A seismic refraction and reflection study across the ...

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the Claremont Fault on the northeast and the Casa Loma. Fault on the southwest. We present a case study of shal- low structure (less than 1 km) in the central ...
GEOPHYSICS, VOL. 61, NO. 5 (SEPTEMBER-OCTOBER 1996); P. 1258-1268, 5 FIGS., 2 TABLES.

A seismic refraction and reflection study across the central San Jacinto Basin, Southern California

Tien-Chang Lee*, Shawn Biehler*, Stephen K. Park*, ‡ and William J. Stephenson

ABSTRACT

basement depth determined for the 1.65km line suggests that an offset in the basement (greater than 260 m) exists around the junction of the two refraction lines (Offset II). By revealing more faults and subtle sedimentary structures, the reflection stack sections confirm the two refraction offsets as faults. Offsets I and III each separate sediments of contrasting structures and, in addition, Offset III disrupts an unconformity. However, the sense and amount of the offset across Offset III contradict what may be expected across the Casa Loma Fault, which has its basinward basement down-thrown to about 2.5 km in the better defined southeastern part of the graben. The Casa Loma Fault trace has been mislinked in the existing geological maps and the trace should be remapped to Offset II where the reflector disruptions spread over a 400-m wide zone. Our Offset III is an unnamed, concealed fault.

The San Jacinto Basin is a northwest-trending, pullapart basin in the San Jacinto fault zone of the San Andreas fault system in southern California. About 24 km long and 2 to 4 km wide, the basin sits on a graben bounded by two strands of the San Jacinto fault zone: the Claremont Fault on the northeast and the Casa Loma Fault on the southwest. We present a case study of shallow structure (less than 1 km) in the central basin. A 2.75km refraction line running from the northeast to southwest across the regional structural trend reveals a groundwater barrier (Offset I). Another line, bent southward and continued for 1.65km, shows a crystalline basement offset (Offset III) near an inferred trace of the Casa Loma Fault. Although a basement refractor was not observed along the 2.75km line, a mismatch between the estimate of its minimum depth and the

Fault and on the southwest by the Casa Loma Fault. Topographically the San Jacinto Basin is well defined on the northeast by the San Jacinto Mountains, but its southwest boundary is poorly delineated except by a subdued escarpment along its southeastern half. According to seismic refraction and gravity modeling (Fett, 1968) the depth to the basement may exceed 2.5 km in the southeastern half of the basin. The basement depth in the northwestern portion has not been determined. In addition, the Casa Loma Fault loses its surface expression toward the northwest. Our seismic lines run across the transition zone in the central part of the basin. [As used here, the San Jacinto Graben is the structural depression in the basement, the San Jacinto Basin refers to the sediment fill, and the San Jacinto Valley is a local geographic term that includes the area over the basin and some area southwest of the Casa Loma Fault .]

INTRODUCTION

The San Jacinto fault zone is the most active member of the San Andreas fault system in southern California, both in terms of slip rate and frequency of earthquakes with Richter magnitudes greater than 5 (Thatcher et al., 1975). Initiated about 1.5 million years ago with a geologically determined long-term average slip rate of about 15 mm/yr, this right-lateral, strikeslip fault zone comprises many disjointed segments or strands including the en echelon, right-stepping Claremont Fault and Casa Loma Fault in our study area (Morton and Matti, 1993). Motion along these two faults (Figure 1) has caused the formation of a pull apart basin in the San Jacinto area (Sharp, 1967, 1975; Crowell, 1974). The San Jacinto Basin is about 24 km long and 2 to 4 km wide and lies in a graben bounded on the northeast by the Claremont

Manuscript received by the Editor October 26, 1994; revised manuscript received November 15, 1995. *Institute of Geophysics and Planetary Physics, Department of Earth Sciences, University of California, Riverside, California 92521. ‡Branch of Earthquakes and Landslide Hazards, U.S. Geological Survey, Denver Federal Center, Denver, Colorado 80225. © 1996 Society of Exploration Geophysicists. All rights reserved. 1258

