JOURNAL OF GEOPHYSICALRESEARCH, VOL. 95, NO. BI0, PAGES 15,365-15,394,SEPTEMBER 10, 1990

Earthquakes,Faulting, and Stress in theLos Angeles Basin

EGII.L HAUKSSON1

Departmentof GeologicalSciences, University of SouthernCalifornia, Los Angeles

Since1920 fourteen moderate-sized (ML=4.9-6.4) earthquakes have been reported in the Los Angelesbasin. Theseevents axe associated with bothmappable surficial faults and concealedfaults beneath the basinsediments. To determinethe styleof faultingand state of stressin the basin,single-event focal mechanisms for 244 earthquakesof M_>2.5that have occurredduring 1977-1989 have been calculated. Fifty-nine percent of the eventsare strike- slip andare mostly located near two of themajor, northwest striking right-lateral strike-slip faults in the basin, the Newport-Inglewoodfault and the Palos Verdes fault. The 1988 Pasadenaand the 1988 Upland earthquakesshowed left-lateral strike-slip on northeast strikingfaults. Numeroussmall earthquakes in theeastern part of thebasin show left-lateral strike-slipfaulting and form a northeasttrend near YorbaLinda. Thirty-twopercent of the eventshave reverse mechanisms and are distributedalong two broadzones. The farst,the ElysianPark fold and thrustbelt, coincideswith anticlinesalong the easternand northern flank of the Los Angelesbasin extending into SantaMonica Bay. The second,the Torrance- Wilmingtonfold andthrust belt, coincideswith anticlinesmapped on the southwestflank of thebasin and extends from offshore Newport Beach to thenorthwest into SantaMonica Bay. Oblique faulting that could be inferredby the mergingof strike-slipand compressional tectonicsdoes not occur in the basin. Instead,the coexistenceof zonesof thrustingand large strike-slipfaults in the basin suggeststhat the thrustand strike-slipmovements are mostly decoupled. A few normal faulting mechanismsappear to be related to faulting orthogonalto the axesof plunginganticlines. The trend of the maximumhorizontal stress variesfrom NIøW to N31øE acrossthe basinand consistentlyforms high angleswith the fold axes. The stressfield that existsalong the flanksof the basinhas a vertical minimum stressaxis. This stressfield and ongoingfolding and thrustingsuggest that tectonic deformation is concentratedalong the flanks of the deep central basin. Today 'the deformation of the basin consistsof uplift and crustal thickening and lateral block movement to accommodatethe north-southcompression across the basin.

INTRODUCTION thrustingis importantbecause they could causesimilar or The Los Angelesbasin is one of severaldeep Cenozoic even more damagingearthquakes than the 1987 Whittier basinsthat formed on the marginof the Pacificplate, near Narrowseanhq•e. the San Andreas fault in southern California. Located at Data from moderate-sizedevents (M>__4.9) since 1920 and the northernedge of the PeninsularRanges, it abutsthe small earthquakes2.5<_M<4.9 since 1977 throughoutthe southernmargin of the centralTransverse Ranges (Figure Los Angelesbasin have been analyzedto evaluateactive 1). Todaythe Los Angeles basin consists of a deepcentral faulting in the basin. A map of the epicenters of basinwith marineand fluvial sedimentup to 10 km thick earthquakesrecorded between January 1973 andMay 1989 and foldedand upliftedeast, north, and southwestflanks in the greaterLos Angeles basin showsevents scattered [Yerkeset al., 1965; Davis et al., 1989]. This study throughoutwith no simple spatialpatterns (Figure 2). analyzesearthquake data from theLos Angeles basin proper This is in contrastto the clusteringof earthquakesalong andthe adjacent offshore Santa Monica Bay andSan Pedro majorfaults observed in many otherparts of California Bayand the San Gabriel Valley. such as along the San Jacinto fault [Allen et al., 1965; The 1987 (ML=5.9) Whittier Narrows earthquake Sanders,1986]. Thusthe spatial distribution of seismicity occurred on a concealed thrust fault beneath the folded alonecannot be usedto map subsurfaceexpressions of activefaults in theLos Angelesbasin. sedimentsof the north flank of the basin [Davis et al., Single-eventfocal mechanismshave been determined for 1989;Hauksson and Jones, 1989]. Thisstudy attempts to 244 earthquakesof 2.5<_M<5.9in theLos Angelesbasin. determinewhere similar zones of foldingand thrust faulting Althoughthe spatialdistribution of epicentersdoes not existin theLos Angelesbasin. Identifyingsuch zones of delineatespecific faults, the focal mechanismsdo form systematicspatial patterns. Thesepatterns are usedto evaluatethe spatial extent of strike-slip,normal, and thrust 1Nowat Seismological Laboratory, Division ofGeological or reversefaulting. and PlanetarySciences,California Institute of Technology, The Los Angelesbasin forms a transitionzone between Pasadena. the strike-sliptectonics of thePeninsular Ranges and the Copyright1990 by the AmericanGeophysical Union. convergenttectonics of the TransverseRanges. Both strike-slip and compressionaltectonic structuresexist Papernumber 901B00807. withinthis transition zone. Traditionally, the compressive 0148-0227/90/901B-00807 $05.00 structuressuch as folds have been interpreted as secondary 15,365 1:5,366 HArmSSON:LOS ANGELES BASIN

119'00" 118'00"

Fig. 1. Map showinggeographical features, late Quaternaryfaults, and the Los Angelesbasin in southern California. The flanks of the basinas definedin this studyare shownas shadedareas. The eastflank is defined as as the regionin betweenthe 8 km sedimentdepth contour of the basin [Yerkeset al., 1965] and the Puente Hills to theeast. The northflank is definedas the areabetween the 8 km depthcontour and the SantaMonica- Hollywoodfaults to the north. The southwestflank is definedas the m:eabetween the 500 m bathymerric contouroffshore and the Newport-Inglewood fault. The 8 •nd 10 km depthcontours outline the deep central basin.

MAGNITUDES

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O 4.0+ 5.0+

, 40' ':• '"',•...... ?•. . . • ..- o..,.,••..,.. . ß ,o.. • ß •. ,oo•* . 0'••'..6-'• • '" ' ' '"'

