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SanAndreasFaultGeometryatDesertHotSprings,California,andItsEffectsonEarthquakeHazardsandGroundwater

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ImpactFactor:2.32·DOI:10.1785/0120080117

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San Andreas Fault Geometry at Desert Hot Springs, California,

and Its Effects on Earthquake Hazards and Groundwater

by R. D. Catchings, M. J. Rymer, M. R. Goldman, and G. Gandhok*

Abstract The Mission Creek and Banning faults are two of the principal strands ofthe San Andreas fault zone in the northern Coachella Valley of southern California.Structural characteristics of the faults affect both regional earthquake hazards andlocal groundwater resources. We use seismic, gravity, and geological data to charac-terize the San Andreas fault zone in the vicinity of Desert Hot Springs. Seismic imagesof the upper 500 m of the Mission Creek fault at Desert Hot Springs show multiplefault strands distributed over a 500 m wide zone, with concentrated faulting within acentral 200 m wide area of the fault zone. High-velocity (up to 5000 m=sec) rocks onthe northeast side of the fault are juxtaposed against a low-velocity (<2000 m=sec)basin on the southwest side within the upper few hundred meters. Near-surface strandsof the Mission Creek fault dip steeply southwestward and northeastward and merge atdepth to form a narrower, steeply southwestward-dipping or near-vertical (80° to 90°)fault zone. The Banning fault, in contrast, dips northeastward (45° to 70°) toward theMission Creek fault, and the two faults likely merge into a single San Andreas faultzone at depth, indicating a transtensional fault system. Mainshock hypocenters fortwo of the historically largest (M >6:0) earthquakes in the area (in 1948 and 1986)occurred at or near the depths (∼10 to 12 km) of the merged (San Andreas) fault.Large-magnitude earthquakes that nucleate at or below the merged fault will likelygenerate strong shaking from guided waves along both fault zones and from amplifiedseismic waves in the low-velocity basin between the two fault zones. The MissionCreek fault zone is a groundwater barrier with the top of the water table varyingby 60 m in depth and the aquifer varying by about 50 m in thickness across a200 m wide zone of concentrated faulting.

Introduction

The San Andreas fault (SAF) represents the boundarybetween the Pacific and North American tectonic platesand extends about 1300 km from near the U.S.–Mexicanborder to Cape Mendocino in northern California. Generally,the San Andreas fault trends about N35°–40°W and is recog-nized as a relatively narrow (several kilometers wide) zonethat accommodates most of the slip between the two plates(Wallace, 1990). However, an approximately 100 km longsegment of the San Andreas fault within the San BernardinoMountains and the northern Coachella Valley is character-ized by a more westerly strike (∼N70°W) with much ofthe slip distributed over two or more major fault strands(Fig. 1). Our study area is located near the southeasternend of this more complex segment of the San Andreas faultwithin the city of Desert Hot Springs, California, about

160 km east of Los Angeles (Fig. 1). About 20 km southeastof Desert Hot Springs, the San Andreas fault splits into twomajor strands, a northeastern strand, herein referred to asthe Mission Creek fault, and a southwestern strand, hereinreferred to as the Banning fault (Allen, 1957; Proctor,1968; Clark, 1984; Matti and Morton, 1993). It is unclearhow these two major strands of the San Andreas fault zoneare related in the subsurface and how slip is partitioned be-tween the two strands. Although the Banning strand appearsto have generated historically larger earthquakes (Jones et al.,1986; Nicholson, 1996), the Mission Creek strand clearly isactive (Hauksson, 2000; Fumal et al., 2002a). Overall, thesouthern San Andreas fault appears to have generated at least14 large-magnitude earthquakes during the past 1500 yr andis believed to again be near failure (Fumal et al., 2002b).

The geometry (including dip) and velocity structure offaults strongly affect the severity of shaking during largeearthquakes (Oglesby et al., 1998, 2000; Aagaard et al.,2004), and faults are also known to affect the lateral and

*Now at: Weiss Associates, 5801 Christie Ave., Emeryville, California94608.

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Bulletin of the Seismological Society of America, Vol. 99, No. 4, pp. 2190–2207, August 2009, doi: 10.1785/0120080117

vertical distribution of groundwater (Bredehoeft et al., 1992;Caine and Forester, 1999; Anderson, 2006; Anderson andBakker, 2008). In this article, we use high-resolution seismicimaging methods, gravity modeling, and seismicity crosssections to better understand the structure of the MissionCreek fault, its potential shaking hazards, and its affect onthe groundwater aquifer system near Desert Hot Springs(Figs. 1 and 2).

Local Geology and Tectonics

Desert Hot Springs is located within the northern Coa-chella Valley on an alluvial fan of the Little San BernardinoMountains (Fig. 2). Rocks and sediments of the northern

Coachella Valley range in age from upper Miocene toQuaternary (Proctor, 1968). Basement rocks consist of am-phibolite and migmatitic paragneiss, mica–schist, recrystal-lized limestone, and granitic rocks (Matti and Morton, 1993).The basement complex is overlain by a series of consolidatedsedimentary rocks that include conglomerate, sandstone,siltstone, and shale, and both the basement and the sedimen-tary rocks locally contain volcanic intrusives. Sand, gravel,and alluvium overlie the consolidated sedimentary rocks.Proctor (1968) suggested that the entire sequence of conso-lidated and unconsolidated sedimentary rocks may be morethan 4 km thick in the northern Coachella Valley, and Saltusand Jachens (1995) infer a similar basin thickness based ongravity studies.

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Figure 1. Location map of the northern Coachella Valley area with major faults, cities, and the DHS seismic profile (short black line)discussed in this article (map from Rymer, 2000). AA′ and BB′ refer to cross sections shown in Figure 9. Banning fault (BF), Desert HotSprings (DHS), East Wide Canyon fault (EWCF), Garnet Hill fault (GHF), Long Canyon fault (LCF), Mission Creek fault (MCF), PintoMountain fault (PMF), and San Andreas fault (SAF). Asterisks depict the epicenters of the 1948 DHS and the 1986 North Palm Springs(NPS) earthquakes (Nicholson, 1986). Quaternary alluvium shown in white, Pliocene and Pleistocene stratified rock shown in dark gray, andpre-Cenozoic crystalline rocks, shown in light gray. The dotted box shows location of the map in Figure 2.

