometrical relationships of stratigraphic units within growth triangles have
been recognized in industry seismic reflection images at depths greater than 200 m in
numerous fold and thrust belt settings (e.g. Suppe, 1983, Shaw and Suppe, 1994; Shaw and
Suppe, 1996), the incremental growth of the kink bands that make up the growth triangle on
the time-scale of several earthquakes has not yet been studied empirically in the detail with
which we document the late Pleistocene-Holocene evolution of the growth triangle on the
Santa Fe Springs segment. This study reveals the stratigraphic and geometric characteristics
of an actively growing growth triangle in unprecedented detail.
Two end-member models for fault-related folding are kink band migration and limb
rotation. The defining characteristics of growth strata deformed by kink band migration are:
(1) a decrease in distance between axial surfaces such that the growth strata are folded in a
triangular zone that narrows towards the surface and; (2) a constant dip of folded
stratigraphic horizons. In contrast, folds developed by limb rotation show a zone of folded
strata that is of constant width and is characterized by a fanning of dips within this folded
zone with strata dipping progressively more steeply with depth.
Our interpretations of the locations of both the active and inactive axial surfaces for
individual folding events, determination of the incremental fold scarp development, and
discovery of the width and shape of the cumulative zone of deformation, provide insights
into the possible mechanisms operating during fold development above the Santa Fe Springs
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93
segment of the PHT. At Carfax Avenue, the cumulative effect of the four uplift events we
have identified is a zone of south-dipping strata bounded on the north by a north-dipping
anticlinal axial surface. The deformation zone is bounded on the south by the southward
transition from southwards dipping strata to near-horizontal strata. This transition is near or
just south of the southern end of the borehole transect, except for the most recent event,
where the zone narrows upwards from over 200 m wide to 100-160 m wide. The shape of
the deformation zone resembles that of the deeper growth triangle observed on industry and
high-resolution seismic reflection data, suggesting that the dominant mechanism operating
during fold growth above the Santa Fe Springs segment of the PHT is kink band migration.
The incremental folding process, however, shows some characteristics reminiscent of the
limb-rotation model, in that the dip of a paleo-fold scarp increases with each subsequent
uplift event, such that there is a fanning of dips on the time scale of the four earthquakes
observed at Carfax Avenue. The locations of both the active and inactive axial surfaces, as
determined on the basis of stratigraphic growth, record a southward migration of both axial
surfaces between earthquake events. The migration of the inactive axial surface may be
responsible for the downward widening of the growth triangle relative to the width of the
kink band developed at any stratigraphic interval. These observations suggest that
incremental development of folds between earthquakes may involve aspects of both limb
rotation and kink band migration. Macroscopically, however, the fold is developed by kink
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94
band migration. Similar observations have been made by Sabat et al. (1997), in upper
Eocene-late Oligocene growth folding structures in the eastern Pyrenees, where a
combination of kink band migration and limb rotation mechanisms may be operating at
different time and length scales to form a growth triangle structure.
To the best of my knowledge, the paleoearthquake ages and slip per event data that we
have generated at the Carfax Avenue site represent the first direct measurements of these
parameters for an active blind-thrust fault. For over a decade, blind-thrust faults have been
recognized as a major seismic threat to southern California (Davis et al. 1989, Shaw and
Suppe, 1994, 1996; Dolan et al. 1995; Shaw and Shearer, 1999). The paleoearthquake
parameters necessary for accurate probabilistic seismic hazard assessment (e.g. age and
magnitude of most recent event and short-term slip rate), however, have not been known for
these faults because they do not reach the surface and therefore are not amenable to
traditional paleoseismologic techniques. In this study, we have determined that the PHT has
ruptured in at least four earthquakes ranging in size from M 7.1 - 7.4 since latest of
Pleistocene time.
The stratigraphic relationships observed in the Carfax Avenue borehole data suggest that
the uplift and folding along the active anticlinal axial surface occurs during temporally
discrete pulses separated by periods of tectonic quiescence. We interpret these relationships
as evidence that folding occurs during co-seismic uplift large earthquakes events (Figure 19).
