A focused look at the Alpine fault, New Zealand: Seismicity, focal mechanisms,

JOURNALOF GEOPHYSICALRESEARCH,VOL. 106,NO. B2,PAGES2193-2220, FEBRUARY10,2001 A focusedlook at the Alpine fault, New Zealand: Seismicity,focal mechanisms,and stressobservations Beate Leitner • andDonna Eberhart-Phillips institute of GeologicalandNuclearSciences, Dunedin,NewZealand HelenAnderson Ministry of Research, Science, andTechnology, Wellington, NewZealand JohnL. Nabelek College of Oceanic andAtmospheric Sciences, Oregon StateUniversity, Corvallis, Oregon Abstract.TheAlpinefaultis thePacific-Australian plateboundary in theSouthIslandof New Zealand. Thisstudyanalyzes195 earthquakes recorded duringthe6 monthdurationof theSouthern AlpsPassive Seismic Experiment (SAPSE)in 1995/1996 andtwoMr.5.0 earthquakes and aftershocks in 1997,whichoccurred closeto thecentralpartof theAlpinefault.Preciseearthquake locations arederivedby simultaneous inversion for hypocenter parameters, a one-dimensional velocitymodel,andstationcorrections. Togetherwith focalmechanisms calculatedusinga first motionandamplituderatiomethod,theseresultsprovidea pictureof theseismotectonics in the central SouthIslandovera 6 monthperiod.Momenttensorinversions of threeearthquakes provide anindependent meansof comparison to thefocalmechanisms derivedusingtheamplitude/first motion method.To validateourobservations overtime,we comparetheSAPSEseismicity with the seismicity recordedby theNew ZealandNationalSeismicNetwork(NZNSN) anda localnetwork atLakePukakieastof theSouthern Alps (6 monthsversus8 years).Our studyindicatesthatthe Alpinefaultreleases elasticstrainseismically fromthesurfacedownto 10-12km depthbetween MilfordSoundin thesouthandtheHopefaultin thenorth.The seismicityrateof theAlpinefaultis lowbutcomparable to lockedsections of theSanAndreasfault,with largeearthquakes expected. Seismicity decreases northof BruceBay at theAlpinefaultandwithin a triangularregionalongthe AlpinefaultlocatedbetweentheHopeandPortersPassfaultzones.We interpretthisastheresult ofdeformation distributed ontheAlpinefaultandtheHopeandPortersPassfaultzones.Thebase oftheseismogenic zoneis fairlyuniformat 12km _+2kmoverlargepartsof theSouthIsland.The highAlpsregionhasa shallower baseof theseismogenic zone,indicating localizedelevated temperatures eastof theAlpinefault.Mostof thestudyregiondeforms undera uniform stress field withamaximum principal horizontal shortening direction of 110ø-120 ø,similartogeodetic observations andplatemotions. Thisconfirms thattheregionis notundergoing strainpartitioning. Theearthquake datashowthatthedeformation awayfromtheAlpinefaultisdistributed onmainly NNEtrending thrustfaultsandstrike-slip transfer faultswitha maximum seismogenic depth of 12km. behaviorof thisregion.Priorto this study,earthquakedepthand focal mechanisms were only availablein localizedregionsor for The central South Island of New Zealand is a a few eventsand are not resolvedby the permanentnational continent/continent collisionzone, where a large part of the seismicnetwork.The spatialresolutionof the new earthquake transpressional platemotionbetweenthe AustralianandPacific data is betterthan observedprior but is limited by an average platesis accommodated by the Alpine fault (Figure1). The stationspacingof 30-50 km anda 6 monthoperationtime. tectonic modelsfor this regionpredictobliquemotionon the In this paper, new preciseearthquakelocationsand focal Alpinefault, high heat flow eastof the Alpinefault, and mechanismsthroughoutthe centralSouth Island are determined distributed deformation in the adjacentcrust.The studyof and interpretedtogetherwith resultsfrom existingearthquake earthquake depthsand focal mechanisms provideimportant studies, from thermal and strain models across the plate constraints for the thermal structure and the seismotectonic boundary, and from new results of a comprehensive multigeophysical investigationalong two transects.Our data confirm that the Alpine fault is capableof producinglarge andindicatea uniformbaseof theseismogenic zone •Now atGeoSphere Exploration Ltd.,Lower Hutt, NewZealand. earthquakes of 12 km in largepartsof theSouthIsland.The PortersPassfault Copyright 200! bytheAmaerican Geophysical Union. zone and its southwestextensiontowards the Alpine fault is seismicallyactive,hencesuggesting that the Marlboroughfault Paper number 2000JB900303. 0!48-0227/01/2000JB900303509.00 zoneis propagating southward. 1. Introduction 2!93 2194 LEITNER ET AL.: A FOCUSED LOOK AT THE ALPINE FAULT, NEW ZEALAND 168ß 170ø 172ø -41 ø -41 ø -35' '40øAustralian Plate _42 ø -42' 170' 175" 180' .' -43' -43' e_ , ..-.' -44' b,. 7%.--' IN 14' } . -45' a ß .T; ½ -46' ...-¾ . _ 1"i: o I k, / ._7/Pacific plate -45' 0 , ½' 0 km 50 100 L ' 3/,/"" , 168 ø 170 ß -46' 172' Figure 1. Earthquakesand focal mechanisms derivedfrom bodywaveform modelingplotted in lower hemisphere projection[Andersonet aL, 1993;Doseret al., 1999] (seealsoHarvardcenttoldmomenttensor(CMT) catalogat http://www.seismology.harvard.edu) for M,, > 5.4 in the SouthIsland. Shadedcirclesare earthquakes with ML > 5 recordedsince1920. Thin linesshowriversand lakes;thick linesshowmappedactivefaults. Dashedline outlines the study region. Inset shows 1000 m bathymetrycontoursdelineatingthe continentalplateausconverging obliquely along the Australian and Pacific plate boundary.Arrow gives the direction of relative plate motion betweenthe platescalculatedwith the Nuvel 1A rotationpole [DeMetset al., 1994]. The solid box is the map regionshownin the enlargement.HF is Hope fault. 