Bulletin of the Seismological Society of America, Vol. 92, No. 6, pp. 2504–2520, August 2002
Delineation of Faulting and Basin Geometry along a Seismic Reflection
Transect in Urbanized San Bernardino Valley, California
by W. J. Stephenson, J. K. Odum, R. A. Williams, and M. L. Anderson
Abstract Fourteen kilometers of continuous, shallow seismic reflection data acquired through the urbanized San Bernardino Valley, California, have revealed numerous faults between the San Jacinto and San Andreas faults as well as a complex
pattern of downdropped and uplifted blocks. These data also indicate that the Loma
Linda fault continues northeastward at least 4.5 km beyond its last mapped location
on the southern edge of the valley and to within at least 2 km of downtown San
Bernardino. Previously undetected faults within the valley northeast of the San Jacinto fault are also imaged, including the inferred western extension of the Banning
fault and several unnamed faults. The Rialto-Colton fault is interpreted southwest of
the San Jacinto fault. The seismic data image the top of the crystalline basement
complex across 70% of the profile length and show that the basement has an overall
dip of roughly 10⬚ southwest between Perris Hill and the San Jacinto fault. Gravity
and aeromagnetic data corroborate the interpreted location of the San Jacinto fault
and better constrain the basin depth along the seismic profile to be as deep as 1.7 km.
These data also corroborate other fault locations and the general dip of the basement
surface. At least 1.2 km of apparent vertical displacement on the basement is observed across the San Jacinto fault at the profile location. The basin geometry delineated by these data was used to generate modeled ground motions that show peak
horizontal amplifications of 2–3.5 above bedrock response in the 0.05- to 1.0-Hz
frequency band, which is consistent with recorded earthquake data in the valley.
Introduction
Delineating the geometry of the basin sediment/bedrock
interface is a critical step toward evaluating possible basin
effects induced by earthquakes. Numerous recent modeling
studies in southern California have demonstrated the effects
basin geometry can have on ground motions (e.g., Frankel,
1993; Pitarka and Irikura, 1996; Alex and Olsen, 1998;
Graves et al., 1998). Currently, one of the areas of concentrated focus for earthquake hazards is the San Bernardino
Valley (Fig. 1). The valley is bounded between the San Jacinto fault on the southwest and San Andreas fault on the
northeast and between the San Gabriel mountains on the
northwest and the badlands and Crafton hills on the southwest. Estimating the hazard from the San Jacinto fault zone
is critical to an overall assessment of hazard in southern
California because of its historic seismicity rate, which is
high even when compared to the adjacent San Andreas fault
(Working Group on California Earthquake Probabilities
[WGCEP], 1988). Additionally, the San Jacinto fault cuts
across major infrastructure such as railroads, Interstate 10,
and Interstate 215. The Riverside–San Bernardino metropolitan area is home to 3.2 million people, with about 180
thousand of those within the San Bernardino Valley (U.S.
Census Bureau 1999 estimates). It is thus an area that could
be seriously impacted by even a moderate earthquake on the
San Andreas, San Jacinto, or other faults in the area.
Frankel (1993) simulated ground-motion effects across
the San Bernardino Valley from a M 6.5 earthquake on the
San Andreas Fault and noted that basin edge effects such as
reflected surface waves were a major contributor to large
ground motions. However, the basin (throughout this manuscript, “basin” refers to the subsurface nonbedrock region
under the valley) velocity model for that study was not well
constrained at depth by either geological or geophysical
data. The current San Bernardino basin reference threedimensional velocity model proposed by the Southern California Earthquake Center (SCEC version 2) generally has
gently dipping layers between the San Jacinto and San Andreas faults (Magistrale et al., 2000). However, variation in
amplification across the San Bernardino Valley from the
1992 Landers and 1999 Hector Mine Earthquakes suggests
more steeply dipping structure within the basin sedimentary
fill (R. Graves, written commun., 2000; Graves and Wald,
2000).
The seismic reflection method has been successfully
2504
Delineation of Faulting and Basin Geometry along a Seismic Reflection Transect in Urbanized San Bernardino Valley
2505
Figure 1. Geographic setting of the San Bernardino Valley, California. The seismic
line of this study is shown by the heavy black line. Well locations are shown by
triangles (34G4, 34Q1, 22L17, LV3, and CR5). Fault traces are at inferred or mapped
locations prior to this study. Opposing gray arrows on seismic lines indicate the location
for the San Jacinto fault interpreted from new seismic and gravity data. Double line
labeled “R.P.” is the location of refraction profile by Hadley and Combs (1974). Map
modified from Anderson et al. (2000) and Woolfenden and Kadhim (1997).
used in the nearby Los Angeles basin to delineate basin geometry and active faulting in heavily urbanized areas (e.g.,
Pratt et al., 1998; Shaw and Shearer, 1999). High-resolution
sources (typically 50–500 Hz) have proven very successful
at imaging sediment–basement interfaces as deep as 500 m
but generally lack energy needed to image deeper in noisy
urban areas. Although oil industry active seismic sources
such as explosives and vibroseis have sufficient energy to
image the basement surface at many kilometers depth, their
use in a modern urban environment can be both financially
and legally difficult, if not impossible. Doll et al. (1998)
compared eight state-of-the-art noninvasive seismic sources
including hammers, weight drops, Mini-Sosie, variously
sized vibroseis sources, and a land air gun. They concluded
the Mini-vib by Industrial Vehicles, (a smaller-scale vibroseis) provided the best subsurface image at their test site in
Tennessee. Because the Mini-vib is noninvasive and portable and therefore appropriate for an urban setting, we tested
the viability of using this source to image basement at 1 km
or greater depth in the urbanized San Bernardino Valley.
We acquired a single seismic reflection profile that begins and ends near bedrock outcrops (Perris Hill and La
Loma Hills, respectively) (Fig. 1). This profile also traversed
the deepest part of basin as inferred by gravity measurements
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W. J. Stephenson, J. K. Odum, R. A. Williams, and M. L. Anderson
(e.g., Willingham, 1968; Lambert, 1987; Anderson et al.,
2000), and crossed the projected locations of numerous geologically mapped faults including the Loma Linda and San
Jacinto faults. These seismic data, combined with results
from gravity and aeromagnetic modeling, reveal a basin geometry that has implications for seismic hazard in the region.