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Specifically, the growth of stratigraphic units across the buried paleo-fold scarps allows for
the identification of earthquakes that occurred just prior to, or during, deposition of these
growth strata. The lack of an increase in thickness of three units in the section provides
evidence that periods of quiescence punctuate large, fold-forming uplift events. For example,
Events V and W are separated in time and space by a 3-m-thick cohesive section (unit 47
and 46) that does not change thickness significantly across the borehole transect. The
deposition of these units took between 1200 - 1800 years, as determined by the age range of
detrital charcoal samples collected from these units. A sand and cohesive section (units 35
and 30) represents a period of quiescence between Events W and X that was at least 800
years long. It is difficult to determine the time period between Events X and Y, as there is
poor age control and lack of stratigraphic layering between the two event horizons. A soil
(unit 11) developed above the stratigraphic interval representing Event X was, however,
most likely at the surface before Event Y occurred, as no growth occurred in this section.
Thus, discrete sedimentary growth intervals separated by intervals of no sedimentary growth
indicate that uplift events on the PUT occur as temporally discrete events.
The resolution of our data limits the scope of observation to events that generate fold
scarps greater than ~ 50 cm in height. On the Santa Fe Springs segment of the PUT, 50 cm
of uplift corresponds to an earthquake of ~ M 6.8 - 6.9. Paleo-fold scarp height should
exceed the paleo-stream gradient by a sufficient amount for growth in the overlying
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sediments to indicate an increase in accommodation space due to scarp development by
uplift. Another limiting factor is the continuity of stratigraphic units across the width of the
growth triangle. Units 30 and 35 represent a stratigraphic package that is separated into a
northern and southern section by the channelization and subsequent deposition of a
localized, younger sand unit. This spatial separation introduces uncertainty when charting
growth within units 30 and 35. As discussed above, uplift of the contacts below units 30 and
35 is not sufficient to create more than ~ 50 cm of growth in this section, although additional
growth may be masked by the lack of continuity in this section.
The temporally discrete nature of paleoearthquake events and the resolution of the
borehole data favors the interpretation that Events V, W, X and Y are individual M > 7
earthquake events, rather than temporal clusters of smaller earthquakes. For example, Event
W generated an estimated slip of ~ 4 m, corresponding to a M 7.3 -7.4 earthquake. This slip
occurred within an absolute maximum time span of ~ 600 years. It seems much more likely
that this slip occurred in one large earthquake, rather than in, for example, six M 6.7,
Northridge-sized earthquakes, that would be required to equal the amount of uplift due to a
M 7.3. Although we can cannot rule out the possibility that more than one event occurred
during each of the four discrete intervals of uplift defined in this study cannot be ruled out, I
favors the single event scenario. Fold growth during quasi-continuous fault creep is mled out
by the episodic nature of the uplift events.
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The size of past earthquake events suggests that most slip on the PHT occurs during
moderate to large (M 7.1-7.4) earthquakes. These earthquakes are larger than predicted by
the fault plane area of the PHT as estimated by Shaw et al. (in press), utilizing regressions of
moment magnitude to rupture area by Wells and Coppersmith (1994). Their estimates of 370
km2 , 260 km2 , and 380 km2 for the Los Angeles, Santa Fe Springs and Coyote Hills
segments, respectively, were calculated on the basis of length, dip and depth estimates for
these segments as observed on industry reflection data. Shaw et al. (in press) show the Los
Angeles segment extending westward only to the westernmost limit of the seismic reflection
data that they examined, recognizing that the fault may extend farther to the west. Dolan
(1998) suggested, on the basis of geomorphologic observations, that the Los Angeles
segment may, in fact, extend ~ 20 km to the west of the westernmost extent of this segment
as estimated by Shaw et al. (in press). If we assume that the same fault dip and seismogenic
thickness characterize the geomorphically defined western extension of the Los Angeles
segment of the PHT, the total fault plane area of this segment would be ~ 600 km. Again
employing regressions of M to rupture area, this extension of the Los Angeles segment
would sufficiently increase the total fault plane area to produce a M 7.1 earthquake. This
additional fault-plane area does not, however, provide enough fault plane area to generate an
