Triggered Surface Slips in the Salton Trough Associated

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Bulletin of the Seismological Society of America, Vol. 92, No. 4, pp. 1300–1317, May 2002

Triggered Surface Slips in the Salton Trough Associated with the 1999

Hector Mine, California, Earthquake

by Michael J. Rymer, John Boatwright, Linda C. Seekins, J. Douglas Yule, and Jing Liu

Abstract Surface fracturing occurred along the southern San Andreas, Supersti-tion Hills, and Imperial faults in association with the 16 October 1999 (Mw 7.1)Hector Mine earthquake, making this at least the eighth time in the past 31 yearsthat a regional earthquake has triggered slip along faults in the Salton Trough. Frac-tures associated with the event formed discontinuous breaks over a 39-km-longstretch of the San Andreas fault, from the Mecca Hills southeastward to Salt Creekand Durmid Hill, a distance from the epicenter of 107 to 139 km. Sense of slip wasright lateral; only locally was there a minor (�1 mm) vertical component of slip.Dextral slip ranged from 1 to 13 mm. Maximum slip values in 1999 and earliertriggered slips are most common in the central Mecca Hills. Field evidence indicatesa transient opening as the Hector Mine seismic waves passed the southern San An-dreas fault. Comparison of nearby strong-motion records indicates several periods ofrelative opening with passage of the Hector Mine seismic wave—a similar processmay have contributed to the field evidence of a transient opening.

Slip on the Superstition Hills fault extended at least 9 km, at a distance from theHector Mine epicenter of about 188 to 196 km. This length of slip is a minimumvalue, because we saw fresh surface breakage extending farther northwest than ourmeasurement sites. Sense of slip was right lateral; locally there was a minor (�1mm) vertical component of slip. Dextral slip ranged from 1 to 18 mm, with thelargest amounts found distributed (or skewed) away from the Hector Mine earthquakesource. Slip triggered on the Superstition Hills fault commonly is skewed away fromthe earthquake source, most notably in 1968, 1979, and 1999.

Surface slip on the Imperial fault and within the Imperial Valley extended about22 km, representing a distance from the Hector Mine epicenter of about 204 to 226km. Sense of slip dominantly was right lateral; the right-lateral component of slipranged from 1 to 19 mm. Locally there was a minor (�1–2 mm) vertical componentof slip; larger proportions of vertical slip (up to 10 mm) occurred in Mesquite basin,where scarps indicate long-term oblique-slip motion for this part of the Imperial fault.Slip triggered on the Imperial fault appears randomly distributed relative to locationalong the fault and source direction. Multiple surface slips, both primary and trig-gered slip, indicate that slip repeatedly is small at locations of structural complexity.

Introduction

The Mw 7.1 Hector Mine earthquake of 16 October 1999in the Mojave Desert, southern California, was associatedwith primary tectonic rupture on many faults in the epicen-tral area (Treiman et al., 2002; Fig. 1) and remotely triggeredminor slip on many other faults in the Mojave Desert areaat distances of about 30 to 40 km (Treiman et al., 2002).The Hector Mine earthquake also triggered surface slip onfaults in the Salton Trough, from about 100 to 280 km tothe south. The Hector Mine earthquake was only the mostrecent earthquake of many local or regional earthquakes

within the past three decades to have triggered slip on faultsin the Salton Trough.

Triggered surface breaks were documented in the SaltonTrough following the 1968 Borrego Mountain earthquake(Allen et al., 1972), the 1979 Imperial Valley earthquake(Fuis, 1982; Sieh, 1982), the 1981 Westmorland earthquake(Sharp et al., 1986a), the 1986 North Palm Springs earth-quake (Sharp et al., 1986b; Williams et al., 1988), the 1987Superstition Hills earthquake (Hudnut and Clark, 1989;Sharp, 1989), and the 1992 Joshua Tree and Landers earth-

Triggered Surface Slips in the Salton Trough Associated with the Hector Mine Earthquake 1301

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Figure 1. Index map showing Quaternaryactive faults in the greater Salton Trough andsouth-central Mojave Desert areas (modifiedfrom Jennings, 1994). Heavy line in MojaveDesert, 1999 Hector Mine surface faulting.Black bars in Salton Trough show generalizedlocation of surface slip along San Andreas, Su-perstition Hills, and Imperial faults triggeredby the Hector Mine earthquake. Large blackstar marks location of Hector Mine mainshockepicenter (labeled with magnitude 7.1). Smallblack stars with date and magnitude indicateepicenters of earlier earthquakes that also trig-gered slip on southern San Andreas, Supersti-tion Hills, or Imperial faults. BF, Banningfault; DHS, Desert Hot Springs; MCF, MissionCreek fault; TC, mouth of Thermal Canyon;TPR, Thousand Palms Road. Letters A to Hmark location of ends of strip maps of San An-dreas fault shown in Fig. 2; letters I to K marklocation of ends of strip maps of SuperstitionHills fault shown in Fig. 5; letters L to Q marklocation of ends of strip maps of Imperial faultshown in Fig. 6.

quakes (Bodin et al., 1994; Rymer, 2000). Instrumental re-cordings of triggered slip, aseismic surface slip, and afterslipalso were reported for events in the Salton Trough (Goultyet al., 1978; Cohn et al., 1982; Louie et al., 1985; Bilham,1989; McGill et al., 1989).

Within the past three decades, triggered slip has been

documented along the southernmost San Andreas fault inthe southeastern Coachella Valley in four discrete earth-quakes (Allen et al., 1972; Sieh, 1982; Williams et al., 1988;Rymer, 2000) and in many additional slip pulses determinedby creepmeters and repeated surveys (e.g., Williams andSieh, 1987; Williams et al., 1988; McGill et al., 1989), along

1302 M. J. Rymer, J. Boatwright, L. C. Seekins, J. D. Yule, and J. Liu

with episodic dextral creep (Louie et al., 1985; Sieh andWilliams, 1990). If such surface movement has occurredthroughout the period since the last great earthquake, about320 years ago (Sieh and Williams, 1990), then the net dis-placement could add up to tens of centimeters of displace-ment, representing a significant amount of shallow strainrelease.

This report describes the distribution and amount oftriggered slip on the San Andreas, Superstition Hills, andImperial faults within the Salton Trough and associated withthe 1999 Hector Mine earthquake. Our tectonic interpreta-tions are based on field measurements, as well as comparisonwith earlier triggered slips on the same faults and primarytectonic rupture on the latter two of these faults. A secondcomponent of this report compares triggered slip on the SanAndreas fault and strong-motion records associated with the1999 Hector Mine earthquake. Macroscopic features ob-served along the San Andreas fault in 1999 indicated a short-term transient opening or propagating wrinklelike pulse ofthe fault during triggered-slip movement; comparison ofstrong-motion records on either side of the fault suggests theplausibility of this process. We also describe geologic evi-dence of an aseismic surface slip that developed along theSan Andreas fault in the fall of 1997; this slip partially filleda gap in earlier-measured triggered slip and demonstrates thenature of slip on faults within the Salton Trough, if not else-where.