San Andreas Fault Geometry at Desert Hot Springs, California, and Its Effects on Earthquake Hazards 2191

Near Desert Hot Springs, the basement and sedimentaryrocks are cut by a series of faults that collectively form theMission Creek strand of the San Andreas fault zone. At thesurface, the Mission Creek fault is expressed as a zone ofmultiple fault strands (Proctor, 1968) that extends about70 km from its split with the Banning fault in the northernCoachella Valley to the San Bernardino Mountains, where itmerges with several other strands of the San Andreas fault

zone (Fig. 1). In the immediate Desert Hot Springs area,the Mission Creek fault is largely concealed beneath Qua-ternary alluvium, except locally where it disrupts Holocenealluvium along discontinuous scarps (Clark, 1984). South-east of Desert Hot Springs, along Miracle Hill, the MissionCreek fault forms more continuous scarps in Quaternarysediments (Fig. 3), and northwest of Desert Hot Springs,multiple strands of the Mission Creek fault are exposed at

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Figure 2. Geologic map (from Proctor, 1968) of the Desert Hot Springs (DHS) area showing the locations (bold black lines) of our DHSseismic profile (this study), an area of near-surface fault mapping along Mountain View Road (Rymer et al., personal commun., 2008), and aseismic profile (R–DHS) along Long Canyon Road (Rymer et al., personal commun., 2008). Mission Creek fault-a andMission Creek fault-brefer to the two mapped traces of the Mission Creek fault by Proctor (1968). The third mapped fault that crosses the DHS seismic profile is theMiracle Hill fault. The Banning fault is located in the lower left corner of the map. The box shows the approximate area of the photographshown in Figure 3.

2192 R. D. Catchings, M. J. Rymer, M. R. Goldman, and G. Gandhok

the surface in Mesozoic crystalline rocks and Tertiary andQuaternary sedimentary rocks (Proctor, 1968; Fig. 2).

The Mission Creek and Banning strands of the SAF arethe principal faults in the area, however, the Garnet Hill,Long Canyon, and numerous smaller faults are within closeproximity (Proctor, 1968; Rymer, 2000; Fig. 1). Collectively,these faults generate considerable seismic activity in the re-gion (Hauksson, 2000), including ∼M �6:0� earthquakes in1948 and 1986 (Richter et al., 1958; Jones et al., 1986;Nicholson, 1996). Richter et al. (1958) attributed the 4 De-cember 1948 ML 6.3 Desert Hot Springs earthquake tomovement on the Mission Creek fault and suggested that epi-centers of the mainshock and the resulting aftershocksformed a lineament parallel to and several kilometers north-east of the surface trace of the fault. However, Richter et al.’s(1958) locations were determined using a poorly determined1D velocity model and a fixed focal depth of 16 km. Using amore refined velocity model, arrival times from close, por-table stations deployed after both events, and based on themain shock and aftershock focal mechanisms, macroseismic

effects, and hypocenters that relocate between the Banningand Mission Creek fault, Nicholson (1996) attributed boththe 4 December 1948 ML 6.3 Desert Hot Springs and 8 July1986 ML 6.0 North Palm Springs earthquakes to slip on theBanning fault with epicenters between the surface traces ofthe two faults. Uncertainty regarding the causative faults forthe larger earthquakes in the area results from several factors,including the complex velocity structure of the region(Nicholson and Lees, 1992), a lack of surface ruptures fromhistoric earthquakes (Clark, 1984; Matti et al., 1992), anduncertainty in fault geometry among the major faults.

Seismic Survey

Data Acquisition

In March 1998, the U.S. Geological Survey, with assis-tance from Michigan Technological University, acquired theseismic reflection and refraction data used in this study. Our∼600 m long Desert Hot Springs (DHS) seismic profile wascentered on and oriented northwest–southeast across the

Figure 3. Oblique photograph of the study area. The bold white line shows the location of the DHS seismic profile with distances inmeters. Semitransparent white lines show the approximate location of faults mapped by Proctor (1968) shown in Figure 2.

San Andreas Fault Geometry at Desert Hot Springs, California, and Its Effects on Earthquake Hazards 2193

Mission Creek fault zone (Figs. 1–3). We generated the seis-mic sources (shots) with a BETSY-Seisgun™ using 8 gauge,300 grain blanks in 0.3 m deep holes, spaced 5 m apart. Seis-mic sensors consisted of 40 Hz Mark Products™ geophonesthat were colocated (1 m lateral offset) with the shot points.For each shot, approximately 2 sec of data were recordedusing a cabled array of two Geometrics Strataview™ RX-60 seismographs with a total of 120 active channels (Table 1).Shot-point and geophone locations were surveyed usingan electronic distance meter (EDM) with accuracies of about0.01 m.

Seismic Data Processing and Images

During data acquisition, the 120-channel recordingarray remained fixed and recorded all shots. Both seismicreflection and seismic refraction data were simultaneouslyacquired with the stationary recording array. We processedboth types of data to obtain independent information aboutthe subsurface structure and its properties. Representativeshot gathers for the seismic profile are shown in Figure 4.The shot gathers show disruptions in both refracted and

reflected arrivals, suggesting a highly complex subsurfacestructure.

In refraction data modeling, we used a modified versionof a refraction-tomography algorithm by Hole (1992) thatutilizes 2D or 3D raytracing through a gridded starting model

Figure 4. Representative shot gathers for shot points located along the DHS seismic profile. Vertical arrows mark areas with prominentoffsets of reflectors and refracted arrivals. The horizontal arrows point out prominent reflections. The data have been bandpassed between 30to 400 Hz, and automatic gain control (100 msec) has been applied.