A Seismic Case Study, San Jacinto Basin

The San Jacinto Basin lies near the northern end of the southern California batholith. Pre-Tertiary plutons and metamorphic rocks crop out in the hills around the basin but the age and composition of basement rocks below the basin are unknown. Tertiary continental deposits constitute the badlands located north of the basin and northeast of the Claremont Fault, and extend to the foothills of the San Jacinto Mountains. However, there is no documented evidence of Tertiary deposits within the basin. The basin contains Pleistocene and Holocene fluvial deposits. Although their spatial distributions are not well delineated, the deposits are referred to loosely as older and recent alluvium, respectively. Both may contain aquifers, as evident by wells that have been drilled in the southeastern part of the basin to tap high quality water. However, the groundwater quality

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degrades northwest along the San Jacinto River (Schlehuber et al., 1989). Near our study area where the San Jacinto River exits the basin, the water quality is so poor that few deep water wells have been drilled. Consequently, there is only one well log suitable for constraining our sediment-basement interface model. The northwestern half of the San Jacinto Graben is poorly delineated because of the uncertainty in tracing the Casa Loma Fault northwestward. A high-resolution Mini-Sosie reflection study was conducted in 1993 to locate the Casa Loma Fault, identify hidden intrabasin faults, and ultimately explore tectonic implications and assess potential earthquake hazards (Park et al., 1995). The reflection data presented here were collected with the same method during the same period of time. Profile SJ1 along Bridge Street (P8 and P9 on

PIG. 1. Location of seismic lines and simplified geologic map of the San Jacinto Basin. Index maps at the upper right corner show the San Jacinto Basin relative to other prominent features including San Andreas fault zone (S.A.F.) and San Jacinto fault zone (S.J.F.). The basin is bounded by two right-lateral strike-slip faults (Claremont and Casa Loma). The map of the area around Bridge Street is enlarged at the lower left corner to show the refraction profiles P8 and P9 as well as refraction Offsets I, II, and III. Reflection line SJ1 (unmarked) follows the two refraction profiles. Both D/U symbols (down- and up-thrown blocks) and strike-slip symbols are attached to the Casa Loma Fault. Double dashed lines for Offset II indicate the uncertainty in locating a fault. The Older Alluvium in the enlarged map is a compressional ridge. Note that the two geological maps differ slightly. The regional map is modified after a 1:250,000 geological map of California (Santa Ana Sheet, California Division of Mines and Geology, 1965) while the enlarged map is adopted from a recent, unpublished map (scale 1:24,000) by D. Morton.

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Figure 1) intersects the northwestern-most end of the known trace of the Casa Loma Fault (Santa Ana Sheet, Geologic Map of California, California Division of Mines and Geology, 1965). The intent was to characterize the reflection across a presumably well-defined section of the Casa Loma Fault and to use those characteristics for future identification of the fault in poorly mapped areas. A seismic refraction study consisting of two profiles was also conducted along Bridge Street to compare the results of the two seismic methods. The reflection and refraction data were first processed independently. Two vertical offsets in refractors were identified. Offset I is inferred from the opposite dipping water-table refractors as a groundwater barrier. Offset III occurs in the basement and lies near the mapped trace of the Casa Loma Fault. Located between the two offsets, Offset II was postulated from a mismatch of the basement refractors under the two profiles. A preliminary stack section of the SJ1 reflection data did not support the findings in the refraction study, especially in the area around Offset III (Pendergraft, 1993). To resolve the difference, the field reflection data were reprocessed. The results of reprocessing confirm the refractor offsets as faults and provide more details about the layering and lateral structural transitions that are not observable in the refraction data. In the following sections we demonstrate how the refractor offsets are identified as faults and explain why a fault (Offset III) was mislinked with the Casa Loma Fault. For tectonic implications, the interested readers can refer to the work of Park et al. (1995). SEISMIC REFRACTION

Two seismic refraction profiles were established along Bridge Street across the middle part of the San Jacinto Basin (Figure 1). Profile 8 (Figure 2) extended 2.75 km southwestward across the regional structural trend from the Claremont Fault to the bend of Bridge Street (near the crossing of the San Jacinto River). Profile 9 (Figure 3) continued due south for 1.65 km. The refraction study was intended to identify major layering within the sedimentary section and depth to the sediment-basement interface. The refraction interpretation presented here was completed (Biehler and Lee, 1993) before reprocessing the reflection data. The refraction models have been reviewed in light of the revised reflection sections but modification was deemed unnecessary. Methods