San Andreas Fault

Field checks for surface breakage along the San An-dreas fault in the southeastern Coachella Valley began onthe day of the Hector Mine earthquake. Two of us (J.L. andJ.D.Y.) checked the San Andreas fault in the area of SaltCreek, Bat Caves Buttes, and Durmid Hill (Figs. 1, 2) on 16and 17 October 1999. Field checks were made in the IndioHills, Mecca Hills, Box Canyon, and Bombay Beach areas,along with many other localized checks, by M.J.R. on 19–21 October 1999.

Discontinuous surface breaks formed along the San An-dreas fault over a distance of about 39 km (Figs. 2, 3; Table1), representing distances from the Hector Mine epicenter tothe northwest and southeast endpoints of triggered slip ofabout 107 and 139 km, respectively (Table 1). Breaks de-veloped in two broad areas, the Mecca Hills and the SaltCreek–Durmid Hill area; a distance of almost 20 km sepa-rates these two broad areas. Areas where surface breaksformed lie in uplifted Pleistocene fine-grained lacustrine andfluvial deposits. See Bilham and Williams (1985) for dis-cussion of structural relations to triggered slip occurrenceand Rymer (2000) for discussion of distribution of triggeredslip and local geologic materials.

Slip components were determined by measuring the dis-placement between matching irregularities in soil blocks orthin soil crusts along the local strike of the fault. We mea-sured the slip vector, the azimuth of the slip, and the local

strike of the fault; where present, the vertical component ofslip and the direction of relative vertical displacement werealso measured. Offset values were small, nowhere greaterthan 13 mm (Figs. 2, 3). Dextral slip greater than 6 mmapparently was confined to two areas in the Mecca Hills,between the mouth of Thermal Canyon to about 0.6 kmsoutheast of Quarry Canyon and between Red Canyon andPainted Canyon (Fig. 2, panels A–B and B–C).

Transient Opening and Strong-Motion Records

A feature that further reveals the nature of surface rup-ture was observed in association with the 1999 triggered slipalong the San Andreas fault southeast of Quarry Canyon(Fig. 2, panel A–B). Figure 4 shows details of triggered slipat this site, where a thin soil surface was broken and offsetalong an en echelon fracture. Perspective drawings of thefractured soil surface (Fig. 4C and D) illustrate that slip atthis site could not have been as a simple lateral slip parallelwith the fault. Compare Figure 4A–C; simple linear motionof matching features X and X� would have broken the thinsoil chip (X�). Rather, we believe that features shown inFigure 4 indicate a transient opening during right-lateral trig-gered slip. Such an opening would allow the delicate chipsto be preserved; after the opening passed and lateral slip wascompleted, the opening closed as much as possible, givennew fault-surface irregularities.

Brune et al. (1993) cited R. Sharpe [R. V. Sharp] forobservations following the 1976 Guatemalan earthquake (Ms

7.5) that had primary tectonic strike-slip displacement av-eraging approximately 95 cm, and at least locally also hada minimum of 3 to 4 cm of motion perpendicular to the traceof the fault. This was evidenced by the preservation of small-scale jogs in the fault trace, which can only be explained ifthe opposite sides of the fault first moved apart, then movedin a strike-slip manner, then moved together again (Brune etal., 1993). Our observations on the San Andreas fault requiresimilar processes of opening, strike-slip motion, and thenclosing, but as remotely triggered slip about 110 km fromthe causative earthquake.

To test our 1999 observations and inferences of a tran-sient opening, we studied nearby strong-motion records fromthe Hector Mine earthquake. Figure 5 shows the locationsof five strong-motion stations situated near the southern SanAndreas fault and near the feature shown in Figure 4. Thesestrong-motion stations lie at distances from the Hector Mineepicenter of about 90 to 150 km. All of the stations are south-west of the San Andreas fault, except for station FRK; sta-tions 542 and FRK bracket the San Andreas fault, lyingwithin and outside of the Salton Trough, respectively, andare situated at about the same distance from the Hector Minemainshock, 149 km. A time–distance plot of strong motionsfrom the Hector Mine earthquake shows the displacement atthese strong-motion stations (Fig. 6). Figure 6 also overlaysthe displacement of stations 542 and FRK; taken together,these data suggest several periods of relative opening,


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Stone ring site

Figure 2. Strip maps from northwest tosoutheast along the southern San Andreas faultwith location of surface fractures formed in as-sociation with 1999 Hector Mine earthquake.Dotted line, San Andreas fault; solid line, lo-cation of documented 1999 surface fractures.Slip values shown in millimeters for both right-lateral (R) and vertical (V) components. Ver-tical components of slip, where present, are in-dicated with either west (W) or east (E) sideup. Map strips derived using selected contoursfrom topographic maps; location of fault fromClark (1984); location of strips shown in Fig.1. Strip map D�–E� is plot of surface fracturesformed during aseismic creep event in fall of1997 (see text for details).

shaded in dark gray. The largest relative openings, as muchas 4 cm, occurred between approximately 30 and 50 sec.Although records from stations 542 and FRK suggest thatthere may have been opening during the Hector Mine earth-quake, no triggered surface slip was observed immediatelynearby (the San Andreas fault at Bombay Beach and along

the southeastern part of Durmid Hill was checked carefullythree days after the earthquake). The closest stations to thetriggered slip shown in Figure 4 are MEC and INO. We inferthat relative openings similar to those shown in Figure 6 mayhave occurred across the San Andreas fault where triggeredslip did form.


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Figure 3. Right-lateral component of triggered slip as a function of distance alongSan Andreas fault. A, Triggered slip in 1968 (from Allen et al., 1972); B, triggered slipin 1979 (from Sieh, 1982); C, triggered slip in 1986 (from Williams et al., 1988) (slipin Indio Hills not shown); D, triggered slip in 1992 (from Rymer, 2000) (slip in IndioHills not shown); E, triggered slip in 1999. Distance scale along fault is relative tointersection of Thousand Palms Road and the San Andreas fault (see Fig. 1). Widehorizontal arrow marks direction of rupture for individual earthquakes; seismic wavesfrom the 1968 Borrego Mountain earthquake arrived approximately normal to the SanAndreas fault and thus no rupture direction is shown for that event. Slip measured oncreepmeters not included in these plots. Caltech measurement arrays and creepmeters,some of which are now inactive, located at: Red Canyon, North Shore, Mecca Beach,and Salt Creek (Louie et al., 1985; McGill et al., 1989).