20' •1 ...... ,I o , , J, , 1, J.... 119' 50' 40' 30' 20' 10' 118 ø 50' 40' 30' Fig.2. S,ismici•A $• •s Ag,l• bsin Eom]923 •o ]989r•o•dcd by $e C•SGS SouSeraC•fo•a Seis•c Network•d •c USC •s •geles B•in SeismicNetworkß •e 1979 •d 1989 •dicate •e location of •e two m•nshock-•tershocksequences in S•[a MofficaBay. • 1987•dicates •e 1987 Whittier N•ows s•uen•. HAUKSSON:LosANOELES BASIN 15,367 structurescaused by movementson the primarylate (Ms=7.7) Kern Countyearthquake. It was causedby Quaternarystrike-slip faults. This interpretation is called raptureon a 75-km-longconcealed thrust fault nearthe the wrenchtectonics model [e.g., Wilcox et al., 1973]. southernend of the San Joaquin Valley [Stein and The wrench faulting models assumelayers of soft Thatcher,198!]. The surfacerapture that was reported sedimentsthat deform in responseto motion on a strike- alongthe White Wolf fault poorlyfits the geodeticdata slipfault at depthbelow them. The Newport-Inglewood[Stein and Thatcher,1981] and may havebeen secondary faulton the southwest flank of thebasin is oftenpresented faulting. If thethrust faults beneath the flanks of thebasin asthe classic example of wrenchfaulting [Wilcox et al., are capableof generatingsimilar earthquakes,the 1973].Recently, Wright [1987] has pointed out the strong earthquakehazards in the Los Angelesbasin have been disagreementbetween the actual distribution and orientation underestimatedin the past becauseestimates of the of fold axesalong the Newport-Inglewoodand Whittier earthquakepotential of thesethrust faults have not been faultsand those predicted by the wrenchfaulting model. included[e.g., Ziony and Yerkes,1985]. Similarly,the folding along the north flank of the basin cannotbe associatedwith a knownstrike-slip fault. Thus T•CTON•½ S•z'rrtNa thefolding along the flanksof the basincannot easily be explainedas secon• effectsfrom strike-slip faulting. The Los Angeles basin has gone through several If boththe strike-slipand compressional tectonics in the episodesof tectonicdeformation since it beganforming in basinare of similar importance,their coexistencecould the middleMiocene (16-11 Ma) [Daviset al., 1989]. The suggestthat the faulting in the basinshould be mostly inceptionof the basinprobably included volcanism, block oblique.Alternatively, the mergingof thesetwo tectonic rotation, and extensionalblock faulting on northwestto stylescould take placeas decoupledstrike-slip and thrust northstriking faults [Campbell and Yerkes,1976; Wright, faulting. Suchdecoupling occurred on a smallerscale in 1987]. The Newport-Inglewoodfault zone may already the1987 Whittier Narrowssequence where the mainshock havebeen a zone of weaknessduring this time, becauseits ruptureda gently dipping thrust fault and the largest southwestflank was uplifted prior to middle Miocene aftershockruptured a steeply dipping strike-slip fault (Figure I) [Wright, 1987]. Toward the end of middle [Haukssonand Jones, 1989]. One of the goalsof this Miocene, Tarzana and Puente fan deposition initiated. studyis to explorethe possiblerole of alecouplingof fault Today sedimentfrom the Santa Monica Mountains have slipin the tectonicdeformation of the basin. replacedthe flux of sedimentfrom the Tarzana fan. Some Therelative importance of strike-slipand compressionalof the old Tamana fan sedimentoutcrop in the Elysian tectonicsis alsoevaluated by determiningthe present stress Hills northof downtownLos Angeles.Sediment from the field. The focal mechanismsare used in this study to Puente fan, however, continue to be depositedinto the determinethe orientationsof the threeprincipal stress axes basinthrough the Whittier Narrows [Wright, 1987]. and their relative magnitudes.Compressional structures In late Miocene and earliest Pliocene(8-4 Ma), the Los such as folds are finite strain markers, which reflect the Angelesbasin went throughits main phaseof subsidence stressfield at the time of formation. In this study the and depositionand acquiredits presentform [Yerkeset al., relationshipbetween the present stress field andorientation 1965]. During this period, the sedimentation was of fold axesare analyzed. If the stressfield had changed primarily turbidity currentstravelling down to the basin floor at 1-2 km depthbelow sea level [Yerkeset al., 1965]. sincethe formation of the folds, the orientation of fold axes Toward the end of this time, the basinfilled morerapidly could be expected to be in part inconsistentwith the presentstress field. Furthermore,within regionsof strike- thanit subsided.Concurrently, left-lateral faulting along slip faultingthe intermediateprincipal stress is vertical, the southernmargin of what is now the east-westtrending TransverseRanges dominated deformation in the northLos while within regions of thrust faulting the minimum Angelesbasin [Lamar, 1961]. principalstress is vertical. If strike-slipand thrust faulting The openingof the Gulf of California in late Pliocene areof similarimportance in thebasin, the intermediate and minimumprincipal stresses should be of very similar time (4-2 Ma) (Pasadenandeformation) had a profound effect on the neotectonicsof southernCalifornia [Atwater, magnitudeand hence could interchangein orientation 1989;DeMets et al., 1987]. In theLos Angelesbasin, the dependingon localperturbations in thestress field. type of deformationchanged from northwest-southeast In addition to the 1987 (ML=5.9) Whittier Narrows extensionto north-southcompression [Wright, 1987]. eanhqu•e, numerousmoderate-sized or largeeanhq•es Since the onsetof compressionaltectonics, the upper haveshown that folds locatedabove thrust faults can grow Pliocenesediments have been uplifted, folded, and in some duringthe earthquake. The 1980 (Ms=7.3) E1 Asnam, casesoverturned by thenorth-south compressive stress field Algeria,earthquake [Nabelek, 1985], the 1983 (Ms=6.5) [Yerkeset al., 1965]. Coalingaearthquake [Eaton, 1985;Namson and Davis, Todaythe Los Angeles basin includes the deep basin and 1988],and the 1985 (Ms=6.7, 6.9) Nahanni, Canada, east,north, and southwestflanks (Figure 1). The deep earthquakes [Wetmiller eta!., 1988] all occurred on basinforms a northwest-southeastelongated synclinorium concealedthrust faults and causedcoseismic growth of filled with marineand fluvial sediments,while the flanks surficialfolds. Althoughnumerous earthquakes that have are upliftedand folded. Using the retrodeformablecross- occ•ed on concealed thrust faults have been documented in section technique,Davis et al. [1989] interpreted detail, no simple methodsexist for estimatingthe compressionalstructures along two profilescrossing the maximumsize of an earthquakeon a buriedthrust fault. basin.One profile crosses the middle of thebasin, along a Oneof thelargest instrumentally recorded em'thquakes to north-southline, and anotherprofile provides a derailed occur on a thrust fault in California was the 1952 structuralinterpretation of the WhittierNarrows area. Both 1$,368 H•u•ssoN: Los ANG•S B^StN of these profiles, constructed using the fault-fold are usuallyfollowed by few if any aftershocksof M_>2.5. relationshipstechnique of Suppe[1985], modelthe near- In Figures 3, 4, and 5, dependingon the rake for the surfacefolding as caused by thrustfaulting at depth.Thus selectedplane, the focal mechanisms have been grouped Davis et al. [1989] infer thrustfaults at depth,although intostrike-slip, normal, and thrust faulting, respectively. they are not able to tell if thesefaults are seismogenicat Normalmechanisms have rakes ranging from -45ø to -135ø . present. Thrustmechanisms have rakesranging from 45ø to 135ø. In additionto the compressionalgeologic structures, the Five eventswith nodal planesdipping less than 20ø and basin is also crosscutby northwestto north-northwest rakes close to 0ø or 180ø are also classified as thrust strikingright-lateral strike-slip faults of lateQuaternary age faulting. Strike-slipevents have rakes ranging from 44 ø to includingthe Whittier fault, Newport-Inglewood fault, and 44 øand 136ø to 224ø . In mostcases the grouping is clear, the PalosVerdes fault. The northernmargin of thebasin is althoughin a few casesthe eventmay be strike-slipand boundedby the range front faults, such as the Santa normalor thrustdepending on which plane is selected.If Monica,Hollywood, and Sierra Madre faults [Crook et al., the first motion data could be fit with more than one 1987; Morton and Yerkes,1987]. Someof theseexposed solution,the other solutions are also shown and are flagged lateQuaternary faults may be seismogenic,while others are with a star. thoughtto be mostlyaseismic [Ziony and Yerkes,1985]. A stress inversion technique developed by Michael [1984] was used to invert the focal mechanismsfor the EARTHQUAKEDATA AND .AdWALYSIS stateof stress.The inversionminimizes the misfitangle (B) betweenthe directionof the shearstress on the fault Earthquakesoccurring within the area extendingfrom plane and the observedslip direction on that plane latitude33ø26'N to 34ø16'Nand from longitude117ø28•¾ determined from focal mechanism. The inversion assumes to 118ø48'Wwere analyzedin this study(Figure 2). The that the regionalstress field is a constanttensor, all slip earthquakedata from 1977 to 1989 wererecorded by both eventsare independent,and the magnitudeof the tangential the California Institute of Technology/U.S. Geological traction (ITI) applied to each fault plane is similar. The Survey(C!T/USGS) SouthernCalifornia Seismic Network third assumptionis equivalentto assumingthat ITl=l, and the University of Southern California (USC) Los becauseonly relative stressmagnitudes can be calculated. AngelesBasin Seismic Network. Digital seismogramsof The inversion solves for the orientation of the three all 244 earthquakesof M>2.5 discussedin this studywere principal stressaxes and the • value, a measureof the analyzed either specifically for this study or for other relativemagnitude of theprincipal stresses defined as studies [Hauksson, 1987; Hauksson and Saldivar, 1989; Haukssonand Jones,1989]. Eanlaquakesfrom two periods •=(S2 - S3)/(S1- S3) (May 1980 to March 1981 and Februaryto June 1983) are not includedbecause the CIT/USGS digital seismicdata whereS1, S2, andS3 are the maximum,intermediate, and havenot yet beenprocessed. minimumcompressive principal stresses. Four setsof velocitymodels and stationdelays were used One plane mustbe selectedfrom eachfocal mechanism to calculatehypocenters and takeoff anglesin this study. as the actual fault plane [Michael, 1987a]. In this study, Thesemodels were determined by invertingP andS arrival the planes listed in Table 1 are those selected for the time datawith the VELEST computerprogram [Roecker inversions. In general,north to northweststriking right- and Ellsworth, 1978]. In the eastLos Angelesbasin and lateralplanes in strike-slipfocal mechanismswere chosen. alongthe Newport-Inglewoodfault themodels and delays In a few instances,where the strike-slipmechanisms align derived by Hauksson and Jones [!989] and Hauksson alonga northeasttrend, the eastto northeaststriking planes [1987] were used,respectively. Events located offshore in were chosen. The northdipping planes in the reverseor SantaMonica Bay werelocated using models derived by thrust mechanisms were chosen because these are assumed Haukssonand Saldivar [!989]. As a part of this studya to be more consistentwith the geologicaldeformation than new setof modelsand delays were derived for theSan Pedro the south dipping planes [Davis et al., 1989]. In San Bay area. Final locations were calculated using PedroBay, however,the southdipping planes were chosen HYPOINVERSE [Klein, 1985] and the velocity models for the thrust eventsbecause the trend of hypocenters and stationdelays for the respectiveepicentral region. A appearsto dip to the south. The planesindicating faulting distanceweight of one in the distancerange 0-100 km and down into the Los Angeles basin were chosenfor the linearlydecreasing weight from one to zero in the distance normalfaulting mechanisms. range 100-120 km was applied. In mostcases both the Althoughthe geologicalinformation makes it possible epicenterand the focaldepth are constrainedto betterthan to selectsome planes correctly, other planes may be picked 1_+1kin. For events located offshore, however, the focal incorrectly. This is accounted for in the bootstrap depthsare lesscertain. technique used to calculate the confidence limits by The 244 single-event, lower hemisphere focal assumingthat 30% of the planesare picked incorrectly mechanisms were determined from P wave first motion [Michael, 1987b]. polaritiesusing a grid-searchingalgorithm and computer programsby Reasenbergand Oppenheimer[1985]. To EARTHQUAKES avoid possiblebias in the state of stressfrom major aftershocksequences, only the focal mechanismsof the Since1920, 14 moderate-sized (ML--4.9-6.4) mainshock- mainshockand largestaftershock for M>5.0 mainshocks aftershocksequences have occurredin the greaterLos are includedin thisstudy. No systematiceffort was made Angelesbasin (Figures6a and 6b and Table 2). These to exclude aftershocksof M<5.0 mainshocksbecause these earthquakesdo not occur along a single fault but are HAUKSSON:LosANGEl.ES B^SlN 15,369 associatedwith manylow slip-rate,late Quaternary faults value for the set of events for which focal mechanisms thatare distributedover a large area [Ziony and Yerkes, have beendetermined (Figure 7). The strike-slipand 1985;Ziony and Jones, 1989]. Researchon Los Angeles normalfaulting events show slightly higher b valuesof basinearthq•es wasinitiated by Taber [1920], whowas 1.06+0.17and 0.94+0.39, respectively, as compared to the commissionedby the SeismologicalSociety of Americato b valuefor thrustfaulting events of 0.70-k_0.15.The error studythe 1920 Inglewood(ML=4.9) earthquake.He barsare the approximate95% confidencelimits. The attributedthe earthquake to slipon theNewport-Inglewood averageb valuefor thewhole data set is 0.94_+0.12, similar faultjust west of thetown of Inglewood,because the event to theb valueof 1.02determined by Allenet al. [1965]for cause3concentrated heavy damage in thetown. theLos Angeles basin. This similarity shows that the dam Thethree largest historic earthquakes that have caused the setis a representativesample. The strike-slipand normal most damage in the Los Angeles area are the 1933 eventsthus seem to causethe high b valuein the basinas (ML=6.3)Long Beach, 1971 (ML=6.4) San Fernando, and comparedto b valuesin therange 0.8-0.9 for otherparts of 1987(ML=5.9) Whittier Narrows earthquakes. The 1933 southernCalifornia [Allen et al., 1965]. If Scholz• [1968] LongBeach earthquake occurred on the southern segment of correlationof low b valueswith high stressis correct,the the Newport-Inglewoodfault, from Newport Beach to low b valuefor thrustfaults could imply higherstresses on SignalHill, andshowed right-lateral strike-slip movement thrustfaults as comparedwith the stresseson strike-slip [Wood, 1933; Richter, 1958; Woodward-Clyde faults. Consultants,1979]. The 1971 San Fernandoearthquake showedreverse faulting with a componentof left-lateral Acriv• F^on•NO strikeslip and rapturedalong the San Fernandomember of the Sierra Madre fault zone [Whitcomb et al., 1973; The styleof activefaulting in the Los Angelesbasin is reflectedin the focal mechanismsof the local earthquakes. Heaton, 1984]. The 1987 Whittier Narrowsearthquake, whichoccurred on a previouslyunrecognized concealed The 244 single-event,lower hemisphere focal mechanisms thrustfault, showedpure thrustmotion on a gently north plottedat theirepicenters in Figure8 andlisted in Table 1 dippingplane [Haukssonand Jones, 1989]. Only three show that strike-slip,thrust, and normal faulting are all focal mechanismsin Figure 6b from a data set of nine occurringin the basin. The events(2.5 <_3/< 5.9) are distributed throughout the Los Angeles basin and the (ML>4.9) eventsshow mostly strike-slip movement, while adjacentSanta Monica Bay and SanPedro Bay. six eventsshow mostly reverse or thrust faulting. The The style of active faulting is also recordedas slip on focalmechanisms of the moderate-sizedearthquakes reflect mappablesurficial faults. The major mappablesurficial the local geologybut are too few to resolvethe detailsof faults in the Los Angelesbasin are the northweststriking theboundary between the thrustregime m the northand the Newport-Inglewood,Whittier, and PalosVerdes faults and strike-slipregime to the south. the west striking faults located within and along the The rate of small earthquakesin the Los Angelesbasin southernmargin of the centralTransverse Ranges, such as andthe adjacent offshore areas is similarto the averagerate the Santa Monica-Hollywood and Sierra Madre faults for southernCalifornia [Allen et al., 1965]. In Figure 2, (Figure 9a). The late Quaternaryfaults in Figure 9a are all the earthquakelocations in the CIT/USGS catalogfrom from Zionyand Jones[1989] and Za'onyand Yerkes[1985], January1973 to May 1989 are plotted. Since 1973 the who point out that more than 95 late Quaternary faults dense CiT/USGS and USC networks of seismograph have been mappedin the greaterLos Angelesarea. Many stationshave been in operationin and aroundthe Los of thesefaults are capableof generatingmoderate-sized and Angelesbasin, providingexcellent coverage of seismic large (M>6.7) earthquakes,although some may be too activity. The densestclusters of eventsin Figure 2 are small or not have a favorableorientation to the present mainshock-aftershocksequences such as 1979 and 1989 stressfield for generatinglarge earthquakes. The presence (ML=5.0) Malibu [Haukssonand Saldivar, 1986] and 1987 of nearbysmall earthquakesmay assistin evaluatingthe (ML=5.9) Whittier Narrows [Haukssonand Jones, 1989] type of faulting and whether a fault is active, while the sequences.The smallearthquakes do not showany simple absenceof smallearthquakes near a fault doesnot exclude correlationwith late Quaternaryfaults. As pointedout by thepossible future occurrence of smallor largeearthquakes Hauksson [1987], a broad zone of activity around the [Allen et al., 1965; Ziony and Yerkes,1985]. Newport-Inglewoodfault can be seenin Figure 2. The The surficialfolding of sedimentsabove concealed thrust absenceof epicentersto the north of the southernmost faults is yet anotherexpression of active faulting in the north dipping frontal faults (e.g., the Santa Monica- basin. Mappedfolds in thebasin, represented by theiraxes Hollywoodand Sierra Madre faults) suggests that the small in figures,form two majorbelts of folding. The first fold earthquakesare not associatedwith these faults. The belt is locatedalong the east and north flank of the Los concentrationof small earthquakesto the southof these Angelesbasin and the secondalong the southwestflank of faults, the zone west of the Whittier fault, and the zone the basin (Figure 9b). Most of the long fold axes in partlyoffshore to the westof theNewport-Inglewood fault Figure 9b are from Yerkes et al. [1965], who did a suggeststhat the deformation is concentratedalong the comprehensiveanalysis of theLos Angelesbasin geology. flanks of the basin. Additional fold axes for the offshore area are taken from To determine if the 244 earthquakes provide a Jungerand Wagner [ 1977],and Narch'n and Henyey [ 1978]. representativesample over the 2.5-5.9magnitude range and t!ardingand Tuminas [1988], and Wright [1987] provided if theirb valuedepends on the focalmechanisms of the numerousfold axesderived from oil exploration. The earthquakes,b valueshave been calculated. The maximum Santa Monica Mountains Anticlinorium (SMMA) is likelihoodmethod [Page, 1968] was used to calculatethe b shown with a dashedthick curve [Davis et al., 1989]. 15,370 HArmssos:Los ANGm.ES B^sIN HAUKS$ON:Los ANGELESBASIN 15,371 15,372 I-I•uxssos: Los ANO•LF.SB^stN HA•SSON:Los ANGELES B^SIN 15,373

7701241135 781030 740 821027 1021 870605 325 Strike-SlipFaulting Z.l• ¾- 2.?• Z-17.78 u. 2.50 Z- B.91 M- 2.• Z- 7,55 M,,,2.60 To examinestrike-slip faulting in the Los Angeles O• basin,late Quaternaryfaults and strike-slipfocal mechanismsare plottedtogether in Figure 10. The northeasttrending depth cross section AA' in Figure10 showsthat strike-slip faulting occurs from the near surface

770529 1558 781209 2301 821027 f021. 870707 2107 downto depthsof 14-17km. Z= 5,5•g ¾m2.D(• Z-ltj•2 I&l,•2.•0 Z• 8.94 M= 2.9• Z- &.ag M• 3,,20 Thecluster of right-lateralstrike-slip mechanisms along theNewport-Inglewood fault can be clearlyseen (Figure 10). The largestevent (ML=4.6) on the Newport- Inglewoodfault sincethe 1940swas strike slip and occurredon April 7, 1989,just 2-3 km to thesouth of the epicenterof the 1933(ML=6.3) LongBeach earthquake 770709 2022 791251 805 821127 1752 881217 2546 Z- •.5S U- 2.50 Zl 9.7• •,- 2.70 [Richter,1958]. The Newport-Inglewoodfault hasmore seismicactivity associatedwith it than other late Quaternaryfaults within the Los Angeles basin. Thespatial correlation between the surficial trace of the Palos Verdes fault and the distribution of strike-slip mechanismsis much less clear than along the Newport- 770805 2208 800414 1155 841016 1749 Z= ?.17 •,,• 2.50 Z- g.la U= 3.50 Z= ?.?O ¾= 3.10 Inglewoodfault. Over more than60 km on the Palos VerdesPeninsula and in San PedroBay, only four strike- slipmechanisms are located near the surface trace of the fault. In SantoMonica Bay, a smallcluster of strike-slip events coincides with the surface trace of the Palos Verdes fault. Severalstrike-slip events are located farther west in