Table 1Acquisition Parameters for the DHS Seismic Profile

Parameter Value

Profile lengthShotsSensors

699.42 m589.23 m

Orientation Southwest to northeastSeisgun (source)Shot spacingDepth

5 m0.3 m

Geophone spacing 5 mGeophone type 40 HzRecording system 2 GeometricsTM

RX-60 seismographsNumber of channels 120Sample rate 0.5 msecAcquisition filters NoneTrace length 2 secTiming Electrical

2194 R. D. Catchings, M. J. Rymer, M. R. Goldman, and G. Gandhok

until acceptable correlations among observed and calculatedfirst-arrival travel times are obtained. For the DHS velocitymodel, we used a 5 by 5 m gridded starting model thatwas determined from 1D analysis of shot gathers (Table 2).In the final 2D velocity model, some 5 by 5 m grids containedmore than 400 rays (Fig. 5). Checkboard tests (Zelt, 1998)performed on the final 2D model infer resolutions varyingwith depth from less than 40 m near the surface to about200 m at the base of the model. On the northeastern sideof the DHS seismic profile, seismic velocities range from600 m=sec near the surface to as much as 5000 m=sec at lessthan 50 m depth (Fig. 5a). However, on the southwest end ofthe profile, velocities vary from about 600 m=sec near the

surface to only about 2500 m=sec at 175 m depth. Theprominent change in velocities occurs over a distance ofabout 200 m (∼ 150 to 350 m of the seismic profile) andincludes the mapped surface traces of the Mission Creekfault (Proctor, 1968; Clark, 1984).

Table 2Starting Model for the DHS Refraction-Tomography

Velocity Model

Distance (m) Velocity (m=sec) Depth (m)

0–700 800 00–700 6000 2900–700 5900 300

Figure 5. (a) P-wave refraction-tomography velocity model inverted from first-arrival refractions along the DHS seismic profile. Ve-locities are in meters per second, and depth is in meters. The 1500 m=sec contour that correlates with the top of the groundwater table isdrawn in white. The white circles show the approximate depth of the top of the groundwater table as determined from wells along the seismicprofile, and the yellow squares denote the calculated depths to the top of the groundwater table from a coincident resisitivity survey by Mayeret al. (2007). MCF-a and MCF-b refer to mapped traces of the Mission Creek fault, and MHF refers to the mapped trace of the Miracle Hillfault (see Figs. 2 and 3). (b) Ray-density plot for the seismic velocity model along the DHS seismic profile. Areas of the velocity model andray-density plot with less than two rays per square are shaded in white and black, respectively.

San Andreas Fault Geometry at Desert Hot Springs, California, and Its Effects on Earthquake Hazards 2195

In seismic reflection data processing, we followed pro-cedures similar to those outlined by Brouwer and Helbig(1998). Processing involved the following steps: geometryinstallation, trace editing, timing corrections, bandpass filter-ing, automatic gain control (AGC), F-K filtering, velocityanalysis, elevation statics, moveout correction, velocity in-version, muting, stacking, and deconvolution. For the mi-grated image, we used Kirchoff prestack depth migration.The principal processing parameters used are listed inTable 3. Velocities needed in seismic reflection stacking weredetermined using the refraction-tomography velocity modeland semblance. We used these velocities to convert thereflection time images to depth images and to migrate theseismic reflection images. An unmigrated, seismic reflectionstack of the upper 300 m is shown in Figure 6a. The unmi-grated seismic image shows a series of layered reflectors inthe upper 40 m on the northeastern end of the profile, but thelayered reflectors extend to at least 100 m depth on the south-western end of the profile. Some of the reflectors can betraced laterally for several hundred meters but are verticallyoffset in multiple locations. Low-frequency diffractionsoccur over a range of depths and likely arise from sharpedges at vertically offset layers. We used Kirchoff prestackdepth migration to collapse the diffractions and to move thereflected seismic energy to a more proper location on thereflection image (Fig. 6b).

Interpretation

Lithology

We superimposed our seismic velocity model onto ourmigrated seismic reflection image to better estimate thelithology and structure of observed reflectors along the seis-mic profile (Fig. 7). Near-surface reflectors correlate withvelocities in the range of 600 to 1500 m=sec; these relativelylow velocities suggest that these reflectors consist of un-consolidated sediments. Because the velocity of saturated,

unconsolidated sediments is known to be at least1500 m=sec (Schon, 1996), we use the 1500 m=sec velocitycontour in our seismic model to infer the top of the ground-water table across the Mission Creek fault (Figs. 5 and 7).This correlation is consistent at the locations of wells oneither side of the principal fault zone (Fig. 5a). Empiricalobservations for refraction-tomography velocity images else-where in the region have also shown that the 1500 m=secvelocity contour correlates with the top of the groundwatertable as observed in wells (Catchings et al., 1999a, 2000;Gandhok et al., 1999). On the basis of velocity and reflec-tivity, we suggest that unconsolidated sediments are gener-ally less than 50 m thick northeast of the surface trace of theMission Creek fault and as much as 150 m thick southwest ofthe surface trace (Fig. 7). Prominent reflectors beneath the un-consolidated sediments are probably partially consolidatedsedimentary rocks (conglomerate, sandstone, and siltstone)as described by Allen (1957). Based on refraction-tomography measurements in the region and elsewhere(Catchings et al., 1999a,b, 2000, 2002, 2006, 2007, 2008a,b,c), the partially consolidated sediments likely have veloci-ties in excess of about 2200 m=sec. The imaged strata are alsoconsistent with strata observed in outcrop. For example,where consolidated sedimentary rocks are exposed onMiracleHill (Fig. 2), approximately 1 km northwest of the DHS seis-mic profile, Rymer et al. (personal commun., 2008) reportnear-surface strata dipping at about 10 degrees to the north-east, consistent with the local dip of our imaged reflectorsacross the corresponding part (400–575 m) of Miracle Hill(Fig. 7). We interpret the underlying, less reflective rocksnortheast of the Mission Creek fault zone to be crystallinerocks (>4 km=sec) like the granitic and metamorphic rocksexposed in the nearby Little San Bernardino Mountains(Fig. 2). Similar velocities (∼4 km=sec) are observed withinnear-surface granitic rocks elsewhere in the region (Catchingset al., 2000, 2002, 2008b).