Geophone spacing was 61 m (200 ft) on profile 8 and 30.5 m (100 ft) on profile 9. Seismic sources were 5.5 to 9.5 kg of high detonation pressure (HDP) primers detonated in drill holes 10 cm in diameter and 5 m in depth (multiple holes were used for profile 8). Each profile was shot forward and reverse. The signals were recorded with two 24-channel analog receivers. Elevation and shot-point depth corrections (Dobrin and Savit, 1988) were made by using the mean ground surface velocity and by setting the reference elevation at the highest geophone site along each profile. The corrections were generally very small except for 30 ms near the northeastern end of profile 8 (see vertically exaggerated topography under the traveltimedistance plots, Figures 2 and 3).

During iterative forward modeling, the time-distance data were displayed on a monitor for least-squares fitting of line segments with data points. Arrivals from each refractor, forward or reverse, were marked with the same symbol. A square symbol at the junction of two line segments denotes an arrival shared by both segments. The velocity, dip and depth of each planar interface (refractor) were determined from the apparent velocities and intercept times. Offsets in refractors were located, then refined by iterative ray tracing. Because refraction signals have sampled the central portion of an interface only, the structural section below each end point of a profile represents the result of extrapolation rather than direct determination. In the following discussion, all distances refer to an origin at the bend of Bridge Street. Profile 8 (from northeast to southwest) and Offset I

Figure 2 shows the time-distance plots of the first arrivals for the forward (northeast shot point) and reverse shots as well as the resulting velocity-structure model. As ordered from the lesser to greater arrival times, the first pair of line segments represents direct arrivals with an average velocity of 600 m/s. The second pair (marked by x’s) represents an interface of unknown nature. The third pair (dashed lines for both arrow and diamond symbols) links arrivals from the water table with an apparent velocity of 1900 m/s for the northeast shot and of 2200 m/s for the southwest shot. There are no arrivals from basement. Each line segment in the first pair is determined by connecting one geophone point to the shot point. Therefore the determination of the direct wave velocity is constrained poorly by the data. However, the uncertainties in velocity and thickness of this layer will not affect the determination of other interfaces significantly because it is only 5 to 15 m thick. Misfits along the two dashed lines are not randomly distributed. Relatively large misfits appear around points C, D, and E (Figure 2), suggesting the existence of an offset across the third interface (the ground surface is counted as the first interface). Hence the arrivals from the third interface are split into two pairs of subsegments: the southwest pair (marked by squares) representing a down-thrown interface and the northeast pair (marked by arrows) representing an up-thrown interface. The offset was located and refined in position by ray tracing as shown by a few key raypaths. One point marked by a circle between points C and D signaled an arrival through the offset zone and hence was not used in either time-distance segment. The throw (apparent vertical offset) is about 24 m. This determination is not well constrained because of the relatively large (61 m) geophone spacing. The down-thrown third interface dips northeast whereas the up-thrown interface dips southwest. If the third interface indeed represents the water table, the opposing slopes on the water table are not generally permissible. Hence a groundwater barrier is postulated with groundwater flowing along the hydraulic gradient from the northeast and southwest basin edges toward the barrier in the central basin. Our geophone spacings at 61 m were not designed to detect a groundwater barrier or resolve an offset in the water table. The finding of an offset was incidental and the postulation of a barrier could not be ascertained by our refraction data alone. This groundwater barrier in the San Jacinto Basin is marked as Offset I in Figure 1.

A Seismic Case Study, San Jacinto Basin

Its orientation cannot be determined from one refraction line and here is assumed to follow the regional structural trend. The available data cannot resolve whether the offset has extended upward across the second interface. Because this offset occurs across a water table, the amount of offset likely varies with the fluctuation of water table. The relative sense of offset could also be reversed during certain periods of time, depending on the relative water levels across the barrier. (Our data were collected in March, 1992 near the end of a long drought in southern California. As of May 1995, most of profile 8 had

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been under the 1994-95 flood water for several months.) Note that surface ramification of a groundwater barrier, like seepage or unusual vegetative distribution, has not been observed or reported. Profile 9 (from north to south) and Offset III The time-distance plot in Figure 3 indicates that there are four pairs of line segments including one from basement. The first three segments for the forward (shot point north) and

