Earthquake locations using single-station deep borehole recordings: Implications for microseismicity on the San Andreas fault in southern California

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1 JOURNAL OF GEOPHYSCAL RESEARCH, VOL. 100, NO. B12, PAGES 24,003-24,014, DECEMBER 10, 1995 Earthquake lcatins using single-statin deep brehle recrdings: mplicatins fr micrseismicity n the San Andreas fault in suthern Califrnia Rachel E. Abercrmbie Department f Earth Sciences, University f Suthern Califrnia, Ls Angeles Abstract. We have lcated and estimated surce parameters f 109 earthquakes (0 t 5 M) using seismgrams recrded at 2.5 km depth in the Cajn Pass brehle, suthern Califrnia. The brehle is abut 4 km frm the San Andreas fault, at the bundary between the lcked, almst aseismic Mjave and San Bernardin segments, where the San Jacint fault appraches the San Andreas. This area is f interest bth n accunt f its tectnic cmplexity and its high ptential seismic hazard. The clear, relatively unattenuatedwnhle recrdings are rtated t determine the incming azimuth f the P wave, and the delay time between the P and S arrivals is used t estimate the hypcentral distance. The difference between the hypcenters lcated this way and the Suthern Califrnia Seismic Netwrk (SCSN) lcatins f the same earthquakes increases with hypcentral distance frm the brehle. Of the earthquakes within 20 km f the brehle, 95% are within 1-2 km f the SCSN epicenters and all are within 5 km f the SCSN depths. All but three f the 58 earthquakes lcated here which the SCSN did nt recrd were within 20 km f the brehle and s the lcatin errrs are similar t thse f SCSN A and B quality events. Mst f the small earthquakes ccurred within the relatively active San Jacint fault zne, but at least eight are lcated clse t r within the San Andreas fault zne. The stress drps fr these events range frm 0.1 t 18 MPa. These earthquakes appear similar t the ther events in this study suggesting that the San Andreas fault is similar t ther faults at the scale f these small earthquakes. The variatin in stress drp and slip rientatin f these small earthquakesuggests that larger scale hetergeneity cntinues t these small scales. Als, the lcatin f these small earthquakes in majr established fault znes implies that earthquake surce dimensins are nt gemetrically cntrlled by fault zne width. ntrductin A triaxial, 10 Hz seismmeter was installed at a depth f 2.5 km in the Cajn Pass scientific drill hle in August The hle, which was drilled t investigate the state f stress and heat flw n the San Andreas fault [Zback and Lachenbruch, 1992], passed thrugh 500 m f sediments, 200 t 500 m f highly fractured granite and then int largely uninterrupted crystalline granite [Silver and James, 1988]. During their 2 years f peratin, the gephnes recrded several thusand lcal earthquakes. Apprximately 90% f the earthquakes recrded dwnhle within abut 20 km were nt large enugh t trigger the Suthern Califrnia Seismic Netwrk (SCSN) [Abercrmbie, 1995]. The SCSN is cmplete abve abut 1.8 ML in this area (E. Haukssn, persnal cmmunicatin, 1994). As the brehle is nly 4 km frm the San Andreas fault (Figure 1), it is pssible that sme f these small earthquakes are ccurring within the San Andreas fault Zealand. Cpyright 1995 by the American Gephysical Unin. Paper Number 95JB /95/95JB zne. This majr fault has been largely aseisrnic in suthern Califrnia abve the netwrk threshld during the mre than 10 years f dense shrt-perid netwrk peratin. The crustal strain is apparently almst all released in large (>7 M) earthquakes, such as ccurred in 1857 [Sieh, 1978], and s there is little infrmatin available abut the structure f the fault zne and the stresses n it frm instrumentally recrded seismicity. t has been prpsed, n the basis f reginal stress and heat flw measurements, that the San Andreas fault is weak, supprting little shear stress, with respect t the surrunding rck [Zback et al., 1987]. Stress drps were determined fr 105 earthquakes recrded by the deep instrument (within 60 km) by Abercrmbie and Leary [1993], and Abercrmbie [this issue]. Only 51 f these events were large enugh t trigger the SCSN. Lcating the ther 58 earthquakes wuld therefre nt nly address whether significant micrseismicity des ccur within the San Andreas fault zne, but als enable investigatin f the spatial distributin f earthquake stress drp in the cmplex tectnic area arund the Cajn Pass drill hle. This is f interest because the brehle is situated at the junctin f three majr thult segments (the Mjave and San Nw at nstitute f Gelgical and Nuclear Sciences, Wellingtn, New Bernardin Muntains segments f the San Andreas fault and the 24,O03 San Bernardin Valley segment f the San Jacint fault) that have been assigned sme f the highest prbabilities in suthern Califrnia f rupturing in a damaging earthquake in the next 30 years [Ward, 1994].

