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The 1988 and 1990 Upland Earthquakes Leftlateral Faulting

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The 1988 and 1990 Upland Earthquakes Leftlateral Faulting JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 96, NO. B5, PAGES 8143-8165, MAY 10, 1991 The 1988 and 1990 Upland Earthquakes: Left-Lateral Faulting Adjacentto the Central TransverseRanges EGnI• HAUKSSON SeismologicalLaboratory, Division of Geological and Planetary Sciences, California Institute of Technology,Pasadena Lucn.• M. JONES U.S. GeologicalSurvey, Pasadena, California Two earthquakes(ML=4.6 and ML=5.2 ) occurred at almost the same location in Upland, southern California, in June 1988 and February 1990 and had similar strike-slip focal mechanismswith left-lateral motion on a northeaststriking plane. The focal mechanismsand aftershocklocations showed that the causativefault was the San Jose fault, an 18-kin-long concealed fault that splays west-southwestfrom the frontal fault of the central Transverse Ranges. Left-lateral strike-slipfaults adjacentto the frontal faults may play an important role in the deformationof the TransverseRanges and the Los Angeles basin as suggestedby these Upland earthquakes,the left-lateral strike-slip 1988 (ML--4.9) Pasadenaearthquake on the Raymondfault, 30 km to the west of Upland, and scatteredbackground seismicity along other active left-lateral faults. These faults may transfer slip away from part of the frontal fault toward the south. Alternatively, these faults could representsecondary faulting related to the termination of the northwest striking right-lateral strike-slip faults to the south of the range front. The 1988 and 1990 Upland earthquakesruptured abutting or possibly overlapping segmentsof the San Jose fault. The edges of the overlapping aftershock zones, which are sharplydefined, togetherwith backgroundseismicity, outline a 14-kin-long aseismic segment of the San Josefault. The 1988 mainshockoriginated at 9.5 km depth and caused aftershocks between 5 and 12 km. In contrast, the 1990 mainshock focus occurred at the top of its aftershockzone, at 5 km, and causedaftershocks down to 13 km depth. These deep aftershocks taperedoff within 2 weeks. The rate of occurrenceof aftershocksin magnitude-timespace was the same for both sequences. The state of stressreflected in the focal mechanismsof the aftershocksis identical to that determinedfrom backgroundactivity and did not change with time during the aftershocksequence. The constantstress state suggeststhat the 1988 and 1990 eventsdid not completelyrelease all the storedslip on that segmentof the fault. The presence of 14 km of unbroken fault, the abrupt temporal termination of deep aftershocks, and the constantstress state all suggestthat a future moderate-sizedearthquake (ML=6.0-6.5) on the San Josefault is possiblewith a rupturelength of at least 14 km and possibly18 km. I_NTRODUCrlON The hypocentersof the 1988 and 1990 earthquakes occurred at almost the same location, less than 2 km to the The 1988 (Mœ=4.6) and 1990 (Mœ=5.2) Upland south of the surface traces of the reverse faults that define earthquakesare separatedby only 2 km horizontallyand 4 km vertically. Their similar focal mechanismsand the southernfront of the centralTransverse Ranges. These reversefaults extend from SanFernando Valley in the west overlappingaftershock zones suggest that the two events to the San Andreas fault in the east [Crook et al., 1987; occurredon abuttingor possiblyoverlapping segments of Morton and Matti, 1987]. A few late Quaternaryleft- onefault. Their locationsuggests that they occurredon the little known San Josefault. The purposeof this paperis lateralstrike-slip faults, such as the Raymondfault and the San Jose fault, splay off the front southwestward. In to analyzethe spatialand temporaldevelopment of both addition to these faults, several northeast trending sequencesto determineif thesetwo sequencesindeed did seismicityalignments that are not associatedwith known overlap. Furthermore,these 1988 and 1990 earthquakes faults are also reportedto the south of the range front have only ruptureda 4-km length of the San Jose fault, (Figure 1). Both events showedleft-lateral strike-slip which is at least 18 km long. It is therefore of motionon northeaststriking planes and occurredon oneof considerableinterest to know if there is any evidencefor a these left-lateral faults, the San Jose fault. In addition to continuationof this damagingearthquake sequence along this fault. the two Upland earthquakes the ML=4.9 Pasadena earthquakewas also causedby left-lateral strike-slip faultingnearby, on the Raymondfault in December1988 [Joneset al., 1990]. Copyright1991 by the AmericanGeophysical Union. How these northeast striking left-lateral faults and Papernumber 9 lIB00481. seismicitytrends are relatedto the deformationalong the 0148-0227/91/91 IB-00481 $05.00 thrustfront is not clear. It is possiblethat thesefaults 8143 8144 I-[AUKS/IONAND JONES: UPLAND EARTHQUAKES East Los Angeles Basin M>l.5 1981 - May 1990 20' oo %oo "•,%•o o o / '/ SAN GABRIELMOUNTAINS o • '-..