Geologic Setting of San Bernardino Basin
As defined by Morton and Matti (1993), the San Bernardino basin is a triangular-shaped region framed by the
San Jacinto and San Andreas faults. Topographically, the
basin narrows to the northwest between the San Gabriel and
San Bernardino Mountains and is bounded on the south by
the San Timoteo Badlands and on the southeast by the Crafton Hills. The tectonic history and structural development of
the basin is a direct result of its location within the convergent region between the San Andreas and San Jacinto fault
zones. This convergence is expressed as an area of compression and uplift (eastern San Gabriel Mountains) that contrast with the San Bernardino basin, which is an area of
extension and pull-apart subsidence (Morton and Matti,
1993). The major structural blocks that compose the physiographic region around San Bernardino consist of the San
Bernardino Mountains block to the north, the Peninsular
Ranges block to the southwest, and the San Gabriel Mountains block to the west (Matti et al., 1985).
Prior to basin development, this area was a topographic
high consisting of exposed pre-Tertiary igneous and metamorphic rock. From approximately 3.8–4(?) Ma to about
0.8–1.3 Ma, the San Bernardino high shed coarse-grained
clastic material to the west and southwest. These sediments
form the extensive San Timoteo Badlands, (Dutcher and
Garrett, 1963; Morton and Matti, 1993). The transformation
of the San Bernardino high into a basin began approximately
1.5 Ma with the inception of the San Jacinto fault zone. The
San Jacinto fault is one the most active in the region, with
an average slip rate of 20 mm/yr over the past 1.5 m.y. (Morton and Matti, 1993). Angular convergence of the evolving
San Jacinto fault zone with the main strand of the San Andreas fault resulted in an area of extension between the fault
zones and the initiation of San Bernardino basin subsidence
at approximately 1 Ma (Morton and Matti, 1993).
The San Bernardino basin stratigraphic section reflects
its youthful geologic age. In general, other than the oldest
fill materials, basin fill stratigraphy consists of unconsolidated, coarser grained, alluvial-fan detritus derived from
the surrounding mountains. These deposits interfinger with
coarse to finer grained clastic fluvial deposits (Dutcher
and Garrett, 1963). The underlying basement complex is
composed of pre-Tertiary igneous and various grade metamorphic rocks including the Pelona schist. Examples of
basement complex rocks can be found in remnant hills
within the valley (e.g., Shandin Hills, Perris Hill, La Loma
Hills, and Crafton Hills), (Fig. 1), which protrude above the
basin fill.
There is little documented information on the oldest basin fill lithology. As the San Bernardino area transformed
from a sediment source area to one of sediment accumulation, deposition of the Pliocene San Timoteo Formation in
the nearby badlands ceased at approximately 1 Ma (Morton
and Matti, 1993). A thin accumulation of continentally derived material that is pre–San Timoteo Formation unconformably overlies the basement complex and locally occupies low areas of the subsiding topography. Based upon
water-well drilling logs, Dutcher and Garrett (1963) described discontinuous units as much as 150–200 m thick of
continentally derived, calcareous indurated clays, sands, and
conglomerates that may represent these oldest basin fill deposits. Deposited upon this unit, or more commonly, directly
upon the basement complex, are Tertiary to Quaternary
weakly to well-compacted units consisting of interfingered
gravel, sand, silt, and clay. Although little information exists
on the areal extend of these units, logs from sparse drill holes
indicate that locally the section may be 200–300 m thick.
Eckis (1934) suggested that these units are equivalent to the
upper part of the San Timoteo in age and composition.
A Pleistocene unit referred to as “Older Alluvium” by
Dutcher and Garrett (1963) unconformably overlies these deposits. The Older Alluvium unit consists of unconsolidated
coarse gravel, sand, silt, and clay deposited primarily by
fluvial and alluvial fan processes and may be as thick as 150–
200 m. These units are in turn overlain by “Younger Alluvium,” which consist of up to 35 m of unconsolidated, coarse
gravel, sand, silt, and clay. Whereas the distinction between
coarse-grained, permeable, and fine-grained hydrogeologic
stratigraphy is important for groundwater modeling within
the San Bernardino basin, the majority of the Younger and
Older Alluvium are believed to be seismically homogeneous
based on their lithologic description. These hydrogeologic
units should not be confused with the seismic–stratigraphic
units defined in the following section.
Seismic-Reflection Data Analysis and Interpretation
Acquisition and Processing
Approximately 14 km of seismic-reflection data were
acquired northeast to southwest through the city of San Bernardino, from Perris Hill to La Loma Hills. The continuous
seismic profile crossed approximately the southernmost 75%
of the San Bernardino basin including the projected traces
of the San Jacinto, Rialto-Colton, and Loma Linda faults
(Fig. 1). The San Andreas fault, approximately 3 km north of
Perris Hill, was not traversed. Previous near-surface seismicreflection studies in urbanized areas of the Los Angeles
region have typically imaged no deeper than about 500 m
using earth impacting sources (e.g., Stephenson et al., 1995;
Pratt et al., 1998). A small vibrator source, or Mini-vib, was
utilized for this work because it provided the capability to
image deeper than was done in these previous near-surface
investigations. The Mini-vib was also selected because it
Delineation of Faulting and Basin Geometry along a Seismic Reflection Transect in Urbanized San Bernardino Valley
was more portable (about 50% smaller) than conventional
industry vibroseis trucks and was capable of sweeping to
frequencies as high as 500 Hz. Based on noise tests, however, a 10- to 100-Hz linear sweep was ultimately used for
acquisition.
We acquired 120 channels of data per shot point, using
a 10-m shot and receiver-station interval. An asymmetric
shooting geometry was used, where the source was walked
through the first 30 stations of each receiver array before the
array was moved forward. This approach was chosen because the recording system did not have 120-channel rollalong capability. A single, vertical-component, 8-Hz resonant frequency geophone was used at each receiver station.
Data signal-to-noise ratio was low to moderate, as revealed
on typical shot records that have only automatic gain correction (AGC) and applied bandpass filter of 10–20–100–200
Hz (Fig. 2). The low-cut ramp of the filter of 10–20 Hz (0%
and 100% pass points, respectively) effectively eliminated
Figure 2.