Near-Surface Fault Locations

We interpret faults in the reflection images as abruptlateral terminations or vertical displacements of a series ofreflectors over a range of depths. In many cases, these inter-preted faults appear as diffractions over a range of depths onthe unmigrated seismic image (Fig. 6a). Vertically offsetreflectors occur along the length of our seismic profile, sug-gesting that the fault zone is at least 500 m wide (Fig. 8a, b),consistent with surface exposures of at least four fault strandsnorthwest of Desert Hot Springs (Proctor, 1968; Fig. 2).However, along our seismic profile, the principal near-vertical offsets associated with the Mission Creek fault inthe near surface occur over an ∼200 m wide zone (∼ 150to 350 m of the seismic profile) with at least two prominenthigh-angle fault strands and associated minor faults.

This 200 m wide zone (Fig. 8a,b) includes the transitionfrom shallow-depth, high-velocity rocks northeast of the faultto deeper, low-velocity sediments southwest of the fault.

Table 3Reflection Processing Parameters for the DHS

Seismic Profile

Parameter Value

Maximum fold 116Common depth point spacing 2.5 mAGCPrestackPoststack

200 msec500 msec

Bandpass filterPrestackPoststack low

40-80-200-400 Hz40-80-600-1200 Hz

Frequency wave number filter 90–400 m=sec (reject)1–600 Hz

Deconvolution(Predictive distance/Filter length) 15 m/100 mMigration(Angle, aperture) 90°, 200 m

2196 R. D. Catchings, M. J. Rymer, M. R. Goldman, and G. Gandhok

Figure 6. (a) Unmigrated seismic reflection stack along the DHS seismic profile. Depth and distance are in meters. Prominent diffractionsare located beneath and southwest of the surface trace of the Mission Creek fault. MCF-a and MCF-b refer to mapped traces of the MissionCreek fault, and MHF refers to the mapped trace of the Miracle Hill fault (see Figs. 2 and 3). (b) Migrated reflection depth section along theDHS seismic profile, plotted as in Figure 6a.

San Andreas Fault Geometry at Desert Hot Springs, California, and Its Effects on Earthquake Hazards 2197

Reflectors with similar velocities on either side of the zone offaulting appear to be offset vertically by at least 150 m, withthe deeper basin on the southwest side of the fault zone. Atabout 100 m depth, the basin edge is located at about250 m distance, but the most prominent basin-bounding faultapproaches the surface farther to the northeast, near 300 mdistance (Fig. 8a,b). Near about 200 m distance of the seismic

profile, a near-vertical (∼89° northward) fault coincides withone of the mapped traces of the Mission Creek fault (Figs. 2and 8a,b), but this fault appears to join with a southwest-dipping (∼83°) fault at several hundred meters depth, withthe combined fault dipping slightly southwestward belowthat depth. A similar structure is associated with anotherset of fault strands between 400 and 450 m of the seismic

Figure 7. Migrated seismic reflection image (from Fig. 6b) with the refraction-tomography velocity model (from Fig. 5a) superimposed.The white line shows the 1500 m=sec velocity contour. MCF-a andMCF-b refer to mapped traces of the Mission Creek fault, and MHF refersto the mapped trace of the Miracle Hill fault (see Figs. 2 and 3).

2198 R. D. Catchings, M. J. Rymer, M. R. Goldman, and G. Gandhok

SW NE

SW NE

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600500400300200100

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Figure 8. (a) Migrated seismic reflection image (from Fig. 6b) with interpretative faults. (b) Interpretative cross section based on ourmigrated seismic reflection image (Fig. 6b), velocity model (Fig. 5a), seismicity plots (Fig. 9), and gravity model (Fig. 10) along the DHSseismic profile. MCF-a and MCF-b refer to mapped traces of the Mission Creek fault, and MHF refers to the mapped trace of the Miracle Hillfault (see Figs. 2 and 3).

San Andreas Fault Geometry at Desert Hot Springs, California, and Its Effects on Earthquake Hazards 2199

profile, where a steeply northeast-dipping near-surface tracemerges with a southwest-dipping trace in the upper 100 m.Previously measured near-surface dips of ∼80° to the north-east for the Mission Creek fault (Proctor, 1968; Clark, 1984)may have been measured on similar northeast-dipping (anti-thetic) faults. Similarly, Rymer et al. (personal commun.,2008) mapped multiple faults within a road cut along Moun-tain View Road, about 1 km northwest of our seismic profile.Their mapping shows predominantly southwest-dipping(∼30° to 70°) faults in the near surface and only one north-east-dipping fault (∼45°) that merges with a southwest-dipping fault.

Comparison with a Nearby Seismic-Reflection Image

Rymer et al. (personal commun., 2008) acquired anapproximately 1 km long seismic reflection survey acrossthe Mission Creek fault zone along Long Canyon Road,approximately 1 km southeast of our seismic profile (Fig. 2).Their seismic profile utilized similar (5 m) shot and geo-phone spacings, but they used data acquisition and proces-sing methods that do not yield coincident seismic velocities.As a result, their seismic images do not resolve structures inthe upper 50 m. Below 50 m depth, however, both Rymeret al. (personal commun., 2008) and our seismic imagesdepict similar structures, including a fault-bounded basinsouthwest of the surface trace of the Mission Creek fault,distributed faulting along the length of the profile and im-aged fault strands that dip predominantly to the southwestor near vertically. Near the mapped surface trace of theMission Creek fault, Rymer et al. (personal commun.,2008) imaged both a northeast-dipping strand and a near-vertical (slightly southwest-dipping) strand, but the north-east-dipping strand merges with the southwest-dippingstrand at several hundred meters depth. Thus, both Rymeret al. and our reflection images are consistent in imaginga slightly southwestward dip for the principal strand ofthe Mission Creek fault.