2. Background 2.1. Seismicity Seismicityin the region is moderate,and large eventshave occurredmainly in the regionsadjacentto the subductionzones to the north and south (referred to in this paper as transition zones),where focal mechanismsderived from teleseismicbody waveformmodeling[Andersonet aL, 1993;Doser et al., 1999] are available(Figure 1). Earthquakedepthsand mechanismsin the central part of the South Island were only available from microseismicity studiesnear Lake Pukaki tReynets,1988] and along parts of the Alpine fault [Scholzet aL, 1974] and are poorly constrainedby the New Zealand National Seismic Network (NZNSN) owing to the sparsestationdistributionand low magnitudesof earthquakes[Andersonand •Vebb, 1994; Eberhart-Phillips,199.5].Seismicityin thecentralSouthIslandis confined to the crust [Allis and Shi, 1995; Reyners, 1988; Reynerset al., 1983'RynnandSchoh,1978;Scholzet al., 1974], with the exceptionof a small number of 50-100 km deep earthquakes beneaththe SouthernAlps [Reyners,1987].Here earthquakes outlinea westwarddippingseismiczonein theupper mantle,parallelin striketo the observedBouguergravitylow oriented17ø counterclockwise to the strikeof the Alpine fault (Figure2). Our newdatasetdefinesthedepthof thebaseof the seismogenic zone throughout the centralSouth Islandand provides precise locations andfocalmechanisms fortectonic and stressanalysis. 2.2. Alpine Fault The Alpine fault marksthe plate boundarybetweenthe Australian andPacificplates,whichareconverging obliquely at a rateof--36 mm/yrparalleland10 mm/yrnormalto thefault (37 mm/yrrelativeplatemotionat 43.5ø,170 0ø,calculated with NuvelIA rotation pole[DeMets etal., 1994]).TheAlpinefault LEITNER ETAL.: A FOCUSED LOOK AT THE ALPINE FAULT, NEW ZEALAND 166' 167' 168' -40': • Ill 169' 170' 171' I II I .,, ' - II 172' . 173' II • Challenge Plateau -•3" > • (, -44' • ,_•BR,, •'/•({ •ascaoe R•v•_ i George Sound• -45'•(f/ •;'. • • . [ •'.•'• • •L •' '•.'> x '• ' •_&'• V '-43" -44' Chatham Oh;Rise "• -45' . '.. -46' ,.,••'"• :; -47' • •, 167' _ o -41' • • '"., .2%',','- . -48 ø •-. FJ Hare Mare R,ver •.• •,:.• Fox•lacier , • .•;', ,..•j '• •< '• /Milford Sound *-•' 166' 174' . •in_40' ,7" .o ; -41' 2195 168' •-48' -47' i 169' 170' 171' -48' 172' 173' 174' Figure 2. Overview ofgeology andgeologic sitelocations. Schist ismarked ingay,higher •adenear theAlpine fault.Darkgrayalsoincludes uplifted igneous rocks intheFiordland andBuller regions. Graywacke isthewhite region bordered bytheAlpine faulttothewest, schist tothesouthwest, andCanterbury Plaintotheaoutheast. Bathymctry contours shown everyI0{10 m.Bouguer gravity lows aremarked bythedashed lines andarethe-50 and-70 mGalcontours. Circlesmarksitesmentioned in textwithknownupliftratesin mm/yrgixeninsidethe symbols (R.Norris, personal communication, 1998). Paleoseismic evidence shows single earthquake slips of4-6m aiong dashed region ofAlpine fault.HF,Hope fault; PPFZ, Porters Pass fault zone; MFZ,Marlborough fault zone' BR,Buller region. Plate motion vectors inmm/year arecalculated withtheNUVELI A rotation pole[DeMets et al., 1994]. The regionwithhighest upliftratesis connected •,•.itha high thermalgradient [AllisandShi, 1995;Koons,1987a;Shiet al., 1996].Thermal modeling [Allis'andShi, 1995;BartandBraun, 1999;Shiet al., 1996],datafromfluid inclusion studies[Craw, continent/continent collisionzone with the Challengerplateauto et al., 1994], thewestandtheChathamRiseto theeast(Figure2). The Alpine 1988;Crawet al., 1994;Holmet al., 1989;JenAins zircon reset ages [Tippett and Kamp, 1993] and heat flow faultaccommodates half to threequartersof the relativeplate connectsthe subduction zones to the north and south, which have opposite-facing convergence directions(inset Figure 1). From Jackson Bayto theHopefaultintersection theplateboundary isa motion [NorrisandCooper,1995:Bearartet al., 1999]. TheAlpinefaultchangesin character alongstrike(Figure2). FromMilfordSoundto the Cascade River,theAlpinefaultis a steeplydipping strike slip fault with very little dip slip component [Hull and Bero'man,1986;Sutherlandand Norris, measurements (R.H. FunnellandR.G. Allis, Thermalregimeof the southeast South Island, New Zealand, submitted to New ZealandJournalof GeologyandGeophysics, 2000)(hereinafter referredto as Funnelland Allis, submittedmanuscript,2000) all indicate•levatedtemperatures andpredicta thermallyweakened 1995]. Thethrustcomponent in thisregionis accommodated by crustcloseto theAlpinefault.Allis andShi [ 1995]pointoutthat nearthe surface, offshore structures andby a widezoneof crustaldeformation thermalmodelspredictelevatedtemperatures but temperatures below 20 km should be depressed asa resultof extending eastinto centralOtago[Norriset al., 1990].Farther Alps.The northit is a moderatelyeastwarddippingobliquethrustfault the buildingof a crustalrootbeneaththe Southern [Berr)wzan et al., 1992;NorrisandCooper,1995]withhighest estimatedbrittle-ductiletransition/one range• from 4 to 12 km relative upliftratesat Paringa River[Simp, on etaL, 1993]and basedon differentmodels.This study providesimportantnee, between FoxGlacier andHareMareRiver[Cooper andNorris, constraintsfor the brittle-ductile transition zone from determination of earthquake depths. 