2507
much of the low-frequency surface-wave noise, but numerous high-frequency linear and random noise events are still
evident. Refracted and reflected wave amplitudes at the far
offsets are often barely above background noise, even using
a 400-msec AGC window.
Seismic data processing was generally conventional by
oil-industry standards, as shown in Table 1. Significant effort
was dedicated to the removal of surface waves and linear
noise events generated by high traffic volume on the I-10
and I-215 freeways and on nearby urban streets. Several processing steps, including surgical muting, dip filtering, and
prestack eigenvector filtering were utilized for this purpose.
Although the data had a moderate to high level of noise from
the urban environment, maintaining a nominal 60 fold
common-midpoint (CMP) stack coverage significantly improved the final interpreted stack.
The final migrated and depth-converted stacked section
is shown in Figure 3A. Depth conversion was performed
Six representative seismic-reflection shot records acquired in the San
Bernardino Valley. Station 180 is near Perris Hill, and station 1419 is near La Loma
Hills (see Fig. 1). Each record contains 120 channels with a 2-sec recording time. Data
have only AGC and a bandpass filter applied. Arrows indicate vibration point locations.
Gray bracket is the receiver array length for each shot. A high urban noise level is
apparent in these records. The relatively high nominal 60-fold coverage was invaluable
in successfully stacking in reflection signal as was stacking eight records per vibration
point.
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W. J. Stephenson, J. K. Odum, R. A. Williams, and M. L. Anderson
Table 1
Generalized Processing Steps for San Bernardino Seismic
Reflection Profile
Processing Step
Data reformat
Vibroseis correlation
Geometry
Trace edit
AGC
Bandpass filter (Hz)
Top Mute
Surgical mute
Elevation statics
Sort to CMP
Adaptive deconvolution
Dip scan filter
NMO
Residual statics
Eigenvector filtering
DMO
Stack
Migration
Eigenvector filtering
Time-to-depth conversion
Description
Convert from SEG 2 to ProMAX威 internal
Correlate on ground force signal, 12-sec
sweep, 2-sec record length
Assign coordinate information to data set
Eliminate bad traces, correct polarity
reversals
Adjust amplitudes using 400 msec root
mean square gain window
10–20–80–100 Hz
Zero all data amplitude before and
including first arrivals
Zero amplitudes of residual surface waves
Correct travel times for variation in station
elevation
Reorder data by common midpoint number
L2-norm predictive, 200-msec operator
length
Velocity filter to remove coherent linear
noise
Correct for normal moveout (with best
velocity function of four analyses); 100%
stretch mute applied
Surface consistent, based on maximum
stack power; three solutions applied
iteratively; 15-msec maximum static shift
allowed
0%–30% eigenimage accepted
Common-offset frequency–wavenumber dip
moveout
Zero-offset mean stack
Steep-dip finite-difference time migration,
using 70% smoothed stacking-velocity
field
0%–30% eigenimage accepted
Digital conversion using best smoothed
stacking velocity function
with a proprietary software algorithm using the best-estimate
smoothed two-dimensional stacking velocities. Typical vertical resolution of the final stacked section varied from 16 m
at 200-m depth to 26 m at 1000-m depth, with a dominant
frequency of 30 Hz through this range. Resolution below
1000 m is poor due to a high ambient noise level. Poor
resolution is also evident in the interval velocity field (Fig.
3B) derived from the smoothed stacking velocities, where velocities below even 800 m are not well constrained by reflection velocity analysis. The interval velocity of 2800 m/sec
is dashed to indicate it is the deepest interval reasonably
estimated from the stacking velocities of these data.
ten comm. 2000). Two of the wells are located within a few
hundred meters of the profile (wells LV3 and CR5; Fig. 1).
Although caution is required when interpreting these borehole data (i.e., do the wells penetrate basement or a granitic
clast?), they are interpreted to constrain the bedrock depth
to the upper 250 m between stations 1100 and 1400. The
seismic profile also passed by an exposed bedrock outcrop
near station 1460 (La Loma Hills). We can trace a strong
reflection from station 1150 to the line end at station 1470
that tie these three points together, and thus we have confidence in our pick of the bedrock surface through this region.
The high amplitude and lateral coherency of this reflection
is also indicative of a sediment–bedrock interface. Between
stations 1050 and 1150, a strong reflection at 220- to 250-m
depth is believed to be the northeastern extension of the bedrock surface, although the reflection is not as continuous as
it is to the southwest (Fig. 4). If the reflection disruption is
caused by a fault, then there is little apparent vertical displacement occurring across it because the reflection occurs
at similar depths across the disrupted region.
The Pelona schist, one of the rock types making up the
basement-rock complex, is exposed in outcrop approximately 100 m east of the northeastern end of the seismic
profile at Perris Hill (Fig. 1). Other hills within the valley
are also composed of basement rocks of various types in
outcrop, suggesting the basement composition and topography may be complex. Bedrock is first imaged on the seismic profile at about 100-m depth beneath the northeastern
end of the profile (Fig. 3A, purple shaded region). The bedrock surface dips at about 13⬚ southwest for roughly 1.5 km
before becoming subhorizontal for approximately 1.5 km, to
station 420. This surface then apparently dips as much as
12⬚ southwest for about 4 km southwest of station 420 into
the deepest part of the basin as inferred from gravity data
(Anderson et al., 2000) between stations 825 and 1075. The
basement surface is not well imaged in the seismic data between stations 650 and 1075. Between stations 650 and 850,
a sequence of coherent but discontinuous reflection packages
that get deeper to the southwest is picked as the basement
surface. This correlation is further guided by analyzing shallower coherent reflections for possible faulting and projecting these faults to depth. Unfortunately, significant noise in
the data, lack of energy from the seismic source, and/or
structural complexity makes interpretation quite ambiguous
between stations 900 and 1010. Based on a subhorizontal
reflection sequence through this station range, we infer this
interface is between 1.4 and 2 km depth, or roughly 1.7 km
deep near the San Jacinto fault.