770925 855 810729 2359 870208 2008 SantaMonica Bay, alongthe westernedge of the Santa Z= g.4n M,,, 2.713 2'- g. E4 Jd- 5.go Z,,,15.61 ld• 2..50 MonicaShelf Projection [Hauksson and Saldivar,1989]. Otheroffshore strike-slip events cannot be associatedwith specificmapped faults. In the easternLos Angelesbasin the strike-slipfocal mechanisms do not cluster around either the Whittier or the

771010 125• 810729 2-•59,, 870220 55 Chinofault (Figure10). A northwesttrending distribution Z= •.al •= 3.DO Z- g.•4 U-- 5.90 Z=l 1.50 of strike-slipmechanisms 5-10 km westof the Whittier fault is too scatteredto correlate with one specific fault suchas the Norwalk fault. An alignmentof left-lateral focal mechanismstrending northeastfrom Yorba Linda (Yorba Linda trend) and crossingthe Whittier fault, however,suggests a northeasttrending left-lateral strike- 760515 155a 810812 2258 870529 1515 Z= •B5 U- 3.•0 Z,,, 5.38 •,. 2.713 Z,- 4.5S •-, slip fault (Figures2 and 10). Lesslikely, thesecould be interpretedto be an en echelonpattern of north-northwest striking faults, which form an apparentnortheast trend. This trendof epicentersand focal mechanismsthat hasnot been pointedout beforein the literaturecrosses the fold belt along the easternflank of the basin. It may also be Fig.5. Normalfaulting focal mechanisms that occurred during associatedwith a clusterof strike-slipevents Iocat• farther 1977-1989 in the Los Angeles basin. See also caption of tO the northeast, 8-16 km east of the Chino fault. This Figure3. trendmay also segment or offsetconcealed thrust faults at depthbecause the focal depthsof someof thesestrike-slip eventsare greater than the depthsof the thrust events. Thesefolds are assumedto be causedby slip on concealed Similar northeasttrending alignments adjacent to the thnastfaults at depth[Suppe, 1985; Davis et al., 1989]. southern San Andreas fault have been identified by The length of the fold axes gives some indicationof Nicholsonet al. [1986]. possiblemaximum earthquake size, •use thelonger the Several recent Los Angeles basin earthquakeshave foldaxis the larger the concealed fault. shownmostly strike-slipmovement. The largeststrike- To comparethe earthquake focal mechanisms to the late slip eventto occurduring the time periodcovered by this Quaternaryfaults and folds, the focal mechanismshave studywas the 1987 (ML=5.3) WhittierNarrows aftershock. beengrouped by rakefor theselected plane into strike-slip, It occurredon a north-northweststriking tear fault that normal,and thrust faulting. More thanhalf or 144of these appearedto limit the western extent of rupture in the eventsshow strike-slip faulting, 78 showreverse or thrust mainshock[Hauksson and Jones, 1989]. This steeply faultingand 22 shownormal faulting. Below, strike-slip dippingstrike-slip fault may segmentthe mainshockthrust mechanismsare compared to mappab!esurficial strike-slip fault at depth. A similarnorth-northwest trending fault is faults,while thrustand normalmechanisms are compared defined by a clusterof strike-slipevents located 10 km tozones of foldingand thrust faulting. farther west near Montebello (Figure !0). The 1988 15,374 HAulssoN: Los A•o• B^s•

TABLE 1. Locationsand Focal Mechanisms of Earthquakesin the Los AngelesBasin

Origin Time, Latitude Longitude Depth, Mag. FocalMechanisms Numberof Day ur N W km M/. Ddir Dip Rake First Motions

Strike-SlipFaulting Earthquakes June 12, 1977 t415 34ø01.67 ' 117ø34.99 ' 6.8 2.7 125 ø 40 ø -40 ø 68 June 13, 1977 0319 33052.37 ' 118037.33 ' 11.9 2.8 245 ø 90 ø 175 ø 66 June 14, 1977 0156 34ø00.62 ' 118ø19.52 ' 8.7 2.8 255 ø 70 ø -164 ø 57 June 20, 1977 1833 34000.66 ' 118018.80 ' 9.4 2.5 59 ø 35 ø 143 ø 48 June 27, 197 7 2059 34005.93 ' 117ø57.85 ' 11.0 2.7 346 ø 73 ø 36 ø 47 Aug. 29, 197 7 1430 33048.80 ' 117051.44 ' 3.5 2.5 37 ø 71 ø 158 ø 29 Sept. 24, 1977 0534 34003.94 ' 117ø58.04 ' 15.4 2.9 325 ø 48 ø 41 ø 51 Sept. 26, 1977 0714 33032.29 ' 118013.58 ' 5.7 2.9 95 ø 35 ø -150 ø 57 Sept. 27, 1977 1810 33ø32.17 ' 118012.97 ' 4.4 3.1 60 ø 80 ø 180 ø 60 Oct. 6, 1977 0816 33059.96 ' 1!8ø11.69 ' 13.6 3.3 310 ø 40 ø 0 ø 57 Oct. 20, 1977 2014 33ø44.31 ' 118001.02 ' 3.8 2.7 255 ø 80 ø -160 ø 54 Nov. 8, 1977 1052 33053.54 ' 117o54.68 ' 3.9 3.4 100 ø 65 ø 20 ø 65 Dec. 7, 1977 1540 33ø47.32 ' 118o04.78 ' 4.6 2.7 170 ø 90 ø 0 ø 48 Dec. 20, 1977 1315 34o00.98 ' 118o12.00 ' 16.9 2.8 60 ø 80 ø 180 ø 51 Feb. 12, 1978 1522 33057.52 ' 117043.06 ' 4.8 2.7 15 ø 80 ø 154 ø 46 April 18, 1978 2242 33ø53.16 ' 117032.53 ' 8.9 3.1 122 ø 56 ø -37 ø 56 April 29, 197 8 0332 33050.42 ' 117ø43.58 ' 9.5 3.3 321 ø 38 ø 43 ø 79 May 9, 1978 1830 33ø48.11 ' 117ø58.41 ' 14.0 2.7 250 ø 60 ø -168 ø 58 May 11, 1978 0547 33059.48 ' 118ø21.36 ' 7.7 2.9 259 ø 60 ø 168 ø 65 May 1!, 1978 1757 34000.76 ' 118ø28.03 ' 8.4 2.8 236 ø 80 ø 159 ø 58 May 27, 1978 1438 34000.79 ' 117036.06 ' 4.5 2.5 245 ø 85 ø 150ø 59 June 14, 1978 0559 34ø00.88 ' 117ø36.07 ' 5.0 2.5 51 ø 80 ø 159 ø 55 Sept. 2, 1978 1815 33ø57.22 ' 117o43.06 ' 4.7 2.7 195 ø 80 ø 170 ø 68 Sept. 3, 1978 1810 33ø57.27 ' 117ø43.32 ' 5.6 3.5 306 ø 64 ø 33 ø 100 Sept. 22, 1978 0313 33o52.07 ' 117ø49.67 ' 1.9 2.7 290 ø 80 ø -10 ø 60 Oct. 6, 1978 1339 33o55.07 ' 117ø45.55 ' 14.4 2.5 315 ø 70* -30 ø 67 Nov. 19, 1978 0355 34ø00.78 ' 118%7.69' 11.8 2.8 64 ø 24 ø 141ø 76 Nov. 19, 1978 0420 33ø51.05 ' 118ø!0.17 ' 7.4 2.6 170 ø 80 ø -10 ø 60 Nov. 19, 1978 1740 33ø50.61 ' 118o10.23 ' 15.2 3.0 230 ø 75 ø -170 ø 71 Nov. 19, 1978 1800 33o50.97 ' 118o09.93 ' 8.5 2.8 170 ø 85 ø 0ø 66 Feb. 27• 1979 0707 33ø56.71 ' 118o19.50 ' 9.1 3.0 34" 72 ø 148ø 66 March 5, 1979 1100 33ø47.96 ' 118007.80 ' 8.6 2.6 95 ø 85 ø -180 ø 60 April 21, 1979 0600 33o47.64 ' 118005.02 ' 7.7 3.1 75 ø 85 ø -150 ø 64 May 16, 1979 0325 33ø51.00 ' 118ø3!.31 ' 10.5 2.7 60 ø 90 ø 160ø 77 May 20, 1979 0442 33ø51.17 ' 118031.46 ' 5.5 2.7 77 ø 60 ø -174 ø 75 July 31, 1979 1251 33050.52 ' 118ø07.81 ' 11.0 2.7 270 ø 70 ø 170ø 69 lan. 8, 1980 1910 34ø02.3! ' 117o33.26 ' 9.8 3.3 35 ø 75 ø -160 ø 92 Jan. 18, 1980 0909 33ø55.95 ' 117o43.25' 11.7 3.1 120ø 54 ø -3! ø 80 Jan. 20, 1980 2054 33ø45.97 ' 117ø31.86 ' 4.3 2.5 345 ø 48 ø 41 ø 65 Feb. 9, 1980 0917 33ø47.87 ' 118ø06.17 ' 7.1 2.7 265 ø 75 ø 170ø 63 April 21, !980 2255 34ø09.21 ' 117o47.28 ' 11.7 2.7 330 ø 75 ø -30ø 70 Feb. 27, ! 981 1511 34ø09.37 ' 118ø35.66 ' !4.3 3.5 255 ø 75 ø -180 ø 84 May 6, 1981 0556 33ø44.05' 1 !8ø01.60' 3.7 3.1 90ø 55 ø -140 ø 78 June 4, 198I 1151 33ø40.81 ' 117022.39 ' 11.7 3.6 215 ø 80ø 150ø 80 June 4, 1981 1426 33o40.74 ' 117ø22.47 ' 12.0 3.0 38 ø 52 ø 154ø 52 June 22, 1981 1803 33o44.92 ' 118ø06.55 ' 2.9 2.5 10ø 90 ø -10ø 72 July 5, 1981 0031 33ø40.86' 117o22.43' 4.1 2.8 35ø 90 ø 175ø 77 July 6, 1981 1953 33ø51.72' 117ø51.90' 4.4 3.1 19ø 81ø 149ø 81 Aug. !4, 1981 0109 33ø57.79' 118ø34.32' 8.3 3.4 225 ø 75 ø 140ø 94 Nov. 4, 1981 1154 33o54.46' 118o37.04' 11.9 2.7 250 ø 65ø -160ø 48 Dec. 5, 1981 2052 34o05.46' 117ø47.59 ' 13.9 2.5 245 ø 8! ø -144 ø 77 Jan. 3, 1982 0037 33o54.72 ' 117ø57.32 ' 5.3 2.8 347 ø 56 ø 37ø 87 Jan. 19, 1982 0535 33o55.97' 1!8ø28.75' 8.2 2.5 52ø 7! ø 158ø 98 Feb. 8, 1982 2353 34014.82' 118ø24.56' 2.7 2.6 280 ø 90ø 175ø 67 Apdl 8, 1982 1503 34ø11.82' 118%8.66' 3.1 2.5 235 ø 80 ø 170ø 65 May 25, 1982 1410 33033.04 ' 1!8ø!2.03 ' 12.1 2.8 39ø 81 ø !49 ø 78 May 25, 1982 2037 33ø33.22' 118o11.83' 5.4 2.6 50ø 85ø 180ø 76 May 26, 1982 1411 33ø38.40' 117ø29.02' 6.1 2.5 32ø 80ø -164ø 84 June 6, 1982 1720 33o28.89' 117ø43.91' 12.5 2.6 70ø 50 ø 180ø 64 luly 2, 1982 0100 33ø49.24' !17ø43.59' 11.7 2.7 205ø 55ø 170ø 87 July 28, 1982 !642 34ø14.31' 118%3.10' 15.6 2.6 28ø 38ø 137ø 63 Aug. 29, 198 2 2123 34ø03.48' !17ø35.96' 15.1 2.6 55ø 70ø -!64 ø 73 Sept. !2, !982 1555 33ø48.18' 118ø14.10' 7.0 2.5 255ø 80" -174ø 67 Oct, 14, 1982 1226 33ø53.18' 117ø41.09' 5.8 2.7 45½ 80ø -140ø 77 Oct. !5, !982 0957 34011.88' 118o38.73' 3.3 3.5 240ø 80ø 150ø 100 Nov. 4, 1982 1613 33o53.27' 117ø54.82' !2.9 2.6 316ø 41ø 15ø 75 Nov. 6, 1982 0737 33ø48.12' ! 18013.92' 8.2 2.8 90ø 75ø -170ø 77 Dec. 13, 1982 0205 33ø42.50' 118004.68' 8.4 2.6 85ø 90ø -170ø 51 Jan. 21, 1983 1450 34ø09.19' 117048.69' 12.1 2.5 323ø 38ø -43ø 65 March 1, 1983 2018 33o55.95' 118ø17,40' 10.8 3.6 250ø 800 160ø 64 April 18, ! 983 2139 34ø01.88' 118012.88' 13.! 2.8 1ø 38ø 43ø 66 July 13, 1983 2353 33041.37' !18ø01.68' 5.4 2.5 261ø 80ø -169ø 41 HAUKSSON:Los ANGm-•-qBASIN 15,375

TABLE 1. (continued)

Origin Time, Latitude Longitude Depth, Mug. Focal Mechanisms Number of Day N w km Mr Ddir Dip Rake First Motions