2 24,004 ABERCROMBE: SNGLE-STATON DEEP-BOREHOLE EARTHQUAKE LOCATONS 40' 30' 20' 10' 35 ø 50' Mjax?e D 30' npas: ehl 40' 20' 10' ø 34 ø 50' 40' 30' Pacific Ocean, 20' 10' 33 ø 50 KM 120ø 40' 30' 20' 10' 119ø 40' 30' 20' 10' 118ø 40' 30' 20' 10' 117ø 40' 30' 20' 10' 116ø 40' Figure 1. Lcatin map shwing the Cajn Pass brehle (slid triangle), the statins f the Suthern Califrnia Seismic Netwrk (small circles), and the principal faults in suthern Califrnia. The bx labeled "a" crrespnds t the area f Figure 4a, and the bx labeled "b" crrespnds t the area f Figures 4b, 5, and 6. This article describes the methd used t lcate the small earthquakes using nly the single brehle statin, and sme f the first estimates f earthquake stress drp n the San Andreas fault in suthern Califrnia. The lcatin technique emplyed is simply t rtate the three-cmpnent seismgrams t btain the directin f the incming P wave, and then cmbine that with the hypcentral distance (calculated ¾m the P and S wave travel time difference) t lcate the earthquake. The spatial distributin f stress drp and seismicity in the cmplex tectnic area arund Cajn Pass is then investigated. Seismgram Rtatin The dwnhle seismgrams are free f the seismic prpagatin thrugh the highly attenuating and scattering near surface rcks encuntered in surface recrdings. Clear P and S wave pulses are seen [Abercrmbie and Leary, 1993; Abercrmbie, this issue]. Figures 2a and 2c shw the three recrded cmpnents f tw earthquakes, cnverted t displacement, with a flat respnse between 2 and 200 Hz. Ntice nt nly the clear P and S pulses, but als the significant P wave energy n the hrizntals, and S wave energy n the vertical. The depth f the seismmeter large enugh that the rays have nt been refracted t vertical incidence, as is usual/'r surface recrdings. Fr each event, a time windw is selected t include nly the first P wave velcity pulse, and the velcity seismgrams are then rtated t find the directin maximizing the P wave energy within these windws (typically, 0.01 t 0.2 s). This directin (termed radial) is taken t be the incming P directin. The tw ther directins are then defined as that in the hrizntal plane, perpendicular t the radial (SH) and the third rthgnal directin (termed SV). Figures 2b and 2d shw the results f the rtatin fr the tw events. t is encuraging that nt nly is the P wave energy cncentrated n the radial cmpnent, but that the S energy is almst purely n the ther tw cmpnents, even thugh the S energy was nt used in the rtatin prcedure. The rtatins were n average stable t within 2 ø t 3 ø fr all events. Using nly the first velcity pulse in the rtatin prevented significant scattered energy frm being included and prduced the best match with the lcatins f thse earthquakes lcated by the SCSN (see belw). T perfrm the rtatins, it is necessary t crrect the seismgrams recrded by ne f the hrizntals fr a gradual

3 ABERCROMBE: SNGLE-STATON DEEP-BOREHOLE EARTHQUAKE LOCATONS 24,005 (a) 2.8 M L, Distance 15 km (c) 1.7 ML, Distance 10 km i i ' i ß i ' i O3 c- O 20 0 H i i i i i Time (secnds) (b) i i ' i i (d), [,, Time (secnds) i i i i i O3 c- O 20 0 Radial _ Radial [ _ SH Time (secnds) -4,,, Time (secnds) Figure 2. Seismgrams recrded at 2.5-km depth (flat t displacement frm 2 t 200 Hz). Ntice the clear P and S pulses. (a and c) The three recrded cmpnents lbr tw lcal earthquakes. (b and d) the seismgrams after rtatin t determine the incming P wave directin. decay in the amplitude respnse f the gephne. This began alter the first 6 mnths f peratin and was caused by the lng perid f time at such an elevated temperature (abut 105øC). Prlnged expsure t such high temperatures can cause the cil insulatin t decay, gradually shrting ut parts f