-..--..L.-•oo • o o o • % •) 1õ' o o o o • o --• o•o o o 0o Z* 10' 1988o •*••erro Modr• •oul•• ø • "'• e .'--. •-' '•- =• •" o• o _. .-'*'•'......... ß ...... :.:....L].• -• -- • .••?..oo•o o" o o.... o .-•o o [ ' ........:.;:: ........ ... ..> :_ ... •'2 •• • -- o o ' [ ..... ß-. •.•• UDlO•O ..' o •o% o• • ß 0 •?":o • •.• o . o o ..-•.•.&o.:•::..... o ••',••' o• ..• I•0 • o ; o Ooo oJ%o oø • "'..•'.. •, _- o.ø•oo o-9•• o•o o ..'•'..'• o ø• go ....•'• ..' o oOß o•o øo o ' ':::.. • 0 0 o oo o _ •. •[•... •... o '•. •••o , o _ o øo1•87- ." : ß . _... oo ••o, • • a• o-- .......... •.... •o ø q/ o • _o %o.... o o oo• o o •o • o".,". *.. o o • o o o o o 0- • •/.o• •o • •0 o o- o o oo Ooooooø •o•o •/••-••bo;,,Oo ;• øo oo ø o oXg.• X•o o o •, oo o . .. o • _•m o'o• o/ •o •/o••'• ••7 o ••• • • o o o .. '-. _- •oxo • /•• •• o '•x"'.• • o o "o.• o oo ß o •o oo o •o '. •o o o _ o o o " ....... o o oo oo•,o %o o •_, • ... 0. ooo o o o .9oo7• :o%•. • ... o o •o0•øoOo , - . ":. • Oo •o•ø3gooX : • ,0' - - •'o ' •{ •g ø' ' •%•'" o Oo - •*,,-,, ' o o •.• .• - '*%%• . ' '5 ' k•' I ' ' ' ' I ' ' * o' Io ' •o• • ' I '"•• ' ' • I 'o•. ' ' ' I o ' '•i'•o,•'.. ' ' I * * *ß * I o ' ' ' I * ' ' o* I ' ' ' [ 10' 5' 118 ø 55' 50' 45' 40' 35' 30' 25' 20' 15' Fig. 1. A map of the easternLos Angelesbasin showing major faults (dotted if inferred)from Jennings [ 1975] and the locationsof earthquakesM L > 1.5 from 1981 to May 1990 recordedby the SouthernCalifornia Seismic Network. Earthquakesof M L > 4.0 are shownas stars.Dates of earthquakesM L > 4.9 are given. transferslip awayfrom partof the front to the south. Such many of these barriers are covered with undis• a tectonicmodel requires that bothconcealed thrust faults Holocene surfaces and in most cases are not associated with and northweststriking strike-slip faults exist to the south significantearthquake activity, their earthquakepotential and westto transfermotion back to the westernpart of the remains difficult to quantify [e.g., Ziony and Yerkes, front to accountfor the total slip budget. Alternatively, 1985]. Analyzingthe relationshipbetween the 1988 and theseleft-lateral faults could be secondary,related merely to 1990 Upland earthquakesand the San Jose fault thus the termination of the northweststriking right-lateral provides some of the first evidence of the earthquake strike-slipfaults to the south. The Elsinoreand the Chino potential of these concealedwater barriers in the Los faults terminatesouth of the range front near the San Jose Angelesbasin. fault (Figure 1) without any clear indicationof how the Two previousstudies of backgroundseismicity showed fault motion is transferredfrom the right-lateralstrike-slip that bothstrike-slip and thrust faulting occur in the Upland regime to the reverse faults along the range front. area,in the northeasterncomer of the Los Angelesregion. Hauksson [1990] suggesteddecoupling of right-lateral The backgroundseismicity of the Upland region was strike-slipand dip-slipmotion as a meansof mergingthe studiedby Cramerand Harrington [1987] who deployeda two differentregimes of faulting. Sucha tectonicmodel is portablenetwork around the Cucamongafault in springof alsoconsistent with the presenceof theseactive left-lateral 1977. They recorded one left-lateral strike-slip focal strike-slipfaults. mechanism near the northern end of the San Jose fault and The SanJose fault has beenmapped solely on the basis a thrust mechanism near its southern end. Hauksson of a water barrier [California Department of Water [1990] analyzedthe backgroundseismicity during 1977- Resources(DWR), 1970]. Concealedwater barriers similar 1989 and reportedseveral strike-slip and thrust fault focal to the San Josefault have beenmapped elsewhere in the mechanismsin the Upland area. Recent studiesof the Los Angeles basin [e.g., Yerkeset al., 1965]. Because 1988and 1990Upland earthquakes have focusedmostly on HAUK•ON AND JONES:UPLAND EARTHQUAKF• 8145 modelingon-scale waveforms. Mori and Harnell [1990] from mostly horizontal slip offsetting basement usedrupture directivity to showthat the 1988 earthquake topography. was causedby rupture on a fault with strike and dip consistent with the left-lateral plane of the focal DATA AND ANALYSIS mechanism,which is presentedin this study. Using three- The arrival time and P wave first-motion data from the componentdata from the Pasadenastation (PAS), Dreger SouthernCalifornia Seismic Network operated by the U.S. andHelmberger[1990 and 1991] foundthat the waveforms Geological Survey and the California Institute of of both the 1988 and 1990 events could be modeled with Technology(USGS/CIT) were usedto obtainhigh-quality left-lateral
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