event of M 7.4. Thus, as suggested by Shaw et al. (in press), it is possible that during large
slip events the PHT may rupture together with other faults, such as the south-dipping
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98
Montebello thrust fault, which branches from the PHT above the Santa Fe Springs segment.
The borehole data at Carfax Avenue thus support multi-segment and possibly multi-fault,
rupture scenarios for large events on the PHT.
We have determined late Pleistocene - Holocene slip rates at both the Carfax
Avenue and Trojan Way sites (Table 3). These short-term slip rates range from 0.95 - 2.1
mm/yr and are, on average, slightly faster than the long-term Quaternary slip rates of 0.44 -
1.7 mm/yr determined by Shaw et al. (in press). These results indicate that slip on the PHT
has been relatively constant since the onset of fold development in the early Quaternary, but
that slip may have accelerated slightly in the past 20-30 ka. The slip rate that we determined
on the Santa Fe Springs segment of 0.95 - 1.0 mm/yr is averaged over at least four
earthquake events that have occurred during in the past ~ 12,000 years. At the Trojan Way
site, the minimum slip rate is averaged over the past 25,000 years. These are robust slip rates
that provide a critical parameter for probabilistic seismic hazard assessment.
The range of slip rates determined in this study provides a means for comparing
geologically determined slip rates for one fault in the Los Angeles basin, the PHT, to the
geodetically determined shortening rates across the metropolitan region. The 0.95 to 2.1
mm/yr late Pleistocene-early Holocene thrust slip rates translate into a horizontal shortening
rate of 0.85 to 1.87 mm/yr along a north-south azimuth. This is 17 - 37% of the geodetically
determined rate of ~ 5 mm/yr (Walls et al., 1998; Argus et al., 1999). Bawden et al. (2001)
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estimate the azimuth of maximum horizontal shortening across the region as N 36° E. Along
this azimuth, the PHT accommodates 15 - 34% of the 4.4 mm/yr horizontal shortening rate
estimated by Bawden et al. (2001). The actual shortening rate above the PHT is probably
somewhat faster than the minimum estimates of 15% and 17%, since the slip rates calculated
for the Santa Fe Springs segment at Carfax Avenue may only represent 65% of the
maximum fault segment slip (Shaw et al., in press). These estimates of horizontal shortening
for the PHT demonstrate that there are other faults are contributing to contraction across the
basin and pose additional seismic threats to the Los Angeles metropolitan area. The Sierra
Madre fault is one such fault. This fault contributes 0.4 to 1.2 mm/yr of horizontal
shortening (Crook et al, 1987; Walls et al., 1998; Walls, 2001). Together, the PHT and Seirra
Madre account for ~ 25% to 60% of the horizontal shortening rate. Shaw et al. (in press)
suggests that the Montebello thrust may also contribute to contraction across the basin but
how much is not known.
Our successful delineation of the geometry and evolution of the of the growth triangle
in the near-surface, our determination of paleoearthquake ages, slip per event, and late
Pleistocene - early Holocene slip rates at the Carfax Avenue site proves that the multi
disciplinary methodology developed and tested in this study is suitable for determining these
critical data from active blind-thrust faults. By identifying the location of the growth triangle
and the active and growth axial surfaces in the sub-surface using high-resolution and
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100
industry seismic reflection imaging, borehole transects can be used to directly sample the
growth stratigaphy, and to collect datable material. This methodology is applicable to other
active blind-thrust faults that generate fault-related folds, and can be used to assess the
seismic hazards of these potentially dangerous faults in populated areas. The methodology
developed herein also reveals the geometric and stratigraphic details of the development of
the growth triangle over the latest Pleistocene - Holocene time, providing unprecedented
resolution of the kinematic evolution of fault-related folds.
Conclusion
The multi-disciplinary methodology developed in this study has proven to be a
successful means for extracting paleoseismologic information from the locus of deformation
on a fold related to slip on a deeply buried blind-thrust fault. We tested our methodology on
the active Puente Hills blind-thrust fault, which generated the 1987 M 6.0 Whittier Narrows
earthquake. The PHT lies directly beneath the heart of the densely populated Los Angeles
metropolitan area, making it one of the most potentially dangerous faults in the United
States. Previous investigations of the PHT and its related folds at the scale of industry
seismic reflection profiles have revealed the structural details of Quaternary fold growth and
fault slip. These data demonstrate the presence of discrete growth triangles on the forelimbs