Basin Sediment
Basement (Bedrock) Depth
Although deep borehole information is sparse within the
San Bernardino Valley, four water wells that encounter granitic bedrock constrain the interpreted depths near the southeastern end of the seismic profile (well information from
W. Danskin, written comm. 2000, and L. Woolfenden, writ-
Several water wells near the seismic profile help constrain the interpretation of the sedimentary deposits in the
upper 300 m over much of the profile length. Well data from
Izbicki et al. (1998) showed that the middle water-bearing
unit (MWB) within the basin tends to correlate on average
with a 30%–40% increase in electrical resistivity (three of
2509
Figure 3. (Caption on next page)
2510
Figure 3. (A) Stacked seismic-reflection profile acquired across the San Bernardino Valley. The San Bernardino seismic-reflection
profile covers over 14 km of the basin, from Perris Hill on the northeast to La Loma Hills on the southwest (see Fig. 1). The continuous
profile crossed numerous busy streets through the city of San Bernardino. Several wells (vertical white bars on upper section) and
outcrops on each end of the profile constrain the location of the sediment–bedrock interface. Bedrock (basement) is the purple shaded
region. White dashed boxes outline areas shown in Figures 4 and 5. The sediment–basement interface is queried where interpretation
is not well constrained. Shallow sediment region within the San Jacinto fault zone is also queried. The basin is asymmetric about its
axis, with its northeastern flank dipping at about 10⬚ southwest. Numerous faults are interpreted along the seismic transect, including
the San Jacinto, Loma Linda, possibly the Banning, and several unnamed faults (red lines, top section). The main strand of the San
Jacinto fault zone is interpreted at the truncation of the bedrock and sedimentary reflections near station 1070. The basin sediments are
divided into two seismic–stratigraphic units (gold region is younger sediment, orange brown region is older sediment) based on the
strong, generally continuous reflection observed along the northeastern half of the profile. All of the sediments discussed here are
undifferentiated Quaternary and Tertiary deposits. (B) Depth-converted final migrated stack overlain by smoothed interval velocity
contours (in m/sec). These velocities were derived from the best-estimate stacking velocities. Dashed 2800 m/sec contour is the deepest
interval velocity reasonably estimated from these data.
2511
Delineation of Faulting and Basin Geometry along a Seismic Reflection Transect in Urbanized San Bernardino Valley
Figure 4. Gray-scale image of seismic data
from stations 1000 to 1300. Disruption of reflections, with little apparent vertical displacement, may be the location of the Rialto-Colton
fault as it converges toward the San Jacinto
fault to the south. Black lines represent interpreted faults.
Station 950
900
SW
Depth (m)
100
ult?
a fa
ind
aL
Lom
the well locations, 34G4, 34Q1, and 22L17, are shown in
Fig. 1). Empirically, the seismic velocity of sediments, and
thus impedance, is often proportional to resistivity (e.g.,
Rudman et al., 1975) (by this nonlinear relationship, a 30%
increase in resistivity is roughly a 5% increase in velocity
in the 100 ohm m range). It appears that the MWB correlates
with the strong, continuous reflection observed over most of
the northeastern side of the seismic profile, particularly in
the regions between stations 200–440 and 640–810 (Fig. 3a).
This unit has thus been interpreted as a seismic–stratigraphic
boundary between shallow and deep basin sediments (in Fig.
3A, the orange brown shaded region represents sediments
below this boundary). The velocity contrasts between the
various deposits within the entire basin sediment package,
even across this inferred seismic boundary, do not appear to
be greater than 5%–10% based on reflection velocity analysis, which is less than what one might expect across the
sediment–basement interface.
By correlating the strong boundary reflection with available water well logs, the top of the deeper basin sediments
is mapped from station 180 to at least station 1150. The
deeper sediments generally increase in thickness southwest
of station 150 toward the San Jacinto fault zone and accommodate faulting by upwarping, downwarping, and tilting. An
apparent tilted block is imaged between stations 820 and 930
(Fig. 5). Reflection character within the block is not as continuous as it is to the northeast (station numbers less than 820).
The strongest reflection, believed to be from the seismic–
stratigraphic boundary and shown by the gray line with teeth
in Figure 5, gets markedly shallower from stations 840 to
900. A borehole log from Izbicki et al. (1998) projects into
the seismic profile at about station 940 and roughly corroborates this interpretation. Although the block is tilted at
about 15⬚, there is no known clear evidence of deformation
at the ground surface (e.g., Dutcher and Garrett, 1963). Thus,
it appears that much of this block rotation occurred prior to
deposition of the youngest basin deposits.
850
800
750
NE
200
300
400
500
600
200 m
Figure 5. Gray-scale image of seismic data from
stations 740 to 950 showing interpreted faults and a
tilted sedimentary block. Heavy gray lines with teeth
represent seismic–stratigraphic contact. This block,
tilted at about 15⬚ northeast, is probably caused by
tectonic deformation between the San Jacinto and
Loma Linda faults.
The seismic–stratigraphic sedimentary boundary is not
strongly imaged between stations 940 and 1070, in the projected vicinity of the San Jacinto fault (Fig. 3A). The boundary was thus picked primarily on water-well-log descriptions
through this region rather than lateral correlation of the reflection. We speculate that the deeper seismic–stratigraphic
unit terminates beneath station 1190. Overall, the shallower
basin sediments are not imaged as continuously as the deeper
deposits; however, this may be caused more by lower-fold
coverage from the selected station interval than by a generally nonreflective nature of the sediments.
Faulting/Basin Structure
Faulting is evident at numerous locations along the seismic profile (Fig. 3A). However, only faults that we believe
2512
W. J. Stephenson, J. K. Odum, R. A. Williams, and M. L. Anderson
are the most prominent features are discussed here. On the
southwestern end of the profile, we interpret a fault disrupting bedrock near station 1140 and infer that it is the southern
extension of the Rialto-Colton fault (Fig. 4). This is based
on the southern projection of the fault intersecting the seismic profile in the vicinity of the disrupted location. The region between I-10 and I-215 (approximately stations 950–
1050) has a very high cultural noise level, and it is unclear
whether the region is nonreflective because of extensive
faulting or poor signal quality. We believe it is caused by
both and thus interpret this region from stations 950 to 1070
as the San Jacinto fault zone. This region may contain several additional fault strands within the sedimentary deposits
that are not clearly imaged in the seismic data. The main
strand of the San Jacinto fault zone is interpreted at the truncation of the bedrock and sedimentary reflections near station 1070. This location is more than 1 km southwest of the
projection of the fault from its mapped location to the southeast in the Loma Linda Hills (Sharp, 1972), and places it
west of the I-10 and I-215 interchange (Fig. 1). At least 1.2
km of apparent vertical displacement is inferred across the
main strand of the San Jacinto fault zone.