FIG. 2. Refraction profile 8. Top: Traveltime curves of first arrivals for forward (northeast) and reverse refraction shots. Topography

along the profile is shown at the bottom of the time-distance plot (vertical exaggeration =10). Numbers next to line segments are apparent velocities in units of m/s; numbers in parentheses represent apparent velocities for the dashed line segments, which are not used in the refraction modeling. Letters A through G denote locations of ray emergence for the raypaths shown in the structural section. A square at the junction of two line segments signifies an arrival used by the two segments. Bottom: Refraction model of structural section. Lines with arrows are raypaths.

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FIG. 3. Refraction profile 9. Top: Traveltime curves of first arrivals. See caption for Figure 2. Note that misfits along the fourth line segment (dashed line) of the south-shot are unacceptably large while misfits along the fourth segment of the north-shot are much smaller. Bottom: Refraction model of structural section. Basement between points 3 and 4 are constrained by both northand south-shots. Basement marked with question marks are estimated without arrivals from the north-shot. Note a subtle scarp located north of Offset III. The notch-like feature near 250-m distance is the levee of the San Jacinto River.

A Seismic Case Study, San Jacinto Basin

reverse shots appear to fit the data well. Like those under profile 8, the unsaturated sediments (second layer) have a velocity of 1100 m/s and saturated sediments (third layer) have a velocity of 2000 m/s. The top layer under profile 9 is slower by 100 m/s but is insignificant because the determination under profile 8 was constrained poorly. (We conducted a hammer seismic refraction survey in May 1995 and confirmed the top layer velocity of 500 m/s under profile 9.) The fourth segment of the north-shot appears to fit the data well also. However, misfits along the fourth segment (dashed line) of the south-shot are too large to have originated from one continuous interface. An offset in the basement is postulated to explain the misfit for the south-shot line and good fit for the north-shot line. In Figure 3 the basement arrivals for the south-shot are split into two subsegments, one marked by arrows and the other marked by overbarred x’s. Between the two subsegments are three offline arrivals marked by circles. The entire basement arrivals for the north-shot are treated as one segment CNG because there is no significant misfit in the fourth segment. The forward and reverse line segments in the south (the pair denoted by arrows) imply a north-dipping basement in the southern half of the structural section. The interpretation of an offset basement is illustrated with a few key raypaths. For the south-shot, the first refraction from the basement is observed at point E and the last one at point CS; for the north-shot, the arrivals from the basement spread from points CN to G. This pair of forward and reverse shots together have interrogated a common portion of the sediment-basement interface between points 3 and 4 (Figure 3) and the basement is well defined between these two points. A driller’s log from a water well that reaches the basement near point F confirms our determination of depth to the basement there. Also, extrapolation of the basement updip (southward) intercepts a point that is within 40 m from the outcrop of the Lakeview Mountains pluton. A driller’s log reveals a sequence of sand, silt, and clay but no significant seismic boundaries can be inferred from the log except the depth to the basement. North of point CS for the south-shot, the first arrivals do not follow the fourth line segment but arrived much sooner than expected. Those early arrivals along the northern half of the south-shot line indicate that the high-velocity basement in the northern half has been up-thrown against the southern half. Subsegment BA represents an offset basement refractor, while the off-line CSB represents rays that travel through a transition zone. The transition must have occurred north of the refracted raypath that reaches point CN (north-shot) and south of the refracted raypath that ends at point CS (southshot). This offset is marked as Offset III in Figure 1. Along the north-shot plot, the basement arrivals do not have any significant positive or negative time delay. There is no subsegment to correspond with the BA subsegment of the southshot because basement arrivals for the north-shot come from the southern, down-thrown side only. Hence, the dip and velocity of the up-thrown basement cannot be determined. Assuming that the up-thrown basement has the same velocity as the down-thrown basement and that subsegment BA of Figure 3 has yielded a true down-dip velocity, the up-thrown basement interface is found to dip north at about 20°, slightly steeper than its counterpart in the south. Because this northsouth section of Bridge Street runs at an oblique angle to the regional structural trend, the dips for the disrupted basement