1994; Wellman, 1979]. precise 2196 LEITNER ET AL.: A FOCUSED LOOK AT THE ALPINE FAULT, NEW ZEALAND Paleoseismic evidencealong the 600 km long Alpine fault suggests thatit hasrupturedin largeearthquakes (M > 7.5) with recurrence intervalsof-200-300 years[BullandBrandon,1998; Norris and Cooper,1998; Yettonand Wells,1998]. The most recenteventis datedat -1720 [NorrisandCooper,1998;Wright et al., 1998;Yettonand Wells, 1998]. Paleoseismic recurrence intervalsare shorterin the northernpart of the Alpine fault (dashed regionof Alpinefault in Figure2) with a slip of 4-5 m per earthquake [Berrymanet al., 1992; Wright et al., !998; Yettonand Wells,1998]. Along the southern part of the Alpine fault, single-event offsetsare 8-12 m [Berrymanet al., 1992; Cooperand Norris, 1990]. The fact that earthquakes in the northernpart of the Alpine fault occur more frequentlywith smallersingle-event slip offsetscomparedto the southernsection is possiblyrelatedto the Marlboroughfaultsystem.Or it maybe causedby a possiblythermallyweakenedcentralsectionof the Alpine fault, which could form a barrier to southward propagatingearthquakes.This paper investigatesif the differencesin earthquakefrequencyobservedalong the Alpine fault are associatedwith differences in the regional seismic activityandthe depthof seismicity. The Alpinefault northof JacksonBay doesnot partitionstrain accommodate thelocalstress fieldat thejunction of theHope andAlpinefaultsystems [Robinson andMcGinty, 2000;Walcott, 1998].Southeast of theHopefaulttheincipientPortersPassfault zone [Cowanet al., 1996] marksthe southeastern limit of the Marlborough fault system[Carterand Carter, 1982;Herzerand Bradshaw,1985;RynnandScholz,1978]andpossibly joinsthe AlpinefaultnearFoxGlacier[CoxandFindlay,1995]. At the southernend of the SouthIslandthe changefrom transform to subduction occurs between Fiordland and the Puysegurtrench.The Alpine fault continuesoffshoreat Milford Soundand follows the continentalmargin.The northeastward subducted slabrelatedto the Puysegursubductionzonereaches as far north as Milford Sound. Here the slab is almost verticalat depths> 40 km. Crustalseismicity is diffusein thisregion [Anderson and Webb,1994;Eberhan-Phillips, 1995;Moore, 1999],andcrustal shortening extends eastward to centralOtago [Yeats,1987]. 3. Earthquake Data and Locations We usefour differentcomplementary datasetsto evaluatethe seismicityin the centralSouthIsland(Figure3). The Southern Alps Passive SeismicExperiment (SAPSE)givesunprecedented high-quality earthquake locations and focal mechanisms as occursalongthe San Andreasfault [Steinand Yeats,1989; Yeatsand Berryman,1987], but instead,it accommodates both thrustand strike-slipcomponentsof the relative plate motion throughoutthe central South Island. The NZNSN and Lake alonga singlefault [Berrymanet al., 1992;CooperandNorris, Pukakinetwork dataprovide insightintothelong-term seismicity 1997 1994; Norris et al., 1990; Simpsonet al., 1994]. GPS (8 yearsversus6 monthsfor SAPSE).The September observations along two transectsnear Haast [Pearsonet al., Mount Cook earthquakes and aftershocks, recordedby the 2000] and Fox Glacier [Beavan et al., 1999] confirm NZNSN and a temporaryaftershockdeploymentof three accumulation of obliqueplate motion at the Alpine fault. The stations,outlinethe tectonicsat the southernmarginof the geodeticdata show that between50% and 70% of the relative Marlborough faultsystem. The locationtechniques appliedand plate motion between the Australian and Pacific plates is errorsof the final locationsare data set-dependent and are outlined below. modeledasstableslip on theAlpine fault below5-8 km nearFox Glacier[Bearartet al., 1999]and 10 km nearHaast[Pearsonet al., 2000].A further10-30%of therelativeplatemotionis inferred tobeaccommodated eastoftheAlpine fault[Beavan et al., 1999,Pearson etal.,2000]. 3.1.SouthernAlps Passive Seismic Experiment FromNovember 1995to April 1996,SAPSEoperated 40 temporary stations whichwereaugmented by 15 permanent national seismic network stations, resulting in anaverage station 2.3. Transition Zones distance of30-50km[Anderson etal., 1997].The14temporary At the northernend of the SouthIslandthe transitionfrom and15 permanent short-period stations (EARSSrecorder and transpression on the Alpinefault to westwardsubduction of the 1Hz, 3 component, L4-Cinstruments [Gledhill andChadwick, Pacific platebeneath theAustralian platehasgenerated a broad 1991])operated in trigger mode.The26 temporary broadband zoneof activedeformation [Berryman et al., 1992;Lamband stations wereequipped withSTS2sensors (except twowith Bibby, 1989;Walcott, 1978].Northwest oftheAlpinefaultsome CMG-3instruments at stations LakeMoeraki(LAMA)and shorteningis accommodated on reversefaults in the Buller Gillespies Beach(GLAA)(Figure3b)) andReftekrecording region [Anderson andWebb, 1994;Rattenbury, 1986].In the unitsandoperated continuously. About5491 earthquakes northeast theMarlborough faultsystem, a region100kmwide (Figure 3a)triggered twoormoreshort-period stations andwere and300 km longwith four majorsubparallel activedextral routinelylocatedwith HYPOELLIPSE[Lahr, 1999]. strike-slip faults,transfers deformation fromthe HikurangiMagnitudes givenforall earthquakes in thisstudyarederived margin subduction zoneto theAlpinefault[Bibby et al., !986; from the short-period stationrecordings and are Richter Van Dissenand Yeats,1991].The mostactivefault of the magnitudes corrected for regionally observed characteristics in Marlborough system atpresent istheHope fault,which joints the energy propagation in thesamefashion asdoneroutinely forthe