Strike.Sl•o Faulting Earthquakes (continued) July 14, 1983 1459 34006.35 ' 118ø16.16' 11.0 2.8 56 ø 61 ø 162 ø 62 Sept. 26, 1983 0408 33o54.83 ' 118o13.65 ' 11.9 2.7 85 ø 75 ø -180 ø 51 Oct. 24, 1983 2050 33ø55.47 ' 118ø28.63 ' 7.8 2.6 220 ø 80 ø !54 ø 64 Nov. 23, 1983 1201 34ø01.84 ' 118ø34.15 ' 8.1 2.5 71 ø 50 ø 167 ø 55 Jan. 29, 1984 2006 33042.66 ' 118ø03.51 ' 7.7 2.7 85 ø 90 ø !70 ø 47 Jan. 29, 1984 2019 33ø43.00 ' 118ø03.23 ' 7.8 2.7 271 ø 80 ø -169 ø 51 March 21, 1984 2352 33038.94 ' 118ø23.14 ' 0.1 2.8 75 ø 70 ø -170 ø 84 March 27, 1984 0546 34ø15.49 ' 118ø40.29 ' 8.9 2.6 70 ø 45 ø -140 ø 80 April 3, 1984 0626 33ø52.38 ' 118ø15.29 ' 9.8 2.6 60 ø 80 ø 180 ø 76 April 18, !984 13!9 33ø39.46 ' 118ø22.39 ' 4.5 3.1 90 ø 60 ø -140 ø 130 April 21, 1984 1158 33ø38.52 ' 117o57.66 ' 8.0 3.0 227 ø 56 ø 143 ø 82 May !!, 1984 0426 33ø59.77 ' 118ø10.58 ' 14.6 2.7 296 ø 41 ø 15 ø 81 June 25, 1984 2248 34ø09.36 ' 117ø46.59 ' 6.1 2.7 303 ø 31 ø -19 ø 98 luly 8, 1984 1141 33ø42.46 ' 118o03.93 ' 8.6 3.0 58 ø 80* 169 ø 83 July 8, 1984 1309 33042.05 ' 118003.88 ' 7.0 2.8 57 o 80 ø !64 ø 79 Oct. 3, 1984 1249 33ø58.97 ' 118ø39.17 ' 13.2 3.3 337 ø 56 ø 37 ø 123 Oct. 6, 1984 2001 33ø46.81 ' 118044.59 ' 11.2 2.6 239 ø 80 ø 174 ø 82 Oct. 15, 1984 1744 33ø42.00 ' 118ø09.92 ' 8.6 3.7 205 ø 30 ø 140 ø 119 Oct. 18, !984 0057 33ø55.5I ' 118ø01.14 ' 3.0 3.1 45 ø 90 ø !70 ø 99 Nov. 8, !984 0943 33ø53.56 ' ! 18 ø15.63 ' 8.7 3.2 54 ø 70 ø !64 ø 88 Nov. 27, 1984 2237 34ø01.61 ' 117032.47 ' 4.0 2.5 235 ø 90 ø 175 ø 90 Dec. 15, ! 984 0515 33053.06 ' 117049.72 ' 3.9 2.5 353 ø 50 ø 12 ø 80 Jan. 19, 1985 1608 33ø54.31 ' 118o28.32 ' 7.9 2.8 235 ø 70 ø 160 ø 122 Feb. 19, 1985 1726 33ø53.81 ' 118ø26.18 ' 7.4 2.8 45 ø 33 ø 151 ø 98 March 4, 1985 0339 33ø38.57 ' 118ø35.02 ' 10.0 2.6 50 ø 90 ø -150 ø 53 March 4, 1985 1151 33ø59.57 ' 118ø34.47 ' 9.8 3.2 59 ø 48 ø 138 ø 107 March 15, 1985 0503 33o45.66 ' 118ø30.25 ' 4.9 2.6 46 ø 31 o - 160 ø 83 March !8, 1985 0743 33ø59.36 ' 118ø34.56 ' 8.9 2.7 65 ø 60 ø -!60 ø 135 May 4, 1985 0817 33ø50.89 ' 118014.50 ' 9.8 2.6 61 ø 50 ø 167 ø 80 June 10, 1985 1250 33ø41.85' 117ø23.58 ' 6.7 3.1 24 ø 70 ø 164 ø 1 11 Sept. 3, 1985 0258 34o02.96 ' 118ø22.31 ' 7.2 2.8 235 ø 50 ø !60 ø 73 Oct. 9, 1985 0629 34013.62 ' 118023.35 ' 9.8 2.7 230 ø 65 ø 150 ø !06 Feb. 11, 1986 2230 33ø39.34 ' 118000.32 ' 12.8 2.5 64o 40 ø 172 ø 73 Feb. 25, 1986 0852 33ø28.90 ' 118ø12.63 ' 5.9 2.5 61 ø 61 ø 162 ø 67 March 3, 1986 1318 33ø45.12 ' 117031.74 ' 5.5 3.3 336 ø 38 ø 43 ø 135 March 11, 1986 0512 34006.78 ' 117046.04 ' 4.6 2.6 257 ø 60 ø -174 ø 72 May 13, 1986 0116 33o50.67 ' 118%4.35' 8.1 2.5 80 ø 90 ø 180 ø 96 May 13, 1986 1635 33047.20 ' 118o17.75 ' 9.1 2.8 48 ø 38 ø 137 ø 1.20 June 26, 1986 0539 33ø51.83 ' 118026.79 ' 11.5 3.4 50 ø 85 ø 160 ø 1.36 July 7, 1986 0913 34o08.87 ' 1 !7ø44.59 ' 4.7 3.0 320 ø 90 ø -5 ø 127 luly 17, 1986 2136 33ø51.38 ' !17ø49.96 ' 5.2 2.5 300 ø 90 ø 0 ø 87 July 31, 1986 1916 34o00.04 ' 118ø22.91 ' 4.5 2.8 41 ø 74 ø 138 ø 95 Aug. 21, 1986 1143 33ø27.85 ' 117ø49.12 ' 7.0 2.8 63 ø 71 ø -153 ø 69 Oct. 14, 1986 0149 33ø53.15 ' 118000.39 ' 2.4 2.7 45 ø 70 ø -150 ø 86 Oct. !4, 1986 0759 33ø53.!8 ' 118o00.40 ' 2.8 2.6 35 ø 80 ø -160 ø 89 Oct. 15, 1986 0546 34009.33 ' 118ø16.32 ' 3.9 2.5 234 ø 80 ø 174ø 79 Jan. 13, 1987 0803 34002.29 ' 117ø38.39 ' 8.9 2.5 105 ø 35 ø -30 ø 67 Feb. 6, 1987 0532 34009.23 ' 117049.35 ' 14.5 2.6 357 ø 71 ø -21 ø 96 March 28, 1987 1928 33058.29 ' 118ø19.61 ' 8.4 3.1 38 ø 73 ø 143 ø 42 May 10, 1987 2008 34ø00.06 ' 117ø35.53 ' 4.6 2.8 240 ø 90 ø 175ø 99 May 25, 1987 1816 33ø51.74 ' 117053.29 ' 4.7 3.1 200 ø 80 ø 170 ø 94 July 2, 1987 1854 33ø55.07 ' 118ø38.39 ' 12.8 2.8 30 ø 70 ø 180ø 95 July 22, 1987 0559 33ø54.I0 ' 117ø48.32 ' 3.5 2.6 300 ø 90 o 0o 99 Aug. 28, 1987 1922 33038.66 ' 118ø33.62 ' 16.2 3.1 70 ø 50 ø - ! 80 ø 101 Oct. 4, 1987 1059 34ø03.60 ' ! 18ø06.21 ' 13.2 5.3 240 ø 70 ø 140 ø 164 Oct. 5, 1987 1355 33o53.99 ' 118ø42.71 ' 14.7 2.5 75 ø 60 ø 180 ø 81 Oct. 30, 1987 0445 33ø26.59 ' 118ø08.97 ' 5.5 2.5 4 ø 72 ø -31 ø 75 Nov. 24, !987 1252 33ø51.43' 118ø02.61' 8.0 2.5 243 ø 74 ø -138 ø 83 Dec. 1, 1987 0703 33ø37.80 ' 1!7ø55.28 ' 11.9 3.1 48 ø 73 ø 143 ø 57 Feb. 5, 1988 !33! 33ø44.83 ' 118ø02.00 ' 9.5 2.6 255 ø 80 ø -174 ø 90 Feb. 7, 1988 0532 34014.74 ' 118o29.77 ' 11.7 2.5 50 ø 54 ø 148ø 89 Feb. 19, 1988 1242 33ø52.43 ' 1 !8ø11.71' 17.4 2.7 105 ø 40 ø -160 ø 98 May 4, 1988 !116 33ø55.84 ' 118017.48 ' 17.8 2.5 331 ø 41 ø 15ø 105 May 18, I988 1722 33ø36.64 ' 118ø26.91' 8.1 2.6 75 ø 75 ø -160 ø 72 June 20, 198 8 1020 33ø41.45 ' !18ø13.47 ' 3.8 2.8 65 ø 50 ø !73 ø 1 11 June 26, 198 8 !504 34007.88 ' 117ø42.77 ' 6.0 4.6 292 ø 42 ø -22 ø 162 Aug. 31, 1988 1623 33ø26,96 ' 118ø01.36 ' 5.4 3.2 70 ø 70 ø -150 ø 95 Sept. 2, 1988 1726 33ø30.'63 ' 118ø04.29 ' !1.0 3.0 19ø 50 ø 6o 89 Sept. 12, 1988 !454 33ø51,31 ' 118ø26.91 ' !!.0 2.5 205 ø 30 ø -180 ø 61 Sept. 12, 1988 1714 33ø5!.59 ' 1 !8ø26,76 ' 12.0 2.7 238 ø 61 ø -162 ø 75 Dec. 3, ! 988 1138 34ø08.57 ' 118ø08.02 ' 15.5 5.0 335 ø 70 ø 0 ø 148 April 7, 1989 2007 33ø37.90 ' 117ø55.87 ' 11.6 4.6 40 ø 90 ø 160ø 126 15,376 I•UKSSON: Los ANGgLESBASIN

TABLE 1. (continued)

Origin Time, Latitude Longitude Depth, Mag. Focal Mechanisms Number of Day W km Mi, Ddir Dip Rake First Motions