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surfaces could be a few degrees more. The throw from the intercept times is determined to be about 130 m. Ray tracing at a few paths yields the computed traveltimes that are within 5 ms of the observed traveltimes and therefore support the general structural section shown in Figure 3. From one single refraction line, it cannot be ascertained whether Offset III is a fault or basement relief. Its location lies within 100 m of the presently mapped trace of the Casa Loma Fault. However, the apparent sense of vertical offset contradicts what is expected across the Casa Loma Fault. As a well-defined graben bounding fault in the southeastern part of the San Jacinto Basin, the Casa Loma Fault has its basin side down-thrown rather than up-thrown as implied by Offset III. In addition, the throw of 130 m is about 20 times less than the offset (about 2.5 km) across the Casa Loma Fault to the southeast. These inconsistencies will be discussed further in the section on seismic reflection. Offset II The structural sections drawn from the two refraction profiles are combined in Figure 4 (top) with a vertical exaggeration of 2.66 for comparison with an interpreted reflection section. Along profile 8, the depth to the basement is not determinable. However, the highest possible elevation for the basement surface can be estimated (Figure 4, dashed line) by assuming that the arrival immediately after the recorded arrivals from the water table would have refracted from the sediment-basement interface and that the velocity of the basement is the same as that under profile 9. This assumption for basement velocity is made for convenience only and does not necessarily imply that the basements under the San Jacinto Basin and outside of it are the same. The composite profile indicates that the sediment-basement interfaces do not match at the junction of the two profiles (at the Bridge Street bend). A fault is hence postulated to exist around the bend but its location cannot be determined from the refraction data. The mismatch between the basement elevations would be 260 m if the fault ran through the street bend. This estimate of mismatch represents a minimum estimate of offset because the basement under profile 8 was not detected and a maximum posssible elevation was used. The inferred sense of vertical offset across Offset II is compatible with that across the Casa Loma Fault. SEISMIC REFLECTION

High-resolution reflection data were acquired with the MiniSosie method (Shedlock et al., 1990; Stephenson et al., 1992). Data acquisition and processing are described in Park et al. (1995) for three reflection lines in the northwestern part of the San Jacinto Valley. The acquisition parameters are listed in Table 1, and the processing used to generate the stack time migration section (Figure 5, top) are listed in Table 2. A pseudodepth section was then constructed (using Promax software) after steep-dip, finite-difference time migration for 50% interval velocities (Figure 5, bottom). The dominant frequencies in the reflection data are in the range from 30 to 59 Hz. Vertical resolution is generally 16 to 30 m while the horizontal resolution (Sheriff’s Fresnel radii) range from 20 m at shallow depths (100 m) to 200 m at greater depths (around 800 m) for the interval velocities used (Park et al., 1995).

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It is noted that the signal-to-noise ratio in the MiniSosie records is much lower than that in typical Vibroseis records. Hence our velocity model is less certain than in the conventional seismic reflection data. We have used 30-70% interval velocities to migrate the data and visually examined the resulted sections. The 30% velocities tended to undermigrate the records when concave downward events appeared, and at 70% velocities, over-migration occurred. We have chosen 50%

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interval velocities for migration of shallow reflection although the usage is not a standard practice. Our choice has satisfactorily collapsed diffractions while minimizing over-migration and further enhanced data for interpretation. The field filter was set at 40-180 Hz with a gentle rolloff of 24 db/octave but significant wave energy at 20 Hz or lower still appeared in the data, and the 28 Hz geophones still permitted recording of waves in the 20-28-Hz range. The application of a prestack

FIG. 4. Top: A composite structural section formed by joining the refraction models in Figures 2 and 3 with a vertical exaggeration of 2.66. Because of mismatch in basement elevation at the junction, Offset II is assigned but its location is uncertain. Bottom: Interpreted pseudodepth section (vertical exaggeration = 2.66). Faults A and C are the locations of the Claremont and the Casa Loma fault traces, respectively. Fault B is interpreted to act also as a groundwater barrier. Fault D is a newly discovered fault, which has disrupted an unconformity (marked by the dotted lines). At the south end, disruption in reflectors near an adjacent pluton may reflect basement relief rather than a fault E. Two long sloping solid lines mark the displaced refraction-basement surface for comparison.