1997 Aseismic Creep Event

Here we briefly diverge from the 1999 Hector Minetriggered slip for a short description of aseismic slip thatoccurred about two years earlier along part of the same areaof the southern San Andreas fault. We present this infor-mation to further describe the range of surface slip behaviorin the region. In the fall of 1997, a localized strain eventdeveloped along the San Andreas fault in the Mecca Hills

that resulted in discrete cracks with consistent right-lateraloffset. Surface slip in this event was restricted to the BoxCanyon area (Fig. 2, panel D–E), where triggered slip oc-curred in association with the 1992 Landers earthquake(Shifflett and Witbaard, 1996; Rymer, 2000) and later withthe 1999 Hector Mine earthquake (this article).

Surface breakage along the San Andreas fault occurredsome time between 25 September 1997 and 15 November


Table 1Characteristics of Triggered Slip along San Andreas, Superstition Hills, and Imperial Faults

in the Salton Trough*

Year of Magnitude Approx. Azmuthal†

Extent ofTriggered Slipalong Fault‡ Distance to Epicenter§

MaximumSlip Value

Event (Mw) Direction of Waves (km) (km) (mm)

San Andreas Fault1968 6.5 005�–053� 41 35–45 241979 6.4 328�–333� 39 100–136 101981 5.6 — — — —1986 6.0 125�–131� 25§ 44–86 91987 6.6 —� — — —�

1992 7.3 146�–152� 54§ 54–105 201999 7.1 161�–170� 38 107–139 13

Superstition Hills Fault1968 6.5 125� 23 28–56 251979 6.4 309�–312� 23 41–60 221981 5.6 200�–252� 16 20–25 141986 6.0 — — — —1987 6.6 Causative fault1992 7.3 �155� # �145–165 81999 7.1 163�–165� 9** 188–196 18

Imperial Fault1968 6.5 119�–125� 22 63–83 201979 6.4 Causative fault1981 5.6 154�–160� 17 23–39 81986 6.0 — — — —1992 7.3 —†† — — —1999 7.1 158�–160� 22 204–226‡‡ 19

*Each of these events also triggered slip on other faults or on other parts of the San Andreas fault zone (fordetails see reports on individual earthquakes).

†Measured from epicenter to endpoints of triggered slip.‡Measured from southeast and northwest endpoints of slip; each slip event is associated with discontinuous

slip.§In Figure 3 we do not show location or amount of slip in the Indio Hills that occurred in 1986 (Williams et

al., 1988) and 1992 (Rymer, 2000).�McGill et al. (1989) instrumentally measured minor distributed slip, but no discrete surface fault slip

occurred.#Triggered slip along the Superstition Hills fault was not investigated following the 1992 Landers

earthquake.**Surface slip along the Superstition Hills fault in 1999 extended more than 9 km, but was not mapped in

detail any farther (see text for details).††R.V. Sharp (oral communication, 1992) stated there was no triggered slip on the Imperial fault associated

with the 1992 Landers earthquake.‡‡Instrumentally determined triggered slip also occurred on the Imperial fault in the Mexicali Valley, 275–

280 km from the 1999 Hector Mine epicenter (Glowacka et al., 2002).

1997. The earlier date coincides with Hurricane Nora, whichwas associated with heavy precipitation and runoff. Precipi-tation was heavy enough to have reset the ground conditionsand thus remove evidence of any earlier surface slip. Thelatter date is the timing of a South Coast Geological Societyfield trip to the San Andreas fault and subsequent coinci-dental observations of the new surface slip. Another partic-ipant on the field trip (G. R. Roquemore, Irvine Valley Col-lege) accompanied one of us (M.J.R.) and walked out partsof the surface breakage near the stone ring site (Fig. 2, panelD–E) of Shifflett and Witbaard (1996).

A dedicated search for surface breakage began three

days later, extending from 18 to 21 November 1997, whenM.J.R. walked out the new surface breaks near the mouth ofBox Canyon in the Mecca Hills and walked out or checkedother sites along the San Andreas fault that have slipped inthe past. These searches extended about 50 km, from Bom-bay Beach northwestward to the mouth of Thermal Canyon(Fig. 1). Surface breaks were mapped for a distance of onlyabout 2.5 km; greater continuity of slip and greater amountsof slip were located southeast of the main wash of Box Can-yon (Fig. 2, D�–E�). The surface breaks occurred in the ac-tive alluvial wash of Box Canyon, in old alluvial depositsforming small hills, and in other, less active alluvial areas


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Figure 4. Triggered slip on San Andreas fault inmodern wash tributary to Quarry Canyon, with 7 mmof right-lateral component of displacement and novertical component. A, photograph of fracture andoffset features; X and X� are matches across fault. B,Line drawing of features shown in A. C, Schematicperspective drawing of features X-X�. Feature X� is1- to 1 1/2-mm-thick tab of surface clay. D, Inferredrelative motion of southwest (X�) side of fault. Frac-ture must have opened at least length of tab X� be-fore right-lateral motion, otherwise tab would havechipped or broken. Photograph taken 21 October1999.

north of the Coachella Canal; two short, isolated surfacebreaks occurred northwest of State Highway 195 (Fig. 2, D�–E�). Everywhere present, the surface slip formed left-step-ping en echelon breaks with a right-lateral component ofdisplacement. Figure 7 shows en echelon fractures in allu-vium along the trace of the San Andreas fault, east of themain wash of Box Canyon. A right-lateral component of slipis readily apparent because of separation of matching fea-tures subparallel to the local trend of the fault.

Horizontal slip was greatest in Box Canyon, southeastof the main wash, and in small hills east of Box Canyon(Fig. 2, D�–E�). The right-lateral component of slip on thenew breaks was small, from 1 to 5 mm; no vertical com-ponent of slip was measured.

Movement on the southern San Andreas fault in the fallof 1997 apparently was aseismic. In the period of 25 Sep-tember to 15 November 1997 there was no earthquake largeenough to cause primary tectonic rupture. (During this pe-riod, a small cluster of M �2 earthquakes formed northeastof the San Andreas fault in the northern Mecca Hills, ap-proximately 10 km away; L. M. Jones, U.S. Geological Sur-vey, oral commun., 1997.) Also, there were no local, re-gional, or global earthquakes of any significance during thisperiod to trigger slip on the San Andreas fault.