and backlimbs of the folds related to the PHT that extend upward up to within 200 m of the
surface (the upper limit of most industry seismic reflection data). In this study, in order to
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101
bridge the gap between the top of the industry data at 200 m and the shallow sub-surface, we
first employed Mini-Sosie and hammer-source high-resolution seismic reflection imaging to
delineate the location and geometry of the growth triangle at depths of 10-200 m. Borehole
transects along the line of the seismic reflection profdes were then used to sample the
deformed sediments from 0-50 m depth, overlapping with the shallowest depths imaged in
the high-resolution reflection profiles. The boreholes allowed us to collect datable material
and to document in unprecedented detail the stratigraphic and structural relationships
associated with fold growth. This information allowed us to determine paleoearthquake ages,
slip per event data, and late Pleistocene - early Holocene slip rates for the Santa Fe Springs
segment of the PHT as well as a late Pleistocene - Holocene slip rate for the Coyote Hills
segment. The detailed borehole results demonstrate that the growth triangle extends upwards
to within a meter of the present-day ground surface in a localized, upward-narrowing zone of
deformation zone.
The Carfax Avenue borehole transect, along the Santa Fe Springs segment of the PHT,
reveals the geometric and stratigraphic details of incremental fold development at the tip of
an active growth triangle. By measuring changes in thickness of growth strata that onlap
now-buried paleo-fold scarps, the locations of both the anticlinal and synclinal axial surfaces
have been determined for each of the four discrete folding events discovered at Carfax
Avenue. These axial surfaces migrate towards the foreland (i.e. southward) with each
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subsequent uplift event, resulting in a dip panel that widens with depth and narrows upward
towards the most recent event. Bedding dip is acquired incrementally with each uplift event,
such that the oldest paleo-fold scarp dips more steeply than the overlying scarps. Our
borehole data suggest that both kink band migration and limb rotation mechanisms may be
responsible for fold growth at the time and length scale of four earthquake events (0-11 ka).
The four paleoearthquake events discovered at Carfax Avenue are temporally discrete,
and are separated from one another by periods of no detectable deformation (i.e. stratgraphic
intervals with no discemable growth). These observations demonstrate that uplift along the
active anticlinal axial surface is not caused by quasi-continuous fault-creep, but rather by
large, fold scarp-forming earthquakes of magnitudes greater than 7. The large size of these
earthquakes suggests that the four paleoearthquakes ruptured all three segments of the PHT.
Although we cannot rule out the possibility that the temporally discrete intervals of fold
growth that we identified could each record brief clusters of multiple, moderate-magnitude
earthquakes, we consider this possibility less likely than single, large magnitude events.
Uplift in the four documented paleoearthquakes accounts for essentially 100% of the total
fold growth during the past 11 ka. This observation indicates that if any other, smaller
earthquakes have occurred that they contributed little to the total uplift, and thus must have
been of relatively small magnitude. The 1987 M 6.0 Whittier Narrows earthquake, for
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103
example, caused maximum uplift of only a few centimeters (Hauksson and Stein, 1989),
well below our detection limit.
To the best of my knowledge, this is the first study in which specific and direct
paleoearthquake age and slip per event data have been determined from a blind-thrust fault.
These are critical parameters necessary for accurate seismic hazard analysis. We have
identified four paleoearthquake events at the Carfax Avenue site. Event V occurred between
10.2 - 11.5 ka and had an estimated moment magnitude range of M 7.1-7.3, with an
estimated ~ 2.6 m of slip on the PHT. Event W, the largest event found at this site, has a
preferred age range of 7.2 - 7.8 ka with an estimated size range of M 13-1.4, with an
estimated slip of ~ 4 m. Events X and Y were approximately the same size, M 7.0 - 7.1 with
an estimated slip of -1.8 m. Event X probably occurred - 1.5 - 5.0 ka and Event X may
have occurred as recently as 1769, when a moderate to large earthquake struck Los Angeles
(Toppozada et al, 1981; Ellsworth, 1990). Based on our magnitude estimate of Event X,
however, we suspect that is was considerably larger than the 1769 event. Thus, Event X is
probably prehistoric.
Slip rates generated during this study are critical for assessing the constancy of strain
accumulation across the Los Angeles region. The late Pleistocene - early Holocene slip rates