A Seismic Case Study, San Jacinto Basin

zero-phase, band-pass filter, 20-30-180-200 Hz, further reduces the undesired components. The poststack filter, 20-30-100120 Hz, acts essentially as a high-cut filter. Numerous disruptions in reflectors are recognized in the reflection sections. For correlation with refractor offsets and geologically mapped faults, those reflector disruptions are grouped into five faults, named in the sequence of A through E from the northeast end of Bridge Street to the south end. While the time section and the pseudodepth section are presented in Figure 5 without interpretation for the readers to reach their own conclusions, an interpreted section is drawn from the pseudodepth section and presented in Figure 4 below the refraction section for direct comparison. Fault B (Offset I)

Fault B is a zone of several minor faults with short vertical extent spaced about 40 to 80 m apart. The disruptions in

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reflectors appear near refraction Offset I (Figures 1 and 4) and splay further southwest. On the northeast side of fault B (subsection AB), the sediments have strong and continuous reflectors that are almost horizontal near the ground surface but warp downward at depth. In contrast, the sediments southwest of fault B have been disrupted severely and the reflectors are too fragmented to trace laterally beyond 100 m. The existence of fault B has not been reported in the literature. Across the fault the surficial sediments do not appear to change characteristics. There are no fault-indicating topographic changes or vegetative features. Cultural noise was investigated and found not to be the cause of the disruption at the reflectors. Thus fault B is hence interpreted as a concealed fault. The top reflector in subsection AB (marked by an inverted triangle) coincides in depth with the water table of refraction profile 8. It (along with two other shallow strong reflectors) dips gently southwest, consistent with the dip of the third

FIG. 5. Top: A final stack of time-distance section. Letters A through E mark locations of faults. Bottom: A pseudodepth section with 50% variable interval velocities.

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refractor. Southwest toward point B the top reflector drapes upward. Obviously the entire top reflector cannot represent the water table because there is no recharge of water near point B. Hence the three reflectors including the top one represent sedimentary layers. Across fault B, subsection BC appears to have been upthrown against subsection AB as shown by the change in the reflector slope (Figures 4 and 5). This relative sense of reflection offset in sedimentary layers contradicts the sense of relative offset across the refraction water table. Because the water table has not been detected by reflection in subsection BC and no layering below the water table has been observed by the refraction, the contradiction cannot be resolved by the available data. We maintain that both interpretations of the relative offset are acceptable because the water table can fluctuate with Table 1.

Acquisition parameters of reflection data.

Parameter Source type Source array Source point interval Source duration Geophone type Geophone array Geophone group interval Shooting spread Field filters Recording system Sampling rate Trace length Table 2.

Earth tampers (Wackers) 3 Wackers spaced 4.57 m (15 ft) apart, parallel to seismic line 15.24 m (50 ft) 2000 Wacker impacts summed per source point Litton 28-Hz resonance frequency, vertical component 12 geophones, summed in l-m point array 15.24 m (50 ft) 24 channels, end-on, with 46 to 396 m offset 60 Hz notch, 40-180 Hz, 24 db/octave rolloff Input/Output DHR 2400 1 ms 1s

Generalized flow of reflection data processing.

Processing step Geometry Trace edit Adaptive deconvolution Band-pass filter Resample Elevation statics Normal moveout AGC FK filter Residual statics AGC Band-pass filter Stack Migration

Description

Parameterization/comments Surface coordinates, elevations to trace headers Bad traces removed from processing L2-norm predictive, 80 ms operator length, 20 ms prediction distance Zero-phase, 20-30-180-200 Hz (0%-100%-100%-O% filter points) pass band 2 ms sample rate 915 m/s weathering velocity Correction picked by constant velocity analysis 995 ms operator length Multiple polygon, accept regions interpolated between selected shots Apply best solutions of two surface-consistent passes, 10 ms maximum time shift allowed 500 ms operator length Zero-phase, high cut, 20-30-100-120 Hz (0%-100%-100%-O% filter points) pass band Fold normalized Steep-dip, finite-difference time migration