AlpinefaultnearArthur's Pass[Beavan et al., 1994;Bourne et NZNSN[Haines,1981].The magnitude of the earthquakes al.,1998a, 1998b; HoltandHaines, 1995; Pearson etal.,1995]. recorded bySAPSEranges from-2 to4.2M•:. Several large earthquakes haveoccurred onandneartheHope Some230 earthquakes wereselected withina 150 km wide faultsince 1881(seesummary byGledhill etal., [2000]). The regionneartheAlpinefaultin thecentralSouthIslandof New 1994M•,6.7Arthur's Pass [Abercrombie etal.,2000;Arnadottir Zealand (solid blackbox,Figure 3a).In thisstudy, emphasis is etal.,1995; Robinson andMcGinty, 2000;Robinson etal.,1994] giventothelong-term regional seismicity; hence aftershocks of and1995M,• 6.2 Cass[Gledhill et al., 2000]earthquakes theArthur's Pass andCass earthquakes areexcluded. Using the occurred south of theHopefaultandhada largecomponent of preliminary eventorigintime,all availablebroadband station reverse slipin thispredominantly strike-slip zone,suggesting datawereextracted andcombined withtheshort-period data to complex,diffusedeformation andpossible blockrotationto calculate earthquake locations usingall available P andS LEITNER ET AL.: A FOCUSEDLOOK AT THE ALPINE FAULT, NEW ZEALAND a) 2197 b) 166" 168ø 170: 172ø 174" 66" 168' 170' 172' 174' -40" ß ". BROADBAND STATIONS ' ø., [] SHORT PERIOD STATIONS ß ,• 23SHOTS -41 ' ß ß' .,,"' _•e ß . Black ß. -41 ø > 0.5 s Dark Grey 0.5 to 0.1 s ß 5461 EARTHQUAKES TRIGGlaRED' O 1997 MT COOK EQ+AFTE!•HO•K'6 "/d"!1,,,,. ;,•.,,,,•,,•,..•,,• White 0,1to-0.1 s -42' ..• . r) Light Grey -0.1 to-0 5s -42 ' '.: 'ro•, n, ) •• "-;s• ....( •-42' ø43 ø ' ;.2. .. -44' -44' T2 -45' ß ., '"e [] • -4,5" ß LU• ' r•"'•'/' "'.'i'_.L' -46" ß '""""' ) "" . km "" 0 ..ß -47' r• ' 166ø 50 6•oo 100 -47' i iii 168" 170ø 172' 174- • i 166' 168" C) 168' 16.9' 170" 171' 172" d)168' 170" 172' • - -47 174' 16.9' 170" 171' 172' t. 2343 ' ....... "" / ' •'42' -42'• EARTQUAKES /,•1• [] 42 PUKAKI NETWORK STATIONS II •- II' ...:.o... - ' 'y"..•..-._• ß ' m .. I ' ' -. •-• ,,. ,..... :. .-..•-..-.,.: ' ,',&•._., ;,'-;?__i '.. '•. .'.,• -,•, .•..•,• x---,,,-..'x ME>:4.0 • ,..' . •. .-• & ß ' :•" ,•"•:•"• ':'-? : -- ß •,, .,'. I , • -•" ,'"• ' . '.A ,. i 'l $ • ß•:"-.',,,.."., ...,, :: .'... •,,.' ,•.lt.J,$..' z.•o "IIi 1•9' ' ß•-.,•,.' v'• .,, ,:•,• .•z_• ,." ;: "i, / ','. el* ß I ¾,---'".-•-----, ) '""• '.. 168ø -•-•' "' :¾,.; m%/ .:>'-..•..,'•.•;"-,'m . .- !•-, ,s .'•. ,j.., :,:,.,...... ./,,.. ß---,, ,• e;,. ./.:;,,..• .• ,•. ,s ø' It q,": -'. '.-ß ':,:--•.": Im ,'., ;, ß / '"',, x ,,.x .•. :- _'¾.. m ß •,,.:,...:..:,.,.?, •:...'.:•.•..'",•...-.; •..,..;. I / •. Ir ? m ' ß'• :. ':.':'.,,__;:•L',.'fi•._'d: • "/. -•" ß 1975 - 1983 170" so loo '- 171" 172" !-.,. m-•-, , m,, , ) ' ':--, •, ,/ ii'4s' , ! ,;--- ..... --- m K.o so lOOi .,•.!t.' ".•-_*,', -•,,11• •- -__-_1_,•. 168" 169" 170' 17'!' 172" Figure3. SouthIslandseismicity andstation distribution maps: (a)Station distribution of broadband andshortperiodinstruments of the Southern AlpsPassive Seismic Experiment in 1995/1996. Duringthe 6 month deployment, 5491earthquakes wererecorded based on twoor moretriggers on theshortperiodinstruments. Shadedstarsindicatethe locationof refractionshots,T1 is the locationof the northerntransect,and T2 is the locationof the southerntransect. White starsmarkthe locationof the Arthur'sPassandCassearthquakes. (b) Stationcorrections in seconds derivedthroughsimultaneous inversion of hypocenter locations and a one- dimensional velocity modelareindicated by theshading of thesymbols. (c) NewZealand National Seismic Network (NZNSN)(1990-1997) and(d) LakePukaki network seismicity. Network stations areindicated by squares. arrivals.The mean characteristicsof the data used for the significantly relativeto theinitiallocations basedon theshortstation recordings, especially whentheazimuthal gapand earthquake locations areazimuthal gap133"(range50ø-200"), period number of picks16 (range8-30) anddistance to the nearest distanceto the neareststation decreased. by invertingsimultaneously for station 24 km (0-50 km). Single-event locations improved We relocated195earthquakes 2198 LEITNER ET AL.: A FOCUSED LOOK AT THE ALPINE FAULT. NEW ZEALAND the hypocenter locations,a one-dimensional velocityprofileand thestationcorrections usingthecodeVELEST[Ellsworth.1977; Kisslinget al., 1994;Krodolfer,1989;Roecker,1981].Events with at lea, eight phasesand a RMS travel time misfit smaller than0.8 s v,ere relocated.This techniqueimprove•,the singlee,•entlocationby absorbinglocalchangesin thevelocitythrough stationcorreciions.Our dataset hasa spreadof severalhundred P wave velocity (km/s) 2 4 6 8 0 kilometerso`èr the central South Island, but velocities castof the ; Alpinefaultare fairly homogenous [Sternet al., 1997],andthe calculated station corrections are stable and within reasonable bounds(Figure3b). Nine well-recordedreft-action shots(Figure 3a) in thestudyregionareusedin the inversion and proxide test datato estimatethe locationerror.We startedthe inversionusing single exent location, and the velocit5 model of EberhartPhillips[1995].Stationele`àtions x•,eresetto theaveragestation height;thereforestationdelaydue to topography is includedin T2 our .,,tation corrections. We testeddifferentdampingx,alue.,,in the im,ersion.different weightsfor S picks and the influenceof fixing the shotsby comparing, the locationsof the,best resol`èdearthquakesand shot, Pickqualitic•arequalitativelyassigned b), analysts,using the range0 to 4. v,,here0 