determined at both the Trojan Way and Carfax Avenue sites are similar to the long-term
average Quaternary (1.6 Ma to present) slip rates determined by Shaw et al. (in press), which
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suggests that the slip rates have been relatively constant since early Quaternary inception of
slip on the PHT. When our slip rates are compared with short-term, geodetically determined
horizontal shortening rates across the Los Angeles basin, the contraction accommodated by
the PHT represents ~ 15% - 37% of the total north-south to northeast-southwest contraction.
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35-134A
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Appendix A:
Key to Color and Texture Borehole Logs
110
Key to Color Log
nam e Hue Value Chrom a
Pateo liv eS Y 6 3 -4
0 liv e 5 Y 5 4 /3 -6
Olive g ray 5Y 5 -4 /2 ™ ,
Dark olive gray 5Y 3 /2 Key to Texture Log
W M Gray SY-2,5 6-5/1
H D ark g ray G ley 4 /N
^■oartgrji5Y,2i,!0yr4/i c o a rse g ra in e d sa n d
l H Very d a * gray s y , 2 ,5 ,lorn 3 /1 c o a rse to m e d iu m g ra in e d sa n d
V e ry d a rag ra y G iey 3 /N m e d iu m g ra in e d sa n d
U ght brow nish gray 25 y - i0y r 6 /2 m e d iu m to fin e g ra in e d sa n d
Light yellow ish brow n 25y-1 Oyr 6 / 3-4 fin e g ra in e d sa n d
Olive yellow 2.5y-5y 6 /6 -8 fin e to v ery fin e g ra in e d sa n d
Grayish brow n 2.5y-!0yr 5 /2 v ery fin e g ra in e d sa n d
L ig h to liv e b ro w n 2 ,5 y 5 /3 -6 v e r y f m e g ra in e d sa n d to silt
Dark grayish brow n 2.5Y-1 Oyr 4 / 2 ciay a n d sa n d
Olive brow n 2.5y 4 /3 -4
Very d a rk grayish brow n 25 y -1 0 y r 3/2 s l l t t 0 c a Y
D ark o liv e b ro w n 2 ,5 y 3 /3 c ' a y
I
" B la d s 5 y ,2 ,5 y ,2 i/1 -2
Black lOyr 2/1
Black G le y 2 i/N
Very d a rk brow n lO yr 2 /2
Pale brow n 10yr 6 /3
Brownish yellow 10yr 6 / 6-8
B row nlO yr 5 -4 /3
Brown 7 i y r 5-4/ 2-4
Yellowish brow n lOyr 5 /4 -8
Dark yellow ish brow n 10yr 4 -3 / 4-6
|D a r k b r o w n 1 0 y r 3 /3
I Dark bro w n 7 J y r 3 / 2
js ," < ,* | G reenish gray, Gley 5/5G
D ark g reen ishgrayG ley4-3/!0y,5gy,10gy,5g
G reenish black 2 i/1 0 y ig y ,1 0 g y ,5 g
}
}
Friable strata
Cohesive strata
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Carfax Avenue
Borehole 10/sp 479
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112
Carfax Avenue
Borehole 11/sp 437
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113
Carfax Avenue
Borehole 12/sp452
o.
c u
" O
3o
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114
Carfax Avenue
Borehole 13/sp 421
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Carfax Avenue
Borehole 14/sp 383
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Carfax Avenue
Borehole 15/sp 404
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Carfax Avenue
Borehole 16/sp462
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118
Carfax Avenue
Borehole! 7/sp 445
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Carfax Avenue
Borehole 18/sp 318
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Carfax Avenue
Borehole! 9/sp 185
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121
Carfax Avenue
Borehole 20/sp 260
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122
Carfax Avenue
Borehole 21/sp 493
and Borehole 22/sp 496 (right)
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Carfax Avenue
Borehole 23/sp 354
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Carfax Avenue
Borehole 24/sp 428
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Appendix B: Southern California Edison (SCE) Right-of-Way Boreholes
SCE 1
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126
SCE 2
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127
Q .
Q J
■ O
SCE 3
T
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128
SCE 4
10 T
i
a
C L )
" D
20
30 j
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129
SCE 5
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130
SCE 6
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131
SCE-7
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Appendix C: Trojan Way Borehole Data
TW-1
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134
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Asset Metadata

Creator Christofferson, Shari Ann (author)

Core Title Determination of paleoearthquake age and slip per event data, and Late Pleistocene-Holocene slip rates on a blind-thrust fault: Application of a new methodology to the Puente Hills thrust fault,...

School Graduate School

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)

Advisor Dolan, James (committee chair), [illegible] (committee member), Sammis, Charles G. (committee member)

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c16-297765

Unique identifier UC11336926

Identifier 1414835.pdf (filename),usctheses-c16-297765 (legacy record id)

Legacy Identifier 1414835.pdf

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Rights Christofferson, Shari Ann

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

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