time. In other words, an offset of water table across a fault does not have to conform in either direction or magnitude with the geologic offset if the fault also serves as a groundwater barrier. Fault B lies along the southeast projection of the Farm Road Fault in the northwest part of the San Jacinto Fault (Park et al., 1995). The genetic relation between the two faults separated by 7 km is not clear at present because the dipping and folding characteristics of sediments in the two areas of the basin are not obviously correlatable. Fault D (Offset III) Fault D coincides in location with Offset III along refraction profile 9 (Figures 1 and 4). On the north side of the fault, an unconformity at a depth of 150 m separates a set of deeper, north-dipping reflectors from an overlying set of horizontal reflectors. Another unconformity appears south of fault D at a depth of 200 m. The reflectors above this 200-m unconformity dip south away from fault D toward a pluton while the reflectors beneath the unconformity, although being relatively weak, seem to dip gently north. The reflectors across fault D are not correlatable because the reflector spacings on the two sides of the fault do not match. The two unconformities are displaced by about 50 m, which is less than the throw (130 m) estimated for the refractor basement but is compatible in the sense of relative offset. As discussed before, Offset III lies near the presently mapped trace of the Casa Loma Fault but the sense and the amount of offset are incompatible with what are known about the Casa Loma Fault in the southeastern part of the San Jacinto Basin, where the Casa Loma Fault can be delineated along a scarp. That scarp becomes less conspicuous northwestward and disappears at about 5 km southeast from the Bridge Street bend (Figure 1). From there northwestward, the trace follows the southwest edge of a pressure ridge and extends northwestward for about 3.3 km, then it bends and shifts west by 0.7 km to resume a northwest course along a subtle scarp-like feature (California Division of Mines and Geology, Geologic Map, Santa Ana Sheet, 1965; Morton, 1977). That feature crosses Bridge Street approximately at 100 m north of Offset III (see a subtle scarp along the topographic profile in Figure 3). On the other hand, the Casa Loma Fault in a strike-slip tectonic setting could have juxtaposed two basement blocks with their overlying sediments to account for the structural features near fault D. The drag folds around fault D, the opposite dipping reflectors, and the unequal reflector spacings across the fault could be indicative of such juxtaposition. In this case, fault D does not need to have a similar sense and amount of offset known for the Casa Loma Fault. If fault D is indeed the Casa Loma Fault, another fault with a large throw in the basement is needed around the Bridge Street bend. That scarp-like feature being a fault was not confirmed during an earlier trenching for water pipe lines along Bridge Street (D. M. Morton, 1994, personal communication). It could be a relic of an erosional feature made by the nearby San Jacinto River rather than a fault. Around fault D, the reflectors from depths 0 to 30 m are not discernible (Figure 5). Hence, our data cannot show whether fault D has extended upward to the ground surface. Because fault D was not detected by the trenching either, we concluded that fault D is a concealed fault.

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A Seismic Case Study, San Jacinto Basin

Fault C (Offset II, Casa Loma Fault) Offset II has been inferred to exist but it is not locatable with our refraction data. If the tracing of the Casa Loma Fault mentioned above were continued northwestward without the bending and westward shift, the fault trace would have intercepted Bridge Street near point C. Here, the disruptions in reflectors are not so intense as in the vicinities of faults B and D. Around the bend of the reflection profile, the reflectors dip basinward without disruption, flatten out with minor disruptions, then dip again toward the basin (Figures 4 and 5). We interpret those disruptions as evidences for the presence of a fault. Fault C and its associated splay faults may spread over a 400-m wide zone. The location and minor disruptions together are not sufficient to link fault C with the Casa Loma Fault. It is the incompatibility between fault D and the Casa Loma Fault that favors relocating the 1965-mapped Casa Loma Fault trace. In addition, linking fault D with the Casa Loma Fault requires finding another major fault that can account for a large offset across Offset II. Aligning the Casa Loma Fault trace with fault C can meet two conditions for being the Casa Loma Fault: the basinward side is down-dropped and the throw across the fault exceeds several hundred meters. Fault A (Claremont Fault) The Claremont Fault lies near the northeastern end of our seismic lines. It is definitely not locatable with our refraction data, and the distribution of splayed faults associated with the Claremont Fault cannot be explored satisfactorily by our reflection data either because the last 12 reflection traces have low stacking fold. Near point A, the shallow set of reflectors disappears northeast but the deeper set extends further. We interpret fault A as the Claremont Fault mainly because of its proximity to the geologically mapped fault. Fault E Near point E at the southern end of our seismic lines, the reflectors are disrupted. The pluton of Lakeview Mountains is nearby. Faults may exist there but the influence of basement relief may contribute to some lateral disruption. Passing traffic and activity in a store near point E could have affected the data quality and contributed to the uncertainty in interpretation. Therefore, the suggestion of a fault E needs to be substantiated by additional data. CONCLUSION

