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Characterizing the recent behavior of the Ventura blind thrust fault, Brookshire Avenue study site: implications for mutifault ruptures in the western Transverse Ranges of southern California
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Characterizing the recent behavior of the Ventura blind thrust fault, Brookshire Avenue study site: implications for mutifault ruptures in the western Transverse Ranges of southern California
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Content
CHARACTERIZING THE RECENT BEHAVIOR OF THE VENTURA BLIND THRUST
FAULT, BROOKSHIRE AVENUE STUDY SITE: IMPLICATIONS FOR MUTIFAULT
RUPTURES IN THE WESTERN TRANSVERSE RANGES OF SOUTHERN CALIFORNIA
By
Jessica Rose Grenader
A Thesis Presented to the
FACULTY OF THE USC GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
MASTER OF SCIENCE
(GEOLOGICAL SCIENCES)
August 2016
1
Table
of
Contents
Abstract
.......................................................................................................................................................
3
Introduction
.................................................................................................................................................
5
Regional
Geology
.........................................................................................................................................
6
Observations
..............................................................................................................................................
10
Brookshire
Avenue
Study
Area
..............................................................................................................
10
High-‐Resolution
Seismic
Reflection
Data
...............................................................................................
11
Borehole
Excavations
.............................................................................................................................
12
Stratigraphic
Observations
....................................................................................................................
13
Age
Control
............................................................................................................................................
14
IRSL
Dating
of
Sediment
.....................................................................................................................
14
Radiocarbon
Dating
...........................................................................................................................
16
Interpretations
...........................................................................................................................................
17
Folding
and
Sedimentary
Growth
..........................................................................................................
17
Uplift
Measurements
and
Fault
Displacement
Estimates
.....................................................................
18
Latest
Pleistocene-‐Holocene
Slip
Rate
of
the
Ventura
Fault
.................................................................
20
Earthquake
Ages
....................................................................................................................................
21
Estimates
of
Paleo-‐Earthquake
Magnitude
...........................................................................................
21
Discussion
..................................................................................................................................................
22
Implications
for
Seismic
Hazard
in
Southern
California
.........................................................................
29
Conclusions
................................................................................................................................................
31
References
.................................................................................................................................................
33
Figure
Captions
..........................................................................................................................................
39
Tables
..…………………………………………………………………………………………………………………………………………………43
Figures..…………………………………………………………………………………………………………………………………………………45
Supplementary
Figures
…..…………………………………………………………………………………………………………………….53
2
ABSTRACT
We use continuously cored borehole and cone penetrometer test (CPT) data, together
with high-resolution seismic reflection data collected and analyzed in companion studies
(Hubbard et al., 2014; McAuliffe et al., 2015), to study the evolution of young folds developing
above the eastern part of the Ventura fault, a major reverse fault in the western Transverse
Ranges. These new data allow us to identify four periods of stratigraphic growth that record 11
m, 4 m, 2 m, and 3.5 m of uplift that we attribute to slip on the eastern part of the Ventura fault
during five paleo-earthquakes (the largest growth phase is interpreted to record two
stratigraphically indistinguishable events). In total, our Brookshire Avenue study site documents
~21 m of uplift since 15.3 ka. This yields an uplift rate of ~1.4 mm/yr, and assuming a 50+ 5°
fault dip (Hubbard et al., 2014), an average five-event reverse-slip rate of 1.6-1.9 mm/yr. This
rate is considerably slower than the 4.4-6.9 mm/yr slip rate determined by Hubbard et al. (2014)
for the central part of the Ventura-Pitas Point fault system. Our study site is located within a
“soft” segment boundary in which slip is transferred eastward from the Ventura fault onto the
blind, southern San Cayetano fault, and the discrepancy between our eastern Ventura fault slip
rate and that determined for the central part of the system suggests that most Ventura fault slip
has been transferred eastward onto the southern San Cayetano fault at the Brookshire Avenue
site. Thus, our measured uplifts, along-fault displacements, slip rate, and estimated magnitudes
significantly underestimates that of the Ventura-Pitas Point fault system as a whole. Minimum
estimates of displacements in each paleo-earthquake at Brookshire Avenue are >2.4-8.5 m.
Paleo-earthquake magnitude estimates based on our per-earthquake displacements are M
w
= 7.3-
8.0. Such large displacements near the far eastern end of the Ventura fault strongly suggest that
these ruptures may have extended through the Ventura-Southern San Cayetano fault segment
boundary, and more generally, support the notion of the occurrence of large-magnitude events
3
with large rupture areas that extended across multiple faults in the western Transverse Ranges..
Although the recurrence intervals of the earthquakes we document are on the order of thousands,
rather than hundreds of years, the magnitudes generated by these large multi-fault ruptures have
the potential to rival those expected on the San Andreas fault system. It is therefore of critical
importance that the Ventura fault and the other structurally connected faults of the western
Transverse Ranges are accurately considered in future modeling efforts and regional seismic
hazard assessments.
4
INTRODUCTION
The 1994 M
w
=6.7 Northridge earthquake, one of the costliest natural disasters in U.S.
history, is just one of a number of recent earthquakes (e.g., 1999 M
w
=7.6 Chi-Chi, 2005 M
w
=7.5
Kashmir, 2008 M
w
=7.9 Wenchuan) that have called attention to large-magnitude thrust
earthquakes and the potential seismic risks they pose to cities worldwide. The 2008 M
w
=7.9
Wenchuan earthquake, in particular, highlighted the potential for multiple faults to link together
to generate very large, multi-segment thrust fault earthquakes. In southern California, the
likelihood of large multi-segment thrust fault earthquakes is not yet clearly understood.
However, the rapid uplift and horizontal shortening observed in the Transverse Ranges, together
with the presence of a > 200-km-long system of large, rapid slip-rate thrust faults, suggest that
this region could be susceptible to such earthquakes.
The Ventura fault forms the middle section of this >200-km-long network of east-west-
striking reverse and oblique-slip faults whose uplift formed the Transverse Ranges of Southern
California. Subsurface structural analysis shows that the Ventura fault, in addition to continuing
offshore as the Pitas Point fault, forming what is commonly referred to as the Ventura-Pitas Point
fault system, is structurally linked by a shared decollement below 7.5 km to other major regional
reverse faults, such as the San Cayetano, Red Mountain, and Lion faults (Hubbard et al., 2014).
This is supported by recent modeling of geodetic data which suggests that longer faults or a
series of connected fault surfaces do a better job of fitting current global positioning system
(GPS) rates than individual, mechanically separate faults (Marshall et al., 2013). In the
Transverse Ranges fault system, each fault represents a major seismic source in its own right, but
this fault interconectivity heightens the threat of large, multi-segment earthquakes.
Recent research efforts aimed at understanding the seismic hazards associated with the
Transverse Ranges thrust faults have focused on documenting the subsurface geometry,
5
paleoseismology, and slip rates of the many fault segments that comprise this system. McAuliffe
et al. (2015), for example, documented the occurrence of two large uplift events at their Day
Road study site on the eastern Ventura Fault that they interpret as the most recent event (MRE)
and the penultimate event for this section of the Ventura fault. This study aims to extend this
record of fault slip and paleo-earthquake occurrence further back in time.
In this paper, we use continuously cored boreholes and cone penetrometer test (CPT)
data, together with high-resolution seismic reflection data collected and analyzed in companion
studies (Hubbard et al., 2014; McAuliffe et al., 2015), to study the evolution of young folds
developing above the Ventura fault, a major reverse fault in the western Transverse Ranges.
These new data allow us to assess the geometry of buried fold scarps and identify periods of
stratigraphic growth that record discrete uplift events along the Ventura fault. We use these data
to determine the slip rate of Ventura fault, as well as the timing and displacements of Ventura
fault paleo-earthquakes. We discuss these results in light of their implications for assessing the
prospects for multi-segment ruptures in the western Transverse Ranges, and more generally for
seismic hazard in southern California.
REGIONAL
GEOLOGY
The Transverse Ranges of Southern California are formed by uplift along a >200-km-
long network of east-west-striking reverse and oblique-slip faults that run “transverse” to the
general northwest-southeast structural grain of coastal California. These reverse faults and
associated folds accommodate significant north-south shortening and form due to compressional
forces that have characterized the region since early Pliocene time (e.g., Luyendyk et al., 1985).
Deformation of Upper Pleistocene- to Holocene-aged alluvial fans and marine terraces,
combined with recent geodetic and seismologic data and structural modeling suggest that this
6
style of deformation is ongoing (e.g., Rockwell, 1988; Hubbard et al., 2014; Marshall et al.,
2013; McAuliffe et al., 2015).
Mid-Miocene clockwise rotation of the western Transverse Ranges block facilitated
opening of one of the deepest Pliocene-Pleistocene basins in the world, the Ventura Basin, which
is >10 km thick at its deepest point (Figure 1; Yeats, 1977, 1983; Huftile and Yeats, 1995). The
Ventura Basin is situated ~75 km northwest of Los Angeles and has undergone left-lateral
oblique, north-south shortening since the Pliocene (e.g., Hornafius et al., 1986; Jackson and
Molnar, 1990; Luyendyk, 1991). Shortening was likely accommodated by reverse faults of the
western Transverse Ranges that bound the basin, such as the Oak Ridge fault to the south and the
San Cayetano fault system and the Ventura-Pitas Point fault system to the north (Figure 1). The
Ventura Basin extends east-northeast for ~50 km from its widest point on the coast to its eastern
edge, where it narrows and the north-dipping San Cayetano fault overrides the south-dipping
Oak Ridge fault (Huftile and Yeats, 1995). The Ventura Avenue anticline, the fastest uplifting
region in southern California with an uplift rate of 5 mm/yr, extends along the northern side of
the western part of the basin (Rockwell, 1988; Hubbard et al., 2014). The Ventura fault underlies
the Ventura Avenue Anticline (VAA) and continues offshore as the Pitas Point fault (Sarna-
Wojcicki et al., 1976; Yeats, 1982; Yerkes and Lee, 1987; Yerkes et al., 1987; Dahlen, 1989;
Kamerling and Nicholson, 1995; Kamerling and Sorlien, 1999; Hubbard et al., 2014).
The city of Ventura is built atop the western edge of the Ventura Basin at the base of a
south-facing mountain front that marks the forelimb of the VAA (Figure 1). Upper Pleistocene to
Holocene alluvial fans deposited from drainages on the southern limb of the VAA make up the
foundation of the city (see supplementary figure S1). To the south of the range front, a prominent
south-facing topographic scarp runs east-west through Ventura, marking the surface expression
7
of the Ventura fault. The scarp extends for ~12 km onshore from the eastern end of the city,
where the mountain front steps ~2 km northward to the eastern edge of the active channel of the
Ventura River (Figure 1). To the west, slip transfers onto the offshore Pitas Point fault, and to the
east slip is transferred eastward onto the blind southern San Cayetano fault across a left-stepping,
en echelon segment boundary (Figure 1; Hubbard et al., 2014; McAuliffe et al., 2015).
The subsurface geometry of the Ventura fault and the depth to which it extends downdip
have been disputed (e.g., Sarna-Wojcicki and Yerkes, 1982; Yeats, 1982; Huftile and Yeats,
1995; Hubbard et al., 2014). One model proposes that the fault is rooted at ~300 m depth in the
syncline at the southern edge of the VAA (Yeats, 1982; Huftile and Yeats, 1995). An alternative
model suggests the Ventura fault extends to seismogenic depth beneath the VAA (Sarna-
Wojcicki et al., 1976; Sarna-Wojcicki and Yerkes, 1982; Hubbard et al., 2014). Work by
Hubbard et al. (2014) supports the latter interpretation, as does evidence for the occurrence of
large-magnitude paleo-earthquakes on the Ventura fault (McAuliffe et al., 2015). Specifically,
Hubbard et al. (2014) documented: (a) fault offset clearly imaged down to 2 km below sea level
(kmbsl); (b) a planar down-dip projection of the fault to ~5 kmbsl suggested by interpreted fault
intersections in deep petroleum wells; and (c) industry seismic-reflection images showing
interpreted fault offset increasing down-dip, rather than increasing up-dip as would be expected
for a shallowly rooted fault. Together, these observations suggest a breakthrough fault-
propagation fold model for the evolution of the VAA and the underlying Ventura fault (Hubbard
et al., 2014). This interpretation provides an explanation for the observation by Rockwell et al.
