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Earthquakes, Faulting, and Stress in the Los Angeles Basin

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Earthquakes, Faulting, and Stress in the Los Angeles Basin 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 the major,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 the easternpart 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 the PeninsularRanges 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 ß 0.0+ ß 1.0+ o 2.0+ o 3.0+ 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 have beena 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
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