Several faults dipping at about 30⬚ to the southwest are
evident within the tilted sedimentary block between stations
810 and 950 (Fig. 5). At the change in slope of the sedimentary reflections (about station 810), the Loma Linda
fault, mapped south of the valley, projects across the seismic
profile. If this change in slope is related to fault deformation,
then these data may provide the first evidence of the Loma
Linda fault continuing at depth into the San Bernardino Valley. This change in slope is also associated with 50 m of
apparent vertical displacement at 300 m depth. Approximately 2 km to the northeast near station 625, another fault
crosses the seismic profile that has over 70 m of apparent
vertical displacement on the basement (because the sense of
motion is different on the shallow horizon than at depth,
significant out-of-plane motion must have occurred). Matti
et al. (1985) suggested the western continuation of the Banning fault projects into the San Bernardino Valley (Fig. 1)
in the general area of this imaged fault. We therefore speculate this structure may be related to the Banning fault beneath the San Bernardino Valley. Finally, another fault near
station 450 accommodates apparent downwarping of the
sedimentary deposits and basement rocks as well as about
30 m of apparent vertical displacement. Major deformation
of the deeper basin sediments and the thickening of the stratigraphic section to the west begin across this fault.
Gravity and Aeromagnetic Data
Anderson et al. (2000) documented data from 611 recently acquired gravity stations throughout the region in and
around the San Bernardino Basin. The seismic profile traverses within a few hundred meters of 35 of the new stations
(Fig. 6), indicating good overlap between the two data sets.
Gravity data were processed using conventional routines
Figure 6. Map view of the seismic reflection profile, shown in gray in upper figure, overlain by 35
gravity stations acquired within several hundred meters of the seismic profile (black diamonds). Isostatic
gravity values projected onto the seismic reflection
profile are shown in the lower part of the figure. Every
fifth point is correlated across the line of projection
by a dashed vertical line. The basin gravity anomaly
is over 30 mGal. These gravity data were gridded in
conjunction with a large regional data set (Anderson
et al., 2000), and the resulting data were used to extract two-dimensional gravity profiles for modeling.
outlined by Telford et al. (1990). Terrain corrections were
carried out to 166.7 km, and isostatic corrections were made
using an Airy-Heiskanen model of compensation. Isostatic
gravity data for the nearest 35 stations projected onto the
seismic profile in Figure 6 reveal that there is over 30 mGal
variation along the transect. The entire newly acquired data
set and previously acquired data were gridded (Anderson et
al., 2000), and the resulting grids were used to extract twodimensional gravity profiles.
Densities and magnetic susceptibility information for
the potential-field modeling were derived from data of Anderson et al. (2000). Typical bulk densities for rock samples
around the valley include 2520–2620 kg/m3 for Pelona
schist, 2690–2850 kg/m3 for gneiss and tonalite, 2630–2790
kg/m3 for diorite, and 2440–2640 kg/m3 for Tertiary sandstone. Unconsolidated basin fill is assumed to have a bulk
average density between 2000 and 2400 kg/m3, typical for
Quaternary–Tertiary basin fill. Magnetic susceptibilities (SI
units times 10ⳮ7) for these rock types range from 8 to 16
for Pelona schist, 16 to 190 for gneiss, 630 to 995 for tonalite, 72 to 613 for diorite, and 56 to 477 for Tertiary sandstone. Unconsolidated basin fill susceptibility is assumed to
be zero.
The gridded gravity data along three transects are generally parallel with the trend of the seismic profile and about
2.5 km apart (Fig. 7). These three two-dimensional transects
Delineation of Faulting and Basin Geometry along a Seismic Reflection Transect in Urbanized San Bernardino Valley
2513
Figure 7. Map view of isostatic gravity across San Bernardino Valley and vicinity.
The data are shown with 1 mGal contour interval (from Anderson et al., 2000). The
heavy black line is the seismic profile location and gray triangles are gravity station
locations. Medium black lines are previously mapped faults as shown in Figure 1. Three
gravity modeling transects were extracted from the gridded three-dimensional gravity
data set along lines aa⬘, bb⬘, and cc⬘. The three gravity transects are generally parallel
with the trend of the seismic profile and about 2.5 km apart. Profile bb⬘ approximately
overlies much of the seismic profile location.
were then forward modeled in conjunction with aeromagnetic profiles along the same transects. The resulting models,
to 2-km depth and to a few hundred meters northeast of the
San Andreas fault, are shown in Figure 8. The aeromagnetic
data provide a useful constraint on intrabedrock structure
when used in conjunction with the gravity data during the
two-dimensional modeling. These data further delimit
the interpreted deformation and fault locations observed in
the seismic data. Anderson et al. (written comm., 2001) presented a detailed analysis of these aeromagnetic data, as well
as the complete gravity data set.
Bedrock outcrops on opposite ends of the seismic transect serve as ties for the gravity and magnetic modeling, and
their variable composition suggests that basement gravity
and magnetic properties should vary along the profile. Overall, the modeling results reflect this basement rock variability
on each of the three transects, with each requiring denser
bedrock material on the southwest side of the valley in the
Peninsular Range block. Highly magnetic bodies are necessary to match the magnetic anomalies within the basin,
even though their inclusion appears at first to be somewhat
ad hoc. Possible error in the model basement depths can be
up to several hundred meters over the deepest model regions.
The best-fit model for each of the three profiles suggests an
asymmetric basin, with the southwest flank dipping more
steeply than the northeast flank (Fig. 8). On each of the pro-
2514
W. J. Stephenson, J. K. Odum, R. A. Williams, and M. L. Anderson
Figure 8. Three two-dimensional gravity and aeromagnetic forward-modeling results along transects shown in Figure 7. Densities and magnetic susceptibilities were
derived from data of Anderson et al. (2000). SAF and SJF are San Andreas and San
Jacinto faults, respectively. D, density, in kg/m3, of body polygons; S, magnetic susceptibility in SI units times 10ⳮ3. Basin sediments are modeled as a single body of
2120–2170 kg/m3. Vertical exaggeration is approximately 2:1. All three of the profiles
indicate an asymmetric basin, with the southwest flank steeper than the northeast flank.