ThrustFaulting Earthquakes Sept. 14, ! 977 2135 33ø52.82 ' 117ø49.51 ' 2.0 2.9 357 ø 61 ø 78 ø 49 Oct. 16, 1977 1702 33o45.14 ' 117ø42.51' 7.6 2.6 200 ø 5 ø 170 ø 35 Oct. 24, 1977 1936 33o32.98 ' 118ø14.53 ' 5.6 2.8 255 ø 65 ø 110 ø 36 Dec. 27, 1977 0607 33ø54.31 ' 118ø32.80 ' 8.9 2.7 14 ø 51' 47 ø 75 March 8, ! 978 1449 33ø49.34 ' 117ø53.43 ' 5.7 3.0 52 ø 39 ø 63 ø 82 March 14, 1978 2359 34ø00.07 ' 118ø40.44 ' 13.6 3.1 3 ø 35' 59 ø 28 April 26, 1978 0608 33ø56.29 ' 118o20.49 ' 7.5 2.8 320 ø 45 ø 90 ø 59 May 1, 1978 1824 33ø56.84 ' 118o44.20 ' 12.9 2.5 320 ø 41 ø 49 o 37 June 4, 1978 0357 33ø55.21 ' 117ø50.32 ' 12.4 3.7 327 ø 54 ø 52 ø 81 Dec. 13, 1978 0952 33ø35.44 ' 118 ø02.04' 7.0 2.6 235 ø 60 ø 60 ø 72 Jan. 1, 1979 2314 33ø57.05 ' 118o40.35 ' 12.1 5.0 20 ø 52 ø 106 ø 80 April 2, 1979 2128 33ø41.92 ' 118ø11.78 ' 5.4 2.6 225 ø 80 ø 130 ø 67 April 19, 1979 1422 34ø00.72 ' 118ø10.37 ' 13.6 '2.5 7 ø 50 ø 98 o 53 June 1i, 1979 1810 33ø50.38 ' 118ø32.35 ' 11.0 2.6 100 ø 90 ø 50 ø 67 June 20, 1979 0530 33ø59.62 ' 118ø21.23 ' 6.9 3.0 342 ø 26 ø 110 ø 69 Aug. 13, 1979 0816 33ø54.83 ' 118ø29.82 ' 8.0 2.5 24 ø 31 ø 106 ø 52 OCt. 17, I979 2052 33ø55.84 ' 118ø39.25 ' 12.8 4.2 29 ø 41 o 78 ø 94 Dec. 18, 1979 0318 33o56.68 ' 118039.39 ' 11.8 2.8 20 ø 31 ø 73 ø 89 April 1, 1980 0402 34ø00.52 ' 118ø39.74 ' 14.0 2.8 34 ø 41 o 130 ø 31 April 12, 1980 233! 34ø03.12 ' 118'43.03' 11.0 2.9 37 ø 39 ø 116 ø 16 May 25, 1982 I344 33o33.44 ' 118ø12.21 ' 10.9 4.4 230 ø 50 ø 100 ø 121 May 25, 1982 1435 33ø32.4I ' 118o11.83 ' 5.5 2.9 125 ø 90 ø 90 ø 81 May 26, 1982 0405 33033.40 ' 1 !8ø12.22 ' 11.2 3.0 205 ø 55 ø 90 ø 81 Aug. 27, 198 2 0425 33055.80 ' 117ø48.84 ' 12.3 3.1 20 ø 35 ø 90 ø 92 Sept. 7, 1982 2258 34ø15.11 ' 117ø56.86 ' 12.2 2.5 140 ø 10 ø -180 ø 56 Sept. 11, !982 1355 33ø48.33 ' 118ø14.54 ' 4.6 2.6 200 ø 35 ø !00 ø 71 Sept. 17, 1982 1057 33055.59 ' 118018.59 ' 15.1 3.3 45 ø 40 ø 90 ø 72 Oct. 11, 1983 1448 33ø53.01 ' 117ø50.02 ' 5.7 2.6 333 ø 27 ø 46 ø 71 Feb. 27, 1984 1018 33028.65 ' 118ø03.84 ' 9.3 4.0 230 ø 45 ø 90 ø 119 March 24, 1984 1440 34o07.44 ' I18ø32.73 ' 14.9 2.6 300 ø 57 ø 57 ø 75 May 8, 1984 2216 33033.70 ' 118ø14.73 ' 11.2 3.2 225 ø 15 ø 60 ø 99 Aug. 26, 1984 1512 33ø47.23 ' 118ø30.48 ' 8.2 2.7 20 ø 41 ø 49 o 82 Sept. 12, 1984 0413 33ø52.32 ' 117ø56.57 ' 10.9 2.7 19 ø 43 ø 112 ø 90 Oct. 22, 1984 2212 33ø41.91 ' 118o09.44 ' 8.0 2.7 135 ø 20 ø 130 ø 78 Dec. 14, 1984 2128 33ø52.91 ' 117ø49.80 ' 6.8 3.1 357 ø 48 ø 108 ø 105 Feb. 14, 1985 2322 33ø41.94 ' 118ø09.75 ' 9.8 3.3 235 ø 85 ø 130 ø 120 March 9, 1985 0955 33ø48.38 ' 118%1.51' 9.8 2.6 347 ø 39 ø 63 ø 90 Sept. 26, 1985 0125 33ø56.65 ' 118%5.44' !0.3 2.5 42 ø 48 ø 108 ø 108 Dec. 19, 1985 1303 34o02.18 ' 118ø28.55 ' 14.0 2.8 56 ø 45 ø 80 ø 120 March 9, 1986 2241 34ø06.84 ' 117ø46.23 ' 5.6 3.5 343 ø 27 ø 46 ø 123 March 19, 1986 0247 33'44.14' 118023.49 ' 4.7 2.7 324 ø 51 ø 47 ø 110 March !9, 1986 1938 34ø05.!9 ' 118ø25.14 ' 8.8 2.5 315 ø 10 ø 00 78 March 20, 1986 0649 33ø46.94 ' 118ø18.42 ' 12.4 3.3 11 ø 45 ø 80 ø 137 March 24, ! 986 0514 33ø47.21 ' 118ø18.11 ' 10.0 2.8 52 ø 36 ø 103 ø 102 April 5, 1986 0650 33ø44.05 ' 118ø00.74 ' 4.0 3.9 235 o 50 ø 110 ø 149 April 5, 1986 1238 33ø59.48 ' 118ø43.28 ' 12.4 2.7 354 ø 51 ø 47 ø !21 May 13, 1986 1155 33ø47.37 ' 118ø17.84 ' 9.0 2.8 22 ø 39 ø 116 ø 74 May 19, 1986 0412 33'53.48' 118 ø23.44' 11.9 3.1 2 ø 36 ø 76 ø 121 May 20, 1986 071I 33'56.39' 118%9.54' 12.4 2.8 30 ø 45 ø 90 ø 110 July 11, 1986 1625 33ø59.55 ' 118o41.95 ' I1.6 2.6 14 ø 43 ø 112 ø 72 July 14, 1986 0024 33%8.90' 117ø32.16 ' 4.0 2.7 350 ø 40 ø 90 ø 108 July 23, 1986 1936 33ø51.16 ' 118ø29.42 ' 83 2.8 22 ø 39 ø 1!6 ø 80 July 30, 1986 0114 34ø00.02 ' 118ø22.91 ' 4.5 2.8 39 ø 67 ø 135 ø 95 Aug. 24, 1986 1633 34ø06.77 ' 117ø45.76 ' 5.1 2.6 9 o 52 ø 116 ø 98 Sept. 5, !986 0604 33ø59.27 ' 118ø32.92 ' 4.6 2.5 4 ø 57 ø 122 ø 98 Oct. 6, 1986 1156 33ø35.95 ' 117051.89 ' 13.4 2.5 180 ø 60 ø 60 ø 83 Oct. 14, 1986 0240 33o51.40 ' 118'33.66' 8.0 2.5 5 ø 20 o 90 o 80 May 5, 1987 1429 33ø46.85 ' 117ø33.48 ' 5.5 2.7 56 ø 27 ø 133 ø 83 June 8, 1987 1229 33o46.46 ' 118ø10.94 ' 12.8 3.2 205 ø 35 ø 90 ø 107 July 8, 1987 !655 33ø41.37 ' 118ø13.91 ' 7.9 3.6 175 ø 50 ø 60 ø 118 July 8, 1987 I857 33ø41.!2 ' 118ø14.01 ' 4.7 2.8 190 ø 50 ø 70 ø 85 luly 9, ! 987 0042 33ø41.34 ' 1!8ø14.13 ' 8.8 3.3 175 ø 50 ø 100 ø 120 Aug. 19, 1987 0940 33ø35.59 ' 118ø00.10 ' 5.8 3.0 245 ø 50 ø 100 ø 100 Oct. 1, 1987 1442 34ø02.96 ' 118ø04.86 ' 14.6 5.9 0 o 25ø 90 o 159 Oct. 5, 1987 1437 33ø53.86 ' 118o42.67 ' 14.8 2.7 57 ø 36 ø 103 ø 104 Oct. 17, 1987 0925 33ø59.32 ' 118041.24 ' 9.0 2.7 355' 43 ø 67 ø 104 March 26, 1988 1454 33ø59.38 ' 118ø42.05 ' 13.1 3.7 27 ø 55 ø 83 ø 127 April 12, 1988 1210 33ø59.93 ' 118ø10.61 ' 14.4 2.7 5 ø 40 ø 90 o 100 April 13, 1988 0209 33ø43.!7 ' 118ø44.56 ' 9.4 2.8 335 ø 40 ø 90 ø 94 April 26, 1988 1726 33ø33.94 ' 118016.99 ' 10.0 2.5 210 o 20 o 0 o 61 HamssoN: Los ANo__m_-•sB^siN I5,377

TABLE 1. (continued)

Origin Time, Latitude Longitude Depth, Mag. FocalMechanisms Numberof Day LIT N W km 'ML Ddir Dip Rake First Motions

ThrustFaulting Earthquakes (continued) 3une 9, 1988 1723 34ø01.17' 118ø13.66' 4.5 2.6 15ø 51'' 132ø 109 Sept. 2, !988 1748 33ø30.19' 118"04.46' 11.0 2.5 225ø 70`' 100ø 32 Sept. 9, 1988 0452 33`'58.46' 118''45.73' 12.5 2.5 358`' 20 ø 14ø 55 Sept. 12, 1988 1324 33051.42 ' 118ø26.12' 12.3 3.9 22`' 56 ø 112ø 118 Nov. 20, 1988 0539 33`'30.44' 118ø04.46' 11.7 4.5 220ø 45 ø 90ø 121 Jan. 19, 1989 0653 33`'55.04' 118037.37' 13.8 5.0 19ø 45 ø 99ø 139 June 12, 1989 1657 33ø59.77 ' 118ø10.71' 14.9 4.4 340 ø 41 ø 78ø 70 June 12, 1989 1722 33ø59.82 ' 118010.33 ' 15.1 4.1 346 ø 31 ø 73ø 108

NormalFaulting Earthq• Jan. 24, 1977 1135 33ø53.03 ' 118ø11.64 ' 10.4 2.7 110ø 50 ø -80 ø 60 June 29, 1977 1358 33`'35.08' 117ø38.04 ' 5.5 2.8 235 ø 41 ø -130 ø 48 July 9, 1977 2022 33028.07 ' 118"01.86' 6.5 2.5 110ø 40" -110 ø 46 Aug. 3, 1977 2208 33ø49.86 ' 118ø08.56' 7.1 2.5 135'' 40 ø -70ø 54 Sept. 26, 1977 0835 33ø32.52 ' 118''13.93' 9.4 2.7 180ø 70 ø -60ø 53 Oct. 10, 1977 1238 33ø36.45 ' 118ø04.30 ' 8.8 3.0 31ø 45 ø -99 ø 78 March 13, 1978 1638 33ø55.40 ' 117059.23 ' 3.8 3.3 287 ø 39 ø -63 ø 58 Oct. 30, 1978 0740 33o38.09 ' 118ø24.14 ' 17.7 2.5 285 ø 5 ø -60 ø 56 Dex:. 9, 1978 2301 33ø29.14 ' 118ø16.88 ' 11.8 2.6 150ø 50 ø -110 ø 58 Dec. 31, 1979 0603 33ø39.32 ' 117054.90 ' 8.7 2.7 90 ø 50 ø -130 ø 63 April 14, 1980 1153 33ø50.19 ' 118ø25.83 ' 9.1 2.5 277 ø 48 ø -71ø 44 July 29, 1981 2339 33o48.00 ' 118o42.75 ' 9.6 3.9 235 ø 5 ø -70 ø 108 Aug. 12, 1981 2258 34ø07.54 ' 118ø36.49 ' 3.3 2.7 129ø 67 ø -69 ø 67 Oct. 27, 1982 1021 33ø52.75 ' 118011.92 ' 8.8 2.9 80ø 55 ø -120 ø 75 Nov. 27, 1982 1752 33ø33.21 ' 118ø12.39' 5.8 3.1 230 ø 0ø -89ø 87 Oct. 16, 1984 1749 33o41.79 ' 118010.04 ' 7.7 3.1 260 ø 5ø -50 ø 96 Feb. 8, 1987 2008 33ø55.05 ' 118ø16.80 ' 13.6 2.5 95ø 40 ø -130 ø 75 Feb. 20, 1987 0033 33o45.84 ' 118042.82 ' 11.5 2.5 235 ø 45 ø -130 ø 41 May 29, 1987 1516 33o41.53 ' 118ø09.48' 4.5 3.1 33ø 66ø -99ø 103 June 5, 1987 0325 33ø27.97 ' 118ø15.61 ' 7.5 2.6 85ø 45 ø -130 ø 78 July 7, 1987 2107 33ø49.77 ' 118ø09.91 ' 6.8 3.2 90ø 50 ø -110 ø 50 Dec. 17, 1988 2346 33ø41.45 ' 118ø07.00 ' 4.2 3.2 8ø 78 ø -56ø 130

TABLE2. SignificantEarthquakes in the Greater Los Angeles Basin and Adjacent Offshore Regions

Date Earthquake ML Reference June21, 1920 Inglewood 4.9 Taber [19201 Aug. 30, 1930 SantaMonica Bay 5.2 Gutenbergeta/. [ 1932] March 10, 1933 LongBeach 6.3 Wood [1933] May 31, 1938 Elsinore Fault 5.5 Hileman et al. [1973] Oct. 22, 1941 Gardena 4.9 Richter [1958] Nov. 14, 1941 Torrance-Gardena 5.4 Richter [i958] Sept. 12, 1970 Lytle Creek 5.4 Jones [1984] Feb. 9, 1971 San Fernando 6.4 Whitcombet al. [1973] Feb. 21, 1973 PointMugu 5.9 Stiermanand Ellsworth [1972] Jan. 1, 1979 Malibu 5.0 Haukssonand Saldivar [1986] Sept. 4, 1981 SantaBarbara Island 5.3 Corbett [1984] Oct. 1, 1987 Whittier Narrows 5.9 Haukssonet al. [1988] Dec. 3, 1988 Pasadena 4.9 Joneset al. [1990] Jan. 19, 1989 Malibu 5.0 E. Hauksson[manuscript in preparation,1990]

(ML=4.9) Pasadenaearthquake showed left-lateral Themappable surficial late Quaternary faults and the 144 movementon the Raymond fault [Joneset al., 1990]. strike-slipfaulting events of M>_2.5show that strike-slip Fartherto the northwest,three fight-lateral strike-slip faultingis a significantmode of deformationin the Los eventshave occurred along the northweststriking surface Angelesbasin. The strike-slipfaults appear to trace of the Verdugo fault. Similar to the Pasadena accommodatelateral movements of crustal blocks. earthquake,the 1988(ML=4.6) Upland sequence showed mostlyleft-lateral strike-slip faulting, on the northeast Thrift Faulting strikingSan Jose fault (Fig• 10). The SanJose fault As can be seen in Figure 11, the thrust focal thusmay provide for a leftstep in thefrontal fault system. mechanismsarenot located adjacent to thenorth dipping 15,378 HAWssoN: Los ANGELF..SBASIN

(A)

119'00' 118'00' (B)

SAN FERNANDO 1971(ML =6 4) PASADENA 1989(ML= 4. 9) LYTLE CREEK I POINTMUGU 970(ML = 5.4) 1973 (ML=5 9) WHITTLER NARROWS 34* 00' Angeles [987(ML=5.9)

MALIBU 1979(ML=5,0)

MALIBU 1989(ML= 5.0')

Sta. BARBARA is \ LONG BEACH 1981{ML=5.3) %%\\ \ 1933(ML=6.3) \ \\ 0 I0 20 •0km •