(1988) of accelerating Quaternary terrace uplift rates on the flanks of the VAA starting at ca. 30
ka. Breakthrough, which involves upward propagation of the fault tip to the near surface
(Hubbard et al., 2014), extended the tip line of the Ventura fault up-dip and to the south, where
8
the locus of active uplift is expressed at the surface by the prominent monoclinal fold scarp that
extends east-west across the city of Ventura (Figures 2 and 3). At our Brookshire Avenue study
site, the fault is buried by ~200 m of sediment, but further west where the fault is less deeply
buried strands have been directly observed by geologists in trenches and large-diameter bucket-
auger holes (Sarna-Wojcicki et al., 1976; Prentice and Powell, 1991).
There is no historical record of major earthquakes on the Ventura fault, but McAuliffe et
al. (2015) documented the occurrence of two large, stratigraphically discrete uplift events
marked by 4.5- to 6-m-high fold scarps at their Day Road study site just east of downtown
Ventura (Figures 1 and 2). They inferred that these two fold scarp-forming uplift events record
the most recent and penultimate events on this eastern section of the Ventura fault. Based on the
large uplifts they measured, they interpreted these folding events as evidence for large-
magnitude (M
w
=7.5-8.0) paleo-earthquakes. To the west of Ventura, along the Pitas Point part of
the Ventura-Pitas Point fault system, uplifted beach shorefaces and marine terraces at Pitas Point
record the occurrence of four large Holocene earthquakes, each of which caused 5-8 m of uplift
at the study site (Rockwell et al., in press 2016). Mapping of marine terrace elevations to the
west of Pitas Point indicates that uplift in each of these four events was ~7-8 m across the crest
of the VAA (Rockwell et al., in press 2016). The very large uplift events and consequent large
fault displacements documented by McAuliffe et al. (2015) and Rockwell et al. (in press 2016),
suggest that these were large-magnitude earthquakes that likely involved multi-segment ruptures
of large sections of the network of reverse and oblique-slip faults in the western Transverse
Ranges. Although the potential for these faults to link and rupture together has previously been
recognized (e.g., Dolan et al., 1995; Hubbard et al., 2014; McAuliffe et al., 2015), relatively little
9
is known about the ages, repeat times, and magnitudes of paleo-earthquakes generated by faults
within the western Transverse Ranges.
OBSERVATIONS
Brookshire
Avenue
Study
Area
We chose the Brookshire Avenue site for a second borehole and CPT transect (Figures 2
and 3), following the methodology of McAuliffe et al. (2015). Unlike the Day Road site 1.4 km
to the west, the Brookshire Avenue site does not receive active alluvial fan input because
sediment eroded off the VAA to the north bypasses the Brookshire Avenue site in drainages that
have incised by 4 to 8 m into the older alluvial fans that have been deposited across this stretch
of the fault trace. We inferred that the resulting slower sediment accumulation rate would allow
us to sample older strata than McAuliffe et al. (2015) within similar borehole/CPT depths and
thus document the longer-term history of fold growth and earthquake occurrence along the
eastern part of the Ventura Fault. Our analysis of recent deformation at the Brookshire Avenue
site also benefitted from the results of an earlier study conducted at a City of Ventura reservoir
site adjacent to Brookshire Avenue, in which a north-south trench and transect of three large-
diameter bucket-auger holes were excavated parallel only ~130 m east of our seismic reflection-
borehole-CPT transect (See Supplementary Figure S2; data from Geotechnical Consultants, Inc.
Job V77151 Report [Quick, 1981] as reported in Yeats [1986] and Sarna-Wojcicki et al. [1976]).
As shown in figure 5, these previous results helped us to constrain subsurface stratigraphic
relationships at our study site along the Brookshire Avenue transect.
10
High-‐Resolution
Seismic
Reflection
Data
We utilized a high-resolution seismic-reflection profile that was collected along
Brookshire Avenue as part of earlier companion studies (Hubbard et al., 2014; McAuliffe et al.,
2015) to characterize the geometry of strata that have been folded above the buried tip line of the
Ventura fault (Figure 4). The profile extended for 1.06 km along Brookshire Avenue, from its
intersection with Woodland Street in the south to its intersection with Kearny Street at the north
end of Brookshire Avenue (Figures 2 and 3; Supplemental Figure S7). The profile provides a
clear image to a depth of ~500 meters. Specifically, the profile reveals coupled anticlinal and
synclinal axial surfaces bounding a panel of south-dipping beds with sub-horizontal strata
flanking the dip panel. The active synclinal axial surface extends upward from the tip line of the
Ventura fault at ~200 mbsl to the surface. The south-dipping strata extend to the surface at the
south-facing fold scarp, which occurs ~500 m south of the topographic range front at this
location (Figures 3 & 4). This surface scarp defines the surface expression of deformation
associated with the most recent folding events on the underlying Ventura fault thrust ramp. Our
structural interpretation of this profile agrees with McAuliffe et al. (2015) and differs slightly
from that of Hubbard et al. (2014) in that we take advantage of the kinematic constraints
provided by our borehole analysis of the youngest folded geometry to refine our estimate of the
dip of the axial surface extending upward from near the tip line of the fault at ~200 m depth. This
interpretation fits the seismic-reflection data equally well, and it provides a kinematically
consistent interpretation of the relationship between the young folding and the tip line of the
fault.
11
Borehole
Excavations
To determine the details of recent fold growth above the Ventura fault tip line at
Brookshire Avenue, we drilled three, 23- to 34-m-deep, hollow-stem auger boreholes that
provided continuous, 8-cm-diameter cores along the section of the Brookshire Avenue transect
that crosses the prominent east-west-striking fold scarp directly above the central part of the
high-resolution seismic reflection profile (Figures 2, 3, and 5). The fold scarp at the Brookshire
Avenue site is ~115 m wide (north to south) and lies south of the intersection of Brookshire
Avenue and Loma Vista Road, and north of the intersection of Brookshire Avenue and Fremont
Street, with Fremont Street extending approximately along the base of the scarp at this site and to
the east (Figure 3 and Supplemental Figure S7). The continuous cores we collected facilitated
sampling for radiocarbon and luminescence dating, and allowed us to observe basic sediment
characteristics, including grain size, Munsell sediment color, and degree of soil development.
These sediment characteristics were used to identify and correlate the subsurface stratigraphy
between the three boreholes. We also acquired six, 15.5- to 31-m-deep CPTs, which provided
detailed measurements of grain-size changes and other sediment characteristics with depth (e.g.
Supplemental Figure S3). Inferred sediment characteristics based on CPT analysis compared
well with the sediment profiles gathered from adjacent cores, and allowed us to improve our
stratigraphic correlation along the transect (Figure 5).
The borehole-CPT transect at Brookshire Avenue extends a total of 275 m from the
northernmost borehole at 34.282000°N, 119.212533°E, which is located ~40 m north of the top
of the fold scarp, to the southernmost CPT at 34.279550°N, 119.212067°E, ~108 m south of the
base of the fold scarp (Figs. 2 and 3; Supplemental Figure S4). The data gap in the center of the
borehole-CPT transect (Figure 5) between CPT-13 and BA-2 is due to the drilling truck’s
inability to operate on the steep (15° slope) incline of the fold scarp at this part of the site. The
12
dip of of the folded strata in the gap between CPT-13 and BA-2 (Figure 5) is approximated using
the surface morphology of the fold scarp correlations of distinctive units to the north and south,
and the dip data from the 1981 Reservoir site trench, which were measured directly by geologists
lowered into the large-diameter bucket-auger boreholes (Yeats et al. 1997; Supplemental Figure
S2).
Stratigraphic
Observations
The Brookshire Avenue borehole and CPT data reveal a sequence of alternating silt and
fine- to coarse-grained sand beds that thicken southward across the surface fold scarp. A number
of distinctive strata can be correlated between the boreholes and CPTs across most of, or in some
cases, the entire length of the transect, as shown in figure 5. Laterally continuous sand units
greater than one meter thick are numbered by intervals of 10, from youngest to oldest (10, 20,
30, 40, 50A, 50B, 50C, and 60). Similarly, >1 m thick, fine-grained (silt and clay) units that can
be correlated across the transect are marked with odd numbers (5, 7, 25, 37, 45, 53, and 65).
The uppermost part of the section (ground surface down to unit 7) is almost entirely fine-
grained, consisting mostly of silt, silty sand, and sandy silt (Unit 5; Figure 6). An ~2 m thick
package of fine- to coarse-grained sand with gravels is interbedded within this section in
borehole BA-2, but the absence of similar coarse-grained beds within this interval in other
borehole and CPTs suggests that this likely represents a local channel that flowed along the base
of the scarp (Supplemental Figure S4). The bottom of the stratigraphic section that we sampled is
marked by a distinctive basal clay-rich layer (Unit 65) of unknown thickness (our boreholes on
the southern side did not expose the base of this unit), similar to that observed in the 1981
Reservoir site transect to the east. Between the thick upper fine-grained unit (Unit 5) and the
distinct basal clay layer (Unit 65), the section consists mostly of alternating fine- to coarse-
13
grained sand, silt, and thin clay beds, with local pebbles and gravels present in several layers.
Sand units are significantly coarser on the down-dropped, southern side of the fold (See BA-2
and BA-3 in Supplemental Figure S4). Units 20 and 30 are fine- to coarse-grained sands wtih
gravels that thicken notably downslope. In BA-2, these sands are the thickest sand intervals in
the entire transect. Three >1-m-thick sand units that are restricted to the southern, downthrown
side of the transect are labelled 50B, 50C, and 60. Two dark gray-brown to black, organic-rich
layers that we interpret as buried soils (7 & 37) can also be correlated across the transect. The
presence of these soils is evidence of hiatuses in sediment deposition, and the depths and relative
stratigraphic positions of these two paleosols compare well with two paleosols documented in
the reservoir transect just 130 m to the east, suggesting that these soils record laterally extensive
periods of slow to no sediment accumulation across the alluvial fan beneath Brookshire Avenue
and the Reservoir sites (Supplemental Figures S2 and S5).
Age
Control
To constrain the ages of the strata exposed in our boreholes, we collected five
radiocarbon and 19 infrared stimulated luminescence (IRSL) samples from the Brookshire
Avenue boreholes. We report the IRSL and calibrated radiocarbon ages in Table 1. Most
radiocarbon samples were reworked and evidence suggests that some IRSL samples were
transported without light exposure and therefore were deposited before being bleached.
IRSL
Dating
of
Sediment
The 19 luminescence samples were dated using the recently developed post-IR IRSL
225
dating approach for potassium feldspar (Buylaert et al., 2009, 2012; Thiel et al., 2011), modified
for single-grain application (Brown et al., 2014; Rhodes, 2015). Samples were collected at
various depths using 15 cm aluminum tubes that were placed inside the core drill, thus ensuring
that they were not exposed to light during sampling. Once filled with sediment and brought to
14
the surface, the samples were covered in foil and stored inside light-proof bags for added
shielding from light exposure. A complete description of the IRSL dating procedure is
documented in Brown et al. (2015).
Although some samples yielded stratigraphically plausible ages, in general the IRSL
dates did not work out as well at this site as they did at the Day Road study site of McAuliffe et
al. (2015). We do not yet fully understand why this is so. The Brookshire Avenue luminescence
ages range from Holocene to latest Pleistocene, with the youngest sample having an age of 4.54
+ 0.64 ka, and the oldest an age of 25.0 + 8.4 ka. Oddly, although all samples were processed at
the same time in the same way (Brown et al., 2015), in some samples only 2-8 grains yielded
results, while others yielded 50-80 results. It is as if the source is entirely different for these two
categories of samples, where one source contains feldspar that is sensitive and the other contains
insensitive feldspar. An alternative possibility is that some samples were transported and
deposited without light exposure, making them appear older than the strata from which they were
sampled, although we think this is unlikely to fully account for the confusing IRSL results. We
considered the possibility that the stratigraphic correlations across the transect as we interpreted
them (Figure 5) are incorrect, and that there were actually regional “islands” of material sticking
up locally on the southern side of the transect. This is unlikely, because our stratigraphic
correlations across the transect show similar subsurface geometries to neighboring transects from
previous studies (Supplemental Figures S5 and S6). For example, as noted above, the Brookshire
Avenue transect is only ~130 m to the west of the Reservoir trench and borehole transect, in
which the down-hole dips of the folded strata were measured directly (Supplemental Figure S2),
and ~1.4 km to the east of the Day Road borehole-CPT transect. The correspondence between
our interpretation and those revealed by the 1981 trench and bucket-auger holes, as well as
15
presence of what appear to be the same soil horizons at similar depths in both transects indicates
(Supplemental Figure S5) that this possibility is not correct. Lab error is also an unlikely
explanation for these problems, given that all samples were prepared identically in a short period
of time in the same laboratory. These IRSL age discrepancies are the subject of ongoing research
in order to determine why some samples yielded problematic ages.