the cil and resulting in a decrease in resistance f the gephne and f its amplitude respnse (L. Lutringer, Mark Prducts persnal cmmunicatin, 1993). Cmparisn f recrdings n this cmpnent with the ther tw shwed n variatin with frequency respnse ver time. Daily crrectin hctrs were reliably determined frm regular measurements f the resistance f the gephne made at the surface during the perid f peratin. They increase frm 2 t 4.5 ver the 13 mnths during which the earthquakes cnsidered here were recrded. Determining the Orientatin f the Dwnhle Hrizntals After btaining an incming directin fr each event t the brehle seismmeter crdinate system, it is necessary t determine the rientatins f the tw hrizntals with respect t nrth. Obviusly, when the instrument package was being lwered int the brehle it was impssible t prevent it frm rtating. Withut the aid f expensive cntrlled surces, there are tw simple appraches t try, utilizing the SCSN lcatins f the larger events. (t shuld be nted that hitting the wellhead with a sledgehammer was insufficien t trigger the deep instrument.) The first methd is t cmpare the angle (0) between the first hrizntal (H) and the incming directin determined by the seismgram rtatin, with that between nrth and the knwn incming directin frm the SCSN lcatin (q)) fr thse earthquakes large enugh t trigger the SCSN. The difference between 0 and q then gives the angle between H1 and nrth. This angle (0 was determined t be 18.4 _+ 3.4 ø (1 standard deviatin) fr the 51 earthquakes studied. n an attempt t cnfirm and cnstrain this estimate f r, the first-mtin plarities f the P waves n all three dwnhle cmpnents f the earthquakes recrded by the SCSN are used. Figure 3 shws hw they shuld be arranged abut the three-cmpnent directins. Frtunately, there were earthquakes lcated very clse t the directins f the

4 24,006 ABERCROMBrE: SNGLE-STATON DEEP-BOREHOLEARTHQUAKE LOCATONS H1 30 Figure 4 cmpares the lcatins f the 51 earthquakes determined using the single brehle statin with the lcatins f the same events determined by the SCSN. The differences between 300 ) ) the tw lcatins are nt dependent n incming azimuth, but d shw sme dependence n the distance f the event frm the brehle. The'depth f the mst distant events (>30 km, Figure 4a) is verestimated by the single statin. Nne f the small events W 'l ' E lcated here withut SCSN lcatins are this far frm the brehle, hwever, and Figures 5 and 6 d nt include these earthquakes. There is als a tendency fr earthquakes >20 km frm the brehle t be lcated here clser t the brehle than by the SCSN, and the very nearest events are here lcated further frm the brehle than by the SCSN (Figure 4b). These errrs are what wuld be expected if the actual velcity structure was ne f gradually increasing velcity with depth and the cnstant velcities used here were average (mid depth range) values. Using a gradually varying velcity structure wuld therefre remve these biases, but the cnstraints such a velcity wuld be small, Figure 3. Rse diagram shwing the incming azimuths f the 51 earthquakes recrded by the SCSN. The letters represent the and the small pssible imprvement in accuracy des nt warrant it. Using the simple, cnstant velcity structure assumed here, all three cmpnent P wave first mtins in the different quadrants. the epicenters determined in this study, within 30 km f the The three cmpnents are Z (vertical) and H1 and H2 brehle are within 3.5 km f their SCSN lcatins and the depths (hrizntals), respectively. D indicates dwn and U indicates up. are within 5.5 km. Of thse earthquakes within 20 km f the Fr example, UUD means up n the vertical and H, and dwn brehle, all the epicenters are within 2 km f the crrespnding n H2. The selectin