On each of the profiles, a knob of bedrock protrudes toward the surface several kilometers northeast of the San Jacinto fault.
Delineation of Faulting and Basin Geometry along a Seismic Reflection Transect in Urbanized San Bernardino Valley
files, a knob of bedrock protrudes toward the surface several
kilometers northeast of the San Jacinto fault. The knob appears to decrease in height along profile bb⬘ relative to the
adjacent two profiles. Although the knob is reflected prominently in the magnetic data, it is only a subtle feature in the
gravity, and there is a height uncertainty of Ⳳ100 m.
Profile bb⬘ most closely overlies the seismic profile location, and these two profiles are directly compared in Figure
9. The heavy black line representing the best-guess sediment–
bedrock interface on the seismic data, when overlain on the
gravity profile, shows a close match between the two data
sets. Although the seismic interface was not used to constrain the gravity and magnetic modeling beyond providing
an initial first guess for the basin shape, the disagreement
Figure 9.
2515
between the model and interpretation is generally 200 m or
less. The two results disagree most where the seismic interpretation is most weakly constrained, in the central part of
the transect and northeast of the main San Jacinto fault. Disagreement between these results is in part not surprising because the transects are not exactly collocated (Fig. 7). Maximum depth is not well constrained on the seismic data, but
the inferred depth nonetheless is consistent with the gravity
result.
The largest discrepancy occurs where the bedrock knob
is modeled from the gravity and magnetic data (Fig. 9). The
seismic data are very noisy at depth in this area, but it is
interesting to note the upwarped sedimentary package between stations 850 and 940, southwest of the interpreted
Comparison of seismic reflection profile to gravity and aeromagnetic
modeling profile bb⬘, shown in Figure 8. Bedrock is simplified into single body for
comparison. Seismic interface between bedrock and basin sediment (heavy black line)
is overlain on the gravity result. The two results disagree most where the seismic
interpretation is weakest, in the central part of the transect and northeast of the San
Jacinto fault; however, the disagreement is generally 200 m or less. Although depth is
not well constrained on the seismic data, it is generally within 10% of the gravity result.
The largest discrepancy occurs where the bedrock knob is modeled in the gravity/
aeromagnetic data. The seismic data are very noisy at depth in this area, but it is
interesting to note that the upwarped sedimentary package southwest of the Loma Linda
fault directly overlies the bedrock knob. It is possible this knob is an uplifted block of
basement caused by faulting.
2516
W. J. Stephenson, J. K. Odum, R. A. Williams, and M. L. Anderson
Loma Linda fault, directly overlies the gravity knob. It is
possible this knob is an uplifted block of basement formed
by reverse or thrust faulting. Alternately, the knob may be
a horst with normal faulting occurring on both flanks. In
either case, the overlying sediments may be warped as result
of the same faulting that created the basement block uplift.
Discussion
Basin Geometry
Hadley and Combs (1974) detected a high-velocity
southwest-dipping layer from seismic refraction data at
about 1.1-km depth near the intersection of the refraction
profile and the inferred Loma Linda fault. Their velocity of
5.3 km/sec is indicative of metamorphic and igneous rocks
of the basement complex. Results of Anderson et al. (2000)
and of this study suggest the basin may be as deep as 1.7 km
west of the refraction profile. The general southwest dip of
the basement interface is consistent between these studies.
Previous gravity studies suggested that the western part of
the San Bernardino basin may be between 1 and 2.5 km deep
(Willingham, 1968; Lambert, 1987), also consistent with this
study. Given the low signal level in the seismic-reflection
data over the deepest part of the basin, the gravity modeling
results presented in this study are probably more accurate in
this area. Both the gravity and seismic-reflection data reveal
an asymmetric basin that deepens to the southwest. This differs from the more gently dipping and symmetric San Bernardino basin reference velocity model currently proposed
by SCEC (e.g., Magistrale et al., 2000). The gravity contours
in Figure 7 (from Anderson et al., 2000) suggest the basin
geometry imaged in the seismic profile and modeled in the
gravity or magnetic data may be extrapolated for 10–15 km
northwest of and up to 10 km southeast of the profile location.
Basin Ground-Motion Effects
The two-dimensional finite-difference SH code of Frankel and Clayton (1986) was used to estimate the effect of
the imaged asymmetric basin geometry on ground motion.
Three separate simulations were performed: one using only
the seismic interpretation to estimate basement (model I),
the second incorporating the gravity-derived basement knob
(Fig. 8) merged with the seismic result (model II), and the
third using a version of the SCEC basin reference model
(model III). For models I and II, the two-dimensional basin
density and S-wave velocity structures used in the finitedifference modeling were reasonably well constrained by the
seismic-reflection/refraction data, gravity data, and surface
geology. These models were comprised of three layers,
whose overall shapes were derived primarily for the San
Bernardino seismic-reflection profile (Fig. 10A). Geologically, the layers represent (1) younger Quaternary/Tertiary
basin deposits; (2) older Quaternary/Tertiary sediments as-
sociated with the lower seismic–stratigraphic unit; and (3)
basement rock of metamorphic or granitic composition. The
velocity structure for the model III was developed in a manner similar to Graves and Wald (2000) from the SCEC reference model (Magistrale et al., 2000) (Fig. 10B).
The S-wave velocities for models I and II were derived
from P-wave reflection/refraction velocities and assumed
Vp /Vs ratios for representative rock types in the model layers.
These velocities ranged from 400 m/sec at the surface to
2500 m/sec at depth. Layer 1 had a 400–800 m/sec vertical
velocity gradient included. The velocity model was extended
beyond the ends of the seismic line to minimize model edge
effects across the region of interest. Density was varied
within the layers using values for representative materials
that were consistent with the densities used in gravity modeling. S-wave velocities and densities for model III were
derived from SCEC version 2.2 code. Attenuation was not
incorporated in the simulations. Because the goal was to
determine the general effects of the basin geometry, no attempt was made to model geologic variation below about
2-km depth.