Our refraction and reflection methods complement each other in constructing the structural framework of the central San Jacinto Basin. In areas near basement outcrop, a refraction offset can be caused by faulting, basement relief, or some out-of-plane phenomena. The nature of our refraction offsets could not be ascertained if a high-resolution reflection profile were not available. For example, the refraction Offset III could be interpreted alternatively as basement relief. Furthermore, in the absence of reflection sections, a groundwater barrier (Offset I) would have not been recognized as a fault B that separates sediments of contrasting structures. On the other hand, our processing of reflection data has become more fruit-

ful by observing the refraction models, which tend to be crude in comparison. Fault D might nave been missed in the stack section if Offset III were not discovered first by refraction; and the reflection fault C as a major fault (Casa Loma Fault) might not be appreciated without the inference of a large refraction offset (Offset II). The refraction Offset I is interpreted as a groundwater barrier to account for water tables with opposing dips. Also, this interpretation allows the time-dependent relative offset across the water table to be detached from the relative sense of displacement across fault B in the reflection section. Perhaps bearing regional tectonic significance, fault B divides the basin into two parts: one with well defined and undisrupted reflectors on the northeast and the other with weak and disrupted reflectors on the southwest. Because of different reflector spacing, reflectors across fault D are not correlatable and accordingly the relative offset cannot be ascertained. A disjointed unconformity, however, substantiates the sense of relative offset in the basement refractor (Offset III, upthrown on the basinward side). The presently mapped Casa Loma Fault trace passes within 100 m from fault D but the sense of offset determined from the seismic data contradicts what is expected of the Casa Loma Fault. In the southeastern part of the San Jacinto Basin, the Casa Loma Fault is well delineated along a scarp and has its basinward side down-dropped. Furthermore, the estimated offset across Offset III is about 20 times less than the offset known for the Casa Loma Fault as a graben boundary fault. The incompatibilty between fault D (Offset III) and the Casa Loma Fault necessitates retracing of the Casa Loma Fault to reflection fault C (refraction Offset II). Large offset (greater than 260 m) within the uncertain zone of Offset II was not determined directly by refraction data but by the inference of mismatch in the extrapolated basement elevations around the junction of the two refraction lines. Unlike the intense disruption around faults B and D, fault C is characterized by minor disruption of reflectors. If the retracing of the Casa Loma Fault is acceptable, fault D is an unnamed fault located outside the basin, southwest of the Casa Loma Fault. We have identified several faults in the reflection section by disruption of reflectors. The sense of relative displacement and the amount of vertical throw cannot be ascertained in many instances because of the difficulty in tracing reflectors across those faults. Quantification of relative offset may not be possible from a single profile because the study area has been dominated by the strike-slip tectonics, resulting juxtaposition of different reflectors to give a false sense of vertical displacement. Perhaps drag folds, reflectors with opposing dips, and the difference in the reflector spacing across fault D are indicative of such juxtaposition. In case of a strike-slip juxtaposition (a proposition we do not favor), fault D is still the Casa Loma Fault and Offset II (fault C) is an unnamed fault with large vertical offset in the basement. ACKNOWLEDGMENTS

The seismic refraction study was funded by the Eastern Municipal Water District under Task Order R91-014-04 to TCL and SB. The reflection study was funded by the US Geological Survey through NEHRP grant 1434-92-2182 to SKP and TCL. Access permits by Richard Devuyst, Everett List, Boris

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Pirin, and Bert J. Verger as well as A & G, Inc., Agri Empire, and Riverside County Transportation Department are appreciated. We have benefited from field assistance by UCR geophysics students and Paul Davis and his students at UCLA. We thank Rick Adair, James L. Allen, Brian Damiata and David W. Eaton for their valuable comments. REFERENCES

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