Radiocarbon
Dating
We collected detrital charcoal from numerous different stratigraphic levels in cores BA-1
and BA-3. These samples were processed at the Keck Carbon Cycle Accelerator Mass
Spectrometry (AMS) facility at the University of California, Irvine. However, only five of the 23
radiocarbon samples submitted yielded recordable ages (Table 1; Figure 5). The other samples
were not measured, because either the samples were too small, or no organic material was left
after the standard acid-base-acid pretreatment. All the radiocarbon results in Table 1 have been
corrected for isotopic fractionation according to the conventions of Stuiver and Polach (1977),
with d14C values measured on prepared graphite using the AMS.
Two of the five radiocarbon samples that provided ages have very large uncertainties due
to small sample size (BA14-14C-02 and BA14-14C-22). Moreover, three of the five samples
appear to be reworked, because their ages are much older than those found deeper in the strata.
Specifically, samples BA14-14C-02, BA14-14C-03, and BA14-14C-22 are tens of thousands of
years older than underlying samples BA14-14C-04 and BA14-14C-042. Samples BA14-14C-04
and BA14-14C-042 were both collected from the same organic-rich clay bed at 34 m depth near
the base of BA-3, and their ages have very small uncertainties. The 14,799-15,284 calendric year
(cal. yr) before A.D. 2015 age of sample BA14-14C-042 and the 15,256-15,724 cal. yr before
A.D. 2015 age of sample BA14-14C-04 are within error of each other and provide a robust
16
maximum age for the oldest part of the stratigraphic sections we sampled at Brookshire Avenue
transect.
Interpretations
Folding
and
Sedimentary
Growth
Several observations reveal the details of recent folding beneath the Brookshire Avenue
study site. Most basically, as shown in figure 5, the tops of sedimentary units are deeper on the
southern end of the Brookshire Avenue transect than the northern end, and the structural relief of
sedimentary units increases with depth as a result of thickening of some of the strata on the
southern, down-dropped side. This southward sedimentary thickening (or “growth”) is restricted
to discrete stratigraphic intervals separated by sedimentary units with constant thickness across
the transect. When a fold scarp forms due to slip on an underlying blind thrust fault, in order to
return to the pre-earthquake fan gradient, sediments will preferentially deposit on the
downthrown side of the scarp, leading to growth of a thickened sedimentary package until the
pre-folded fan gradient is restored (e.g., Dolan et al., 2003; Leon et al., 2007; 2009). Whereas
cohesive silts and clays can drape an existing topographic slope (e.g., fold scarps), sands and
gravels will not. Instead, such coarse-grained deposits will buttress (i.e., onlap) existing
topography, or exhibit extreme thinning of these strata on the upthrown side. Thus, any sand unit
with constant thickness across the transect must have been deposited at the fan gradient in the
absence of a topographic scarp during a time of structural quiescence. In the Brookshire Avenue
transect, we find four intervals of sedimentary growth separated by three such sand units with
constant thickness across the transect (Units 10, 40, & 50A). As discussed below, we infer that
these four growth intervals record uplift during five folding events – the shallowest growth
interval likely records two events.
17
Uplift
Measurements
and
Fault
Displacement
Estimates
Using the inclined shear-restoration method of Novoa et al. (2000), we incrementally
restored (i.e., “unfolded”) the strata to more precisely determine the uplift history of the
Brookshire Avenue section (Figure 7). In this method, the tops of deformed units are sequentially
restored to the undeformed, assumed depositional geometry, from youngest to oldest, by
restoring all points parallel to the dip of the active axial surface of the fold forming above the
buried tipline of the Ventura fault. In this case, the undeformed, depositional geometry is
inferred to be the regional slope of the alluvial-fan surface (dashed black lines in Figures 5 and
7); the regional slope of the alluvial-fan surface was measured over a distance of ~500 m from
the northern end of the Brookshire Avenue borehole-CPT transect northward to near the base of
the mountain front. These reconstructions provide an incremental record of sediment growth and
the evolution of folding during stratigraphically discrete uplift events.
We restored a total of four event horizons. The three sand units of constant thickness
(Units 10, 40, and 50A) that we infer were deposited in the absence of a scarp provide the three
shallowest restoration horizons. Pinchout (i.e., onlap) of sand units 50B, 50C, and 60
demonstrates that a scarp existed at the site during deposition of these sands (i.e., after the
deposition of unit 65). We therefore use the top of unit 65 as a fourth restoration horizon based
on the inference that this horizon was deposited at the alluvial fan slope. Use of the top of unit 65
is not as well supported as the use of sands 10, 40, and 50A, because: (a) unit 65 is a clay; and
(b) we do not know whether unit 65 thickens southward, as we were unable to drill deep enough
at the southern end of the transect. Although we are unsure of the maximum depth extent of this
growth section, we can definitively say a scarp existed after deposition of unit 65 and that this
scarp was subsequently completely buried before the deposition of unit 50A.
18
Reconstructions of these four inferred event horizons (i.e., paleo-fan surfaces) are shown
in figure 7. The reconstructions do not take into account any erosion that may have occurred on
the northern upthrown side of the scarp, and therefore our uplift measurements (paleo-scarp
heights) are minima. Since its deposition, unit 10 has sustained 11 m of uplift (Figure 7), based
on measuring paleo-scarp height. This is similar to the ~11.2 m of combined uplift produced
during the most two most recent folding events (~6 m [0.2-0.8 ka]; ~5.2 m [4.0-4.8 ka]) at the
Day Road study site (McAuliffe et al., 2015). This suggests the shallowest growth interval at
Brookshire Avenue (i.e., the ground surface to unit 10), with 11 m of uplift, records the
combined uplift that was generated by both the MRE and the penultimate event seen at Day
Road (Figure 8).
Unit 40 is the next-shallowest sand unit below unit 10 that displays constant thickness
across the transect, indicating deposition at the fan gradient in the absence of a fold scarp. The
shear restoration indicates that in the interval between these two sands there is ~4 m of
stratigraphic growth. Major sand units 20 and 30 pinch out to the north, suggesting that they
were deposited against a south-facing paleo-fold scarp that existed at that time (Figures 5 and 7).
This indicates uplift in a third earthquake that occurred prior to deposition of unit 30 and after
deposition of unit 40.
The constant thickness of sand unit 50A across the transect, in combination with the ~2 m
of stratigraphic growth between unit 40 and unit 50A, suggests that somewhat smaller uplift
occurred during a fourth earthquake that occurred after deposition of 50A at the fan gradient and
prior to deposition of unit 40 at the fan gradient. Lastly, pinch-out of sand units 50B, 50C, and 60
demonstrates that a paleo-fold scarp at least 3.5 m high was generated by an earlier fifth
earthquake (Figures 5 and 7). In summary, these inclined shear reconstructions reveal minimum
19
measured uplifts of 11 m (likely two events), 4 m, 2 m, and 3.5 m for the four growth intervals at
this site (Figures 5 and 7).
Our uplift measurements can be used as a proxy to estimate fault slip at depth (e.g.,
Novoa et al., 2000; Dolan et al., 2003; Leon et al., 2007; 2009; McAuliffe et al., 2015). To
estimate reverse displacements on the underlying Ventura fault, we divide the scarp height (i.e.,
measured uplift) by the sine of the 50° ± 5° dip of the fault documented by Hubbard et al. (2014).
This yields thrust fault displacements of 13.4-15.6 m from the shear restoration of unit 10, 4.9-
5.7 m from the shear restoration of unit 40 (i.e., uplift recorded by the second growth interval
between unit 40 and the base of unit 10), 2.4-2.8 m from the shear restoration of unit 50A, and
4.3-5.0 m from the shear restoration of unit 65 (the range in displacement is due to the
uncertainty of fault dip). These displacement estimates are based on the conservative
assumptions that: (a) coseismic slip is constant on the fault ramp, rather than increasing with
depth, as is observed for the total slip that has accumulated over geologic time scales (Hubbard
et al., 2014); and (b) that all folding in the near-surface is accommodated within the zone of
folding that we document, and that none is consumed in distributed deformation outside of this
zone. These assumptions render our displacement measurements minima.
Latest
Pleistocene-‐Holocene
Slip
Rate
of
the
Ventura
Fault
The continuously cored boreholes, CPT data, and radiocarbon ages, together with high-
resolution seismic reflection data collected and analyzed in companion studies (Hubbard et al.,
2014; McAuliffe et al., 2015), allow us to determine the average latest Pleistocene-Holocene slip
rate for this eastern section of the Ventura fault. The rate is constrained by the ages of
radiocarbon samples BA14-14C-042 (14,799-15,284 cal. yr before A.D. 2015) and BA14-14C-
04 (15,256-15,724 cal. yr before A.D. 2015), which are within error of each other and provide a
20
maximum age of ca. 15.3 ka for the oldest part of the stratigraphic sections we sampled (Unit 65)
at Brookshire Avenue. As discussed above, the Brookshire Avenue transect records ~21 m of
uplift in five possible events that we attribute to slip on the Ventura fault during paleo-
earthquakes since ca. 15.3 ka. Together, these events produced a total of ~24-30 m of slip along
the eastern part of the Ventura fault, yielding a minimum average slip rate of 1.6-1.9 mm/yr
since 15.3 ka at Brookshire Avenue.
Earthquake
Ages
Determining the ages of paleo-earthquakes at Brookshire Avenue has proven to be
problematic because, as discussed above, three of the five radiocarbon samples we dated were
reworked, and the discrepancies among the IRSL ages are the subject of ongoing research. Once
we resolve why some luminescence samples yielded problematic ages, we hope to reanalyze key
samples near the potential paleo-folding event horizons in order to determine paleo-earthquake
ages and incremental slip rates for this section of the Ventura fault.
Estimates
of
Paleo-‐Earthquake
Magnitude
We use our estimates of displacement per event to determine a conservative range of
paleo-magnitudes, using published empirical equations based on global regressions that relate
earthquake magnitude, fault area, and average displacement (Wells and Coppersmith, 1994;
Biasi and Weldon, 2006). As previously mentioned, it is likely that the 13.4-15.6 m of slip
recorded in the shallowest growth interval (above Unit 10) represents two uplift events (as
recorded at Day Road) rather than one, but this cannot be confirmed or refuted at Brookshire
Avenue due to the lack of distinctive growth strata deposited across the scarp since the
deposition of unit 10 that could record the formation of any paleo-folding events. We therefore
do not calculate paleo-magnitude estimates for this growth interval, and instead show paleo-
21
magnitude estimates for the two most recent earthquakes documented at Day Road by McAuliffe
et al. (2015) in Table 2.
We assume that uplift measured in each of the three deeper growth intervals was
produced in single earthquakes, rather than multiple events, and make the simplifying
assumption that the calculated displacements at this site are representative of the average slip
during rupture of the Ventura-Pitas Point fault. Wells and Coppersmith’s (1994), regressions of
average displacement-versus M
w
for all events yields paleo-magnitude estimates of M
w
= 7.49-
7.55 (3
rd
event back), M
w
=7.25-7.30 (4
th
event back), and M
w
=7.45-7.50 (5
th
event back). Biasi
and Weldon’s (2006) regression of M
w
with average displacement yields paleo-magnitudes of
M
w
= 7.73-7.80 (Unit 40), M
w
=7.38-7.45 (Unit 50A), and M
w
=7.66-7.73 (Unit 65). These
estimates are summarized in Table 2.
DISCUSSION
Although the potential for the major reverse faults of the central and western Transverse
Ranges to link and rupture together in large-magnitude earthquakes has been recognized (e.g.,
Dolan et al., 1995; Hubbard et al., 2014; McAuliffe et al., 2015), relatively little is known about
the ages, repeat times, magnitudes, and spatio-temporal patterns of paleo-earthquakes generated
by faults within the Transverse Ranges. The few available data from the Ventura-Pitas Point and
adjacent faults, including those from this study, are summarized in Figure 8 and discussed
herein.
Several observations suggest that the MRE and the penultimate event recorded at Day
Road also ruptured through the Brookshire Avenue site. Unit 10, a fine-grained sand unit in the
Brookshire Avenue transect, maintains relatively constant thickness across the transect,
suggesting that it was deposited at the gently southward-sloping alluvial fan gradient during a
22
time of structural quiescence. As noted above, our shear restorations indicate that unit 10 has
sustained ~11 m of uplift since its deposition (Figure 7). This value is similar to the ~11.2 m of
combined uplift produced during the most recent folding event (~6 m; 0.2-0.8 ka) and the
penultimate event (~5.2 m; 4.0-4.8 ka) at the Day Road study site (McAuliffe et al., 2015). In
addition, one of our potentially more reliable luminescence samples from the Brookshire Avenue
transect yielded an age of 4.54 + 0.64 ka from unit 10, in agreement with the 4.0-4.8 ka age of
the penultimate event from the Day Road site measured by McAuliffe et al. (2015). This
luminescence age, combined with the close proximity of the two study sites (1.4 km) and the
similarity in the amount of uplift seen in the upper section of each site, strongly support our
inferred interpretation that the growth interval above unit 10 at the Brookshire Avenue site
records uplift in the same two most recent events (the MRE and the penultimate event) as
recorded at Day Road (Figure 8).