A vertically incident SH-plane wave at 3-km depth was
used as the source in all simulations. Nominal peak frequency for the simulation was 10 Hz using a grid spacing
of 2.5 m. The resulting displacement seismograms were
differentiated to velocity and bandpass filtered from 0.05 to
1.0 Hz for comparison with ground motions in an approximate frequency band where interpreted basin amplification
was previously observed (Graves and Wald, 2000). Because
the seismograms for both simulations II and I are visually
quite similar, only those for the model I are presented in
Figure 10C. Peak horizontal velocity for these simulations
indicate the basement knob derived from gravity modeling
has very little effect on the relative ground motion across
the basin (Fig. 10D). Amplification of over 3 in velocity is
observed over the deepest part of the basin in both simulations, northeast of the San Jacinto fault.
The moderate dip of the basin toward the San Jacinto
fault does not seem to cause dramatic variation in amplification once a factor of about 2.5 has been attained, with the
exception of the deepest basin between the Loma Linda and
San Jacinto faults. Likewise the variation in the interface
between layers 1 and 2 does not appear to have much effect
on amplification in the 0.05- 1.0-Hz frequency band (intuitively, this shallow interface should have more effect at
higher frequency). A slight decrease in amplification at
traces 8 and 9 seems to be related to the sliver of layer 2
sediment overlying bedrock southwest of the San Jacinto
fault. Generally, the amplifications from these models are
consistent with the amplification of 2–4 for recordings in the
basin from the 1999 Hector Mine earthquake on stations
within a few hundred meters of the seismic profile (e.g.,
Graves and Wald, 2000).
Amplification from the model III simulation is about a
factor of 2 above the bedrock reference ground motion (as
observed in the model I simulation), but it is generally con-
Delineation of Faulting and Basin Geometry along a Seismic Reflection Transect in Urbanized San Bernardino Valley
2517
Figure 10. Results of two-dimensional
finite-difference simulations along seismic reflection transect and comparison with seismograms of Graves and Wald (2000). Filtered
transverse-horizontal velocity seismograms recorded at stations ehos, 5339, and hldr from
the 1999 Hector Mine earthquake are shown in
E. Peak horizontal velocities (cm/sec) for these
seismograms are shown by the station name in
parentheses. Peak velocity amplification for
three simulations along the seismic reflection
transect are shown in D. Amplification curves
are predicted peak values caused by basin geometry as interpreted from seismic data only
(model I, solid black line), as modified by gravity/aeromagnetic modeling results (model II,
dashed black line), and by basin derived from
the SCEC version-2 reference model (model III,
dashed gray line). For all simulations, the
source is a vertically incident SH-plane wave.
Only the synthetic seismograms for model I are
shown in C, after bandpass filtering from 0.05
to 1 Hz. Model III (shown in B) has a relatively
flat response in amplification of about 2 across
the seismic profile location. The shape of the
basin generally causes a factor of 2–4 amplification in the simulations of models I and II
(shown in A), relative to bedrock response at
trace 1.
stant across the part of basin underlying the seismic transect.
Seismic energy from the Hector mine event did not enter the
basin as a vertically incident plane wave because it occurred
at 5-km depth and 100 km to the northeast (e.g., Dreger and
Kaverina, 2000). Amplification factors will change with angle of incidence of the wave field. Nonetheless, the variability seen across the basin in the tangential horizontal component (Fig. 10E; from Graves and Wald, 2000) is more
consistent with amplification observed in the simulations for
models I and II than with the roughly constant amplification
seen in the model III simulation.
Basin Fault Structures
The Rialto-Colton and Banning faults are two of the
more speculative faults interpreted in Figure 3a. Dutcher and
Garrett (1963) inferred a groundwater obstruction from water well data southwest of the San Jacinto fault they referred
to as the Rialto-Colton barrier. This barrier has been interpreted as the Rialto-Colton fault in more recent groundwater
studies (Woolfenden and Kadhim, 1997) and gravity studies
(Anderson et al., 2000) 2–10 km west of the San Jacinto
fault zone. Based on structural restoration arguments, the
Rialto-Colton fault was interpreted as a strand of the San
Jacinto fault zone by Anderson et al. (2000). The RialtoColton fault apparently converges southeastward toward the
San Jacinto fault. While there is no apparent vertical offset
observed at the reflector disruption on the seismic profile
(Fig. 4; stations 1130 to 1170), we infer by its location that
it may be the southward extension of the Rialto Colton fault
or some ancillary fault linking it to the San Jacinto fault.
2518
As an alternate interpretation, the main strand of the San
Jacinto fault may not be on the southwestern edge of the
interpreted fault zone but rather within the zone or on its
northeastern edge, at station 950 (Fig. 3a). The fault truncating bedrock at station 1070 might then be the RialtoColton or another associated fault. With this interpretation,
the deepest part of the basin would presumably fall between
the San Jacinto and Rialto-Colton faults. However, this scenario seems less likely given the arguments of Anderson et
al. (2000), who interpret only 2 km of right-lateral offset on
the Rialto-Colton fault based on magnetic anomalies and
only 600 m of vertical offset based on two-dimensional modeling. The gravity data also suggest the fault at station 1070
is the main San Jacinto fault because of the size of the gravity
anomaly associated with it. Another possible scenario is that
the Rialto-Colton fault merges onto the San Jacinto fault
near or northwest of the seismic transect. Resolving this
question will require additional seismic reflection data.
Matti et al. (1985) inferred that the Banning fault continues westward past the Crafton Hills into the San Bernardino basin. A fault can be inferred, in fact, along this projection from the gravity data (e.g., Anderson et al., 2000)
(Fig. 7). Faulting and deformation of basin sediments also
exist in the seismic data within a few kilometers of its projection; however, whether it is due to the Banning fault cannot be determined definitively from a single seismic reflection profile. The Banning fault would have to cut through,
or be associated with, significant faulting observed in the
Crafton Hills (Fig. 1), which is certainly possible at depth.
It is equally plausible that the deformation observed in the
seismic data is related to secondary faulting associated with
the San Jacinto fault.
Park et al. (1995) investigated the San Jacinto basin,
about 40 km southeast of the San Bernardino basin, for intrabasinal fault structures. This basin was formed by a dilatational right step of the San Jacinto fault zone. They present evidence for a possible flower structure and multiple
faults within the graben associated with the San Jacinto fault.
The tilted sedimentary blocks between stations 810 and 950
(Fig. 3, 5) are reminiscent of the tilted block adjacent to the
interpreted flower structure of Park et al. (1995) and could
represent a similar kind of structure in the San Bernardino
basin.