f up and dwn directins f H1 and H2 SCSN lcatin, and tw thirds are within 1 km. Similarly the was initially arbitrary, but the same fr all events. Up is fund t depth discrepancies are less than 5 km, less than 2.5 km fr tw be the directins indicated by the arrw heads n the preferred rientatins f the dwnhle hrizntal gephneshwn. The thirds f the events. These 'small differences in lcatin are fact that the first mtin frm the earthquake depends n its fcal mechanism means that tw cmbinatins f plarities are cmparable with the errrs in the SCSN lcatins and s imply that the lcatins f earthquakes within abut 20 km f the pssible fr all fur quadrants defined by the gephne brehle can be estimated reliably frm the single deep rientatins, as labeled. The rientatins f the gephnes are instrument. The lcatins f the 58 earthquakestudied by cnstrained t be where the bserved earthquake plarity Abercrmbie [this issue] which were t small t trigger the SCSN cmbinatins change. are then determined the same manner. Of these, nly three were mre than 20 km frm the brehle, and the vast majrity were hrizntal cmpnents, and s using this technique, ct was cnstrained t be between 14 ø and 20 ø with n incnsistent plarity bservatins. The cmpnent H1 is therefre taken t be 18 ø east f nrth in determining the earthquake lcatins. assuming that all earthquakes are deeper than the brehle instrument (2.5 km belw the surface, 1.5 km belw sea level), which is true fr all the SCSN-lcated events. within 15 km. Spatial Distributin f Earthquakes and Stress Arund the Brehle Cmparisn f the Single-Brehle Statin and SCSN Lcatins The delay between the P and S arrival times is used t determine the hypcentral distance t each event, using V, = 6 km/s and Vs = Vv/ /3 km/s. The earthquakes are then simply lcated by using the incming directins, ct = 18 ø, and straight ray paths. The assumptin f this simplestype f velcity structure is thught reasnable as many studies are able t mdel the crust as a half space belw the very near surface sedimentary layers. This mdel is als within the reslutin f the three dimensinal velcity inversin in this area by Magistrale et al. [1992]. The 180 ø ambiguity inherent in the rtatin methd is slved by using the incming plarities (Figure 3) n the three cmpnents and Figure 5 shws all the earthquakestudied by Abercrmbie [this issue]. Thse t small t trigger the SCSN are assigned 0 ML tbr pltting purpses. The earthquakes which were large enugh t trigger the SCSN are pltted at the netwrk lcatins which shuld be cnsidered superir, when available. Ntice that many f the small earthquakes lie n r between the San Jacint and San Andreas faults. Sme earthquakes lie beneath the surface trace f the San Andreas fault, suggesting that they may be ccurring within the San Andreas fault zne if it is fairly vertical. T investigate the spatial distributin f stress drp in the area, the earthquakes shwn in Figure 5 are pltted in Figure 6 with the symbl size prprtinal t 2xlgt(stress drp, bars) frm Abercrmbie [this issue. These stress drp estimates were btained frm mdeling displacement spectra t determine the seismic mment and surce radius. An attenuatin crrectin f Qt, = Qs =