Following these two uplift events, the scarp was likely never completely buried nor the
fan’s gradient restored, as demonstrated by the current 8 m high fold scarp (Figure 5; measured
vertically from the northward and southward projections of the average far-field ground surface
slope). Based on the Day Road data, we infer that most of the Brookshire Avenue scarp (~6 m)
records uplift during the MRE. Restoration of the current ground surface by 6 m suggests that an
~2 m-tall scarp associated with the penultimate event likely still existed at Brookshire Avenue
when the MRE occurred. Given the likely several-thousand-year-long interval between the most
recent and penultimate events observed at Day Road (McAuliffe et al., 2015), the fact that the
scarp from event 2 was not completely buried at Brookshire Avenue prior to the MRE suggests
very slow sediment accumulation rates of only ~1 mm/yr on the upthrown, northern block,
relative to the pre-4.0-4.8 ka upthrown block sediment accumulation rate of ≥1.5-1.6 mm/yr (unit
23
10 to unit 65; 15.3 ka to 4.0-4.8 ka). Above unit 10 the sediments are exclusively very fine-
grained (clays and silts), in contrast to the deeper section, which comprises fine- to coarse-
grained sands interbedded with fine-grained silt and clay units (Figure 6). The much finer-
grained nature of the sediments above the unit 10 sand, together with the slower late Holocene
sediment accumulation rate, suggest that uplift in event 2 may have led to initial incision of the
young streams located <1 km east and west of the site. Specifically, incision of the streams
resulted in sediment bypassing that isolated the Brookshire Avenue site fan from alluvial fan
deposition. This sediment bypassing in the incised channels explains the slower sediment
accumulation rates and the absence of any significant sand or gravel deposition across the
Brookshire Avenue transect since unit 10 time. Unit 7 soil development likely occurred after this
incision event. Reduced sediment accumulation rates also likely account for the greater height of
the fold scarp at Brookshire Avenue (8 m) relative to scarp height at Day Road (6 m;
Supplemental Figure S6).
In addition to documenting sedimentary growth and fold scarp development that occurred
in what we infer to be the same most recent and penultimate events seen at Day Road, we
suggest the occurrence of at least three additional folding and uplift events, for a total of five
uplift events since ca. 15.3 ka at Brookshire Avenue. Together, these events produced a total of
~24-30 m of slip along the fault, which yields an average slip rate of 1.6-1.9 mm/yr since 15.3 ka
at Brookshire Avenue. At the Day Road site, the total of 12.8-15.8 m of slip along the fault in
the two most recent events yields a late Holocene slip rate of 2.7-4.0 mm/yr for the past 4.0-4.8
ka on the eastern Ventura fault. The Brookshire Avenue site recorded 13.4-15.6 m of slip in the
same two events, yielding a very similar mid to late Holocene slip rate of 2.8-4.2 mm/yr for the
24
past 4.0-4.8 ka [age constraint from Day Road site (McAuliffe et al., 2015)], or 2.6-4.4 mm/yr if
we use the 4.54 + 0.64 ka luminescence age from unit 10, which pre-dates the two uplift events.
Interestingly, our five-event slip rate is slower than the 2.8-4.2 mm/yr slip rate from just the two
most recent events at this site and Day Road.
Farther west, along the central part of the Ventura-Pitas Point fault system, Hubbard et al.
(2014) determined a much faster, longer-term fault slip rate of ~4.4–6.9 mm/yr for the past 30 ±
10 k.y. based on terrace uplift rates from west of Ventura and an updated interpretation of the
fault kinematics. This large discrepancy in slip rates likely largely reflects two factors: (1) the
Brookshire Avenue slip rate is a minimum, both because our displacement measurements are
minima and the 15.3 ka age from detrital charcoal samples at the base of unit 65 is a maximum;
and (2) the relative structural position of both the Brookshire Avenue and Day Road study sites
near the eastern end of the Ventura fault, within the “soft” segment boundary in which slip is
transferred eastward from the Ventura fault onto the blind, Southern San Cayetano fault suggests
that some slip on this part of the system is not manifest on the easternmost part of the Ventura
fault we study, but rather is accommodated either by distributed deformation within the transfer
zone, or more localized slip on faults to the north of the Brookshire Avenue and Day Road study
sites. The eastward-decreasing slip on the Ventura fault in this transfer zone is clearly manifest in
the landscape, as less than 2 km to the east of the Brookshire Avenue site the prominent fold
scarp associated with the Ventura fault disappears. East of this point Ventura fault slip is
transferred to the blind, southern San Cayetano fault (Figure 1).
The inference that the slip-rate discrepancy is predominantly a reflection of the eastward
transfer of slip onto the Southern San Cayetano fault is supported by the available paleo-
earthquake age and displacement data from the eastern Ventura fault sites and sites farther west.
25
Specifically, Hubbard et al. (2014) and Rockwell et al. (in press 2016) pointed out four paleo-
shorefaces, ~16.5 km to the west-northwest of Brookshire Avenue, that they argued were
uplifted 5–8 m in each of the four most recent Ventura-Pitas Point fault earthquakes. These uplift
amounts are significantly larger than the uplift amounts seen in each of the five most recent
events at Brookshire Avenue. These discrepancies in slip rate and uplift per event are likely due
to eastward-diminishing slip on the Ventura fault in the area of the Brookshire Avenue and Day
Road study sites corresponding with a major en echelon left step in the thrust system (Hubbard et
al., 2014; McAuliffe et al., 2015).
Moreover, the detailed mid- to late Holocene paleo-earthquake record generated by
McAuliffe et al. (2015) also supports this inference. As noted above, the Ventura-Pitas Point
fault at depth is likely a continuous fault (Hubbard et al., 2014), suggesting end-to-end rupture in
large-magnitude earthquakes is possible, and we can search for evidence of this in the
paleoseismologic record. McAuliffe et al. (2015) noted that although the age range of the MRE
at Day Road overlaps marginally with the age range of the MRE at Pitas Point (Figure 8), their
IRSL ages for the MRE at Day Road are essentially maxima, suggesting that the MRE at Day
Road could have occurred later than the MRE at Pitas Point. They suggest the alternative
possibility that the most recent events at the Brookshire Avenue and Day Road sites could
correlate with the large-displacement (>~5 m), post-A.D. 1660 eastern San Cayetano fault event
recorded at the Piru trench site of Dolan and Rockwell (2001) ~41 km east of Brookshire Avenue
(Possibility 1 in Figure 8). The very large (7.3–8.5 m) displacements observed for the MRE at
Day Road so close to the eastern end of the Ventura fault are certainly consistent with the notion
that the most recent rupture extended eastward onto the blind southern San Cayetano fault and
onto the eastern San Cayetano fault (Figure 8; McAuliffe et al., 2015).
26
However, although the IRSL age constraints at Day Road (McAuliffe et al., 2015) and
the radiocarbon ages for the MRE at Pitas Point (Rockwell et al., in review 2016) apparently
barely overlap, it seems extremely unlikely that the Pitas Point and Day Road sites do not record
the same MRE on the Ventura-Pitas Point fault, given the very large displacements observed at
both Day Road (7.0 -8.5 m) and west of Pitas Point (~9-16 m), and the fact that these sites are
only ~16 km apart along the same fault. If the radiocarbon results at Pitas Point accurately
constrain the age of the MRE at that site to 970-1210 cal. years A.D., then either: (a) the IRSL
ages at Day Road might be slightly too young; and/or (b) the ~1 m of pre-MRE alluvial fan strata
at Day Road that accumulated after the strata from which the ca. 800 year old IRSL ages were
determined must have been deposited very quickly; and/or (c) no measurable deposition has
occurred across the Day Road fault scarp since the MRE. Although we show both rupture
scenarios are shown in Figure 8, we think it is most likely that the MRE documented at Day
Road, and inferred at Brookshire Avenue, is likely the same event measured farther west at Pitas
Point (Possibility 2 in Figure 8).
The A.D. 970-1210 age range of the MRE at Pitas Point documented by Rockwell et al.
(in press 2016) does not overlap with the A.D. 1660-1780 age range of the MRE at the eastern
San Cayetano fault Piru trench site documented by Dolan and Rockwell, (2001), indicating that
the MREs at these two sites cannot be the same earthquake involving system-wide concurrent
rupture of the Ventura–Pitas Point, southern San Cayetano, and eastern San Cayetano faults. The
similarity in ages of the events at all four study sites, however, indicates that the entire Ventura-
Pitas Point- southern San Cayetano- eastern San Cayetano fault system had ruptured within a
brief period between ca. 900 and 200 years ago (Figure 8). Thus, these two earthquakes must
27
have occurred in the span of a few hundred years, and such an occurrence could indicate laterally
propagating sequences of very large-magnitude events along the Transverse Ranges fault system.
Comparison of the newly published earthquake ages at Pitas Point (Rockwell et al., in
press 2016) with those of McAuliffe et al. (2015) indicates that some ruptures either did not
extend as far east as the eastern Ventura fault, or that they bypassed both the Brookshire Avenue
and Day Road study sites as slip is transferred eastward in diffuse fashion onto the Southern San
Cayetano fault.. For example, although we suggest that the MRE observed at Day Road is likely
the same event as the MRE recorded at Pitas Point, the 2.07 ka age of the penultimate event at
Pitas Point occurred during a well-constrained period of structural quiescence at Day Road
(McAuliffe et al., 2015), indicating that this event did not leave a detectable stratigraphic record
of folding at the Day Road site. Similarly, although the Ventura-Pitas Point earthquake that
occurred ca. 4.4 ka is well recorded at both Day Road (4.0-4.8 ka) and Pitas Point (ca. 4.2-4.7
ka), the fourth event back at Pitas Point occurred during a period of structural quiescence at Day
Road (McAuliffe et al., 2015). Thus, only two of the four most recent events at Pitas Point
ruptured through the Brookshire Avenue and Day Road sites. These “missing” ruptures must
have either terminated to the west of the eastern Ventura fault, or propagated through the soft
segment boundary onto the blind southern San Cayetano fault without rupturing along the
eastern Ventura fault (Figure 8). The Rockwell et al. (in press 2016) ca. 4.2-4.7 ka age range for
the third event back at Pitas Point agrees well with the 4.0-4.8 ka age for the penultimate event at
both Day Road and Brookshire Avenue and it is possible that these two large uplift events
occurred in the same large-magnitude earthquake (Figure 8). Additional paleo-earthquake age
constraints at more sites along this fault system will be needed to resolve more fully the spatio-
temporal patterns of large-magnitude earthquakes generated by these faults.
28
Implications
for
Seismic
Hazard
in
Southern
California
Less than 2 km to the east of the Brookshire Avenue site, the prominent fold scarp
associated with the Ventura fault dies out as slip is transferred eastward onto the blind southern
San Cayetano fault (Hubbard et al., 2014; McAuliffe et al., 2015). In light of the fact that the
Brookshire Avenue site lies within a probable zone of distributed slip transfer at the “soft”
segment boundary between the eastern end of the Ventura fault and the western end of the
southern San Cayetano fault, the 1.55-1.95 mm/yr slip rate estimated at Brookshire Avenue
likely significantly underestimates the slip rate for the Ventura-Pitas Point fault system as a
whole. Likewise, the >2.4-8.5 m fault displacements recorded at Brookshire Avenue, which on
their own point to the occurrence of large-magnitude events with long rupture lengths, may
underestimate displacements that occur on other sections of the fault. This is supported by the
very large displacement events that are observed at the Pitas Point site ~16.5 km west-northwest
from Brookshire Avenue, each of which generated ~5-10 m of uplift (Rockwell, 2011; Hubbard
et al., 2014; Rockwell et al., in review 2016). Such uplifts would necessitate large (M
w
7.7–8.1)
earthquakes, likely involving rupture of multiple faults, such as the Ventura, Pitas Point,
southern San Cayetano, eastern San Cayetano, Red Mountain, and western Santa Barbara
Channel faults, and/or the deeper extents of these faults (Hubbard et al., 2014).