Mission Creek Fault Zone Geometry at Depth

Previous Seismicity Studies

At depth, the dip of the Mission Creek fault zone hasbeen previously inferred on the basis of seismicity and grav-ity studies. Richter et al. (1958) calculated a northward dip of75° for the Mission Creek fault, based principally on epicen-ters related to the 1948 Desert Hot Springs sequence thatlocated northeast of the Mission Creek fault. However,Richter et al. (1958) calculated location (and dip) may haveresulted from systematic location bias caused by velocitycontrasts across the fault (Jones et al., 1986). More refinedlocations suggest that the 1948 earthquake did not occur onthe Mission Creek fault but instead on a northeast-dippingBanning fault, with the epicenter between the surface loca-tions of the Mission Creek and Banning faults (Nicholsonand Lees, 1992; Nicholson, 1996). Thus, one of the principal

indicators previously used to infer a northward dip of theMission Creek fault at depth is inconsistent with these morerecent studies.

Hypocenters recorded between 1980 and 2000, includ-ing those of the 1986 North Palm Springs earthquakesequence (Jones et al., 1986; Nicholson, 1996), do not alignalong a northeast-dipping plane from the Mission Creekfault. Instead, the hypocenters align along an ∼50°-northeast-dipping plane that extends from the Banning fault, with themajority of hypocenters terminating beneath the surface traceof the Mission Creek fault. Although there are a few hypo-centers that locate north of the surface trace of the MissionCreek fault, those hypocenters likely result from locationbias associated with the 1D velocity model used to locatethe earthquakes (Jones et al., 1986; Nicholson, 1996), orthose hypocenters likely occurred on faults other than theMission Creek fault (Rymer, 2000; see Fig. 1). Furthermore,the seismicity cross-sections of Jones et al. (1986) andNicholson (1996) may not accurately display the relationshipbetween the fault zone and the pattern of hypocenters be-cause their cross sections do not account for the appreciablewidth of the Mission Creek fault zone, and their crosssections include hypocenters from as far away as 10 km.The prominent bend in the Mission Creek fault causes someof the distant hypocenters to project slightly north of thefault trace.

Relocated Seismicity

To better infer the dip of the Mission Creek fault at seis-mogenic depths near our seismic line, we used relocatedhypocenters and double-difference-located hypocenters(Hauksson, 2000) to plot more localized seismicity crosssections (Fig. 9). The localized cross sections include onlyrelocated hypocenters (1980–1992; 1993–2000) within�3 km of the cross sections, and the cross sections excludemost hypocenters clearly associated with other highly activefaults, such as the East Wide Canyon, Long Canyon, Moron-go Valley, and Pinto Mountain faults. We also excluded seis-micity associated with the two major earthquakes in theregion that were not on the San Andreas fault (the M 7.31992 Landers andM 6.2 Joshua Tree earthquakes; Haukssonet al., 1993). Our localized cross sections show that between1980 and 2000, most hypocenters occurred beneath or south-west of the Mission Creek fault and were contained withinplanes that dip about 50° to 70° northeast from the surfacetrace of the Banning fault and about 80° to 90° southwestfrom the surface trace of the Mission Creek fault (Fig. 9).Assuming that the Mission Creek fault plane is near themiddle of the hypocenter distribution, as assumed for the50° northeast-dipping Banning fault (Jones et al., 1986;Nicholson, 1996), the alignment of hypocenters in thevicinity of the Mission Creek fault infers a fault that dipsbetween 80° to the southwest and vertical (Fig. 9c).

2200 R. D. Catchings, M. J. Rymer, M. R. Goldman, and G. Gandhok

Gravity Modeling

Griscom and Jachens (1990) inferred a northward dipfor the Mission Creek fault on the basis of gravity modeling,but we suggest that the northward dip is not required by thegravity data. To determine if the gravity field permits a ver-tical or southward dip of the Mission Creek fault, we usedconstraints provided by our seismic images, geologic data ofProctor (1968), and geophysical data of Nicholson (1996) tomodel the Bouguer gravity anomaly determined by Ponceand Langenheim (1992) along our seismic profile (Fig. 10).We developed three gravity models with the basin-boundingMission Creek fault dipping at 80° southwest, 80° northeast,and 50° northeast (models 1, 2, and 3, respectively). Wechose the 80° southwest dip (model 1) on the basis of thedips observed in our seismic profile (Fig. 8), the 80° north-east dip (model 2) on the basis of nearby surface measure-ments reported by Proctor (1968), and the 50° northeast dip(model 3) on the basis the interpreted velocity model andseismicity data of Nicholson (1996). Density values forour gravity model were modified from those of Griscomand Jachens (1990). We assumed a minimum thickness ofabout 2 km for the low-density sediments of the northernCoachella Valley because a nearby well shows that sedimentsare in excess of 2 km deep slightly southwest of our profile(Proctor, 1968).

Our gravity modeling indicates that a northward dip ofthe Mission Creek fault causes the gravity field to be under-

estimated northeast of the Mission Creek fault due to low-density rocks in the footwall of the fault zone. Our modelingindicates that the misfit between the calculated and observedgravity anomaly further increases as the thickness of low-density rocks is increased beyond 2 km when a northeast-dipping fault is used. A 4 km deep basin, as suggested byProctor (1968), and a moderate northeast-dipping MissionCreek fault are, therefore, inconsistent with the gravity data.However, we obtain progressively better fits to the observedgravity data as the dip of the Mission Creek fault varies fromnortheast to southwest because a southwesterly dip placeshigher density rocks southwest of the main fault zone(Fig. 10). We found the gravity data to be most consistentwith our calculated anomaly when we used a southward(∼80°) dip for the Mission Creek fault and a 50° northwarddip for the Banning fault in the upper few kilometers.Although we do not consider our gravity model to be con-clusive, the cumulative data (seismic images, gravity model-ing, and seismicity data; see following section) suggest thatthe Mission Creek fault dips steeply southwestward or nearvertically (∼80°–90°), and the Banning fault dips about ∼50°to 70° northeastward.