The presence of aseismic surface slip events along thesouthern San Andreas fault, as described above, comes aslittle surprise. Louie et al. (1985) stated that for the 18-yearperiod from 1967 to 1985 the San Andreas fault in the areahad an average slip rate, excluding triggered slips, of about1.5 mm/yr. Thus, one may expect creep events. The remotenature of the desert in the region makes for infrequent visitsto the fault traces, and thus similar creep events may occurwithout being noticed. Of greater interest is the localizationof slip near the mouth of Box Canyon, where triggered slipwas not reported in 1968 (Allen et al., 1972) or 1979 (Sieh,1982), but was noted in 1992 (Shifflett and Witbaard, 1996;Rymer, 2000). Also, there was additional slip, albeit small,in the same area associated with the 1999 Hector Mine earth-quake, described above (Fig. 2, panels D–E and D�–E�). Ap-parently, the area presently is in a decade-plus period ofepisodic slip. This observation, combined with the geodeticand creepmeter measurements and conclusions of Louie etal. (1985), suggests that creeping faults do not necessarilyslip at a constant rate, but rather slip can occur in phases ofa few years or longer, interspersed with periods of either noslip or decreased slip.

We were careful to take into account possible relationsof the new surface fractures along the San Andreas fault andrainfall associated with Hurricane Nora. We still concludethat cracks formed near the mouth of Box Canyon in the fallof 1997 indeed are from aseismic slip, as opposed to a swell-and-shrink process associated with moisture increase fol-lowed by desiccation. In this latter process, clays in faultgouge, or entrained clay-rich sediment in a fault, swell withincreased moisture content during rainfall and runoff. In des-ert environments, as in the Coachella Valley and many otherareas of southern California, shortly after rainfall the areareturns to dry conditions wherein the clays shrink. Where afault containing abundant clays is at or close to the surface,desiccation cracks then form along the fault. This phenom-enon is most common in canyons or washes, where rainfallaccumulates. For geologists looking for faults, the resultingdesiccation cracks may aid in identification of fault location,but these same features may mislead an investigator intothinking there has been surface slip or that, somehow, the


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Figure 5. Map of Quaternary-active faultsin the Salton Trough and south-central MojaveDesert areas (same area as Fig. 1). Map alsoshows inferred subsurface boundary of SaltonTrough (heavy gray line), Brawley seismiczone (short diagonal lines between Imperialfault and San Andreas fault at Bombay Beach),and location of strong motion stations INO,MEC, NRT, FRK, and 542 near southern SanAndreas fault (see description in text and Fig.6). Boundary of Salton Trough from Saltus andJachens (1995), for the Coachella Valley area,and Fuis et al. (1982), for the Imperial Valleyarea.

desiccation cracks are a measure of overall activity of thefault. For strike-slip faults in the San Andreas fault system,an easy way to discern surface-slip events from paleorainfallindicators is that surface slip will have a horizontal com-ponent of offset (with a consistent sense of slip), whereaswith swell-and-shrink processes there is no horizontal offset,

rather cracks with only opening across them. Figure 7 clearlyshows a right-lateral component of slip associated with the1997 observations along the San Andreas fault near BoxCanyon. Further indications that the 1997 slip was not as-sociated with rainfall include the following: (1) cracksformed not only in the mouth of Box Canyon, but also in


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Figure 6. Plot of strong motion records from the1999 Hector Mine earthquake at stations Indio (INO),Mecca (MEC), North Shore (NRT), Frink (FRK), andstation 542, located at Bombay Beach. All of thesestrong motion sites are located southwest of the SanAndreas fault, except FRK, which is to the northeast.Station FRK is approximately the same distance fromthe 1999 Hector Mine epicenter as station 542 and isonly 4 km from that other station. Plots shown arebasically those of a time–distance series, except thatplots for stations NRT and FRK-542 are moved apartfor clarity. Plots shown are the northeast–southwest(NE) and northwest–southeast (NW) components ofmotion, normal and parallel to the 1999 Hector Minewaveform, respectively. Dark areas in the comparisonof recordings from stations FRK and 542 mark pe-riods of relative opening in a northeast–southwest di-rection between these stations.

Figure 7. Photograph of fracture along San An-dreas fault east of the main wash in Box Canyon withright-lateral component of slip. Fracture formed asaseismic slip in fall of 1997; pencil is oriented parallelwith local trend of fault. Arrows point to matchingbreaks in soil crust; arrows are oriented parallel to slipazimuth. Photograph taken 22 November 1997.

hills composed of older alluvium to the east of the canyon;(2) the largest amount of slip occurred southeast of the wash;and (3) locally on slopes the right-lateral motion moved acanyon bottom uphill, which would not happen with a des-iccation mechanism alone.

Discussion

Triggered slip in 1999 along the San Andreas fault inthe southeastern Coachella Valley generally occurred whereit had in previous moderate to large earthquakes. The fourprevious documented slip events, in 1968, 1979, 1986, and1992, were triggered by earthquakes in various azimuthaldirections away from the fault, and with varying magnitudes(see Fig. 1; Table 1). Likewise, slip that occurred in 1999

added to the range of source directions and magnitudes, al-though it was most similar to that of the 1992 Landers earth-quake. Earlier slips, in general, formed along the fault in theDurmid Hill and Mecca Hills areas; two of the earlier events,1986 and 1992, also triggered slip in the Indio Hills (Rymer,2000, his Fig. 8, presents a more detailed description of ear-lier triggered slip events).

Gross similarities and differences between earlier trig-gered slip events and that in 1999 along the southern SanAndreas fault are abundant. Of the documented triggeredslip events, slip most commonly forms in the central MeccaHills and occurs there in the most continuous stretches. Fur-thermore, the most pronounced slip occurs in the MeccaHills regardless of the direction from the associated earth-quake (Fig. 3), except in 1986, when slip was greatest in theIndio Hills (Williams et al., 1988). Two areas in the MeccaHills where slip developed with relatively large amounts ofdisplacement and more continuous stretches are betweenThermal Canyon and Quarry Canyon and between Red Can-yon and Painted Canyon (Figs. 2, 3). Also, in four of theevents, 1968, 1979, 1992, and 1999, slip stopped, or started(depending on azimuth from source), within a few meters ofthe same spot near the mouth of Thermal Canyon. This topicwas discussed by Sieh (1982) following the 1979 ImperialValley earthquake, but by now there have been two addi-tional events, in 1992 and 1999, that also had slip that prob-ably initiated at the same location and then extended intoareas commonly associated with triggered slip.