Based on currently available paleo-earthquake data, only two of the four uplift events
observed at Pitas Point could correlate with uplift events seen at Brookshire Avenue or Day
Road. This suggests that either the Ventura–Pitas Point fault system does not always rupture end-
to-end, or that Ventura fault ruptures propagate through the soft eastern segment boundary onto
the blind southern San Cayetano fault without rupturing along the eastern end of the Ventura
fault. It is also possible that the fault could rupture in smaller events at times between the larger-
magnitude multi-segment ruptures. This level of rupture complexity is consistent with the
29
apparent variations in latest Pleistocene and Holocene slip rates revealed by our study and that of
McAuliffe et al. (2015). Specifically, the late Holcoene, 2.80-4.24 mm/yr slip rate determined by
McAuliffe et al. (2015) for the most recent two events is significantly faster than the longer-term,
1.6-1.9 mm/yr latest Pleistocene five-event average slip rate we determine at Brookshire Avenue.
These data suggest that the Ventura fault slip rate is variable, reflecting either: (a) variations in
displacement per event; and/or (b) variations in earthquake timing. It is also possible that the
slower slip rate we determine at the Brookshire site 1.4 km east of the McAuliffe et al. (2015)
Day Road site reflects diminishing slip eastward as slip is progressively transferred onto the
Southern San Cayetano fault. Arguing against this possibility, however, is the fact that we
observe nearly identical uplift in the most recent growth interval at Brookshire to that observed
by McAuliffe et al. (2015) in their two most recent uplift events.
The data from the Brookshire Avenue (this study) and Day Road (McAuliffe et al., 2015)
sites along the Ventura fault, the Pitas Point site along the Pitas Point fault (Rockwell, 2011;
Hubbard et al., 2014; Rockwell et al., in review 2016), and the Piru trench site along the eastern
San Cayetano fault (Dolan and Rockwell, 2001) imply complicated patterns of large-magnitude
earthquake occurrence along the major reverse faults of the western Transverse Ranges (Figure
8). In addition, a number of these faults bound the deep Ventura basin, and rupture on any of
these faults could cause amplification of seismic waves significantly increasing ground motions
in the region. If such large ruptures continued onto connected offshore faults, there is the added
risk of tsunami generation, although the relatively shallow water depths traversed by the offshore
part of the Ventura-Pitas Point fault suggest that any such tsunami would likely involve
relatively small volumes of water (Hubbard et al., 2014; McAuliffe et al. 2015). Even though the
inter-event times of the earthquakes documented on the Ventura fault are on the order of
30
thousands of years, the apparently very large magnitudes of these potentially multi-segment
ruptures require that this fault system be properly accounted for in seismic hazard assessments
for southern California.
CONCLUSIONS
Continuously cored boreholes and cone penetrometer test (CPT) data, together with high-
resolution seismic reflection data collected and analyzed in companion studies (Hubbard et al.,
2014; McAuliffe et al., 2015), reveal the structural and stratigraphic evolution of young folds
developing above the Ventura fault, a major reverse fault in the western Transverse Ranges.
These new data from the Brookshire Avenue study site allow us to identify four periods of
stratigraphic growth that record 11 m (likely two events), 4 m, 2 m, and 3.5 m of uplift, which
we attribute to slip on the Ventura fault during five paleo-earthquakes. In total, the Brookshire
Avenue study data document ~21 m of uplift since 15.3 ka. This yields an uplift rate of ~1.4
mm/yr, an average five-event reverse-slip rate of 1.6-1.9 mm/yr (assuming a 50+ 5° dip for the
fault; Hubbard et al., 2014), and paleo-earthquake magnitude estimates of M
w
= 7.3-8.0. The
study location lies near the eastern end of the Ventura fault, within a soft-segment boundary in
which slip is transferred eastward from the Ventura fault onto the blind Southern San Cayetano
fault. Thus, our measured uplifts and estimated displacements, slip rates, and paleo-magnitudes
all may underestimate the behavior of the Ventura-Pitas Point fault system as a whole.
Comparisons of paleo-earthquakes records at the Day Road and Brookshire Avenue sites with
the record at Pitas Point from Rockwell et al. (in press 2016) reveals that two of the four most
recent events observed at Pitas Point did not extend through the eastern Ventura fault sites. This
could reflect either ruptures that terminated to the west of the eastern Ventura fault sites or that
propagated through the “soft” segment boundary at the eastern end of the Ventura fault and onto
31
the blind southern San Cayetano fault in diffuse fashion without leaving a detectable
stratigraphic and structural record. The large (>2.4 to ~8.5 m) minimum displacements we
estimate for the Ventura fault at the Brookshire Avenue study site add to a growing body of data
indicating that the Ventura fault has generated very large-magnitude (M
w
=7.5-8.0) earthquakes
during latest Pleistocene to Holocene time. Structural modeling (e.g., Hubbard et al., 2014)
confirms potential linkages between the major reverse faults of the western Transverse ranges
including the Ventura fault. Together these data suggest the likelihood of future multi-fault
ruptures that will encompass large sections of the western-central Transverse Ranges fault
system. The proximity of this large reverse-fault system to major population centers, including
the greater Los Angeles region, and the potential for basin amplification and tsunami generation
during ruptures extending offshore along the western parts of the system highlight the
importance of understanding the complex behavior of these faults for future modeling efforts and
regional seismic hazard assessments.
32
REFERENCES
Biasi, G.P., and Weldon II, R.J., 2006, Estimating surface rupture length and magnitude of
paleoearthquakes from point measurements of rupture displacement: Seismological
Society of America Bulletin, v. 96(5), p. 1612-1623.
Bronk Ramsey, C. (2009). Bayesian analysis of radiocarbon dates. Radiocarbon, 51(1), 337-360.
Brown, N., Rhodes, E., Antinao, J., and McDonald, E., 2015, Single-grain post-IR IRSL signals
of K-feldspars from alluvial fan deposits in Baja California Sur, Mexico: Quaternary
International, v. 362, p. 132-138.
Buylaert, J.-P., Murray, A. S., Thomsen, K. J., and Jain, M., 2009, Testing the potential of an
elevated temperature IRSL signal from K-feldspar: Radiation Measurements, v. 44, no. 5,
p. 560-565.
Buylaert, J. P., Jain, M., Murray, A. S., Thomsen, K. J., Thiel, C., and Sohbati, R., 2012, A
robust feldspar luminescence dating method for Middle and Late Pleistocene sediments:
Boreas, v. 41, no. 3, p. 435-451.
Dahlen, M. Z., & Anonymous. (1989). Late Quaternary history of the Ventura mainland shelf,
California; implications for late Pleistocene sea levels. Abstracts with Programs -
Geological Society of America, 21(5), 71.
Dibblee, T.W., and Ehrenspeck, H.E., 1992, Geologic Map of the Saticoy Quadrangles, Ventura
County, California: Dibblee Geological Foundation Map DF-42, scale 1:24,000.
Dolan, J. F., Sieh, K., Rockwell, T. K., and Yeats, R. S., 1995, Prospects for larger or more
frequent earthquakes in the Los Angeles metropolitan region: Science, v. 267, no. 5195,
p. 199.
Dolan, J.F., and T. Rockwell, T.K., 2001, Paleoseismic evidence for a very large (Mw >7), post-
A.D. 1660 surface rupture on the eastern San Cayetano Fault, Ventura County,
33
California; was this the elusive source of the damaging 21 December 1812 earthquake?:
Seismological Society of America Bulletin, v. 91(6), p. 1417-1432.
Dolan, J.F., Bowman, D.D., and Sammis, C.G., 2003, Paleoseismologic evidence for long term
and long range elastic interactions: Proceedings and Abstracts of the 2003 Southern
California Earthquake Center annual meeting, v. XIII, 49
Hornafius, J. S., Luyendyk, B. P., Terres, R., and Kamerling, M., 1986, Timing and extent of
Neogene tectonic rotation in the western Transverse Ranges, California: Geological
Society of America Bulletin, v. 97, no. 12, p. 1476-1487.
Hubbard, J., Shaw, J. H., Dolan, J., Pratt, T. L., McAuliffe, L., and Rockwell, T. K., 2014,
Structure and Seismic Hazard of the Ventura Avenue Anticline and Ventura Fault,
California: Prospect for Large, Multisegment Ruptures in the Western Transverse
Ranges: Bulletin of the Seismological Society of America, v. 104, no. 3, p. 1070-1087.
Huftile, G. J., and Yeats, R. S., 1995, Convergence rates across a displacement transfer zone in
the western Transverse Ranges, Ventura Basin, California: Journal of Geophysical
Research, v. 100, no. B2, p. 2043-2067.
Jackson, J., and Molnar, P., 1990, Active faulting and block rotations in the western Transverse
Ranges, California: Journal of Geophysical Research: Solid Earth, v. 95, no. B13, p.
22073-22087.
Kamerling, M., & Nicholson, C. (1996). The Oak Ridge fault and fold system, eastern Santa
Barbara Channel, California: Southern California Earthquake Center Annual Report, v.
11, p. C26-C30.
Kamerling, M., & Sorlien, C. (1999). Quaternary slip and geometry of the Red Mountain and
Pitas Point-North Channel faults, California. Eos. Trans. AGU, 80(46), 1003.
34
Leon, L. A., Christofferson, S. A., Dolan, J. F., Shaw, J. H., & Pratt, T. L. (2007). Earthquake-‐
by-‐earthquake fold growth above the Puente Hills blind thrust fault, Los Angeles,
California: Implications for fold kinematics and seismic hazard. Journal of Geophysical
Research: Solid Earth, 112(B3).
Leon, L. A., Dolan, J. F., Shaw, J. H., & Pratt, T. L. (2009). Evidence for large Holocene
earthquakes on the Compton thrust fault, Los Angeles, California. Journal of Geophysical
Research: Solid Earth, 114(B12).
Luyendyk, B. P., 1991, A model for Neogene crustal rotations, transtension, and transpression in
southern California: Geological Society of America Bulletin, v. 103, no. 11, p. 1528-
1536.
Luyendyk, B. P., Kamerling, M. J., Terres, R. R., and Hornafius, J. S., 1985, Simple shear of
southern California during Neogene time suggested by paleomagnetic declinations:
Journal of Geophysical Research: Solid Earth, v. 90, no. B14, p. 12454-12466.
Marshall, S.T., Funning, G.J., and Owen, S.E., 2013, Fault slip rates and interseismic
deformation in the western Transverse Ranges, California: Journal of Geophysical
Research, v. 118, p. 4511–4534.
McAuliffe, L. J., Dolan, J. F., Rhodes, E. J., Hubbard, J., Shaw, J. H., and Pratt, T. L., 2015,
Paleoseismologic evidence for large-magnitude (Mw 7.5-8.0) earthquakes on the Ventura
blind thrust fault; implications for multifault ruptures in the Transverse Ranges of
Southern California: Geosphere (Boulder, CO), v. 11, no. 5, p. 1629-1650.
Perry, S.S., and Bryant, W.A., 2002, Fault number 91, Ventura fault, in Quaternary fault and fold
database of the United States. U.S. Geological Survey website: http:
//earthquakes.usgs.gov/regional/qfaults (accessed May 2011).
35
Peterson, M.D., and Wesnousky, S.G., 1994, Fault slip rates and earthquake histories for active
faults in southern California: Seismological Society of America Bulletin, v. 84(5), p.
1608–1649.
Prentice, C.D., and Powell, J.R., 1991, Ventura fault, in Blake, T.F., and Larson, R.A., eds.,
Engineering Geology along the Simi–Santa Rosa Fault System and Adjacent Areas, Simi
Valley to Camarillo, Ventura County, California: Southern California Section,
Association of Engineering Geologists, 1991 Annual Field Trip Guidebook, Volume 2, p.
288–295.
Reimer, P. J., Bard, E., Bayliss, A., Beck, J. W., Blackwell, P. G., Bronk Ramsey, C., Grootes, P.
M., Guilderson, T. P., Haflidason, H., Hajdas, I., Hatt , C., Heaton, T. J., Hoffmann, D.
L., Hogg, A. G., Hughen, K. A., Kaiser, K. F., Kromer, B., Manning, S. W., Niu, M.,
Reimer, R. W., Richards, D. A., Scott, E. M., Southon, J. R., Staff, R. A., Turney, C. S.
M., & van der Plicht, J. (2013). IntCal13 and Marine13 Radiocarbon Age Calibration
Curves 0-50,000 Years cal BP. Radiocarbon, 55(4).
Rhodes, E. J., 2015, Dating sediments using potassium feldspar single-grain IRSL: Initial
methodological considerations: Quaternary International, v. 362, p. 14-22.
Rockwell, T. K., 1988, Neotectonics of the San Cayetano Fault, Transverse Ranges, California:
Geological Society of America Bulletin, v. 100, no. 4, p. 500-513.