.... •o-• ---•-• X k', •3-9 ..... <1 A

Fig. 6. (a) Significantearthquakes of M_>4.9that have occurred in the greaterLos Angelesbasin area since !920. Aftershockzones are shaded with crosshatching. Dotted areas indicate surface rupture, including the ruptureof the 1857 earthquakealong the San Andreasfault. (b) Single-eventlower hemispherefocal mechanismsof the M_>4.9events in the greaterLos Angelesbasin. Focalmechanisms are from Woodward- Clyde Consultants[1979], Jones [1984], Whircombet al. [!973], Stierrnanand Ellsworth [1976], Corbett [1984],Hauksson and Saldivar [1986], Hauksson and Jones [1989], and Jones et al. [1990]. surficialreverse faults, suchas the Santa Monica and Sierra sequence.This wasdemonstrated by the coseismicuplift Madrefaults that define the north edge of thesedimentary of sedimentin the 1987Whittier Narrows earthquake [Lin basin. Insteadthe thrustor reversefocal mechanisms are, and Stein,1989; Davis et al., 1989]. Thus,combining the in general,south of thesereverse faults, forming broad geologicalevidence for folding and the seismological clustersalong the flanksof the basin. This coincideswith evidencefor thrustfaulting, two fold andthrust belts can be mostof the foldingof the basinsediments (Figure 9b). identifiedalong the flanksof thebasin (Figure I 1). These Thisspatial coincidence of the folding and thrust faulting beltsare called here the ElysianPark and the Torrance- can be explainedwith fault-bendor fault-propagationWilmington fold andthrust belts and are 1ocatexlalong the folding models [e.g., Suppe, 1985], where the thrust east and north and southwest flanks of the basin, faultingearthquakes at depthcause folding of the cover respectively. HAUKSSON:Los ANGELESBASIN 15,379

(a) (c)

S'!•!KE- •P MECHANISMS ALL laECHANISMS LOGN=4.67 -- (1.O6 +- 0.17)-MB LOGN--4 62 - (0.94 +- 0.12)-M8 NUMBER OF QUAKE'• : q 4.4 NUMBER OF QUAKES : 242 MAGNIl'UOE RANGE : 2.5 - 5.4 I,iAGNII'UOE RANGE

g

g

3.00 4 O0 5.00 •.00 2.00 3.00 4,00 5.00 !.OO MAGNITUDE MAGNITUDE

(b) (d)

THRUSTtaœCHANISUS NORMAL MECHANISMS LOGN=3.53 - (0.70 +- 0.15),UB LOGN =5.64 - (0.94 +- 0.39}-UB NU.MBERNUMBER OF' QU•QUAKES : 78 NUMBER OF QUAKES : 22 MJ•NITUDE RANGE : 2.5 - 5.g MAGNITUDE RANGE : 2.5 --

MAGNITU DE MAGNITUDE

Fig. 7. (a)The b value from the Gutenberg-Richterrelationship for the 244 eventsof M>2.5 from 1977 to 1989. The numberof eventsincluded in the b value determinationand magnituderange are also shown. The three lines are the best fit, the best fit minus one standarddeviation in the b value, and the best fit plus one standarddeviation in the b value. (b) The b value for only the thrustfaulting events. (c) The b value for only the strike-slipfaulting events. (d) The b value for only the normalfaulting events.

Within the easternpart of the Elysian Park fold and boththe shallowthrust events and the surfaceexposure of thrustbelt the thrustfaulting focal mechanismsform a thePeralta Hills thrustfault near Yorba Linda suggest that zonedipping 25ø-35 ø north(Figure !2). It is not po•sib!e in rare cases the concealed thrusts can cause surface to tell, basedon hypocentraldepth distribution and focal ruptures. Recently Wright [1990] has suggested mechanismsalone, if this zone consistsof one continuous independentlythat the Peralta Hills thrust may be faultor manysmall abutting or overlappingfaults. This associatedwith theElysian Park thrustzone. zoneof thrustfaulting is closestto the surfaceat 4-6 km The largest earthquake in this data set is the 1987 depth,near Yorba Linda. The southernmostmapped thrust (ML=5.9)Whittier Narrows earthquake that ruptured a 5- faultalong the eastern flank of theLos Angeles basin, the km-longsegment of the ElysianPark fold and thrustbelt. PeraltaHills thrustfault is located in this area, 5 km south The ElysianPark fold andthrust belt extendsto thewest of of YorbaLinda [Bryant and Fife, 1982]. Thepresence of theWhittier NmTOWS, beneath downtown Los Angeles, and 15,380 HAUKSSON:Los ANGELESBASIN

1977 - 1989 Focal Mechanisms M•,2.5

MAGNITUDI•S

0 2.5+

$.0+

3.5+

•..o+

4.5+

5.0+

5.5+

Fig. 8. The 244 single-eventlower hemispherefocal mechanismsdetermined for earthquakesfrom 1977 to 1989 of M>2.5. The solid nodal plane in eachmechanism is the one chosenfor stressinversions, while the dashednodal plane is the auxiliaryplane. continuesinto SantaMonica Bay, where it is found at a In SantaMonica Bay the two thrustbelts merge into a depth of 10-15 km (Figure 11). It may be offset or northdipping imbricate thrust zone. The ElysianPark segmentedby the northernend of theNewport-Inglewood thrustzone is on the north side, beneaththe Malibu shelf. fault,because the maximum focal depths to theeast of the On the southside, the northdipping thrust zone of the intersectionof thetwo faultsare 3-4 km deeperthan to the Torrance-Wilmingtonbelt changesstrike from northwestto west(Figure 12). westas it mergeswith the ElysianPark zone. The Shelf The ElysianPark fold and thrustbelt is crosscut by Projectionanticline in SantaMonica Bay is probablya severalstrike-slip faults. The mostprominent of these surfaceexpression of this imbricatethrust zone [Hauksson strike-slipfaults is theWhittier fault that has had up to 30 and Salch'var,1989]. km of right-lateralslip since9-4 Ma [Lamar, 1990]. The The Palos Verdes fault coincides with the Torrance- verticaloffset on it is more difficult to quantifybut is Wilmingtonfold and thrustbelt alongmost of its length muchless and averagesabout 1.0-1.5 km [Wright, 1990]. (Figure9) and hasbeen either interpreted to be a right- The Elysian Park thrustzone has moved5.5-21 km since lateralstrike-slip fault or an obliquesteeply dipping reverse 4.0-2.2 Ma [Davis et al., 1989]. Becauseit is difficult to fault [e.g., Ziony and Yerkes,1985; Davis et al., 1989]. analyzethe coexistence of thesetwo systems of faultingin Although numerous efforts have been made to find the samearea, previous investigators have simply stated horizontalsurficial offsetson the Palos Verdes fault, no that the strike-slipfaulting was primary[e.g., Wilcoxet clearevidence has been found [Darrow and Fischer, 1983]. al., 1973;Lamar, 1990], or thatthe thrusting was primary Verticaloffsets have been documented from uplifted shore [e.g.,Davis et al., 1989]. The evidencepresented in this terracesand show a vertical slip rate of 0.1-0.4 mm/yr papersuggests that both strike-slip and thrust faulting are [Darrow and Fischer, 1983]. Davis et al. [1989] assumed activesimultaneously. negligiblelateral motion since2-4 Ma on the PalosVerdes The Torrance-Wilmingtonfold andthrust belt is outlined fault and modeled a concealed thrust fault beneath Palos by foldaxes and thrust focal mechanisms extending from Vexdeswith a slip rate of 1.9-3.5 mm/yr. In this study, offshoreNewport Beach, across the Palos Verdes Peninsula strike-slipevents have only been found near the Palos into SantaMonica Bay along the southwestflank of the Vexdesfault as it headsoffshore, both in SanPedro Bay and basin(Figure 11). The threecross sections in Figure13 in SantaMonica Bay. Becausealmost no evidencefor showthat the depthdistribution of thrustfaulting events strike-slipmotion has been found (Figure 10) and the within this fold and thrustbelt is more complexthan verticalmotion appears to be small,the Palos Vexdes fault within the Elysian Park fold and thrustbelt. The cross thusappears to accommodatelittle of thetotal convergence sectionC-C' showsa zoneof thrustfaulting dipping north in the southwestflank of the basin. Alternatively, 300-40ø, while the cross sectionsD-D' and E-E' show a moderate-sizedor largeearthquakes on the PalosVetdes steeplydipping zone beneaththe PalosVerdes fault trace faultmay rarely rupture up to thesurface. anda zonein the SanPedro Bay dippingsouthwest 20 ø- Becausemost faults in the studyarea are eithernorth or 30 ø. northeastdipping, the southdipping thrust in San Pedro HX•ssoN: Los ANG•mF..SB^SIN 15,381

40'

20 K•/ 30' I...... I...... •l •

40' 30' 20' 10' 118' 50' 40' 30'

Fig.9. (a)Late Quaternary faults in theLos Angeles basin [Ziony and Jones, 1989]. Shown as solid lines wherethe surface trace has been mapped andas dashed lines where inferred. SFV, San Fernando Valley; SMDF, SierraMadre fault; SGV, San Gabriel Valley; LA Basin, Los Angeles basin; NIF, Newport-Inglewood fault; PVF,Palos Verdes fault; SMB, Santa Monica Bay; MF, Malibu fault; HF, Hollywood fault;RF, Raymond fault; CF,Chino fault; EF, Elsinore fault; NF, Norwalk fault; PHT, Peralta hills thrust fault; and VF, Verdugo fault. (b)Late Quaternary foldaxes in the Los Angeles basin [Yerkes, 1965; Wright, 1987; Harding and Tuminas, 1988;Davis eta/., 1989]. SMMA, Santa Monica Mountains Anticline; RH, Raymond Hill; SJH, San Jose Hills;CH, Chino Hills; AN, Anaheim Nose; PJR, Playa-del Ray;TA, Torrance anticline; WA,Wilmington anticline;BA,Beta anticline; SPS, San Pedro Shelf; MS, Malibu Shelf; SMB, Santa Monica Bay; SP, Shelf Projectionin SantaMonica Bay. 15,382 HAUKSSON:LOS A,'

_ , k••., '••-•_••.••

.30' 20 RM ',...,- • '• 'x,,•_ '•,. •

40' 30' 20' 10' 118 • 50' 40' 30' A PVF NIF WF 1

- -•0 e

-20 ...... 1...... i ...... i'""'..... [ ..... ' .... I ...... i"...... W'"..... "'• ...... I ...... j ...... l ...... i ...... 0 10 20 50 40 50 60 70 80 90 100 110 1 20 130 g•STANC• (•M) •i•. 10. (Top) Map of line •ua[em• fau]• •d s•e-s]ip foc• mech•isms. Compression•qua•[s •e shaded. Mech•is• •a[ overlapsi•nific•fiy •e no• shaded. WF, Whittier fault; CH, China fault; •F, New•rt-In•]ewood fault; PVF, PalosVetdes fault; SMB, SantaMonica Bay; SP, Shelf Projection. Yorba Lind• •end is •e no•e•[ •end of focal mech•Jsms. •oRom) A norbert •endin• dep• sectionA-A'.

x.... '"'•"'•' • ...... "en..• f MAGNITUDES ",•.. -- ,' "-,. lo' ,. ,...... ••-,• •5..•,__..,•..,•.•.;•.,7..1•_..• .....:::. '- ...... • ...... -):.....•,.,,•.. 0 2.5+ --...... '•-t...... :¾-"-"'"' .... ('...... '...... •'•::.::"' :...... " 0 •.o.,.. • s"-• _...• ,- "' ; ,.,•,...... " - ¸ •.s+ 34ø- • .• ' ,.• •_ • ElysianPark 4.o+ . ,o. "x "-,,,_

40' Torrance-/Wliming,on . ,o.,a.,,'.'.'.ru......

40' 30' 20' 10' 118" 50' 40' 30' Fig. 11. Map of fold axes and thrustfocal mechanisms.The Elysian Pa•k and Torrance-Wilmingtonfold and thrustbelts are shaded. Breaks in the shadingare at segmentboundaries. RF, Raymond fault; SMDF, Sierra Madre fault; SMF, Santa Monica fault; and PHT, Peralta Hills thrust. HxcrtcssoN:Los ANOtm• B^s• 15,383

0 0 ,1.0+ " 3,5+ 34 4.0+

•.$+

$.0+

5.5+ SO' •D © •

20 30' I ...... I ......