Merger of the Banning and MissionCreek Faults at Depth

Our modeled dip for the Banning fault (Fig. 10) agreeswith the range of dips (∼50° to 70°) suggested by Jones et al.

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Figure 9. (a) Plot of seismicity along and within 3 km of profile AA′ (see Fig. 1). (b) Plot of seismicity along and within 3 km of profileBB′. (c) Plot of double-difference-located seismicity (Hauksson, 2000) along and within 3 km of profile BB′. Approximate locations of thesurface traces of the Garnet Hill, Banning, Mission Creek, and unnamed faults are shown. Dashed lines are drawn to indicate dips of 50°, 60°,and 70° northeast for the Banning fault, and 80° southwest, 90°, and 80° northeast for the Mission Creek fault.

San Andreas Fault Geometry at Desert Hot Springs, California, and Its Effects on Earthquake Hazards 2201

(1986) and Nicholson (1996). Due to the close lateral proxi-mity of the Mission Creek and Banning faults at the latitudeof our seismic profile, a slightly southwestward or near-vertical dip for the Mission Creek fault (as inferred from ourseismic images and gravity modeling) and a moderate-to-steep dip for the Banning fault (as inferred from seismicityand gravity studies) implies that the two faults join within theseismogenic zone (Fig. 11). Assuming respective minimumand maximum dips of 50° and 70° for the Banning fault and80° to 90° for the Mission Creek fault, the two faults wouldmerge between about 5 and 25 km depth in the northernCoachella Valley (Table 4) with increasing depths ofmerger from the southeast to the northwest (Fig. 11).

Hypocenters of mainshocks for both the 1948 DesertHot Springs and the 1986 North Palm Springs earthquakesappear to have occurred near the depths at which the Banningand Mission Creek faults likely merge into a unified SanAndreas fault (Table 4). The Banning and Mission Creekfaults are laterally separated by about 5 km at the latitude

of the 1948 Desert Hot Springs mainshock (Fig. 1); at thatdepth (∼12 km), Nicholson (1996) suggests that the Banningfault dips northeastward more than 70°, which is consistentwith our model of a moderate dipping (∼50°) Banning faultin the upper few kilometers that steepens (> 70°) with depth(Figs. 9c, 10b, and 11). Assuming an average dip of about70° for the Banning fault (Nicholson, 1996) and a near-vertical Mission Creek fault (as suggested from this study),the Banning and Mission Creek faults would merge at about12 km depth, near the seismically determined 12 kmhypocentral depth (Nicholson, 1996) of the 1948 mainshock(Table 4). Similarly, at the latitude of the 1986 North PalmSprings mainshock, the surface traces of the Banning andMission Creek faults are separated by about 10 km (Fig. 1).There, Nicholson (1996) suggests that the Banning fault dipsabout 50° to the northeast. This 50° dip of the Banning, com-bined with a near-vertical Mission Creek fault (80° to 90°),infers that two faults merge between 9 and 11 km depth,approximately the hypocentral depth (10.4 and 11.3 km)of the 1986 North Palm Springs mainshock (Nicholson, 1996and Jones et al., 1986, respectively). Thus, the two histori-cally largest earthquakes in the region appear to haveoccurred near the depths at which the two main faults likelymerge.

Earthquake Hazards

Among other factors, the crustal structure (velocity ofnear-surface materials, dip and impedance contrast of crustalinterfaces, etc.), and the geometry (dip, rake, rupture direc-tion, etc.) of faults are important factors that affect the lengthand severity of ground shaking in a given location duringmoderate- to large-magnitude earthquakes (Borcherdt andGlassmoyer, 1992; Catchings and Kohler, 1996; Catchingsand Lee, 1996; Olgesby et al., 1998; 2000; Aagaard et al.,2004; Zhang and Chen, 2006). A thorough understandingof the geometry of the San Andreas fault and the crustalstructure of the northern Coachella Valley are, therefore,important in estimating ground shaking from future large-magnitude earthquakes in that region. The San Andreas faultin the northern Coachella Valley is not a simple, near-verticalplanar structure. Instead, the San Andreas is a zone of faults,including the Mission Creek, Banning, Garnet Hill, and otherunknown fault strands, which extends across much of thenorthern Coachella Valley, and each of these major strandsincludes multiple traces in the near surface that are dis-tributed over 0.5 km or more (Proctor, 1968; this article;Rymer et al., personal commun., 2008). Our analysis furthersuggests that the Mission Creek and Banning faults (and pos-sibly other faults) merge at depth into a single San Andreasfault zone, which has implications for the locations of strongshaking in the region.

A single San Andreas fault at depth, composed of theBanning, Mission Creek, Garnet Hill, and possibly otherstrands, can widely distribute seismic waves across thenorthern Coachella Valley, whereby guided waves from

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2202 R. D. Catchings, M. J. Rymer, M. R. Goldman, and G. Gandhok

deep-seated earthquakes (>10–12 km) can be channeled (Liand Leary, 1990) into the individual strands. We theorizethat the wide distribution of strong shaking near major faultzones across the northern Coachella Valley during the 1986

ML 5.9 North Palm Springs earthquake (Sharp et al., 1986)may have been caused by such guided waves, as well as thestrong shaking observed along the Mission Creek faultduring the 1948 Desert Hot Springs earthquake, as described

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Figure 11. Interpretative 3D fault model (gray shading) of the northern Coachella Valley. In this model, the Mission Creek fault is near-vertical, and the Banning and Garnet Hill faults dip northward toward the Mission Creek fault; dip increases with depth. The model assumesthat the Banning fault merges with the Mission Creek fault at progressively greater depths from southeast to northwest. Gray dots showearthquake hypocenters (from Hauksson, 2000) along profile BB′ (see Fig. 1).