Rockwell, T., 2011, Large Co-Seismic Uplift of Coastal Terraces Across the Ventura Avenue
Anticline: Implications for the Size of Earthquakes and the Potential for Tsunami
Generation, 2011 SCEC annual meeting Plenary Session III
Rockwell, T.K., Clark, K., Gamble, L., Oskin, M.E., Haaker, E.C., and Kennedy, G.L., accepted
for publication, Large Transverse Ranges Earthquakes Cause Coastal Upheaval Near
36
Ventura, Southern California: submitted to Bulletin of the Seismological Society of
America
Sarna-Wojcicki, A. M., Williams, K. M., and Yerkes, R. F., 1976, Geology of the Ventura Fault,
Ventura County, California, U. S. Geological Survey, Reston, VA.
Sarna-Wojcicki, A. M., and Yerkes, R. F., 1982, Comment on article by R. S. Yeats on "Low-
shake faults of the Ventura Basin, California", Geol. Soc. Am.
Stuiver, M., and Polach, H. A., 1977, Discussion; reporting of C-14 data: Radiocarbon, v. 19, no.
3, p. 355-363.
Thiel, C., Buylaert, J.-P., Murray, A., Terhorst, B., Hofer, I., Tsukamoto, S., and Frechen, M.,
2011, Luminescence dating of the Stratzing loess profile (Austria)–Testing the potential
of an elevated temperature post-IR IRSL protocol: Quaternary International, v. 234, no.
1, p. 23-31.
Wells, D. L., and K. J. Coppersmith, 1994, New empirical relationships among magnitude,
rupture length, rupture width, rupture area, and surface displacement: Seismological
Society of America Bulletin, v. 84, p. 974-1002.
Yeats, R. S., 1977, High rates of vertical crustal movement near Ventura, California: Science, v.
196, no. 4287, p. 295-298.
Yeats, R. S., 1982, Low-shake faults of the Ventura basin, California: Geological society of
America, 78th Cordilleran Section Annual Meeting, no. Guidebook, p. 3-15.
Yeats, R. S., 1983, Large-scale Quaternary detachments in Ventura Basin, Southern California:
Journal of Geophysical Research, v. 88, no. NB1, p. 569-583.
37
Yerkes, R.F., and Lee, W.H.K., 1987, Late Quaternary deformation in the western Transverse
Ranges, in Recent Reverse Faulting in the Transverse Ranges: U.S. Geological Survey
Professional Paper 1339, p. 71-82.
Yerkes, R., Sarna-Wojcicki, A., & Lajoie, K. (1987). Geology and Quaternary deformation of
the Ventura area. Recent reverse faulting in the Transverse Ranges, California. US
Geological Survey Professional Paper, 1139, 169-178.
38
FIGURE
CAPTIONS
Figure 1. Location map of the western Transverse Ranges adapted from McAuliffe et al. (2015)
and Hubbard et al. (2014). The principal faults and folds in the region are shown in red (open
teeth indicate a blind thrust). Choice cities are marked by green circles. Grey shading designates
the surface extent of the Ventura Basin. Two possible surface projections of the Southern San
Cayetano fault are marked with dashed lines (preferred in red and the other blue). The Pitas Point
fault is the offshore continuation of the Ventura fault. Black lines mark high-resolution seismic-
reflection profiles from the Brookshire Avenue (BA) profile (discussed in text) and the Day
Road (DR) profile. Black inset box marks location of geologic map (Supplemental Figure S1).
Yellow stars show location of the Pitas Point (PP) site of Rockwell et al. (2016 in review) on the
Pitas Point fault and the Piru trench site (PTS) of Dolan and Rockwell (2001) on the eastern San
Cayetano fault.
Figure 2. Oblique aerial northward-looking view of the Brookshire Avenue borehole-CPT
transect (yellow line) and site of the high-resolution seismic reflection profile in Figure 4 (blue
line). The GoogleEarth base image is 3x vertically exaggerated. The Day Road borehole-CPT
transect from McAuliffe et al. (2015) is marked by the red line and the green line shows their
high-resolution seismic profile. The east-west fold scarp developing above the Ventura blind
thrust fault is shown by orange shading. Purple shading shows the surface extent of Holocene
alluvial fan deposits.
Figure 3. Westward-looking perspective of Brookshire Avenue high-resolution seismic-
reflection profile (red line; see figure 4), continuously cored borehole (green ovals), and cone
penetration test (CPT; yellow ovals) locations along the Brookshire Avenue transect. The black
thick line indicates the location of the 1981 Reservoir Site trench. The GoogleEarth base image
39
is 3X vertically exaggerated to highlight the south-facing fold scarp (orange shading) developing
above the Ventura fault.
Figure 4. Brookshire Avenue high-resolution seismic reflection profile. Adapted from Hubbard
et al. (2014) and McAuliffe et al. (2015). (A) Elevation profile with location and depth of
continuously cored boreholes (green) and cone penetrometer tests (orange) marked by vertical
lines. (B) Interpreted seismic profile. Solid red line marks the Ventura fault buried ~200 m below
ground surface. Dashed green line shows active synclinal axial surface. Dashed red line shows
active anticlinal axial surface. Yellow lines show several traced seismic reflections with
downward-increasing displacements. Stratigraphic units noted along the right side of the image
are based on projections of the surface geological mapping of Dibblee and Ehrenspeck (1992).
No vertical exaggeration. (C) Uninterpreted seismic profile.
Figure 5. Stratigraphic interpretations from the Brookshire Avenue study site and the 1981
Reservoir trench site (discussed in text) both shown at the same scale with 3X vertical
exaggeration. (A) Stratigraphic interpretations from borehole and cone penetration test (CPT)
data along the Brookshire Avenue transect. Vertical lines mark location and depth of
continuously cored boreholes and CPTs. Major units are labelled (discussed in text). Growth and
no-growth intervals are indicated on the right of the cross-section with black and red arrows,
respectively. Dotted black line shows the projected far-field surface slopes. Red circles and black
circles denote locations of samples for luminescence (IRSL) and 14C dating, respectively. All
IRSL ages (ka) are shown in red and all 14C ages (calibrated years before A.D. 2015) are showin
in black. To view this transect with detailed borehole and CPT logs, see Supplemental Figure S4.
(B) A simplified drawing of 1981 Reservoir trench site (full figure in Supplemental Figure S2).
Supplementary figure S5 shows Fig. 5B superimposed on Fig. 5A.
40
Figure 6. Stratigraphic column and sample ages centered on borehole BA-3. All age data are
projected onto borehole BA-3 along nearest stratigraphic contacts. Black lines are prominent
paleosols that can be correlated across the transect (units 7 and 37), and dark grey units are
prominent sands and gravels (e.g. 10, 20, 30, etc.) that can be correlated between boreholes. Pale
gray units are fine-grained (clays and silts; units 5, 25, 45, 53, 65) that are correlative between
boreholes. White areas are undifferentiated units of variable grain size. Luminescence (ka) and
radiocarbon (calibrated years before A.D. 2015) ages and sample names shown along right side.
Radiocarbon sample ages calibrated in OxCal 4.2 (Bronk Ramsey, 2009; Reimer et al., 2013).
Figure 7. Incremental record of sediment growth and uplift history at the Brookshire Avenue site
during latest Pleistocene to Holocene time. Made using the inclined shear-restoration method of
Novoa et al. (2000). Stratigraphic units not present at each time step are not shown in each
image. (1a, 2a, 3a, & 4a) Restorations of the four inferred event horizons, units 10, 40, 50A, &
65, at Brookshire Avenue to their undeformed, depositional geometry (the far-field alluvial-fan
slope; thin dashed black lines) by “unfolding” the paleo–fold scarp associated with each uplift
event. (1b, 2b, 3b, & 4b) Uplift measurements based on paleo-fold scarp height. (1a) Deposition
of growth strata, units 20 through 37, and deposition of sand unit 10 in the absence of a surface
scarp. (1b) Uplift and folding of units 10 through 65 by 11 m after the deposition of unit 10
during the most recent event(s); Uppermost 5 to 9 m of section, present-day ground surface, and
8-m-high scarp not shown (see Figure 5). (2a) Deposition of growth strata, unit 45, and
deposition of sand unit 40 in the absence of a surface scarp. (2b) Uplift and folding of units 40
through 65 by 4 m during the third event back. (3a) Deposition of growth strata, units 50B
through 60, and deposition of sand unit 50A in the absence of a surface scarp. (3b) Uplift and
folding of units 50A through 65 by 2 m during the fourth event back. (4a) Deposition of unit 65;
41
thickness of unit 65 is unknown on the southern side of the transect. (4b) Uplift and folding of
unit 65 by 3.5 m during the fifth event back.
Figure 8. Adapted from McAuliffe et al. (2015). Compilation of available paleoseismologic data
from the Ventura, Pitas Point, and San Cayetano faults. Green bars indicate timing of the four
most recent uplift events observed at Pitas Point above the Pitas Point fault near the locus of
maximum uplift along the Ventura Avenue anticline (Rockwell et al., in review 2016). Red bars
indicate timing of events observed at Day Road (McAuliffe et al. 2015). Purple bars indicate
timing of events at Brookshire Avenue (this study). Blue bar indicates timing of most recent
surface faulting at the Piru trench site of Dolan and Rockwell (2001) along the eastern San
Cayetano fault. Dashed black lines indicate possible rupture scenarios discussed in text.
42
TABLE 1A. INFRARED STIMULATED LUMINESCENCE (IRSL) AGES AND CALIBRATED, CALENDRIC DATES OF SAMPLES FROM THE BROOKSHIRE AVENUE TRANSECT
Sample Sample type
UCLA laboratory
code Transect Borehole
Actual
Depth
(m)
Projected
depth to
borehole
BA-3
(m) Unit
Equivalent
dose (Gy, 1σ
uncertainty)
Total dose
rate
(mGy/yr,
1σ
uncertainty)
Age (ka, 1σ
uncertainty)
BA-OSL-2/1 Luninescence J0950 Brookshire Avenue BA-2 4.72 6.55 5 34.0 + 3.0 4.13 + 0.25 7.88 + 0.79
BA-OSL-2/2 Luninescence J0951 Brookshire Avenue BA-2 7.77 8.84 5 20.0 + 2.7 4.26 + 0.26 4.54 + 0.64
BA-OSL-2/3 Luninescence J0952 Brookshire Avenue BA-2 10.82 11.28 - 20.6 + 2.9 4.13 + 0.25 4.90 + 0.72
BA-OSL-2/4 Luninescence J0953 Brookshire Avenue BA-2 13.87 15.24 30 27.0 + 1.7 4.07 + 0.25 6.69 + 0.61
BA-OSL-2/5 Luninescence J0954 Brookshire Avenue BA-2 16.92 18.68 37 43.7 + 2.6 3.99 + 0.24 11.3 + 1.1
BA-OSL-2/6 Luninescence J0955 Brookshire Avenue BA-2 19.96 21.64 - 89.9 + 8.0 3.97 + 0.24 24.2 + 3.0
BA-OSL-2/7 Luninescence J0956 Brookshire Avenue BA-2 23.01 25.00 50A 90.2 + 29.6 3.60 + 0.22 25.0 + 8.4
BA-OSL-3/1 Luninescence J0957 Brookshire Avenue BA-3 2.29 2.29 5 36.2 + 6.3 4.22 + 0.25 8.66 + 1.60
BA-OSL-3/2 Luninescence J0958 Brookshire Avenue BA-3 4.72 4.72 5 23.7 + 2.1 3.38 + 0.20 6.80 + 0.67
BA-OSL-3/3 Luninescence J0989 Brookshire Avenue BA-3 7.09 7.09 5 N/A N/A N/A
BA-OSL-3/4 Luninescence J0959 Brookshire Avenue BA-3 10.06 10.06 - 48.6 + 3.9 4.92 + 0.30 9.66 + 0.93
BA-OSL-3/5 Luninescence J0960 Brookshire Avenue BA-3 11.68 11.68 - 50.3 + 10.2 4.34 + 0.27 11.6 + 2.5
BA-OSL-3/6 Luninescence J0961 Brookshire Avenue BA-3 14.57 14.57 25 40.4 + 19.0 4.11 + 0.25 9.42 + 4.46
BA-OSL-3/7 Luninescence J0962 Brookshire Avenue BA-3 17.75 17.75 - 37.4 + 2.1 3.99 + 0.24 9.68 + 0.88
BA-OSL-3/8 Luninescence J0963 Brookshire Avenue BA-3 20.8 20.8 - 165.4 + 30.2 3.85 + 0.23 43.9 + 8.6
BA-OSL-3/9 Luninescence J0964 Brookshire Avenue BA-3 23.85 23.85 45 65.3 + 7.9 4.84 + 0.31 14.8 + 2.3
BA-OSL-3/10 Luninescence J0965 Brookshire Avenue BA-3 26.82 26.82 50C 11.8 + 1.2 4.16 + 0.25 24.6 + 6.2
BA-OSL-3/11 Luninescence J0966 Brookshire Avenue BA-3 29.93 29.93 - 11.8 + 1.2 4.06 + 0.25 117.2 + 46.6
BA-OSL-3/12 Luninescence J0967 Brookshire Avenue BA-3 34.59 34.59 65 84.8 + 32.8 4.11 + 0.25 22.8 + 9.1
TABLE 1B. RADIOCARBON AGES AND CALIBRATED, CALENDRIC DATES OF SAMPLES FROM THE BROOKSHIRE AVENUE TRANSECT
Sample
Sample
type Transect Borehole
Actual
Depth
(m)
Projected
depth to
borehole
BA-3 (m) Unit
Fraction
modern +
δ
14
C
(
o
/
oo
) ±
14
C age
(yr B.P.) ±
Oxcal
calibrated age
(yr before A.D.