is idealand4 i•, not useable[Lahr, 1999]. Forthefinalin``'crsion we included onlyS andP picksof quality2 and higher,usedan S weightingof 0.8, fixed theshot locations, anddampedtheupperandlowerlayersof thevelocity •xith0.1 versus0.01 for themiddlecrustalla5ers.RigiSreduction (-15%) isreachedmainlyby calculating stationcorrections, with minorchanges of the`èlocity model.Theonlysignificant change between thestartingandfinal`èlocitymodeli,a thickerupper crubandthinnerlowercrust(Figure4). Ourvelocityin theupper crustis0.0km/scompared to 6.2 km/,[KltJ)Snann et al., 1998b] and5.8-6.2km/s[Holbrooket al., 1998]derivedfromtraveltime 2o- ,, EP95 i - .................. This Study [i m ,, 30- ß 40- _ I, _ modelingalongthe two seismictranscots. Our station corrections are the sum of travel time differences dueto velocityheterogeneitie,at the •ite andtopob•aphy. The stationcorrections tFigure3b) arenearlyzeroeastof theAlpine faultin thegeneralareaof thickcrust,asreflectedby theextent of theBouguer gravit3,low (Figure2). Largepositivealelass at theWestCoastandwithintheCanterbury basinarecau.,,ed by thicksediment layersat the surface[KlefJbumn et al., 1998b; Sternet al., 1997]. 50 ......... i i I Figure4. P •,½1ocity modelsdiscussed in text.Solidlineshov, s P wavevelocitiesderivedby simultaneous inversionfor the hypocenter parameters, stationcorrections, and the velocity the Thefinalearthquake locations anderrorsareplottedin Figure model.Dottedline is thevelocitymodelusedto calculate ratiosand has low-velocitylaversat the surface. 5. Thelocation errorfor eachearthquake wascalculated by the amplitude lineisfromEberhar½-Phillips [1995]andwasused asa tbllowingmethod.For eachearthquakethe maximumdistance Dashed and depth difference to the final location between different starting model.Shadedlinc is the velocitymodelat 45 km distance fromtheAlpinefaultontransect 2 [Klef•nann e•½1., inversion runs,,,,ascalculated. The deptherroris defineda• haf 1998b]. the c.,lculated maximumdepth difference.Twenty separate inversionrun,testing differentvelocitymodcls,S pick weighting,and dampingvaluesfor the •,elocit5layersand hypocenter parameters ``',ereperformed. The resultingaverage locationerror in depthis 1.5 km. About 90• of the data havea deptherrorof< 3 km (for furtherdetails, seeLeitner[1999]). Theusefulness of thiscalculation wasverified bycalculating the relativeerrorsfor theshots,usingrunswithoutfixedshots. 3.2. New Zealand National Seismic Network the dataset.Of the 15,800earthquakes recordedduringthis8 year period, over half are aftershocksof the 1994 M,•. 6.7 Arthur'sPassearthquake [Robinson etal., 1994]. We improve thelocations by relocating theearthquakes using the one dimensionalvelocity model and station corrections derivedfromthe inversion of theSAPSEdata.We therebyredo Eberhart-Phillips [1995] studyand includethe now available eightyeardataset.To comparethedatato the SAPSEdata,the followingselection criteriawereapplied:RMS traveltimemisfit Seismicity recorded by the NZNSN overthe period19901997provides longtermseismicity of comparable magnitude to ß.• o theSAI'SEdatabutwithlargeruncertainties of thehypocentcr < 0.8 s, at leasteightphasedata,azimuthal gap< _00, anda parameters dueto anaverage stationdistance of -100 km(Figure minimum distance of 50 km to the nearest station. There is a 3c). Depthsarc:only reliablewhentheneareststationis within25 tradeoffbetween tightening thequalityparameters andthespatial kmof theepicenter, whichapplies toonlya smallpercentage of distribution of datacoverage. We chosea datasetthatre,•ealsthe LEITNER ETAL.:A FOCUSED LOOK ATTHEALPINE FAULT, NEWZEALAND a) •o 2199 50 6O SO 40 FZ 4O 30 3O 20 PO 10 •0 0 0 0 I 2 3 4 5 MAGNITUDE 168 ø 0 169' 10 DEPTH 170' 2O 30 171' 172' -' '- ---•-'•---- "' --: A9SHOTS TO ESTIMATE LOCATION ERROR DEPTH (KM) ERROR (KM) oo_ oo. o_,0 -10-15O3-5 -42' -43' %•"•'-41' ,/ / i // •/'I 42' Franz Josef .... %•%•%• • ••/ •-43 . Glacier •'• u••½) --• • Haas,• ••_ 0 • •/,- k% ' / • ' •. : 68' •--•': 9/ '%1 ,. • / -46' ! '• E_/- ' - ß F ": ' • - ' •'• ß •--} ß/ 169' km '-•, - 170' ' •-• 0 • 50 100 • -46' 171' 172' Figure 5. (a)Histograms showing magnitude anddepth ofearthquakes recorded withSAPSE. Depth distribution is forearthquake depth < 30kin,illustrating thatmost earthquakes occur atdepths shallower than12km.(b)Location ofSAPSE earthquakes relocated byJoint Hypocenter inversion using P andSwave picks oftheshort period and broadband stations. Nineshots, marked astriangles, were used torelocate andtesttheearthquake relocations. Size ofcircles marks thedepth error (small is0-2km;medium is2-3km,large is3-5kin),andcolor indicates thedepth range(white< 5 km;shaded 5-10 km,solid10-15km).Starsmarkearthquakes deeperthan30 kin. Shaded ellipsoid marks region withashallow seismogenic zone of5kmdepth. Shaded rectangle isregion where seismicity located by thePukakinetwork indicates a seismogenic depthof-8 km.Triangular regionhadalmost no earthquakes during SAPSE, NZNSN,andPukaki network operations (seeFigure 11).Lines show position ofcross sections inFigure12.MS,MilfordSound; HF,Hopefault. seismic patterns andhasacceptable quality parameters. It is necessary to verifythatseismic patterns arenotcaused by the 3.3. Lake PukaM Network The Lake Pukaki network provides quality earthquake selection criteria andareapparent in boththerawandselectedlocations(Figure 3d) data. Note that theoffshore seismicity south ofBruce Bayisreal and excluded bytheselection criteria. for