Timing of slip along the southern San Andreas fault in1999 is constrained or inferred by a variety of observations.Field measurements and mapping of slip along the San An-


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Figure 8. Strip maps of Superstition Hills fault with location of surface fracturesformed in association with 1999 Hector Mine earthquake (see Fig. 1 for location ofpanels I–J and J–K). Dotted line, Superstition Hills fault; solid line, location of docu-mented 1999 surface fractures. Slip values shown in millimeters for both right-lateral(R) and vertical (V) components. Vertical components of slip, where present, are in-dicated with either west (W) or east (E) side up. Map strips derived using selectedcontours from topographic maps. Distance scale along fault same as used by Sharp etal. (1986a, 1989); reference point located near western end of Superstition Hills fault.

dreas fault, as stated above, started the day of the HectorMine earthquake. Following the 1992 Landers earthquake,Bodin et al. (1994) stated that slip developed along thesouthern San Andreas fault about 1 min after that mainshock,as the seismic wave passed through the area. We do not havecreepmeter recordings for slip along the San Andreas faultin association with the Hector Mine earthquake; however,similar observations of fault effects associated in time withthe passage of the seismic wave are recorded by Glowackaet al. (2002) for slip triggered by the Hector Mine earthquakenear the southern end of the Imperial fault (discussed belowunder the Imperial fault). Also, the strong-motion recordsdiscussed above and shown in Figure 6 indicate that theseismic wave passed the areas of triggered slip from about20 to 50 sec after the Hector Mine mainshock.

Seismicity also was triggered in the Salton Trough bythe 1999 Hector Mine earthquake (Gomberg et al., 2001;Glowacka et al., 2002; Hough and Kanamori, 2002). Houghand Kanamori (2002) conclude that the largest of the trig-gered events was an earthquake of M 4.7, which occurredbeneath the Salton Sea, in the Brawley seismic zone (seehollow star in Figs. 1 and 5). Could this moderate event havetriggered, or otherwise been causally associated with, thesurface slip on the San Andreas fault? The event was largeenough to be associated with surface slip in the Salton

Trough; for example, Brune and Allen (1967) reported a1966 M 3.6 earthquake on the Imperial fault associated withabout 10 km of discontinuous surface slip. Also, the timingof the small shock, about 30 sec after the Hector Mine earth-quake (Hough and Kanamori, 2002) fits within the 20- 50-sec timing of passage of the seismic wave as measured onthe strong-motion records. However, the time–distance plotshown in Figure 6 indicates that the seismic wave and in-ferred transient pulse moved from north to south, from thegeneral direction of the Hector Mine mainshock and towardthe M 4.7 shock. Thus, we believe the surface slip in theSalton Trough was triggered by the Hector Mine mainshock.

The sum of slip in the Mecca Hills due to triggering inthe past three decades is about 50 mm, but this rate is slowerthan either the measured background creep rate (exclusiveof triggered slip) or the geodetically determined rate. Louieet al. (1985) presented data showing a variable backgroundcreep rate along the southern San Andreas fault for the pe-riod 1967–1985. Their greatest creep rate, 3.1 mm/yr, wasin the central Mecca Hills, the same area where triggeredslip develops most readily and with relatively larger amountsof displacement. Geodetically determined broad-scale sliprates for the southernmost San Andreas fault zone usingGlobal Positioning System (GPS) measurements and a rigidblock model infer about 26 � 2 mm/yr (Bennett et al.,


1996). This, of course, approaches 10 times larger than theeven faster creep rates observed on the southern San Andreasfault during the past few decades. The difference betweenthe combined triggered and creep rates and the broad-scalerates presumably will be released in a surface-rupturingearthquake.

Superstition Hills Fault

Field checks for surface breakage along the SuperstitionHills fault began about 1 month after the Hector Mine earth-quake. One of us (M.J.R.) checked the fault on 11 and 22–23 November 1999. Cracks were mapped and slip was mea-sured along about 9 km of the fault trace, representingdistances from the Hector Mine epicenter to the northwestand southeast endpoints of observed triggered slip of about188 and 196 km, respectively (Table 1). The northwestern11 km of the fault was not checked for surface slip due tologistical constraints, but cracks along the fault trace wereobserved extending farther to the northwest than our north-westernmost slip measurement. Thus, the northwestern ex-tent of 1999 surface slip shown in Figures 8 and 9 is a min-imum. Two other areas of the fault were not checked: (1)the area around Building 5000 (Fig. 8, panel J–K), due to amilitary exercise the week before our field measurementsthat had obliterated any cracks, and (2) the southeast end ofthe central strand, where freshly wind-blown sand coveredany new fault breaks.

Surface slip in 1999 occurred on two of three structuralfault segments associated with the 1987 Superstition Hillsearthquake (Sharp et al., 1989). Superstition Hills fault seg-ments are, from northwest to southeast, the north, central orImler, and the Weinert strands. Fresh breaks associated with1999 triggered slip were mapped along the southeastern endof the north strand and along most of the central or Imlerstrand. Spot checks were made along the Weinert strand,where no new surface slip was found. Surface breaks in 1999formed in areas of uplifted Pleistocene lacustrine deposits.

Slip components were determined in the same manneras along the San Andreas fault, by measuring the displace-ment between matching irregularities in soil blocks or thinsoil crusts along the local strike of the fault. We measuredthe slip vector, the azimuth of the slip, and the local strikeof the fault; where present, the vertical component of slipand the direction of relative vertical displacement were alsomeasured. Offset values were small, nowhere greater than18 mm (Figs. 8, 9). Dextral slip was greatest near the south-eastern end of the central or Imler strand (Figs. 8, 9). Lo-cally, a small vertical component of slip was measured;nowhere did the vertical component exceed 1 mm. Wherea vertical component of slip was measured it was similarin relative proportions of slip and direction of slip relativeto earlier episodes of triggered slip and the 1987 surfacefaulting.

Discussion

There were five episodes of documented triggered sur-face slip (1968, 1979, 1981, 1992, and 1999) and 1987 pri-mary surface faulting (Sharp et al., 1989) on the SuperstitionHills fault. The 1987 primary (coseismic) surface faultingwas associated with an extended period of afterslip that ini-tially occurred as more or less continuous slip and then de-cayed to episodic slip events (Bilham, 1989; Wennerbergand Sharp, 1997).

Timing of surface slip on the Superstition Hills fault in1999 is directly constrained only by the dates of our fieldinvestigations. Field studies began about 1 months after theHector Mine earthquake and, although surface fractures ap-peared fresh at the time of the field checks, other processesmay have produced the cracks. Our field inspections of theSan Andreas fault on the day of the Hector Mine earthquakeand timing of slip along the southern Imperial fault as re-corded in Mexicali Valley by Glowacka et al. (2002) indi-cate that a strong seismic wave from the Hector Mine earth-quake passed through the Salton Trough. These observationsprovide no constraints on the timing of slip on the Supersti-tion Hills fault, nor any proof that surface fractures therewere necessarily associated with the Hector Mine earth-quake. Rather, slip mapped along the Superstition Hills faultmay have developed independently of the Hector Mineearthquake. For example, triggered slip along the Supersti-tion Hills fault associated with the 1992 Landers earthquakewas preceded by an episodic pulse of afterslip from the 1987Superstition Hills earthquake (Bodin et al., 1994; R. V.Sharp, oral commun., 1992). Such a slip event may haveoccurred between the time of the Hector Mine earthquakeand the first field inspections, or at some time shortly beforethe Hector Mine earthquake. Without further evidence onthe timing of slip, we can only infer that slip mapped in 1999was triggered by the Hector Mine earthquake. Closely brack-eting dates of interferometric synthetic aperture radar(InSAR) scenes around the time of the Hector Mine earth-quake may provide InSAR images that will aid in determin-ing the timing of slip on the Superstition Hills fault and thetotal extent of the 1999 slip.