San Andreas Fault Geometry at Desert Hot Springs, California, and Its Effects on Earthquake Hazards 2203

in Nicholson (1996). Although the 1948 Desert Hot Springsmainshock appears to have occurred on the Banningfault, Nicholson’s (1996) summary of reports from obser-vational surveys following 1948 DHS mainshock indicatethat shaking was very strong along the Mission Creek faultwith ground cracking, slumping of cliffs and river banks,increased flow of springs, and changes in the temperatureof well water. In contrast, although the Banning faultmay not have been as closely examined as the Mission Creekfault following the 1948 event, no such strong shakingwas reported for the apparently causative Banning fault(Nicholson, 1996).

In addition to the guided-wave effect, the geometry (par-ticularly the dip) of the Mission Creek fault is an importantfactor in strong shaking, as recent studies have shown thatground motions are typically stronger on the hanging-wallrelative to the footwall of reverse faults (Oglesby et al.,1998, 2000). If, as indicated by previous studies, the MissionCreek fault is a northeast-dipping reverse fault, the areanortheast of the fault, which includes the most populous partsof Desert Hot Springs, would be located on the hanging wallof the fault and subject to stronger shaking. However, ourstudy indicates that the Mission Creek fault is a near-verticalor slightly southwest-dipping strike-slip fault, implying thatalthough the most populous parts of the city of Desert HotSprings would be subject to strong shaking from guidedwaves and from surface ruptures along the distributed faults,that area would not be subject to increased shaking caused bythe hanging-wall effect.

The area southwest of the Mission Creek fault, includingmost of the northern Coachella Valley, would also likely ex-perience strong shaking due to the presence of low-velocitysediments, which are known to amplify seismic waves(Borcherdt, 1970; Borcherdt and Glassmoyer, 1992; Olsenet al., 2006). The amplified shaking could conceivably arisefrom earthquakes on either strand of the SAF. Such strongshaking in the northern Coachella Valley may only directlyaffect a population of about 150,000 people (2000 U.S. Cen-sus); however, because major lifelines (the Colorado RiverAqueduct, natural gas lines, electrical lines, I-10 freeway,etc.) that serve millions of people in larger population centersof southern California are concentrated in the northern Coa-chella Valley, the earthquake hazard would not be confined tocities of the northern Coachella Valley. To better mitigate the

hazard to these lifelines, and therefore, to the larger popula-tion centers of southern California, a more complete under-standing of the fault geometry and shallow basin structures inthe northern Coachella Valley and other valleys along theSan Andreas fault zone is needed.

Groundwater

The northern Coachella Valley is segmented into fourprincipal groundwater subbasins that are separated by themajor fault strands of the San Andreas fault zone (Planertand Williams, 2009). Our seismic profile straddles theboundary between the Desert Hot Springs subbasin northeastof Mission Creek fault and the Mission Creek subbasin to thesouthwest of the fault, with the two subbasins separated bythe Mission Creek fault. Most of the water used in the north-ern Coachella Valley is derived from groundwater wells(Planert and Williams, 2009), and therefore, the structuralcharacteristics of the groundwater basins are important inevaluating and managing the groundwater resources. Inthe near surface, the Mission Creek fault acts as a barrierto flow of groundwater that drains from the western slopesof the Little San Bernardino Mountains, and evidence of thebarrier nature of the fault is seen from surface vegetationlineaments and from wells that show the depth to ground-water is deeper on the southwest side of the Mission Creekfault zone relative to its northeast side (Proctor, 1968; Mayeret al., 2007; Fig. 5a). Our seismic profile was favorablysituated to image the lateral variation in the depth to thetop of the groundwater table across parts of the two sub-basins. Because the P-wave velocity of water-saturated,unconsolidated sediments is known to be approximately1500 m=sec (Schon, 1996), we used the 1500 m=sec con-tour of our velocity model as a proxy for the top of the staticgroundwater table at the time of data acquisition (Fig. 5a).Comparisons among groundwater depth from nearby wells,a resistivity survey (Mayer et al., 2007), and the 1500 m=secvelocity contour of our refraction-tomography model showsthat our tomography model accurately maps the top of thegroundwater table (Fig. 5a). Our velocity model suggests thatthe top of the groundwater table varies by as much as 60 macross a 200 m wide central Mission Creek fault zone withgreater depths southwest of the principal fault zone. Thegreatest vertical variation in the 1500 m=sec contour occursbetween 150 and 350 m, where our seismic images show

Table 4Depths at which the Mission Creek and Banning Faults Merge *

MCF Dip (Southwest)† BF Dip (Northeast)‡1948 DHS (Earthquake)

Merger Depth (km)§1986 NPS (Earthquake)

Merger Depth (km)

90° 70° ∼12:4� ∼24:590° 50° ∼5:4 ∼10:7�80° 70° ∼8:5 ∼16:380° 50° ∼4:5 ∼9:0

*As a function of the dip at the latitudes of hypocenters of the 1948 Desert HotSprings earthquake and the 1986 North Palm Springs earthquake.

2204 R. D. Catchings, M. J. Rymer, M. R. Goldman, and G. Gandhok

faulting to be most pervasive. This observation suggests thatthe change in depth of the groundwater table does not occuracross a single fault trace but a series of faults. Our velocitymeasurements indicate that the groundwater table is about20–30 m below the ground surface northeast of the faultand is about 60–80 m below the ground surface southwestof the principal fault zone (Fig. 5a). Unconsolidated topoorly consolidated sediments with velocities in excess ofabout 2200 m=sec have been correlated elsewhere withrelatively low-permeability sediments, which were not con-sidered part of the principal aquifer system (Catchings et al.,2006, 2007). Thus, assuming that the 1500 m=sec velocitycontour coincides with the top of the water table, andthe 2200 m=sec contour coincides with relatively low-permeability sediments, our velocity image suggests that,along our seismic profile, the most productive aquifer inthe Desert Hot Springs subbasin is about 20 m thick onthe northeast side of the Mission Creek fault and about70 m thick in the Mission Creek subbasin on the southwestside of the Mission Creek fault (Fig. 5a). It is likely, however,that the more permeable sediments appreciably increase inthickness beyond the southwest end of our seismic profile,toward the center of the Mission Creek subbasin.