5 ABERCROMBE: SNGLE-STATON DEEP-BOREHOLEARTHQUAKE LOCATONS 24,007 (a) 34 ø 30' lllilllllllllllllllllllltlllllllllllllllllllllllllllilltlllllll 34 ø 25' 34 ø 20' ':. Brehle./..-' - '- 34 ø 15' 34 ø 10' 34 ø 05' 34 ø 00' [ [ 117 ø 45' 117 ø 30' 117 ø 15' 117 ø 00' 116 ø 45' Cajn Pass ß rehle 10 Km View frm 127 degrees 1.0 < M < < M < < M < < M < < M < 6.0 Figure 4. Cmparisn f the SCSN earthquake lcatins (gray circles) with thse determined thi study (black stars). The crssectin is the view frm the SE, perpendicular t the San Andreas and San Jacint faults, at the brehle. The slid triangle and circle n the crssectin represent the lcatin f the surface traces f the San Andreas and San Jacint faults respectively, clsest t the brehle is used fr all events frm mdel 2 f Abercrmbie [this clse t r within the San Andreas fault zne (marked with an issue]. The errrs in the stress drps are thught be abut a asterisk in Table 1) have stress drps f 0.1 t 18 MPa (2.0 factr f 10 [Abercrmbie, this issue]. N bvius pattern and less). emerges, as has been fund in many similar studies n accunt f the prbable stress hetergeneity and errrs (typically, a factr f Discussin 10) in stress drp measurements. Als the stress drps d nt appear dependent n depth. Perhaps the mst interesting The results f this study shw that if micrearthquake stress bservatin is that the eight micrearthquakes which are lcated drps can be cnsidered in any way representative f the stresses

6 24,008 ABERCROMBE: SNGLE-STATON DEEP-BOREHOLE EARTHQUAKE LOCATONS (b) 34 ø 22' 34 ø 20' Cajn Pass Brehle 34 ø 18' 34 ø 16' 34 ø 14' 34 ø 12' 34 ø 10' 34 ø 08' 34 ø 06' 117 ø 40' 5 Km 117 ø 35' 117 ø 30' 117 ø 25' 117 ø 20' Cajn Pass Brehle 117ø15 ' View frm 127 degrees 1.0 Figure 4. (cntinued) _< M < _< M < 3.0 n the majr thult znes in which they ccur, then there is clearly a its magnitude. This is typically accmplished by cmputing significant amunt f small-scale hetergeneity in the lcal stress earthquake fcal mechanisms. Determining the rientatin f slip field. T understand fully the stress field in this cmplex area, we in the small earthquakes wuld als enable us t discver hw need t determine its rientatin as well as estimating variatin in much f the thult zne micrseismicity is ccurring n faults

7 ABERCROMBE: SNGLE-STATON DEEP-BOREHOLE EARTHQUAKE LOCATONS 24, ø 22' 34 ø 20' --. Cajn Pass Brehle 34 ø 18' 34 ø 16' 34 ø 14' 34 ø 12' 34 ø 10' 34 ø 08' * 34 ø 06' 5 Km 117 ø 40' 5 Km ' 117 ø 35' 117 ø 30' 117 ø 25' Cajn Pass Brehle ß 117 ø 20' ' 20 View frm 127 degrees 0.0 <M < < M < < M < 3.0 Figure 5. Lcatins f the 93 earthquakes within 25 km f the brehle. The SCSN lcatins are preferred if available, and the smaller events are assigned magnitude 0 fr pltting purpses. Crssectin as in Figure 4. parallel t the San Andreas and San Jacint. Unfrtunately, the t cnstrain the slutin (Figure 7). They represent a mixture f dwnhle gephnes are f t high a frequency t recrd any right lateral strike-slip and reverse faulting, as was fund fr the near-field radiatin terms, and withut them it is impssible t area by Jnes [ 1988]. Fr example, tw earthquakes lcated within cnstrain a fcal mechanism using nly ne statin. Fcal 2 km f ne anther n the San Jacint fault have significantly mechanisms are calculated using SCSN first mtins fr the different mechanisms, indicating that there is sme small-scale earthquakes in this study large enugh t have sufficient readings hetergeneity in rientatin f small earthquake slip. One

8 24,010 ABERCROMBE: SNGLE-STATON DEEP-BOREHOLEARTHQUAKE LOCATONS 34 ø 22' 34 ø 20' 34 ø 18' 34 ø 16' Cajn Pass Brehle / 34 ø 14' 34 ø 12' 34 ø 10' 34 ø 08' 34 ø 06' i 117ø40 ' 5 Km ' ' ' ' 117 ø 15' ø ' Cajn Pass Brehle 20 View frm 127 degrees 0.0 < M < < M < _< M < _< M < _< M < _< M < 6.0 Figure 6. Spatial distributin f earthquake stress release arund the Cajn Pass brehle. Crss sectin as in Figure 4. The pentagns are prprtinal t lg10(stress drp, Ac ). earthquake is strike-slip with all three cmpnent first mtins recrded dwnhle being dwn, and the ther is reverse with all three cmpnent dwnhle first mtins being up. This lcal variatin in mechanism cmbined with the general tectnic cmplexity f this area render it undesirable t attempt t determine slip rientatin fr the smaller earthquakes using cmpsite fault plane slutins. The surce parameters f the earthquakes lcated clse t r within the San Andreas fault shw n significant differences frm ther earthquakes in this study, suggesting that earthquakes within