2015)
BA14-14C-02
14
C Brookshire Avenue BA-3 20.42 20.42 40 0.0010 0.0009 -999.0 0.9 >47200
>47267
BA14-14C-03
14
C Brookshire Avenue BA-3 20.50 20.50 40 0.0104 0.0012 -989.6 1.2 36670 970 39478-42894
BA14-14C-04
14
C Brookshire Avenue BA-3 34.13 34.13 65 0.2006 0.0013 -799.4 1.3 12900 60 15256-15724
BA14-14C-22
14
C Brookshire Avenue BA-3 11.43 11.43 20 0.0041 0.0026 -995.9 2.6 >37600
>41832- 42257
BA14-14C-042
14
C Brookshire Avenue BA-3 34.13 34.13 65 0.2076 0.0013 -792.4 1.3 12630 55 14799-15284
43
Table 2. Uplift, Along-fault Displacement, Age Limits and Estimated Moment Magnitude (M
w
) for the Four to Five
Paleoearthquakes on the Ventura Fault from Brookshire Avenue Borehole and CPT Results
M
w
Slip (m)
Wells and
Coppersmith (1994)
All-slip-type
displacement
Biasi and Weldon
(2006)
Event
Age
(ka)
Uplift
(m) Min Max Min Max Min Max
1* <0.8 6 7.32 8.49
7.64 7.69
7.91 7.98
2* 4.0-4.8 5.2 6.35 7.35
7.59 7.64
7.84 7.91
3 - 4 4.88 5.66
7.49 7.55
7.73 7.80
4 - 2 2.44 2.83
7.25 7.30
7.38 7.45
5 15.3 3.5 4.27 4.95 7.45 7.50 7.66 7.73
Note: Table is based on the simplifying assumption that our measured displacements represent the average along-fault slip in each earthquake.
Uplift is based on measured values from Brookshire Avenue transect. Slip is based on a fault dipping 50° + 5°, providing minimum and
maximum slip vales. Biasi and Weldon (2006) incorporated the data of Wells and Coppersmith (1994).
* Uplift, along-fault displacement, age limits and estimated moment magnitude (Mw) for the two most recent events recorded at Day Road by
McAuliffe et al. (2015)
44
Figure 1. Location map of the western Transverse Ranges adapted from McAuliffe et al. (2015) and Hubbard et al. (2014). The principal faults and folds in the
region are shown in red (open teeth indicate a blind thrust). Choice cities are marked by green circles. Grey shading designates the surface extent of the
Ventura Basin. Two possible surface projections of the Southern San Cayetano fault are marked with dashed lines (preferred in red and the other blue). The
Pitas Point fault is the offshore continuation of the Ve ntura fault. Black lines mark high-resolution seismic-reflection profiles from the Brookshire Avenue (BA)
profile (discussed in text) and the Day Road (DR) profile. Black inset box marks location of geologic map (Supplemental Figure S1). Yellow stars show location
of the Pitas Point (PP) site of Rockwell et al. (2016 in review) on the Pitas Point fault and the Piru trench site (PTS) of Dolan and Rockwell (2001) on the eastern
San Cayetano fault.
D
U
km
map area
C A L I F O R N I A
3 4 ° N
34°1 0' N
34°2 0' N
34°3 0' N
119°50' W 1 19°40' W 1 19°30' W 119°20' W 1 19°10' W 119° W 118°50' W
BA DR
Fault
Thrust fault
Anticline
Seismic reflection profile
Ventura Basin
Red Mountain fault
Southern
San Cayetano fault
San Cayetano fault
Lion fault
Ventura Avenue
anticline
Pitas Point
fault
Ventura fault
Oak Ridge fault
PTS
PP
Ojai
Santa Barbara
Oxnard
Fillmore
Santa Paula
Ventura
Western Transverse Ranges
Ventura River
45
Figure 2. Oblique aerial
northward-looking view of
the Brookshire Avenue
borehole-CPT transect
(yellow line) and site of the
high-resolution seismic
reflection profile in Figure 4
(blue line). The GoogleEarth
base image is 3x vertically
exaggerated. The Day Road
borehole-CPT transect from
McAuliffe et al. (2015) is
marked by the red line and
the green line shows their
high-resolution seismic
profile. The east-west fold
scarp developing above the
Ventura blind thrust fault is
shown by orange shading.
Purple shading shows the
surface extent of Holocene
alluvial fan deposits.
Day Road high-resolution seismic reflection profile Ventura Fault scarp
Brookshire Avenue borehole/CPT area
Day Road borehole/CPT area
Arroyo Verde
Fan
Brookshire Avenue high-resolution seismic reflection profile
N
46
Borehole (green)/CPT (yellow) locations
1981 Reservoir Site trench
Day Rd. borehole and CPT locations (summer 2012)
Fold scarp
N
Brookshire Ave
Telegraph Rd
Figure 3. Westward-looking perspective of Brookshire Avenue high-resolution seismic-reflection profile (red line; see figure 4), c ontinu-
ously cored borehole (green ovals), and cone penetration test (CPT; yellow ovals) locations along the Brookshire Avenue transect. The
black thick line indicates the location of the 1981 Reservoir Site trench. The GoogleEarth base image is 3X vertically exaggerated to
highlight the south-facing fold scarp (orange shading) developing above the Ventura fault.
Fold Scarp
47
Figure 4. Brookshire Avenue high-resolution seismic reection prole.
Adapted from Hubbard et al. (2014) and McAulie et al. (2015). (A)
Elevation prole with location and depth of continuously cored
boreholes (green) and cone penetrometer tests (orange) marked by
vertical lines. (B) Interpreted seismic prole. Solid red line marks the
Ventura fault buried ~200 m below ground surface. Dashed green line
shows active synclinal axial surface. Dashed red line shows active
anticlinal axial surface. Yellow lines show several traced seismic
reections with downward-increasing displacements. Stratigraphic
units noted along the right side of the image are based on projections
of the surface geological mapping of Dibblee and Ehrenspeck (1992).
No vertical exaggeration. (C) Uninterpreted seismic prole.
120
80
Quaternary
alluvium
Quaternary
fan gravel &
fanglomerate
deposits
Saugus Fm.
& older
Figure 8.
along Brookshire road.
A1
S2
(b)
115 m
0
500
Depth (m) Depth (m)
0
500
(C)
(B)
(A)
Borehole (green)/CPT(orange) locations
5X vertical exaggeration
48
?
?
?
?
?
?
?
7.88 + 0.79
8.66 + 1.60
6.80 + 0.67
9.66 + 0.93
11.6 + 2.5
>41832- 42257
>47267
39478-42894
15256-15724
14799-15284
9.42 + 4.46
9.68 + 0.88
43.9 + 8.6
14.8 + 2.3
24.6 + 6.2
117.2 + 46.6
22.8 + 9.1
4.90 + 0.72
6.69 + 0.61
11.3 + 1.1
24.2 + 3.0
25.0 + 8.4
4.54 + 0.64
No Growth?
No Growth
No Growth
No Growth
Figure 5. Stratigraphic interpretations from the Brookshire Avenue study site and the 1981 Reservoir trench site (discussed in text) both shown at the same scale with 3X vertical exaggeration.
(A) Stratigraphic interpretations from borehole and cone penetration test (CPT) data along the Brookshire Avenue transect. Vertical lines mark location and depth of continuously cored bore-
holes and CPTs. Major units are labelled (discussed in text). Growth and no-growth intervals are indicated on the right of the cross-section with black and red arrows, respectively. Dotted black
line shows the projected far-field surface slopes. Red circles and black circles denote locations of samples for luminescence (I RSL) and 14C dating, respectively. All IRSL ages (ka) are shown in
red and all 14C ages (calibrated years before A.D. 2015) are showin in black. To view this transect with detailed borehole and CPT logs, see Supplemental Figure S4. (B) A simplified drawing of
1981 Reservoir trench site (full figure in Supplemental Figure S2). Supplementary figure S5 shows Fig. 5B superimposed on Fig. 5A.
0 100 m 200 m
0 meters 20
BA-1
CPT-12
CPT-13
BA-2
CPT-14
CPT-15
CPT-16 BA-3
CPT-17
10
20
5
7
37
50A
50B
50C
40
60
30
25
65
53
45
N
S
~8 m scarp
(B)
(A)
soil
sand
Paleosol
silt, clay layer
(stream channel)
sand
Paleosol
silt, clay layer
49
?
?
>41832- 42257 (BA14-14C-22)
>47267 (BA14-14C-02)
39478-42894 (BA14-14C-03)
15256-15724 (BA14-14C-04)
14799-15284 (BA14-14C-042)
8.66 + 1.60 (BA-OSL-3/1)
6.80 + 0.67 (BA-OSL-3/2)
BA-OSL-3/3)
7.88 + 0.79 (BA-OSL-2/1)
4.54 + 0.64 (BA-OSL-2/2)
4.90 + 0.72 (BA-OSL-2/3)
6.69 + 0.61 (BA-OSL-2/4)
11.3 + 1.1 (BA-OSL-2/5)
24.2 + 3.0 (BA-OSL-2/6)
25.0 + 8.4 (BA-OSL-2/7)
9.66 + 0.93 (BA-OSL-3/4)
11.6 + 2.5 (BA-OSL-3/5)
9.42 + 4.46 (BA-OSL-3/6)
9.68 + 0.88 (BA-OSL-3/7)
43.9 + 8.6 (BA-OSL-3/8)
14.8 + 2.3 (BA-OSL-3/9)
24.6 + 6.2 (BA-OSL-3/10)
117.2 + 46.6 (BA-OSL-3/11)
22.8 + 9.1 (BA-OSL-3/12)
GRAVELY SAND TO SAND
VERY STIFF SAND TO CLAYEY SAND
VERY STIFF FINE GRAINED
0
20 m
30 m
10 m
Figure 6. Stratigraphic column and sample ages
centered on borehole BA-3. All age data are projected
onto borehole BA-3 along nearest stratigraphic
contacts. Black lines are prominent paleosols that can
be correlated across the transect (units 7 and 37), and
dark grey units are prominent sands and gravels (e.g. 10,
20, 30, etc.) that can be correlated between boreholes.
Pale gray units are ne-grained (clays and silts; units 5,
25, 45, 53, 65) that are correlative between boreholes.
White areas are undierentiated units of variable grain
size. Luminescence (ka) and radiocarbon (calibrated
years before A.D. 2015) ages and sample names shown
along right side. Radiocarbon sample ages calibrated in
OxCal 4.2 (Bronk Ramsey, 2009; Reimer et al., 2013).
10
20
5
7
50A
50B
50C
40
60
30
37
25
65
53
45
50
sand
Paleosol
silt, clay layer
50A
50B
50C
60
65
53
50A
50B
50C
40
60
65
53
45
10
20
37
50A
50B
50C
40
60
30 25
65
53
45
0 100 m 200 m
0 meters 20
65
Figure 7. Incremental record of sediment growth and uplift history at the Brookshire Avenue site during latest
Pleistocene to Holocene time. Made using the inclined shear-restoration method of Novoa et al. (2000). Strati-
graphic units not present at each time step are not shown in each image. (1a, 2a, 3a, & 4a) Restorations of the four
inferred event horizons, units 10, 40, 50A, & 65, at Brookshire Avenue to their undeformed, depositional geometry
(the far-field alluvial-fan slope; thin dashed black lines) by “unfolding” the paleo–fold scarp associated with each
uplift event. (1b, 2b, 3b, & 4b) Uplift measurements based on paleo-fold scarp height. (1a) Deposition of growth
strata, units 20 through 37, and deposition of sand unit 10 in the absence of a surface scarp. (1b) Uplift and folding
of units 10 through 65 by 11 m after the deposition of unit 10 during the most recent event(s); Uppermost 5 to 9 m
of section, present-day ground surface, and 8-m-high scarp not shown (see Figure 5). (2a) Deposition of growth
strata, unit 45, and deposition of sand unit 40 in the absence of a surface scarp. (2b) Uplift and folding of units 40
through 65 by 4 m during the third event back. (3a) Deposition of growth strata, units 50B through 60, and deposi-
tion of sand unit 50A in the absence of a surface scarp. (3b) Uplift and folding of units 50A through 65 by 2 m
during the fourth event back. (4a) Deposition of unit 65; thickness of unit 65 is unknown on the southern side of
the transect. (4b) Uplift and folding of unit 65 by 3.5 m during the fifth event back.