the time between 1975 and 1983 [Haineset al., 1979]. The networkmonitoredthe seismicity duringimpounding of LakePukakiduring1976-1979[Reyners, 2200 LEITNER ET AL.: A FOCUSEDLOOK AT THE ALPINE FAULT, NEW ZEALAND approximated. In our datasetthe signalto noise 1988].Owingto thedense stationdistribution nearLakePukaki satisfactorily with magnitude (-30 km averagestationdistance),2660 earthquakes were ratio limitswaveformmodelingto earthquakes Themoment tensorsolutions of the recorded, complete fromaboutmagnitude 1.8upward,providing M•, 3.9(-4.1 Mr) andhigher. threeof its largestaftershocks andone gooddepthresolutionwithin25 km of theneareststation.The M,• 6.2 Cassearthquake, eventof ourdata(Figure6) setwerecalculated usingthecodeof errorsin depthand hypocenter parameters are estimated to 3andXia [1995].Forthreeevents bothtechniques yielded 5 km [Reyners, 1988]. The Lake Pukakinetworkcomplements Nabetek andtheir resultscanbe compared. theSAPSEdatain a low-seismicity regionandprovidesinsight mechanisms, into the seismicityat a lower-magnitudethreshold.The hypoeenter parametererrorsare comparable to thoseof the 4.1. AmplitudeRatio and First Motion Method SAPSE data but are dependenton the distanceto the network stations. We selectedearthquakes with RMS traveltime < 0.8 s, at leasteightarrivalpicks,andM•: > 1.8 (1260earthquakes). 3.4. The 1997 Mount Cook Earthquakesand Aftershocks The MountCook earthquake sequence occurredin September 1997 and includedtwo M• 5.0 main shocksfollowed by eight aftershocks. The first six earthquakeswere recordedby the NZNSN, andthe locationsare not well constrained. A temporary networkof threeshort-periodEARSS stationswas deployedon September22, reoccupyingthe two SAPSE sites GLAA and MountCook (MTCA) and an additionalnew siteat Fox Glacier (FOXA) (Figure 3b). This earthquakesequenceis of special interestto us becauseit occurrednear the Alpine fault, in a regionof low seismicactivity.Eventsrecordedby the temporary stationshavegoodqualitylocationsand focal mechanisms. 4. Focal Mechanisms Earthquakes usedin-thisstudyhave magnitudesrangingfrom aboutMt. 2 to 4.2. We applied a combinedfirst motion and amplituderatio techniqueto our data set [Robinsonand Webb, 1996]. This has the advantageof utilizing both types of informationand is importantbecausefirst motion or amplitude ratios alone do not constrainthe mechanismstightly. The amplitude ratiotechniquecanbe usedat frequencies higherthan 1 Hz, wheretheenergyis concentrated for earthquakes < Mr. 4. As an independent meansof comparison,we calculatedfocal mechanisms by invertingtheregionalwaveformsfor the moment tensor. Three-componentwaveform modeling [Dreger and Helmberger,1993; Fan and Wallace, 1991; Nabelek and Xia, 1995; Ritsemaand Lay, 1993] is performedat much lower frequencies(< 0.1 Hz), where the earth's structurecan be We applya first motionandamplitudetechnique[Robinson and Webb, 1996], which searchesfor the double-couple mechanism whichbestfitsallchosen amplitude ratios(amplitude ratio techniqueafter Schwartz,[1995]) and satisfiesthe maximumpossiblenumberof first motion observations. This techniquehas provided excellent results with aftershockand temporarydeploymentsof short period EARSS instruments [Gledhillet al., 2000;Reyners andMcGinty,1999;Reyners et al.,1997;Robinson etal.,1994].Theselected frequency band (14 Hz), maximum distance of calculated synthetics (75 kin),and useof all amplitude ratioshasgivengoodresultsthroughout NewZealand [Gledhilletal., 2000;Reyners andMcGinty,1999; Reyners etal., 1997;Robinson et al., 1994]andwereadapted for ourstudy.The final hypocenter parameters andvelocitymodel from the simultaneous hypocenterinversionare used in the calculation.Three low-velocitysurfacelayerswere addedto simulate scattering andattenuation of seismicwavesexpected nearthesurface andto steepen theraypath[e.g.,Abercrombie, 1997].Refractedarrivalsfromthe lowercrustarriveat distances of-150 km andcancomplicate the waveforms.Hence,we did not use stationsat this distancerangefor amplitudeor first motions calculations. All possiblesolutionsfitting the maximumnumberof first motions aredetermined by stepping through strike,dipandrake in 7.5øincrements (Figure 7, shaded P andT axes).Onlyclear firstmotions should beincluded sincethebestsolution willtryto satisfyall firstmotions. To determine the amplitude ratios,the envelopes of completetheoretical seismograms [Herrmann, 1991] arecalculated, andthemaximumvalueswithin 1.5 sof the P andS arrivalsare automatically pickedand,if necessary, modifiedby hand(Figure6a). The methodsearches for thefocal mechanism whichbestfitsall sevenlogamplitude ratios(PZ/SZ, PZ/SR,PZ/ST,PR/SR,PR/SZ,PR/ST,andSR/ST)of observed Figure6. (a) Synthetic (dashed line)andobserved (solidline)seismograms forJan.24.174426(year,month,day, andtime;readasJanuary, 24, !996, 1744:26UT). Thefull waveform inversion wasperformed in the14-25spassband.Z, R andT arevertical,radial,andtransverse component for eachmodeledstation.Stationname,azimuthand epicentraldistancearelistedto theleft of theverticaltrace.At thestartof eachtrace,a 1.0 indicatesthatit wasused in the inversion, and0.0 meansit wasexcluded. Beachbail on left showsbestfittingsolutionderivedby the waveform inversion andazimuthal distribution of thestations plotted.Seismogram amplitudes arenormalized to 100kmdistance assuming cylindrical spreading. A strikeslipsolution isobtained whichissimilartothebestfitting mechanism derived by thefirstmotionandamplitude ratiotechnique. (b) Synthetic (solid)andobserved (shaded) seismograms for Jan.24.174426.The amplitude ratiotechnique wasperformed in the !