10'

o

Bayis unexpected and requires a •ial explanation.One canrupture down into the semibrittleregion [Scholz, possibleexplanation for thiszone of thrusting,which has 1988].The 1987 Whittier Narrows earthquake was located twoML--4.0 and 4.5 eventsnear its downdip end, could be nearthe downdipend of the ElysianPark thrustbelt. a backthrust, which often appears at thelater stages of Similarly,the 1979 and 1989 Malibu earthquakes, which thrustfaulting (J. Suppe, personal communication, 1989). arethe largest events to occurin Santa Monica Bay since Usuallya set of thrustfaults is configured such that on a 1932,axe located near the downdip end of theTorrance- regionalscale the upper block is overridingthe lower Wilmingtonthrust belt. Themost shallow thrust events block,while often a conjugateset of thrustfaults, called occurin thedepth range of 4-6 km. Thusthe thrust zones backthrusts, may develop. in SanPedro Bay, this implies extend from shallow depths of4-6 km down to depths of a deepereast or northeast dipping thrust fault, which would 14-16km. If a 30ø dip is assumed thewidth range is20-24 bethe primary member of theconjugate set. km,30-50% wider than the seismogenic width of most To evaluatethe earthquakepotential of thesethrust strike-slipfaults in southernCalifornia. zones, some constraints on their widths are needed. Daviset al. [1989]proposed that most of the thrust Moderate-sizedearthquakes in most cases nucleate near the faultingin thebasin, including the WhittierNarrows bottomof a faultzone, although large or great earthquakes earthquake, occurs on crustal ramps that form structural and 15,384 HAUKSSON:LOS ANOELESBASIN

(A)

MAGNITUDES

0 2.$+ 0 •.0+ 0 •.S+ A.O+

A.$+

$.0+ ß ß $o' % • ee •e

SO' &O' 30'

½ T-W EP

• --20 •"1 ' i•89•'• •197 'I"" '1 (c) D T-W EP D' o .... •"t ...... (,i...... •...... •,•",..... ;•.... ;I,,,• ...... •...... • ' PVP • -10 • ML=4. 5 • 1987ML=5.9 = 4.3 -20 "- ' ..... I' ...... I"' I r...... I" ' (D) • T-W 0 I...... !,• ...... •...... •...... •7 ...... =-10 .• •

-20 • • ...... •...... 0 10 20 •0 40 50

•ap show•g•st •oc• m•h•sms •d ]oc•io• o• dep• s•dons.(•) Nor•-sou•dep• • S•ta Mo•caBay, •c]udes e•qu•es •om • •e Tom•ce-Wi•in•ton •]s. T-W,Tom•ce-Wi•Sm• foldmd •st be]t;EP, E]ysi• P•k fold•d •en•g dep•s•don D-D' •oss •e P•os•des Pe•u]a showinga c]usmr of sineplyd•pp•g ß e P•os Vetdesfault. (J) Eas•cend•g dep• s•don E-E'• S• PeSoBay show•g a zone. topographichighs on a regionalsubhorizontal, detachment accommodatemost of the brittle-elasticcompressional surface.The complexdistribution of thrustfaulting deformation. earthquakesbeneath the flanks of thebasin reported here is not obviously compatiblewith such a model. For NormalFaulting instance,the dipping zone of thrustfaulting seen in the Normalfaulting is observedin 9% of the earthquakes crosssection in Figure12b extendingfrom 16 to 4 km analyzed,a smallfraction of theongoing faulting (Table depth,is toosteep to be the detachment itself and too long 1). Thenormal faulting events all havesmall magnitudes to be a simplecrustal romp. Althougha subhorizontalwith the largestevent of ML=3.9. The normalfaulting regionaldetachment surface without crustal ramps may mechanismsin Figure14 are distributedadjacent to the existbeneath the deepbasin and the flanks,existing Newport-Inglewoodfault and offshorein the San Pedro dipping fault zoneswithin the flanks appearto Bay. The offshoreevents may be lesswell constrained HAUKSSON:LOS aAdqG•.F.3 B^SlN 15,385

MAGNITUDES

0 2.5+ '0 3.0+ Q•'•J 3.5+ Q•<,, •-.0+ ?-") 4.5+

{• 5.0+5.5+

30'

40' 30' 20' 10' 118 ø 50' 40' $0'

A A' i

-20• ...... i"...... I...... :...... l"...... ["..... ""I ...... l.... '"' "" l...... i...... j...... 1...... 0 10 20 30 40 50 60 70 80 90 100 1 • 0 D•ST^NCS (KU) Fig. 14. (Top)Map of foldaxes and normal faulting focal mechanisms. (Bottom) an easttrending depth sectionA-A'. NIF, Newport-Inglewoodfault, and SPB, San Pedro Bay. than the onshore events becausethe distance to the nearest suggeststhat crustalthickening and lateral movementof stationis in somecases larger than one focal depth. The crustal blocks are both important mechanisms for normal faulting may be associatedwith extensional accommodatingcrustal shortening within the basin. The bendingmoments that can causeextensional faulting normalfaulting is secondaryand appears to accommodate duringthe growth of anticlines[e.g., Ramsey and Huber, local geometricaladjustments within compressional 1987]. Alternatively,the normalfaulting events with geologicstructures. nodalplanes trending northwest to north-northwestcould be relatedto faultingon planesorthogonal to theaxes of STATEOF STRESS anticlines.Because the anticlinesusually plunge at the endsto maintaincontinuity with theunfolded beds, normal The seismicity,active faulting, and folding in the Los faultingcould occur to accommodatethis deformation as is Angelesbasin are driven by a regionaltectonic stress field. oftenseen in seismicreflection profiles (J. Suppepersonal In Figure 15, the P and T axes from the 244 focal communication,1989). mechanismsare shownalong with six P axesdetermined If the lower crustbeneath the Los Angelesbasin was from boreholebreakouts by Mount [1989]. Close still subsiding,the normal focal mechanismscould be correspondencebetween the two data sets is shown in the expected to be deep events associated with this consistentlocal variations in the orientationsof the P axes. downfaultingof the lower crust. This is not observedin Forinstance, atthe northern end of theNewport-Inglewood the crosssection in Figure 14. Instead,the foci of the fault both the thrustfocal mechanismsand the borehole normalfaulting earthquakes are shallowand appear to be breakoutdata show northeast toeast-northeast trending P associatedwith geological processes in theupper crust such axes.This suggests that the trends of theprincipal stress asgrowth of foldsor sedimentloading of theshallow crust. axesdo not changesignificantly with depthand that In summary,active faulting in the Los Angelesbasin boreholebreakouts in theLos Angeles basin although includesstrike-slip, thrust, and a few casesof normal takenfrom shallow depth are representative of the tectonic faulting.The coexistence of strike-slip and thrust faulting stress field. 15,386 Haulssos: Los ANG• B^SIN

5.5+

DATA

Fig. 15. Map showingP andT axes from the 244 focal mechanisms.Six P axes from boreholebreakouts determinedby Mount [1989] are shownas openarrows. The 7 km depthcontour of the LA basinis shownfor reference.

The P axes show somespatial variations in orientation Angelesbasin has a maximumhorizontal principal stress with mostlynortherly trending azimuths along the north with an azimuth of 13ø+_3ø and a vertical intermediate stress and easternsides of the basin but more northeasterly axis. The value of 0=0.22 shows that the relative azimuths in the southwest. A similar variation can be seen magnitudesof the threeprincipal stresses axe all different, in the azimuthsof fold axes (Figure 16). The fold axes with S2-$3 smallerthan S1-S3 indicatinga small but were groupedinto three setsof sliding windowsalong significantthrust faulting component. This stressstate latitudes33o42 ', 33ø52', and 34o02'. Theseoverlapping reflectsthe coexistenceof strike-slipand thrustfaulting windows,with a constantradius of 23 kin, are spaced23 regime in the basin. This is also consistent with the and 22 km apartin longitudinaland latitudinaldirection, prominentlate Quaternarystrike-slip faults and the strike- respectively.Azimuths of fold axeswithin thesewindows slip faulting observed for more than half of the focal plottedin rose diagramsshow how fold axesin the north mechanisms. This stress state is also consistent with and the easternparts of the basinhave azimuths that differ previousresults obtained for theNewport-Inglewood fault about 30o-40 ø from azimuths of fold axes in the [Hauksson, 1987], SantaMonica Bay [Haukssonand southwesternpart of thebasin. Saldivar,1989], and San Fernando aftershocks [Gephart and The P and T axes or fold axes alone, however, do not Forsyth, 1984]. This stress state also fits within the adequatelyrepresent the local stressfield, becausethe trend averagestress trajectories of centraland southernCalifomia of theP axescan differ from the maximum principal stress determinedfrom borehole breakouts by Mount[1989]. by 45ø [McKenzie,1969]. To determinethe average deviatoricstress field in theLos Angeles basin, the data set of focal mechanismswas inverted for the orientationsof SpatialStress Variations the principal stressesand their relative magnitude. To determinethe spatialdistribution of stresswithin the Furthermore, to determine spatial variations in the Los Angeles basin, the slip vectorsfrom the 244 focal deviatoric stress field across the basin the data set was mechanismswere grouped into three sets of sliding dividedup into 15 spatialgroups and each group was windows,like the foldaxes, along latitudes 33ø42 ', 33ø52', 'rovertedseparately. and 34ø02'. Theseoverlapping windows, with a constant radius of 23 km, are spaced23 and 22 km apart in The Whole Data Set longitudinaland latitudinaldirection, respectively. The The stereonetplots in Figure 17 showthe dataand the overlappingwindows provide increased stability of the directionsof the maximumprincipal stresses with 95% stressinversion and somesmoothing of the stressfield. confidencelimits obtainedfrom the inversion(see also The stereonetprojections of the selectedplanes and slip Table 3). As discussedearlier thesestresses were calculated vectorsfor eachwindow are shownin Figure 18. Each by selectingone of thetwo nodalplanes from eachfocal projectionis ploUedat the originof the circle of 23 km mechanismas the actualfault plane. The 95% confidence radius that was used to select data for the inversion. The limits werecalculated assuming that 30% of the planes orientationsof themaximum principal stress axes and the werepicked incorrectly. The averagestress state in theLos associated95% confidence limits determined assuming 30% HALrKSSON:LOS ANGEI• BASIN 15,387 15,388 H•UKSSON:Los ANGELES B^s•

NORTH of the planeswere picked incorrectly are shownfor the 15 datasets in Figure19 and listedin Table3. The large misfit anglesfrom the stressinversions for eventsin the southwesternpart of thebasin (Table 3) arecaused mostly by the normalfaulting mechanisms. This suggeststhat the normal faulting mechanismsare causedby localized spatialvariations in thestress field and further supports the interpretationthat the normal faulting reflects local geometricreadjustments. The state of stressvaries significantly acrossthe Los Angelesbasin. The mostimportant spatial variation in the state of stress is the variation from N 1øW to N31 øE in the trendof the maximumprincipal stress. The eastside of the basin has a nearly north trending maximum principal NORTH sn:ess.From east to west, along the northernedge of the basinthe principalstress rotates to the eastfrom 0ø to 13ø. This is also consistent with the orientation of the fold axes alongthe northernflank. The 32ø variationin azimuthof the maximumprincipal stress,from a northtrending axis to a northeasttrending axis, occurs between the northeast basin and the southern inter segmentof the Newport-Inglewoodfault andthe SanPedro Bay. This deviationis statisticallysignificant because the 95% confidencelimits do not overlap, and it showsthat a different stressstate exists in the southwestpart of the Los Angelesbasin. Fold axes in San Pedro Bay have a more northwesterlytrend than observedelsewhere in the Los Fig.17. Datafrom the 244 focal mechanisms and results from Angelesbasin. The agreementbetween average trends of the stressinversion. (a) Lower hemisphereprojection of all fold axes (shownas petals in Figure 19) and the trendof selectednodal planes (one from each focal mechanisms).the minimum horizontal principal stresssuggests that the Location of the slip vector is shown on each nodal plane as a current stressfield has existed over the lifetime of the folds plussymbol (with a normalcomponent) ora star (with a thrustor since 24 Ma [e.g.,Davis et al., 1989]. withcomponent). 95% confidence(b)The areas, orientations determinedofthe assuming principal that stress 30% axes of Thesecond most important spatial variation inthe stress theplanes were picked incorrectly, indicated withsolid, heavy field is the variation in0 orthe relative magnitude ofS2 dashedor lightdashed lines; 1, 2, and3 arethe maximum, and S 3, whichshows where strike-slip faulting and thrust intermediate,and m'mimum principal stress axes. faultingdominate in the basin(Table 3 and Figure 19). In

TABLE 3. StressInversion Results for the Los AngelesBasin Center Number • Maximum Intermediate Minimum Average of of Principal Principal Principal Misfit Angle Region Planes m Stress Stress Stress 13

# Trend Plunge Trend Plunge Trend Plunge Mean S.D.

All data 244 0.22 -167 ø 8 ø 102 ø 4 ø -16 ø 80 ø 30 ø 30 ø

34003 ' 117ø40 ' 26 0.49 -180 ø 2 ø 90 ø 9 ø -77 ø 80 ø 22 ø 18 ø 117o55 ' 40 0.37 -179 ø 6 ø 90 ø 9 ø -56 ø 79 ø 25 ø 21 ø 118ø10 ' 41 0.18 -167 ø 2 ø -77 ø 12 ø 94 ø 78 ø 22 ø 19 ø 118025 ' 53 0.08 -168 ø 1ø 101 ø 30 ø -76 ø 59 ø 22 ø 21 ø 118040 ' 38 0.24 -167 ø 1ø -76 ø 44 ø 101 ø 46 ø 21 ø 17 ø

33o51 ' 117o40 ' 32 0.37 -1 ø 5 ø 90 ø 19 ø -106 ø 70 ø 22 ø 20 ø 117055 ' 50 0.34 9ø 1ø 99 ø 8 ø -87 ø 82 ø 28 ø 29 ø 118ø10 ' 73 0.25 -163 ø 9 ø 105 ø 14 ø 17 ø 80 ø 27 ø 27 ø 118o25 ' 65 0.22 -166 ø 1ø -76 ø 2 ø 82 ø 88 ø 24 ø 25 ø 118ø40 ' 51 0.17 -170 ø 5ø 98 ø 15 ø -63 ø 74 ø 26 ø 24 ø

33o39 ' 117o40 ' 1 1 0.34 -179 ø 6 ø 88 ø 18 ø -71 ø 71 ø 26 ø 14 ø 117ø55 ' 34 0.47 -149 ø 15 ø 120 ø 3 ø 20 ø 75 ø 42 ø 41 ø 118o10 ' 71 0.30 -152 ø 14 ø -61 ø 3 ø 41 ø 76 ø 36 ø 32 ø 118025 ' 32 0.18 -159 ø 17 ø 107 ø 11 ø -15 ø 70 ø 40 ø 44 ø 118o40 ' 10 0.33 -178 ø 19 ø 87 ø 11 ø -31 ø 68 ø 30 ø 25 ø HArmssoN:Los ANG!mESB^SIN 15,389

•4 o --

50'

40'

30'

Fig. 18. Map showingthe 15 data setsinverted for the stressfield. See alsocaption of Figure 17a.