One of the most pronounced similarities between earliertriggered slip events and that in 1999 along the SuperstitionHills fault is the location of surface breakage. In each eventthe fault has slipped at nearly the exact same place. Nailarrays and wooden-peg arrays placed across the SuperstitionHills fault following the 1987 primary surface faulting wereoffset by the Hector Mine earthquake and in exactly the sameplace as in 1987 and in subsequent afterslip.

Another similarity between earlier slip events and thatin 1999 is a propensity for the location of maximum dis-placement to be distributed away from the source of theearthquake, or oppositely skewed, as described by Allen etal. (1972) and Fuis (1982). Of the well-mapped triggeredslip events along the Superstition Hills fault, in 1968, 1979,


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Figure 9. Right-lateral component of slipalong Superstition Hills fault during primarysurface faulting in 1987 and triggered slip in1968, 1979, 1981, and 1999. A, 1968 triggeredslip; B, 1979 triggered slip; C, 1981 triggeredslip; D, surface faulting associated with the1987 Superstition Hills earthquake; E, 1999triggered slip. Detailed measurements were notmade of surface slip in 1951 and 1992 and thusthose events are not included in these plots.Distance scale along fault is relative to north-west end of faulting in 1987. A structural step-over between two fault strands as revealed in1987 primary surface faulting is marked bythin vertical arrow. Same distance and slipscales as Figs. 3 and 11. Wide horizontal arrowmarks direction of rupture for individual earth-quakes; seismic waves from the 1981 West-morland earthquake arrived approximatelynormal to the Superstition Hills fault and thusno rupture direction is shown for that event.Slip measured on creepmeters not included inthese plots.

1981, and 1999, slip was oppositely skewed in three of thefour events (the exception being 1981). The 1981 West-morland earthquake was located such that seismic waves andstatic stress changes arrived approximately normal, about87�, to the Superstition Hills fault. Interestingly, the 1987primary tectonic surface faulting did not show this skewednature of slip distribution, neither in initial slip nor in pro-jected final slip (Fig. 9D).

Mapping the 1999 triggered slip was an opportunity tocarefully measure the distribution and amount of surface slipwithin the area of a structural stepover in the main trace ofthe Superstition Hills fault, as was revealed by the 1987surface rupture (Sharp et al., 1989; Rymer, 1989). As withearlier episodes of triggered slip, the 1999 surface breakageextended through the 50-m-wide (erroneously stated as 100-m-wide by Rymer, 1989) structural stepover (see Fig. 8, I–J). Detailed measurements of the 1999 triggered slip in thestepover area show a decrease in slip on both north andcentral strands as one approaches the stepover and only veryminor (1–2 mm) dextral slip on individual strands extending

past where slip is transferred from one strand to the other(Figs. 8, 9E) (Rymer, 1989). Also, the 1999 triggered slipapparently was transferred from one fault strand to the other,where Rymer speculated that most of the 1987 primary slipwas partitioned through the stepover (gray area in Fig. 8,panel I–J; vertical arrow in Fig. 9).

Imperial Fault

Our study of the Imperial fault is restricted to that withinthe Imperial Valley, north of the U.S.–Mexico border (Fig.1). Glowacka et al. (2002) discuss instrumental measure-ments of triggered creep, tilt, and seismicity associated withthe 1999 Hector Mine earthquake near the southern end ofthe Imperial fault, in the Mexicali Valley.

Field checks for surface breakage along the Imperialfault began 25 days after the Hector Mine earthquake. Oneof us (M.J.R.) checked the Imperial fault on 10 and 23 No-vember 1999. Field checks were made primarily along road-ways and dirt shoulders that cross the fault; locally, where


the fault lies in uncultivated fields, mapping and measure-ments were made while walking along the fault trace.

Discontinuous surface breaks formed along the Imperialfault over a distance of about 22 km (Figs. 10, 11; Table 1),representing distances from the Hector Mine epicenter to thenorthwest and southeast endpoints of the triggered slip ofabout 204 and 226 km, respectively (Table 1). Two factorslimited the continuity and extent of our mapping surfacebreakage. Cultivated fields, especially fields with a densegrowth of crops, were not checked for surface fractures. Lo-cally, fields that recently had been cleared of crops werechecked for surface breaks, but this generally was futile be-cause of abundant tire tracks and general disruption of theground surface. Another area of ground disruption was northof Harris Road (Fig. 10, L–M). There, surface breaks clearlyextended farther north than our northernmost slip measure-ments, but ongoing construction and modification of drain-age canals obliterated evidence of the breaks. We believe the1999 triggered slip extended farther north, because ournorthernmost measurement (10 mm dextral slip) was ratherlarge and there was no indication of slip decay as near theend of a rupture. No slip was observed at Keystone Roadand, thus, the full northern extent of 1999 triggered slip laysomewhere between Harris and Keystone Roads, at an un-known distance north of our measurement sites.

Slip components were determined in the same manneras along the San Andreas fault, by measuring the displace-ment between matching irregularities in soil blocks or thinsoil crusts along the local strike of the fault. We measuredthe slip vector, the azimuth of the slip, and the local strikeof the fault; where present, the vertical component of slipand the direction of relative vertical displacement were alsomeasured. Offset values were small, nowhere greater than19 mm (Figs. 10, 11). Dextral slip was greatest near themiddle of triggered surface breakage (Figs. 10, 11). Locallythere was a minor (�1–3 mm) vertical component of slip;larger proportions of vertical slip (up to 10 mm) occurred inMesquite basin (near kilometers 23–25, Fig. 10), where faultscarps indicate a long-term oblique slip for this part of theImperial fault.

Discussion

The Imperial fault has a rich history of documented sur-face slips. Primary surface faulting was associated with mod-erate to large earthquakes in 1940 (M 7.1) and 1979 (M 6.4);small earthquakes in 1966 (M 3.6) and 1975 (M 4.2); trig-gered slip occurred in 1968, 1981, 1987, and 1999; and therehave been many earthquake swarms and minor aseismiccreep events along the fault (Goulty et al., 1978; Louie etal., 1985; Cohn et al., 1982; Sharp, 1989). Afterslip asso-ciated with the 1979 earthquake continued episodically foryears after that event (Sharp et al., 1982). Furthermore, Al-len et al. (1972) stated that an earthquake on the SuperstitionHills fault in 1971 also triggered slip along the Imperialfault.