-4 Hz passband and obtained thebestamplituderatiofit mechanism shownat thetopleft.Z, R andT arevertical,radial,andtransverse components of thestations. Plotted aretheenvelope functions of synthetic andobserved datanormalized by the maximum amplitude foreachstation. Forthisplot(notforthecalculation of mechanism), synthetics arecalculated to nearest 5 kmwhichcauses a slightoffsetbetween observed andsynthetic seismograms for somestations. Station name,azimuth,andepicentral distance areat startof theZ component. A starmarksstations thatwereusedin the amplitude ratiotechnique. Ontheleft-hand sidethetwooutputs fromthefirstmotion amplitude ratiotechnique are, ontop:P (pluses) andT (opencircles) axeswhichfit all firstmotions shaded, andP andT (bothcrosses) axes which bestfit the amplituderatios(solid) and, on bottom,the focal mechanismthat best fits first motion and amplitude ratios.The first motions are markedby circles,wheresolidis compressional andshadedindicates dilatational first motion. LEITNER ETAL.:A FOCUSED LOOK ATTHEALPINE FAULT, NEWZEALAND 2201 a) HOKA 2• 42' 195km 960124.174426 Mw=3.87 EWZA 61' IZ$ km 14-25s 6km MTJA g4' 78 km1• MT LUMA ß 2o3' 20: kmo.(•• •77' 71km 'rime {s/ LAMA maximumamplitude:0.8 pm z b) R T AMP 75 km 85' / 78 km .... •= - AMP=6 . JACA71 km _ 332' LAMA49 km FMO+AMP • HABA8km ,.,r,! .... 5 1 .... 10 i .... 15 i .... 20 Time(sec) i 25 0 1 i 5 10 15 20 Time(sec) 250 5 10 15 20 Time(sec) 25 2202 LEITNER ET AL.: A FOCUSED LOOK AT THE ALPINE FAULT, NEW ZEALAND 960328.164047 -43.6022 170.6530 S,D,R= 557.5 67.5 -22.5 Depth=10.84ML=3.4 FMO/A 14/8 S,D,R= 76.5 69.5 204 1 a) .. b) C) 960320.210747 -43.5285 170.1152 S,D,R= 300.0 60.0 -60.0 Depth 0.5 ML=3.1 FMO/A 5/3 S,D.R= 70.941.4 229.1 d) 960222.005550 -43.6698 170.7435 67.5 60.0-1,50.0 Depth 3.5 ML=2.2 FMO/A 9/7 S.D.R= S.D.R=321.4 64.3 326.3 -F iO AMP FMO+AMP Figure7. Examples of first motionandamplituderatiomethodfor earthquakes withA-C quality.For eachevent thefocalmechanism is shownin lowerhemisphere projection. To theleft,shadedP (pluses)andT (circles)axesof focal mechanisms satisfythe maximumnumberof first motions,and solid P and T axesof the focal mechanisms whichbestfit theamplituderatios(RMS error> 2 standard deviations belowthemeanerror)areshownin anequal area projection.To the right, P and T axes which satisfyamplitudeand first motion criteria and first motions projectedon bestfitting mechanism are given,wheresolidcirclesrepresent compressional and shadedcircles represent dilatational firstmotions. (a) A qualityand(b) sameearthquake asshownin Figure7a butonlythreefirst motionsand threeamplituderatiosare usedto constrainthe solution.This showsthat few observations can resolve theearthquake focalmechanisms. (c) B quality;(d) C quality,whichhavetwo clustersof P and T axes. LEITNER ETAL.:A FOCUSED LOOKATTHEALPINE FAULT,NEWZEALAND 2203 Table1. FocalMechanisms of A andB QualityEvents Q Date, UT ....... Time, 'Latitude, Longitude, Strikel/Dipl/Rakel Strike2/Dip2/Rake2 StrikeDip A B A A A B B B A A A Nov. 16, 1995 Nov. 18, 1995 Nov. 19, 1995 Nov. 22, 1995 Nov. 22, 1995 Nov. 23, 1995 Dec. 04, 1995 Dec. 09, 1995 Dec. 09, 1995 Dec. 10, 1995 Dec. 11, 1995 B A A B B A A B B A B A A A B A A B B A A B B A A B Dec. 13, 1995 Dec. 17, 1995 Dec. 22, 1995 Dec. 28, I995 Dec. 28, 1995 Jan. 02, I996 Jan. 04, 1996 Jan. 04, 1996 Jan. 05, 1996 Jan. 06, 1996 Jan. 07, 1996 Jan. 13, 1996 Jan. 15, 1996 Jan. 24, 1996 Jan. 26, 1996 Jan. 28, 1996 Jan. 30, 1996 Jan.31, 1996 Feb. 09, 1996 Feb. 11, 1996 Feb. 20, 1996 Feb. 22, 1996 Feb. 23, !996 Feb. 26, 1996 Mar. 04, 1996 Mar. 04, 1996 B A B A B B Mar. 05, 1996 Mar. 09, 1996 Mar. 09, 1996 Mar. 12, 1996 Mar. 13, 1996 Mar. 16, 1996 B A B B Mar. 19, 1996 Mar. 19, 1996 Mar. 19, 1996 Mar. 19, 1996 UT deg. deg. 1250:29 2143:49 0202:52 0111:48 1352:12 0427:43 1153:59 1433:22 2230:00 1723:57 1321:38 0910:16 1641:53 2015:07 1040:58 1059:32 -44.5465 -44.5130 -43.6075 -44.1705 -44.1678 -44.0917 -43.9562 -42.3015 -43.4885 -42.8205 -42.8248 168.7637 168.1897 170.6252 168.7947 168.7972 168.8002 169.5728 171.5728 170.0057 171.8392 171.2878 -43.5910 -43.5910 -43.5975 -43.3002 -43.2643 -42.9918 -43.2580 -44.3082 -44.4020 -43.7048 -43.3230 -44.1687 -43.2253 -44.0507 -43.0120 -44.5570 -43.4360 -43.5950 -44.5352 -43.5590 -43.3032 -44.2507 -44.3507 -43.1775 -44.4453 -44.5715 170.3977 171.5255 170.3928 170.7995 170.7837 171.3697 170.8192 169.7152 169.5932 169.5855 171.5058 168.7993 170.8807 169.4927 171.2122 !68.2488 170.7607 170.2025 168.6242 170.6040 170.9513 168.5455 168.1423 171.9818 168.5937 168.2017 168.7532 170.6782 169.0638 169.5417 168.2132 168.0440 171.5615 168.6262 168.6788 168.4535 1905:35 0942:41 1009:37 2335:36 2216:16 1340:14 2011:06 1353:09 1744:26 0609:19 0105:00 0053:33 0210:14 1340:21 2340:27 0432:51 1242:01 1020:38 1351:39 1006:42 1056:43 1950:22 0424:12 0633:53 1946:12 0714:40 0324:40 -44.7658 -43.4540 -44.2260 -44.2833 -44.5897 -44.5612 0959:02 1421:20 1641:20 1848:04 2107:47 1640:47 i703:09 0123:42 -42.3092 -44.5718 -44.8130 -44.1650 -43.5285 -43.6022 -43.5990 -43.5992 B Mar. 20, 1996 A A Mar. 28, 1996 Mar. 28, 1996 170.1152 170.6530 170.6487 170.6373 A Mar. 30, 1996 A A Apr. 02, 1996 004i:26 -43.9458 169.0297 Apr.06, 1996 1520:47 -43.2417 170.7450 Error 127/83/-30 98/38/105 330/60/-23 360/38/-15 353/68/-38 53/75/-143 255/38/158 353/45/-158 263/53/128 255/75/150 360/68/15 210/45/-120 248/23/113 218/38/-120 8/68/-38 83/83/I13 285/45/-135 90/75/-143 15/30/53 345/38/53 98/60/-143 98/53/-158 360/30/-15 68/45/17 3 360/83/-22. 