In many alluvial-covered basins, active faults are not ap-parent at the surface, the faults generate limited measurableseismicity, and/or cultural noises mask the seismic events. Asa result, many active faults are not detected until moderate- tolarge-magnitude earthquakes occur on them. However, stud-ies of the hydrogeology in alluvial-covered basins can behighly useful in locating such active faults, as studies haveshown that many active faults are groundwater barriers, andthese faults affect the lateral and vertical distribution ofgroundwater (CDWR, 1967; Hill et al., 1979; Bredehoeftet al., 1992; Caine and Forester, 1999; Anderson, 2006;Anderson and Bakker, 2008). Abrupt lateral changes inthe depth to groundwater, such as seen across the MissionCreek fault zone, can be useful in inferring the locationsof blind faults and because refraction tomography can accu-rately map the lateral variations in groundwater, we suggestthat refraction tomography is an ideal tool to map such faultbarriers (Catchings et al., 1999a,b, 2000, 2006, 2008a;Gandhok et al., 1999).

Summary and Discussion

Our analysis shows that the Mission Creek fault zone atDesert Hot Springs is characterized by a zone of distributedfaults that is at least 500 m wide, with a more concentratedzone of faulting that is about 200 m wide near the mappedsurface traces. The principal fault zone separates highervelocity rocks (up to 5000 m=sec) on the northeast side froma thick accumulation of low-velocity (<2500 m=sec) sedi-ments on the southwest side. Although previous studiessuggest that the Mission Creek fault dips northward at highto moderate angles (Richter et al., 1958; Proctor, 1968;Griscom and Jachens, 1990; Nicholson and Lees, 1992;

Nicholson, 1996), the evidence for a northward subsurfacedip is not conclusive. In contrast, our seismic reflectionand velocity images, seismicity cross sections, and gravitymodeling show strong evidence for a slightly southwest-dipping (∼80°) Mission Creek fault at Desert Hot Springs.We further suggest that the Banning fault dips northeastward(∼50°) toward the Mission Creek fault in the upper part of theseismogenic zone and more steeply (∼70°) as it approachesthe Mission Creek fault (Figs. 10b and 11). The proximityand dips of the two faults suggest that the Banning and Mis-sion Creek faults merge into a single San Andreas fault atdepths between about 5 and 25 km in the northern CoachellaValley with increasing depth of merger from southeast tonorthwest (Fig. 11). The overall geometry of the Banningand Mission Creek faults, with a structural depressionbetween the two faults, suggests that fault system is transten-sional in the northern Coachella Valley.

The structural characteristics of the San Andreas faultand the northern Coachella Valley strongly influence theseismic hazard of the area. The San Andreas fault consistsof several fault strands that are widely distributed acrossthe valley, and each of the fault strands is composed of multi-ple fault traces in the near surface. Previous surface mapping(Proctor, 1968) and our seismic imaging suggest that theMission Creek strand consists of a zone of faulting that isat least 0.5 km wide zone, with near-surface traces that un-derlie the northeastern part of the city of Desert Hot Springs.These distributed near-surface fault traces represent a hazardfrom the perspective of potential damage to buildings andinfrastructure that overlie the fault traces and from the per-spective of high-amplitude guided waves that may propagatealong the fault traces. However, because the Mission Creekfault is apparently not a northeast-dipping reverse fault, thenortheastern part of the city of Desert Hot Springs should notexperience strong shaking from the hanging-wall effect. Thethick accumulation of low-velocity sediments southwest ofthe Mission Creek fault will likely amplify seismic waves,resulting in strong shaking in the southwestern part of DesertHot Springs and much of the northern Coachella Valley. Inaddition to the hazard posed to the northern CoachellaValley, strong shaking will likely affect the concentrationof lifelines (electrical, natural gas, water, roadways, etc.)in the northern Coachella Valley that serve millions of peoplein southern California. A more complete understanding ofthe crustal and fault structure in the northern CoachellaValley, therefore, is needed in mitigating the earthquakehazards to the region.

In addition to the earthquake hazards, the geometry ofthe Mission Creek fault also affects the local availability anddistribution of groundwater. The top of the groundwatertable, as inferred by the 1500 m=sec velocity contour ofour velocity model and by well data (Mayer et al., 2007),is about 20–30 m below the surface northeast and60–80 m southwest of the surface trace of the MissionCreek fault. We estimate the maximum thickness of thepermeable shallow aquifer to be about 20 and 70 m northeast

San Andreas Fault Geometry at Desert Hot Springs, California, and Its Effects on Earthquake Hazards 2205

and southwest of the surface trace of the fault, respectively.The principal change in groundwater depth occurs over anapproximately 200 m wide zone, centered near the mappedsurface trace of the Mission Creek fault. The large lateraldistance over which the depth of groundwater changes sug-gests there are multiple groundwater fault barriers, likely dueto clay smearing, grain-scale mixing, or mineral precipitation(Bense et al., 2003) within the 200 m wide zone, consistentwith our reflection images. The presence of multiple, widelydistributed shallow-depth fault barriers suggests that slip onthe Mission Creek fault has not been confined to the few faulttraces mapped at the surface. Long-term estimates of the sliprate should account for the movement on the unmappedfaults.

Data and Resources

The U.S. Geological Survey High-Resolution ImagingGroup (Earthquake Hazards Team in Menlo Park, California)acquired the seismic reflection and refraction data used inthis study. The data are available by contacting the authorsof this study.

Acknowledgments

Wewish to thank the 1998 applied geophysics class of Michigan Tech-nological University, Tom Burdette, Jose Rodriguez, Joe Grow, and AndyGallardo for field assistance. Technical reviews by Tom Brocher, ShirleyBaher, and Robert Sickler are greatly appreciated.

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U.S. Geological SurveyEarthquake Hazards Team345 Middlefield Rd. Mississippi 977Menlo Park, California 94025catching@usgs.govmrymer@usgs.govgoldman@usgs.govgg@weiss.com

Manuscript received 9 June 2008

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