9 _ ABERCROMBE: SNGLE-STATON DEEP-BOREHOLE EARTHQUAKE LOCATONS 24,011 Table 1. Hypcentral Parameters f Earthquakes Used in This Study Event Mi D, Latitude Lngitude Depth, AH, AZ, M0,,, km Deg Min. Deg. Min. km km km Nm e e le+l l e e+1 l e e e l e ll l e e e+l l e e e e e+l l e+l l e e ll e e e e+ l e+1 l e e e e+l e+l e e l.l e e e e e e+1 l e e e e e e e+l e e e e e e+09 MPa * , , , e-02

10 , 24,012 ABERCROMBE: SNGLE-STATON DEEP-BOREHOLE EARTHQUAKE LOCATONS Table 1. (cntinued) Event M L D, Latitude Lngitude Depth, AH, AZ, M0, A j, km Deg Min. Deg. Min. km km km Nm MPa e e+l e e e e e * e e-02' e e ? e e e * e e e e e e e e e e e e % e+l e : e l. e e e :i'e e e e e e e-02 Read 1.29e+11 as 1.29 x 10 SCSN lcatins are given where available, and AH and AZ are the differences in epicenter and depth, respectively, between the SCSN lcatins and thse determined using the dwnhle recrdings. D is the hypcentral distance frm the brehle. The surce parameters mment (M) and stress drp (Ate) are frm Abercrmbie [this issue], mdel 2. * Thse earthquakes cnsidered clse t r within the San Andreas fault zne.

11 , ß ABERCROMBE: SNGLE-STATON DEEP-BOREHOLE EARTHQUAKE LOCATONS 24,013 25' Cajn Pass,. Brehle,,, / 10 f - / //'... S t Figure 7. First-mtin fcal mechanisms calculated frm the SCSN fr the larger earthquakes in thi study (ML> 1.5). Ntice the lcal variatin, including the almst clcatin f reverse and strike-slip mechanisms t the suth f the brehle. the San Andreas fault are similar t, and are ccurring under Abercrmbie and Leary [1993] and Abercrmbie [this issue], and similar cnditins as, thse n ther lcal faults (Figure 8). There lcated here, hwever, ccurred within established, majr fault is a slight dependence f stress drp n seismic mment, znes, principally the San Jacint and San Andreas faults, and especially fr the largest and smallest earthquakes in the data set. therefre the argument abve culd nt explain the surce This variatin is small cmpared t the ttal scatter, hwever, and dimensins f arund 10 m bserved fr earthquakes recrded in the stress drps during the earthquakes within the San Andreas the Cajn Pass brehle. The surce diameters f the events fault zne are typical f the data as a whle. The issue f abslute lcated clse t r within the San Andreas fault zne range frm abut 10 t 70 m. frictinal strength f the San Andreas fault has lng been debated, with measurements f heat flw and principal stress rientatin suggesting thathe fault is weak cmpared t labratry mdels f slip [Zback et al., 1987]. The earthquakes ccurring within the Cnclusins San Andreas fault zne appear n different frm ths elsewhere, suggesting that the San Andreas fault is n different frm ther The earthquake seismgrams recrded in the Cajn Pass faults, at least at this small length scale. brehle are s undistrted by attenuatin and scattering that the The lcatins f the earthquakes determined here als supprt surces can be lcated reliably using the single-statin, threethe cnclusins f Abercrmbie and Leary [1993] and cmpnent recrdings. At least eight f the earthquakes lcated in Abercrmbie [this issue], that there is n breakdwn in earthquake this manner are clse t r within the San Andreas fault zne, and surce scaling abve -1 M, and that the "minimum" surce s the fault is prbably nt aseismic belw the netwrk threshld. dimensin prpsed in a number f studies [e.g., Archuleta al., The surce parameters (including stress drp) f the earthquakes 1982] is simply reflecting the