?
?
?
~3.5 m uplift
65
?
?
?
* Thickness of unit 65 unknown on southern end of transect
~2 m uplift
~4 m uplift
50A
50B
50C
40
60
65
53
45
~11 m uplift
10
20
37
50A
50B
50C
40
60
30 25
65
53
45
(1b)
(2b)
(3b)
(4b)
(1a) Inclined shear-restoration of unit 10
(2a) Inclined shear-restoration of unit 40
(3a) Inclined shear-restoration of unit 50A
(4a) Inclined shear-restoration of unit 65
51
Distance west along strike from Pitas Point (km)
30 010 20 60 50 40
Year
3000
AD/BC
2000
1000
1000
2000
4000
5000
Piru trench site Pitas Point Brookshire Avenue Day Road
?
?
Possibility 1
Possibility 2
Fillmore
Santa Paula
Ventura
Southern
San Cayetano fault
San Cayetano fault
Ventura Avenue
anticline
Pitas Point
fault
Ventura fault
PTS
PP
BA DR
Figure 8. Adapted from McAuliffe et al. (2015). Compilation of available paleoseismologic data from the Ventura,
Pitas Point, and San Cayetano faults. Green bars indicate timing of the four most recent uplift events observed at
Pitas Point above the Pitas Point fault near the locus of maximum uplift along the Ventura Avenue anticline
(Rockwell et al., in review 2016). Red bars indicate timing of events observed at Day Road (McAuliffe et al. 2015).
Purple bars indicate timing of events at Brookshire Avenue (this study). Blue bar indicates timing of most recent
surface faulting at the Piru trench site of Dolan and Rockwell (2001) along the eastern San Cayetano fault. Dashed
black lines indicate possible rupture scenarios discussed in text.
km
map area
C A L I F O R N I A
34°1 0' N
34°2 0' N
119°20' W 119°10' W 119° W 118°5 0' W
52
CA
NV
image
area
AZ
San Andreas Fault
Ventura Avenue Anticline
Pleistocene bedrock (marine)
Late Pleistocene terrace deposit (nonmarine)
Late Pleistocene terrace deposit (marine)
Late Pleistocene old alluvial fan deposits Holocene alluvium
Holocene alluvial fan deposits
Late Pleistocene/Early Holocene older alluvium
Late Pleistocene/Early Holocene landslide deposit
Holocene active young alluvial fan deposits
City of Ventura
Artificial fill
Pacific Ocean High resolution seismic profile transect Holocene undifferentiated deposit- includes artificial fill, beach, & dune deposits
Borehole (green) / CPT (yellow) location
N
Fold scarp
2000 Meters 1000 500 0
1 1
2 2
3 3
Evergreen/Hall Canyon
Day Road
Brookshire Ave.
Arroyo Verde
Fan
Harmon Canyon Fan
Supplemental Figure S1. Geologic map of the Ventura area from McAuliffe et al. (2015) adapted from Sarna-Wojcicki et al. (1976)
53
200’
250’
350’
300’
325’
225’
275’
200’
250’
350’
300’
325’
225’
275’
0 10 20 30 40 50 FEET
SCALE
ALTITUDE
ALTITUDE
330 RESERVOIR SITE TRENCH
-
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1
Paleosol
Paleosol
clay layer
sand
sand
sand
sand
sand
sand, silt
soil
soil
dark sand, soil
Paleosol
Paleosol
Paleosol
Paleosol
silt
gravel, shell fragments
clay layer
clay layer
(stream channel)
Bottom of trench
Bottom of trench
Age: 15,200+850
14
C yrs.
silt
Age: >40,000
14
C yrs.
charcoal
No vertical exaggeration
Age: >40,000
14
C yrs.
charcoal
T.D. 95’
T.D. 96’
T.D. 103’
DH1
DH4
DH3
(Redrawn from Geotechnical Consultants Inc.,
Job V77151; Quick, 1981; Yeats et al., 1997)
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
54
55
?
?
?
?
?
No Growth?
No Growth
No Growth
No Growth
~8 m scarp
OSL SAMPLE BA3-OSL-1
ROCK OSL SAMPLE BA3-OSL-2
w/ white nodules OSL SAMPLE BA3-OSL-3
WELL DEVELOPED SOIL
OSL SAMPLE BA14-B3-4
OSL SAMPLE BA14-B3-5
Gravel 1-5 cm ARGILLIC HORIZON MOSTLY GRAVEL 1-9 CM OSL SAMPLE BA14-BA3-6
Pebbles 1-10 mm
ROCK
SOME PEBBLES 0.5-2 CM
OSL SAMPLE BA14-BA3-7 SOME PEBBLES 1-4 CM
4 0 * - SOME PEBBLES
MOSTLY PEBBLES
BA14-14C-02 BA14-14C-03
OSL SAMPLE BA14-BA3-8
OLD SOIL???? 1 " - & 0 4 0 - GRAVEL, SHELL
MOSTLY GRAVEL/ PEBBLES 0.5-3 CM
( 3 " 7 & - OSL SAMPLE BA14-BA3-9
OSL SAMPLE BA14-BA3-10
GRAVEL 1-4 CM
OSL SAMPLE BA14-BA3-11
OSL SAMPLE BA14-BA3-12
PEAT??
FILL
OSL SAMPLE BA14-BA3-9
OSL SAMPLE BA14-BA3-10
PURPLE-RED STUFF OSL SAMPLE BA14-BA3-11
OSL SAMPLE BA14-BA3-12
5/6 7.5YR
OSL SAMPLE BA3-OSL-1
ROCK OSL SAMPLE BA3-OSL-2
OSL SAMPLE BA14-B3-4
the old 35 ft
OSL SAMPLE BA14-B3-5
OSL SAMPLE BA14-BA3-6 with darker flecks
ROCK
OSL SAMPLE BA14-BA3-7
OSL SAMPLE BA14-BA3-8
OSL SAMPLE BA3-OSL-3
FILL
MOSTLY GRAVEL/ PEBBLES 0.5-3 CM
( 3 " 7 & - OSL SAMPLE BA14-BA3-9 OSL SAMPLE BA14-BA3-9
GRAVEL 1-4 CM
OSL SAMPLE BA14-BA3-11
PURPLE-RED STUFF OSL SAMPLE BA14-BA3-11
Gravels
Gravels
Gravels
Gravels
OSL SAMPLE
OSL SAMPLE
OSL SAMPLE
Rock
Rock pebbles and cobbles 1-3 cm
some FGS pebbles/cobbles 0.5-3.5 cm
OSL SAMPLE
Mostly pebbles 1-1.5 cm
OSL SAMPLE
OSL SAMPLE
Coarsens downward
OSL SAMPLE
Coarsening upward
Coarsening upward
Gravels w/ FGS matrix
Fining Upward
Fining Upward
pebs/cobs orange FGS 5/6 10YR
OSL SAMPLE
OSL SAMPLE
Rock
Rock
OSL SAMPLE
OSL SAMPLE
OSL SAMPLE 6
Dark black purple layer 3/2 10YR 5/4 7.5YR
OSL SAMPLE 7
Gravels w/ FGS matrix
?????
gravel
Gravels w/ some orange sand
5/6 7.5YR
some gravel
some gravel
Some gravels (6/5)/8 some orange SS
Mostly Gravel with some dark silt from above
gravel
no recovery
Mostly
gravel
BA14-14C-22 >37600 bp
BA14-14C-02 >47200 bp
BA14-14C-03 >36670 bp
BA14-14C-04 12900 bp
BA14-14C-042 12630 bp
Figure S4. Stratigraphic interpretations from borehole and
cone penetration test (CPT) data along the Brookshire Avenue
transect with detailed borehole and CPT logs. Borehole logs show
grain size and Munsel color identifications. Growth and no-growth
intervals are indicated on the right of the cross-section with black and red
arrows, respectively. Major units are labelled (discussed in text). Dotted black
line shows the projected far-field surface slope. Red circles and black circles
denote locations of samples collected for luminescence and 14C dating, respectively.
No recovery
Medium - coarse grained sand
Medium grained sand
Fine - medium grained sand
Fine grained sand
Very fine - fine grained sand
Very fine grained sand
Very fine grained sand - silt
Silt
Silt - very fine grained sand
Silt with some clay
Clay
COLOR CODE FOR GRAIN SIZE
Silt - fine grained sand
Fine grained sand - silt
Fine - coarse grained sand
Clay - very fine grained sand
Clay with some silt
ORGANIC MATERIAL
CLAY TO SILTY CLAY
CLAYEY SILT TO SILTY CLAY
CLEAN SAND TO SILTY SAND
GRAVELY SAND TO SAND
CPT LEGEND
SILTY SAND TO SANDY SILT
SENSITIVE FINE GRAINED
VERY STIFF SAND TO CLAYEY SAND
VERY STIFF FINE GRAINED
0 100 m 200 m
0 meters 20
BA-1
CPT-12
CPT-13
BA-2
CPT-14
CPT-15
CPT-16 BA-3
CPT-17
10
20
5
50A
50B
50C
40
60
30
25
65
7
37
53
45
N
S
56
?
?
?
?
?
?
?
No Growth?
No Growth
No Growth
No Growth
Figure S5. Stratigraphic interpretations from borehole
and cone penetration test (CPT) data along the Brook-
shire Avenue transect (grey) overlain by simplified strati-
graphic interpretations from the 1981 Reservoir trench site
(shown in red; full figure in Supplemental Figure S2). Red vertical
lines mark location and depth of large-diameter bucket-auger holes along
the reservoir site. Black vertical lines mark location and depth of continuously
cored boreholes and CPTs along the Brookshire Avenue transect. Growth and no-
growth intervals are indicated on the right of the cross-section with black and red arrows,
respectively. Major units are labelled (discussed in text). Dotted black line shows the projected far-
field surface slope.
0 100 m 200 m
0 meters 20
BA-1
CPT-12
CPT-13
BA-2
CPT-14
CPT-15
CPT-16 BA-3
CPT-17
10
20
5
7
37
50A
50B
50C
40
60
30
25
65
53
45
N
S
~8 m scarp
sand
Paleosol
silt, clay layer
(stream channel)
sand
Paleosol
silt, clay layer
57
Day Road Borehole-CPT Transect
Brookshire Avenue Borehole-CPT Transect
Supplemental Figure S6. Brookshire Avenue and Day
Road borehole-CPT transects shown at the same
scale. Note similarity in subsurface geometries.
58
Supplemental Figure S7. Google earth image of Brookshire Avenue study site with streets labeled.
59
Abstract (if available)
Abstract
We use continuously cored borehole and cone penetrometer test (CPT) data, together with high-resolution seismic reflection data collected and analyzed in companion studies (Hubbard et al., 2014
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Asset Metadata
Creator
Grenader, Jessica Rose
(author)
Core Title
Characterizing the recent behavior of the Ventura blind thrust fault, Brookshire Avenue study site: implications for mutifault ruptures in the western Transverse Ranges of southern California
School
College of Letters, Arts and Sciences
Degree
Master of Science
Degree Program
Geological Sciences
Publication Date
07/28/2016
Defense Date
06/27/2016
Publisher
University of Southern California
(original),
University of Southern California. Libraries
(digital)
Tag
anticline,blind thrust,Brookshire Avenue,multifault,OAI-PMH Harvest,paleoearthquake,paleoseismology,Pitas Point,Rocks,San Cayetano,seismic hazard,slip rate,Southern California,uplift rate,Ventura Avenue anticline,Ventura fault,western Transverse Ranges
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application/pdf
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Language
English
Contributor
Electronically uploaded by the author
(provenance)
Advisor
Dolan, James F. (
committee chair
), Sammis, Charles G. (
committee member
), West, A. Joshua (
committee member
)
Creator Email
grenader@usc.edu,jessica.grenader@gmail.com
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Tags
anticline
blind thrust
Brookshire Avenue
multifault
paleoearthquake
paleoseismology
Pitas Point
San Cayetano
seismic hazard
slip rate
uplift rate
Ventura Avenue anticline
Ventura fault
western Transverse Ranges