Timing of slip on the Imperial fault within the ImperialValley is directly constrained only by the time of field in-vestigations; however, Glowacka et al. (2002) instrumen-tally recorded triggered slip and tilt along the southern endof the Imperial fault, in the Mexicali Valley. They state thateffects of the Hector Mine earthquake reached the southernImperial fault about 1.5 min after the mainshock, as the seis-mic wave passed through the area. The freshness of breaksseen on the Imperial fault in the Imperial Valley combinedwith the data of Glowacka et al. (2002) lead us to believethat the surface breaks were triggered by the 1999 HectorMine earthquake.

As with the San Andreas and Superstition Hills faults,the location of surface breakage in 1999 along individualtraces of the Imperial fault closely matched the location ofearlier triggered slip and primary surface rupture. Fresh sur-face breaks, where they occurred, formed in exactly the sameplace as previous slips; variations of location of breakagefrom one event to another vary by only a few centimeters,at most.

A comparison of the distribution and amount of the fewwell-mapped slips along the Imperial fault indicates a ratherrandom distribution (Fig. 11), unlike slips along the San An-dreas and Superstition Hills faults. Primary surface rupturesin 1940 and 1979 are similar, in both distribution and amountof slip, from kilometer 7 to the northern end of the fault (Fig.11; Sharp, 1982). Farther south of kilometer 7, near theU.S.–Mexico border, the 1940 break attained displacementsof up to 6 m (Sharp, 1982). In contrast, triggered slip in1981, 1987, and 1999 occurred in different locations (thoughwith some commonality) and attained maximum slip at dif-ferent locations. In spite of these differences, there are twoimportant points in common. First, for each of the triggeredslips, surface breakage was discontinuous. Even withdensely vegetated fields that obscure minor surface breaks,enough measurements were made within roadways and bar-ren fields to prove that locally no slip occurred. For the 1999triggered slip, negative evidence of slip is shown with ablack dot along the origin (Fig. 11). Second, discrete zonesalong the Imperial fault commonly are maintained throughtime with little or no slip; this is true for both triggered slipevents and primary tectonic rupture events (Fig. 11). Figure11 shows four shaded zones, marked w, x, y, and z, wherecomplexity exists in the surface trace of the Imperial fault,as became apparent following the 1979 surface rupture(Sharp et al., 1982). Zone w is a dilational jog in the traceof the fault. There, the ground surface is clear of vegetationand has a thin soil crust, in which surface breaks are easilydetected and measured. In all five of the slip events plottedin Figure 11, very little or no slip was detected here. Zonesx and z are areas where multiple strands add complexity tothe Imperial fault (compare Figs. 10 and 11). As with zonew, these two zones commonly (except zone z in 1979) showlittle to no slip (Fig. 11). Zone y has, with minor exceptions,shown little to no slip; zone y is where the Brawley faultzone interacts, or may connect, with the Imperial fault.


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RUPTURE

RUPTURE

Northwest Southeast Figure 11. Right-lateral component of slipalong northern Imperial fault during two pe-riods of primary surface faulting and three trig-gered slips. A, 1940 and 1979 primary surfacefaulting; B, triggered slip associated with the1981 Westmorland earthquake; C, triggeredslip associated with the 1987 Superstition Hillsearthquake; D, triggered slip associated withthe 1999 Hector Mine earthquake. Distancescale along faults is relative to south end offaulting in 1979. Detailed measures were notmade of surface slips in 1966 and 1968 andthus those episodes are not included in theseplots. Slip shown in A for the 1940 earthquakeincludes only that along northern part of Im-perial fault; surface slip in 1940 extended ap-proximately 30 km farther to the south and dis-placement reached a maximum of about 6 mright-lateral slip near the U.S.–Mexico border(Sharp, 1982). Short stretches of the Imperialfault with rupture complexity in the 1979 earth-quake are marked as gray zones w, x, and z.Low slip values or no slip is characterized foreach of these stretches. Another stretch of faultwith low slip in each of the events is foundfrom about 15 to 18 km (zone y), where theBrawley fault zone structurally interacts withthe Imperial fault. Same distance and slipscales as Figs. 3 and 9. Wide horizontal arrowmarks direction of rupture for individual earth-quakes.

Other Faults

The 1999 Hector Mine earthquake may have triggeredsurface slip on faults in the Salton Trough region other thanthe San Andreas, Superstition Hills, and Imperial faults.However, field checks along four faults, two of which havemoved in the recent past, showed no evidence of additionalsurface slip. Faults investigated are the Skeleton Canyon andPainted Canyon faults in the Mecca Hills area of the SanAndreas fault zone; the Brawley fault zone, northeast of theImperial fault; and the Coyote Creek fault, part of the SanJacinto fault zone, northwest of the Superstition Hills fault.The Skeleton Canyon and Painted Canyon faults werechecked only in Box Canyon and Painted Canyon. In con-trast, we carefully searched for new surface slip along theBrawley fault zone, which ruptured in the 1979 ImperialValley earthquake (Sharp et al., 1982). We searched alongKeystone, Harris, and Worthington road crossings of theBrawley fault zone; road pavement, dirt shoulders, and con-crete canal liners were checked. No fresh breaks were ob-served. The Coyote Creek fault was checked by Geoff Fa-neros (San Diego State Univ., oral commun., 1999) for new

surface slip at only one place, near the southeastern end ofthe Ocotillo Badlands. This was the site of afterslip and trig-gered slip from other earthquakes in the region (Burford,1972; Hudnut and Clark, 1989). No new slip was observedin association with the 1999 Hector Mine earthquake; how-ever, surface slip may have occurred elsewhere along thefault.

Discussion and Conclusions

Some questions as to the nature of triggered slip in theSalton Trough now can be answered in light of new datapresented in this paper. Field evidence indicates a transientopening as the Hector Mine seismic waves passed the south-ern San Andreas fault (Fig. 4). That observation is supportedby strong-motion records presented herein, showing relativeopening between strong-motion stations on opposite sides ofthe fault at the time of seismic wave passage (Fig. 6). Bodinet al. (1994) also proposed a model for slip at the time ofthe passage of the seismic waves; however, their proposedmodel incorporates transient increased pore pressures duringfault-normal contraction and reduction of effective normal


stress across the fault. We believe our direct observationsbest support the transient-opening model of fault slip.

An intriguing result of our measurements and compi-lations of repeated earlier slips is that each of the faults inthe Salton Trough with 1999 triggered slip behaves differ-ently. That is, comparison of triggered slips on the San An-dreas fault show the most frequent and largest amounts ofslip in the central Mecca Hills (Fig. 3), regardless of dis-tance, rupture direction, or magnitude of the causative earth-quake. For the Superstition Hills fault, distance and magni-tude likewise show little effect, but rupture directioncharacteristically produces slip distributions skewed awayfrom the source (Fig. 9). The Imperial fault, in further con-trast to the two other faults, shows a more random distri-bution of triggered slip (Fig. 11).