limit f spatial reslutin pssible n the San Andreas fault shw n systematic differences frm the with surface data. t had been suggested [Aki, 1987] that the ther earthquakes studied. The San Andreas faul therefre appears "minimum" surce dimensin was related t the width f the fault similar t ther faults at least at this small length scale. The spatial zne (typically a few hundred meters fr majr faults) and that hetergeneity f earthquake stress release bserved in this study earthquakes with smaller surce dimensins culd nly ccur n implies that the tectnicmplexity f the area cntinues t very small scales. yung, less well develped faults. The earthquakes cnsidered by

12 24,014 ABERCROMBE: SNGLE-STATON DEEP-BOREHOLE EARTHQUAKE LOCATONS All Earthquakes San Andreas Fault m ,- 10' O 10 ' ø TM Seismic Mment (Nm) Figure 8. The earthquake stress drps determined by Abercrmbie [this issue] and used in this study. An apparent slight increase in stress drp with magnitude is mstly attributable t the largest and smallest events and s culd be a result f the finite recrding bandwidth. The earthquakes lcated within less than 1 km f the surface trace f the San Andreas fault (marked with an asterisk in Table 1) are typical f the ttal data set. Acknwledgments. am grateful t P. Leafy and D. Manv fr installing the deep-brehle seismmeters; t M. Rbertsn, A. Martin, and L. Fields fr help with the data recrding; and t L. Teng, P. Davis, the Califrnia Department f Mines and Gelgy, and the Suthern Califrnia Earthquake Center (SCEC) fr generusly laning recrding equipment. J. Vidale, J. Batwright, A. McGarr and the Assciate Editr prvided cnstructive reviews f the riginal manuscript. The brehle seismmeter was installed under NSF award EAR , and R. E. A. was supprted by a SCEC Visiting Fellwship and NSF award EAR SCEC cntributin 114. References Abercrmbie, R. E., The magnitude - frequency distributin f earthquakes recrded with deep seismmeters at Cajn Pass, suthern Califrnia, Tectnphysics., in press, Abercrmbie, R. E., Earthquake surce scaling relatinships frm -1 t 5 Mz, using seismgrams recrded at 2.5 km depth, J. Gephys. Res., this issue. Abercrmbie, R. E. and P. Leafy, Surce parameters f small earthquakes recrded at 2.5 km depth, Cajn Pass, suthern Califrnia: implicatins fr earthquake scaling, Gephys. Res. Lett., 20, , Aki, K., Magnitude-frequency relatin fr small earthquakes: a clue t the rigin ffmax f large earthquakes, J. Gephys. Res., 92, , Archuleta, R. J., E. Cranswick, C. Mueller, and P. Spudich, Surce parameters f the 1980 Mammth Lakes, Califrnia, earthquake sequence, J. Gephys. Res., 87, , Jnes, L. M., Fcal mechanism and the state f stress n the San Andreas fault in suthern Califrnia, J. Gephys. Res., 93, , Magistrale, H., H. Kanamri, and C. Jnes, Frward and inverse threedimensinal P wave velcity mdels f the suthern Califrnia crust, J. Gephys. Res., 97, 14,115-14,135, Sieh, K., Slip alng the San Andreas fault assciated with the great 1857 earthquake, Bull. Seisml. Sc. Am., 68, , Silver, L. T., and E. W. James, Gelgic setting and lithlgic clumn f the Cajn Pass deep drillhle, Gephys. Res. Lett., 15, , Ward, S. N., A multi-disciplinary apprach t seismic hazard in suthern Califrnia, Bull. Seisml. Sc. Am., 84, , Zback, M. D., and A. H. Lachenbruch, ntrductin t the special sectin n the Cajn Pass scientific drilling prject, J. Gephys. Res., 97, , Zback, M.D. et al., New evidence n the state f stress f the San Andreas fault system, Science, 328, , R. E. Abercrmbie, nstitute f Gelgical and Nuclear Sciences, Kelbum Research Centre, 32 Salamanca Rad, PO Bx 1320, Wellingtn, New Zealand. ( R.Abercrmbie@gns.cri.nz) (Received April 22, 1995; revised July 25, 1995; accepted August 2, 1995.)