We believe the southwest-dipping bedrock surface interpreted across the San Bernardino basin is the top of a tilted
block of crystalline basement that has rotated about a horizontal axis in response to extensional stress accommodation
between the San Jacinto and San Andreas faults. Figure 11
is a hypothetical structural diagram of the San Bernardino
Valley between the San Andreas and San Jacinto faults from
the seismic profile location northward. The subsurface gray
dashed line represents a theoretical surface within crystalline
basement that indicates relative sense of vertical displacement believed to have occurred across the San Jacinto fault.
The three hachured patterns all represent undifferentiated
crystalline basement, but they are subdivided into the sepa-
W. J. Stephenson, J. K. Odum, R. A. Williams, and M. L. Anderson
rate major blocks based on their location relative to the San
Jacinto and San Andreas faults. The peninsular ranges block
and San Bernardino Mountain blocks bound the block underlying the San Bernardino basin.
We speculate that the Loma Linda fault is a strand of
the San Jacinto fault that has accommodated some compression as the valley block has tilted under general strikeslip motion. This seems a reasonable assumption, given the
complexity of the San Jacinto fault zone as previously discussed by Morton and Matti (1993). The San Jacinto fault
is mapped as a very linear feature through the hills south of
the San Bernardino basin, where it is observed as a sequence
of small surface ruptures with stepovers of typically no more
than a few hundred meters (Sharp, 1972). We believe this
overall structural pattern probably continues into the valley.
The two other faults interpreted within the San Bernardino
basin northeast of the Loma Linda and San Jacinto faults
offset shallow basin sediments but do not appear to have
accommodated as much displacement. Because of their location within the basin under the urban area, however, they
may pose an equally significant hazard to the region.
Conclusions
Seismic reflection data acquired through the urbanized
San Bernardino Valley reveal that the basin geometry is
asymmetric with its northeastern flank dipping at roughly 8⬚
to 13⬚ southwest and its southwestern flank bounded by the
nearly vertical San Jacinto fault zone. This is in contrast to
the current SCEC version 2 reference velocity model (Magistrale et al., 2000) that consists of a more gently dipping
and symmetric sediment–basement boundary across the imaged part of the basin. The seismic-reflection data show evidence of numerous faults cutting through the Quaternary–
Tertiary basin sediments and basement surface and possibly
provide the first evidence of the Loma Linda fault and the
Banning fault cutting into the valley. At least one additional
previously unidentified fault within the basin has also been
observed north of the Banning fault. The main strand of the
San Jacinto fault is interpreted at a location more than 1 km
southwest of its previously inferred position, and it appears
to be the southwestern limit of an over 1-km-wide zone of
faulting. The Rialto-Colton fault is also interpreted from
these data southeast of the San Jacinto fault zone. Basin
sediments are deformed between the San Jacinto and Loma
Linda faults, with the strata at 200-m depth tilted at 15⬚ to
the northeast. The combined gravity/aeromagnetic modeling
and seismic interpretation indicate the basin near the profile
may be as deep as 1.7 km. Over 1.2 km of apparent vertical
displacement on the basement is observed across the San
Jacinto fault in the gravity/magnetic and seismic-reflection
data. Finite-difference modeling suggests the overall basin
geometry can contribute to a 2- to 3.5-fold amplification of
earthquake waves in the 0.05–1.0 Hz frequency band, consistent with recorded earthquake data within the basin.
Delineation of Faulting and Basin Geometry along a Seismic Reflection Transect in Urbanized San Bernardino Valley
2519
Figure 11. Hypothetical block diagram
showing how structural features delineated in
seismic and gravity/aeromagnetic data might
relate to surface features and at depth. The three
hachured patterns represent undifferentiated
basement in three blocks: the San Bernardino
Mountain block, the Peninsular Ranges block,
and the San Bernardino Valley block. SJF, LLF,
BF, and SAF are San Jacinto, Loma Linda, Banning, and San Andreas faults, respectively.
Heavy black lines represent hypothetical faults,
dashed where inferred on surface. Subsurface
solid gray line is sediment–bedrock boundary
from seismic and gravity analyses. Subsurface
dashed gray line represents theoretical surface
within the crystalline basement that indicates a
relative sense of vertical displacement across
the San Jacinto fault.
Acknowledgments
We greatly appreciate the efforts of Mike Seal, John LaRose, and
Chris Smith of the County of San Bernardino Flood Control District, who
made county property permitting and access go smoothly, and who allowed
us to use County facilities. Deb Underwood helped greatly in permitting
the southern portion of the seismic profile. We thank Richard Dart, Steve
Harmsen, Carlos Mendoza, Nenna Okpara, and Susan Rhea for all their
hard work during field acquisition. We especially thank our field observer
David Worley for keeping the seismic acquisition equipment functional
during some difficult moments. Rob Huggins and Craig Lippus of Geometrics Inc. assisted us immensely with our recording equipment. We very
much appreciate the help of Elmo Christensen and O. B. Velez of Industrial
Vehicles during field acquisition. Rufus Catchings generously lent acquisition equipment. The gravity models developed in this study were produced with GMSYS, interactive software produced by Northwest Geophysical Associates. Thanks to Wes Danskin, Linda Woolfenden, Jon
Matti, Don Hough, and Gene McMeans for their assistance in obtaining
well information. Discussions with Rob Graves and Art Frankel greatly
improved the manuscript. The manuscript was also greatly improved
through reviews by Vickie Langenheim, Harold Magistrale, Bill Savage,
and an anonymous reviewer. Thanks to Lucy Jones for supplemental
NEHRP funding, Wes Danskin for supplemental funding from the U.S.
Geological Survey Water Resource Division, and Jon Matti and Doug Morton for additional funding from the Southern California Areal Mapping
Project (SCAMP). This work was primarily supported by the U.S. Geological Survey National Earthquake Hazard Reduction Program. Use of brand
names is for descriptive purposes only and does not represent a product
endorsement by the U.S. Geological Survey.
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U.S. Geological Survey
P.O. Box 25046, MS 966
Denver, Colorado 80225
wstephens@usgs.gov
(W.J.S., J.K.O., R.A.W.)
Department of Geosciences
University of Arizona
Gould-Simpson Building
Tucson, Arizona 85721-0077
(M.L.A.)
Manuscript received 8 August 2001.