Trends of fold axes 10' e on ß o•

50'

40'

30' 40 30' 20' 10' 118 ø 50' 40' 30'

Fig. 19. Map showingthe stressfield in stereonetprojection at the 15 sites. The largestpetals from each of the rose diagrams in Figure 16 are also shown as petals on the outside of each stereonetcircle. See also caption of Figure 17b. the centralpart of the basin and along its easternedge, 0.25 (i.e., S2 is approximatelyequal in magnitudeto S3), •=0.25-0.49 (i.e., S2 is significantlylarger than S3), and so the stressstate is equivalentto thrustfaulting. These theplunge of S2 is closeto 90ø so thatthe stress state is spatialvariations in the magnitudesand orientationsof the equivalentto strike-slipfaulting. Along the ElysianPark principal stressessuggest that the flanks of the basin ,•ndthe Torrance-Wilmington fold andthrust belt, •=0.08- locally influencethe stateof stress. ! 5•390 H•mcsso•: Los A•o• B^snq

DISCUSSION The Torrance-Wilmington fold and thrust belt is approximately60 km long and can be dividedinto three The Los Angelesbasin is not a passivedepositional largesegments mostly based on the boundariesof oil fields sedimentarybasin but a regionof complextectonics [e.g., within the belt. Like the Elysian Park thrustzone whether Yerkeset al., 1965;Davis et al., 1989]. This complexity has made it difficult to interpretthe ongoingtectonic eachof the largesegments consists of onelarge or many small thrust faults cannot be resolved. One of the deformationand the associated earthquake activity, which, segmentationboundaries is in San Pedro Bay where the in turn, has resultedin "surprise"earthquakes causing Wilmington oil field is offset from the Beta oil field. significantproperty damage [e.g., Hauksson et al., 1988]. This study has contributedtoward understandingthe (Wright [1987] showsthe detailed locationsof the oil fields.) The otheris offshorefrom Torrance,at the western complextectonics through mapping of fold andthrust belts usinggeologic data and earthquake focal mechanisms and endof theTorrance oil field. Thissegmentation boundary is supportedby somestrike-slip focal mechanismsnear the suggestingthat the coexistenceof strike-slipand thrust north offshore extension of the Palos Verdes fault. The faultsis the resultof alecouplingof obliquefault motion. Torrance-Wilmingtonand Elysian Park fold andthrust belts Fold and Thrust Belts appearto mergein SantaMonica Bay and continueto the The 1987 WhittierNarrows earthquake confumed that the west beyondPoint Dume as one fold and thrustbelt [see thrust fault identifiedby Davis et al. [1989] beneaththe also, Davis et al., 1989]. ElysianPark anticlineis an activefault. Otherdamaging Any of these eight large segmentscould rapture in a earthquakessuch as the Coalinga and Kettleman Hills moderate-sized(M>5) or large (M>6.7) earthquake. If earthquakesin central California have also confirmed a severalsegments could rupturesimultaneously, a large causalrelationship between folding and faulting [Stein and (M>6.7) or great (M>7.8) earthquakecould result. The King, 1984; Eaton, 1990]. In this study, the focal compressionaltectonics of the Los Angeles basin are mechanismsof small and moderate-sizedearthquakes similarto the tectonicsof thesouthern San Joaquin Valley (2.5<_M<5.9)are usedto infer the zonesof thrustfaulting. with concealed thrust faults beneath thick folded sediments. If the focal mechanismsshow thrust faulting and are The 1952 (Ms=7.7) Kern Countyearthquake raptured a 75- locatedin an area of folding, a belt of thrustfaulting and kin-longconcealed fault with threedistinct segments near foldingis inferred. the southernend of the San Joaquin Valley [Stein and Two major fold and thrustbelts have been recognized Thatcher, 1981]. Although the concealedfaults in the along the flanks of the basin (Figure 11). In the past the basinappear to be segmented,a largeearthquake that would folds were ascribedto wrench faulting along the major break multiple fault segmentsbeneath the Los Angeles strike-slip faults [e.g., Wilcox et al., 1973]. The thrust basincannot be precluded. focal mechanismsfrom this studyand the fault-bendand fault-propagationfolding modelsby Suppe [1985] and Decouplingof Fault Slip others indicate that these folds are not associated with Decouplingof fault motionor slip partitioningimplies strike-slipfaults but are causedby movementon blind or that two setsof faultswith distinctlydifferent fault motion concealedthrust faults at depth. This interpretationis also coexistand togetheraccommodate oblique fault motionin similar to the conclusionsof Davis et al. [ 1989], who used the sameregion. Decouplingof fault motionhas not been geologicaldata and folding models to interpretthe folds in thoughtimportant in the past becauseshort faults often the basin. exhibit oblique slip and focal mechanismsof small Segmentationof fold and thrustbelts can limit the size earthquakesshowing oblique faulting are also common. of earthquakerapture and is thusimportant for evaluating However,unlike the smallearthquakes in the Los Angeles the earthquakepotential of theseconcealed faults. The basin,the moderateand large earthquakesrarely exhibit available data can not resolve whether the fold and thrust oblique faulting (Figure 6b). Recentstudies of the San belts are underlainby contiguousor segmentedfaults. Andreas fault in central California infer that slip is Furthermore,the seismicityfrom 1977to 1989is a snap alecouplednear the fault suchthat strike-slipmovement shot in time and may not illuminateall of the relevant takes place on the main trace of the fault and thrust or concealedfault structures. Several lines of evidence, normalon smallersubsidiary faults [Zobacket al., 1987; however,suggest that the thrustbelts may be underlainby Mount and Suppe,1987; Mount, 1989]. Similarly,along segmentexlfaults. the San Andreasfault in southernCalifornia, Jones [1988] The ElysianPark fold and thrustbelt is approximately founda goodcorrespondence between off-fault earthquakes, 100 km long andbased on limitedevidence can be divided late Quaternarytectonic deformation, and stateof stress. into at least five large segments(Figure 11). The This suggeststhat the decouplingof strike-slip from geologicaland earthquake data alone cannot resolve if each normaland thrustmotion in somecases is an important of theselarge segmentsare underlainby one continuous part of faulting. thrustfault or manysmall thrustfaults. The segment The Los Angelesbasin is a boundaryzone between the boundariesare identifiedfrom (1) theeastern and western limits of the aftershockzone of the 1987 Whittier Narrows strike-slipfaulting of the PeninsularRanges to the south andthe thrustor reversefaulting of the TransverseRanges. earthq•e; (2) the YorbaLinda seismicitytrend; and (3) The mergingof thesetwo modesof deformationrequires the intersectionwith the Newport-Inglewoodfault where some form of oblique deformation. However, oblique the depth of the hypocentersappears to be offset with faultingis not observedon mappablesttrficial faults such greaterfocal depths found to theeast. as the Whittier and Newport-Inglewoodfaults where the HAL'KSSO•:Los A•G•-ES B^s'N 15,39! verticalcomponent is less than 10% of the horizontal Bend"of the SanAndreas fault. Their modelexplains the componentof motion. Rather, the results of thisstudy differencein convergenceacross the central Transverse suggestthat for moderate-sizedor large earthquakes the Ranges (about 3 mm/yr) and the western Transverse faultslip is alecoupledinto a strike-slipcomponent on Ranges(about 20 mm/yr)by left steppinga zoneof right- steeplydipping northwest to northstriking faults and a lateral shearseparate from the San Andreasacross the thrustfaulting component onnorth dipping, east striking westernTransverse Ranges. The strike-slipmovement in faults.Hence these two different types of faultingneed to the Los Angelesbasin is compatiblewith their model. coexistin the basin. The Newport-Inglewood and the Whittier faults accommodate northwestward right-lateral motion 4ctive Tectonics subparallelto the platemotion of the Pacificplate. The A schematicdrawing of themajor late Quaternary faults Yorba Linda trendand the Raymondfault accommodate andthe fold and thrustbelts (Figure 20) summarizesthe someof the westwardleft-lateral motion required for the complextectonics of the Los Angelesbasin. The westwardmotion of theseblocks around the "Big Bend". maximumhorizontal principal stresses acting on the basin The Weldon and Humphreys[1986] model includes (determinedin this study) and, to the north, on the Mojave motionson strike-slipand reversefaults. The resultsof segmentof the SanAndreas fault [Jones,1988] are also this studycompliment their modelby showinghow the included.The complextectonics responding to this motionon strike-slipand exposed reverse faults are merged regionalstress field show (1) right-lateralmotion on throughdecoupled fault motionin areaswhere these faults northwestto north-northweststriking faults suchas the abut. Weak faultsor communicationthrough elastic strain Whittierand Newport-Inglewoodfaults; (2) left-lateral are possiblemechanisms for decoupledfaulting, which motionon east-northeastto northeast striking faults such allowthe neededoblique motion to be accommodatedby as theRaymond fault and YorbaLinda trend; (3) thrust the two distinct sets of faults. motionon west to west-northweststriking gently dipping faultslocated within the fold and thrustbelts; and (4) reversemotion on exposedreverse frontal faults such as the CONCLUSIONS SierraMadre fault. The strike-slipfaults accommodate The analysisof 244 single-eventfocal mechanismsof lateralmovements of blocks while the thrust and reverse earthquakesin theLos Angelesbasin and available data on faultsaccommodate uplift and crustal shortening. foldingshows coexistence of majorstrike-slip and thrust Weldonand Humphreys [1986] modelthe deformation of faults. Two fold and thrust belts are identified. The southernCalifornia as dominatedby lateral motionof ElysianPark fold and thrust belt extends along the east and crustalblocks to thenorthwest and west around the "Big north flank of the basin from the north end of the Elsinore

TRANSVERSE RANGES Prmc•pel Horizontel I MoximumStress (Mojove) MOJAVE DESERT -.a•. SANGASR/EL Son • • MOUNTAINS Fernondo -• I

•LYS IAN • ver•oL,ndo

• • x•'•. • PENINSULAR•.RANGES

Fig.20. Schematictectonic map of theLos Angeles basin showing mappable surficial late Quaternary faults [Zionyand Jones, 1989] and the Yorba Linda sel. smicity trend. The Elysian Park and Torrance-Wilmington foldand thrust belts are shown as shaded areas. Maximum compressire horizontal principal stresses from thks studyand Jones [1988] for the Mojave segmentof the SanAndreas fault are alsoshown with thickarrows. 15,392 HAUKSSON:Los ANGELESBAS•

fault, throughthe Whittier Narrows,beneath downtown Ranges, California, edited by D. M. Morton and R. F. Los Angeles,and into SantaMonica Bay. The Torrance- Yerkes, U.S. Geol. Surv. Prof. Pap., 1339, 27-64, 1987. Wilmington fold and thrust belt extends along the Darrow, A. C., and P. J. Fischer,Activity and earthquake southwestflank of the basinfrom offshoreNewport Beach potentialof thePalos Vetdes fault, technical report, 90 across the Palos Verdes Peninsula and into Santa Monica U.S. Geol. Surv., Menlo Park, Calif., 1983. Bay. The merging of the strike-slip faulting of the Davis, T. L., J. Namson,and R. F. Yerkes, A crosssection of the Los Angelesarea: Seismically active fold andthrust belt, PeninsularRanges and the thrust faulting of theTransverse the 1987 Whittier Narrows earthquake,and earthquake Rangesdoes not occurthrough oblique faulting but rather hazard, J. Geophys.Res., 94, 9644-9664, 1989. throughslip partitioningor decoupl• strike-slipand thrust DeMets,C., R. G. Gordon,S. Stein, and D. F. Argus,A revised faulting. Moderate-sizeddamaging earthquakes have estimateof Pacific-NorthAmerica motion and implications occurredon boththe strike-slipand the thrustfaults. The for westernnorth America plate boundaryzone tectonics, flanksof the basinthat surroundthe less active deep central Geophys.Res Lett., 14, 911-914, 1987. basin appear to accommodatemost of the deformation. Eaton,J.P., Regionalseismic background of the May 2, 1983 The stress field determined from focal mechanisms is Coalinga earthquake,in Proceedingsof WorkshopXXVII, consistent with stress directions derived from borehole Mechanicsof the May 2, 1983 Coalinga Earthquake,edited breakoutdata. The spatial coincidenceof the minimum by M. J. Rymerand W. L. Ellsworth,U.S. Geol.Surv. Open. File Rep., 85-44, 44-60, 1985. principal stressdirections and the trends of fold axes Eaton, J.P., The May 2, 1983 Coalinga earthquakeand its suggeststhat the presentstress field hasexisted since 24 aftershocksfrom May 2 through Sept. 30, 1983, in The Ma. Furthermore,evaluations of the earthquakepotential Coalinga,California, Earthquake of May 2, 1983, editedby that in the past only took into accountthe mappable M. Rymer andW. Ellsworth,U.S. Geol. Surv.Prof. Pap., surficial faults also have to take into account the concealed 1487, in press, 1990. thrust faults. Gephart,J. W., and D. W. Forsyth,An improvedmethod for determiningthe regional stresstensor using earthquake focal mechanism data: Application to the San Fernando Acknowledgments.I would like to thankLucile Jones, earthquake sequence, J. Geophys. Res., 89, 9305-9320, Ray Weldon, Thom Davis, Andy Michael and an 1984. anonymousreviewer for criticalreview of the manuscript. Gutenberg,B., C. F. Richter,and H. O. 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Ziony, J. I., and R. F. Yerkes, Evaluatingearthquake and E. Hauksson, Seismological Laboratory, Division of surface-faultingpotential, in EvaluatingEarthquake Hazards Geological and Planetary Sciences,Califomia Institute of in the Los AngelesRegion: An Earth-SciencePerspective, Technology,Pasadena, California, 91125 editedby J. I. Ziony, U.S. Geol. Surv.Prof. Pap., 1360, 43- 91, 1985. Zoback, M.D., M. L. Zoback, V. S. Mount, J. Suppe,J.P. Eaton,J. H. Healy, D. Oppenheimer,P. Reasenberg,L. M. Jones, C. B. Raleigh, I. G. Wong, O. Scotti, and C. (ReceivedSeptember 17, 1989; Wentworth, New evidence on the state of stressof the San revised January30, 1990; Andreasfault system,Science, 238, 1105-1111,1987. acceptedMarch 30, 1990.)