Theoretical and laboratory models suggest plausible ex-planations of variations in behavior of triggered slip on faultsin the Salton Trough. Models by Hartzell and Archuleta(1979), Heaton (1990), Brune et al. (1993), Andrews andBen-Zion (1997), Harris and Day (1997), Ben-Zion and An-drews (1998), Anooshehpoor and Brune (1999), Ranjith andRice (2001), and Gerde and Marder (2001), among others,indicate a transient opening as a mechanism of slip along afault and a dependence on velocity contrasts or a low-velocity fault zone. Variations in thickness of fault gouge insome of these models dictates the distribution of slip, espe-cially if the fault is along the margins of the gouge or low-velocity fault zone. A thick zone of gouge, or in the case ofthe San Andreas fault in the Mecca Hills, a thick zone ofmudstone injected into the fault (see Rymer, 1994), may aiddevelopment of slip. The presence of a low-velocity zoneprovides material contrast and aids in a self-sustained prop-agation of the slip pulse (Andrews and Ben-Zion, 1997; Har-ris and Day, 1997).

Oppositely skewed triggered slip on the SuperstitionHills fault following the 1968 and 1979 slip events was ob-served and discussed by Allen et al. (1972) and Fuis (1982).Fuis (1982) proposed two possible explanations for slipskewed away from the source: (1) slip is complementary toearlier slip, or (2) slip adjustments were triggered in a stressfield that varied oppositely along the fault. We favor arupture-runaway explanation, also discussed by Fuis (1982),which is further supported by laboratory and theoreticalstudies. These studies indicate that on coherent breaks (withno structural complications), displacement is skewed in adirection away from the point of initiation. In such models,slip is skewed away from the source due to focusing of stressduring rupture propagation (Hartzell and Archuleta, 1979;Archuleta and Day, 1980). Additional studies presented byHeaton (1990) and Brune et al. (1993) further speculatedthat rupture runaway, producing oppositely skewed slip maydevelop along faults. Slip triggered along the SuperstitionHills fault by the 1999 Hector Mine earthquake likewise isskewed away from the source (Fig. 9E) and thus adds to thehistory of oppositely skewed slip on the fault. The Super-stition Hills fault clearly contains a structural complication

in the stepover between the north and Imler strands (Rymer,1989) (Figs. 8, 9) that at first glance may challenge the con-dition of coherent breaks; however, even though there is lesssurface fault slip in the structural stepover (see Rymer, 1989,and brief discussion below), clearly the stepover has not im-peded propagation of slip through this structural complexityin at least five periods of surface fault slip, both triggeredand primary tectonic slip (Fig. 9).

Another point that comes into clearer focus with ourdata is the decrease of slip in areas of structural stepoversand multiple fault strands. This is not a new idea, but onethat is more apparent now that we have multiple surface slipsalong individual faults. We now have well-mapped slip dis-tributions along the southern San Andreas, SuperstitionHills, and Imperial faults during five events for each of thesefaults (Figs. 3, 9, 11). The structural stepover on the Super-stition Hills fault consistently has shown low slip values(Sharp et al., 1989; Rymer, 1989) (Fig. 9), even after sum-ming slip on minor faults between and around the stepover(Rymer, 1989). Likewise, smaller structural complexities inthe surface trace of the Imperial fault, as revealed in the 1979surface rupture (Sharp et al., 1982), show less slip duringboth primary rupture and remotely triggered slip (Fig. 11,zones w to z). We speculate that instead of having future slipcompensate for past low-slip zones, the areas of structuralcomplexity include off-fault deformation (folding, rotation,and other nonbrittle deformation) that will not be includedin surface displacement.

The location and shape of the Salton Trough may addfurther structural controls on the location and likelihood oftriggered slip. Figure 5 shows the location of triggered slipin 1999 and the subsurface margins of the Salton Trough(heavy gray line). Field (1996) presented data and a modelfor spectral amplification and clear basin-edge-inducedwaves associated with the 1992 Landers earthquake. Hisstudy was located approximately halfway between PalmSprings and Indio. A casual look at the shape of the troughand the source of the Hector Mine mainshock indicates that1999 seismic waves likewise may be associated with basin-edge effects. While this may be the case for the San Andreasfault in association with the Hector Mine earthquake, how-ever, such is not the case for the other two faults with trig-gered slip: the Superstition Hills fault on the opposite sideof the basin and the Imperial fault lying within the basin.Furthermore, given the location and direction of seismicwaves from each of the causative earthquakes for slip alongthe San Andreas, Superstition Hills, and Imperial faults (Ta-ble 1; see black stars in Figs. 1, 5), clearly basin edge effectsare not a controlling factor for development of triggered slip.

These and other speculations on the relations of slip,material properties, and structural control should be pursuedby further comparison of real geologic properties and theo-retical and laboratory models. The Salton Trough is a naturallab for such comparisons, with its multiple repeats of trig-gered slip, aseismic slip (creep), and, in the case of the Coy-ote Creek (not included in this report), Superstition Hills,


and Imperial faults, primary tectonic slip. We agree with thesuggestions of Bodin et al. (1994) for a strain-monitoringarray to help resolve the causative mechanism of triggeredslip. We further suggest installation of two overlappingcreepmeters, set approximately 45� to a fault and set normalto each other. Such an installation could further resolve thedevelopment and nature of a future transient pulse along theSan Andreas fault. A third suggestion is the use of InSARdata to better understand the distribution and amount of sliparound structural complexities, such as the stepover in thecentral Superstition Hills fault. The bare ground and gentleslopes in that area, combined with repeated surface slips andaseismic creep events, make this an ideal place to test someof our hypotheses.

Acknowledgments

We thank S. Myers (Cathedral City) for field assistance in this study.Conversations with USGS colleagues L.M. Jones, K.W. Hudnut, and M.J.S.Johnston aided the investigation of 1997 aseismic creep. Geoff Faneros(San Diego State Univ.) kindly checked the Coyote Creek fault for triggeredslip. K.W. Hudnut, V.E. Langenheim, and H.D. Stenner provided extremelyhelpful reviews of an earlier version of the manuscript.

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U.S. Geological Survey345 Middlefield Road, MS 977Menlo Park, California 94025

(J.B., M.J.R., L.C.S.)

Department of Geological SciencesCalifornia State UniversityNorthridge, California 91330

(J.D.Y.)

Seismological LaboratoryCalifornia Institute of TechnologyPasadena, California 91125

(J.L.)

Manuscript received 31 January 2001.

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Triggered Surface Slips in the Salton Trough Associated