5 83/75/I35 233/45/16 5 15/53/8 38/68/105 353/83/38 225/38/165 180/83/-60 1I3/38/-23 30/38/83 360/60/23 158/75/-38 Error 222/60/189 15 -101/54/79 15 72/71/212 7.5 102/81/234 7.5 99/56/208 15 311/54/341 7.5 3/77/55 15 246/74/313 7.5 31/51/52 7.5 -7/61/17 7.5 -6/76/157 7.5 69/52/297 7.5 43/69/81 22.5 74/58/291 7.5 114/56/208 7.5 -170/24/19 7.5 160/60/305 30 349/54/341 7.5 7.5 7.5 15 15 15 7.5 15 7.5 15 7.5 15 7.5 22.5 7.5 7.5 7.5 30 7.5 -124/67/I09 7.5 -151/61/115 7.5 347/58/32 4 7.5 353/72/320 7.5 I03/83/241 7.5 163/85/45 7.5 93/68/188 7.5 -173/47/21 7.5 -27/80/46 7.5 -80/84/142 7.5 -178/27/58 7.5 -103/53/171 7.5 -33/81/54 22.5 283/31/195 15 221/77/23 5 22.5 -141/53/96 60 -102/71/148 7.5 7.5 7.5 7.5 7.5 15 15 7.5 15 7.5 7.5 7.5 7.5 22.5 15 22.5 22.5 7.5 7.5 7.5 180/60/-13 5 353/83/-53 259/54/199 64/52/321 92/38/192 7.5 15 7.5 15 135/45/-45 98/53/-105 150/53/-15 45/30/173 90/45/90 285/53/105 143/53/-30 330/60/38 60/83/135 300/60/-60 338/68/-23 75/68/-!43 338/75/-38 83/75/128 173/60/-45 38/68/105 260/60/23 5 301/40/289 249/78/219 142/86/60 -90/45/90 81/40/71 252/67/222 -141/58/144 157/46/11 7.5 37.5 15 7.5 52.5 30 7.5 7.5 15 30 15 22.5 15 7.5 7.5 15 7.5 15 22.5 7.5 7.5 15 7.5 15 15 71/41/229 77/69/204 329/56/332 78/54/199 -169/40/24 289/52/219 -178/27/58 15 15 30 30 7.5 7.5 22.5 Dip FMO/AR CC ML 7/5 6/4 10/6 8/4 11/4 6/3 6/4 4/5 11/5 6/5 9/7 0.87 0.68 0.82 0.84 0.81 0.81 0.68 0.88 0.91 0.8 0.7 2.5 3.3 3.3 2.6 2.8 2.8 2.7 2.5 3.4 2.6 3.4 0.81 3.0 0.8 2.7 0.7 3.9 Error 7.5 7.5 15 7.5 15 7.5 22.5 22.5 7.5 7.5 22.5 7.5 30 7.5 COMP 8/6 COMP 7.5 7.5 30 7.5 7.5 7.5 7.5 7.5 7.5 15 7.5 15 7.5 7.5 7.5 7.5 15 22.5 30 30 7.5 7.5 7.5 22.5 7.5 45 5/4 6/3 7/5 6/6 6/3 4/3 7/5 7/4 8/4 9/7 17/6 6/4 13/3 9/7 3/5 6/3 7/6 8/6 3/5 6/3 6/6 13/6 5/4 7/6 15/10 3/3 0.83 0.84 0.9 0.75 0.84 0.96 0.89 0.86 0.81 0.77 0.79 0.87 0.83 0.82 0.85 0.84 0.78 0.74 0.7 0.88 0.68 0.65 0.5 0.6 0.75 0.74 2.2 3.4 3.5 3.6 3.8 3.4 3.3 3.0 3.7 3.5 4.2 2.5 3.9 3.4 3.0 3.7 3.0 3.3 2.2 2.9 4.0 3.7 3.5 3.4 2.0 2.6 22.5 15 30 22.5 7.5 15 22.5 22.5 6/6 4/3 4/3 5/4 8/5 5/7 3/3 7/3 17/9 8/7 6/8 15/5 8/4 5/3 0.68 0.79 0.83 0.92 0.63 0.54 0.9 0.71 0.84 0.89 0.84 0.82 0.81 0.89 3.5 2.6 2.4 3.2 3.0 2.6 3.9 3.1 22.5 45 30 7.5 7.5 22.5 3.4 3.5 2.3 2.0 3.5 3.0 B Sep.23, 1997 0210:14 -43.5950170.2025 Thestrike, dip,andrakeerrors giverange of focalmechanisms whichfit themaximum number offirstmotions andwhichhaveamplitude ratioswith astandard error> 2 standarddeviationsbelowthe meanerror.COMP notesa composite mechanism. Q, Qualityasdefinedin text; FMO, numberof stations with first motionobservations; AR, numberof stationswith amplituderatioobservations; CC, correlationcoefficient. P and T axesof example focalmechanisms areplotted totherightof Figure7.Mt isderived fromshort-period observations (seetext). andsynthetic data(example in Figure6b).All P andT axesof mechanismsthat have a standard error more than 2 standard deviations belowthemeanerrorarepossible solutions (solid black crosses, left-hand sideof Figure7). P andT axeswhich satisfy boththe first motionandamplitude ratiocriteriaare selected asfinalsolutions (P andT axes,right-hand sideFigure 7).Theirrange ofstrike, dip,andrakegivestheerrorbounds of thebest mechanism (Table1).Thebestmechanism (nodal plane, right-hand sideFigure7) hasthelowestamplituderatioerrorand the lowest numberof first motion errors (e.g., fits all first motionsif possible). We applythemethodto seismogram o recordedby broadband and short-periodrecordersand therebymodifiedRobinsonand Webb's[1996] codeslightly.On average,we havefewer first motion and amplituderatio observationsthan in previous investigations. To testthe robustness of the method,we selected 2204 LEITNERET AL.: A FOCUSEDLOOKAT THE ALPINEFAULT, NEW ZEALAND +T_Axis• '"'--'• + b) -t- 168' 170' 172' -42' c) '- + ML2-4 (;•AQuality -4a(•13 Quality i F•.:;,:•,,,. •j Jackson Fig. •r.[ -43' Bay -45' .- 'I' -46' • 168' km -46' 170' 172' +T-Axi• + + ___•_ ++++ P-Axis + Figure 8. Stereonet projection ofP andTaxes. a)P andTaxes forSAPSE ML2-4.2earthquakes north ofJackson Bay. (b)PandTaxes forM..>5.4earthquakes [Anderson etal.,1993' Doser etaL,1999; Harvard CMTcatalog] north ofthedashed linenear Jackson Bay.(c)Lower hemisphere projection ofA quality (compressional quadrant in black)andB quality(compressional quadrant indarkgray)SAPSEmechanisms derived withthefirstmotion and amplitude ratio method and lower hemisphere projections oflarge earthquakes (compressional quadrant inlight gray). (d)P andTaxes forMt2-4.2 earthquakc• south ofJackson Bay. (e)P andTaxes forM.,>5.4earthquakes southof JacksonBay. LEITNER ETAL.:A FOCUSED LOOKATTHEALPINE FAULT, NEWZEALAND eventswith good coverageand tested that the solution was 2205 broadbandstation SNZO (Figure 3b) was available and instrument responses of thetemporarystationscouldbe verified (theGuralpstation needed anadjustment of 50 in gain).We then recoverable withfewerobservations. Forexample, theMarch28, 1996,1640:47UT earthquake is well constrained by fourteen firstmotions andeightamplitude ratios(Figure7a).Thefocal mechanism of the sameeventcalculated with onlythreefirst motions andthreeamplitude ratios(Figure7b)variesonly7.5ø applied the method to the smaller Cass aftershocks: the November25, 1995, 08!6:17 UT 5.2 M,•, November25, 1995, 0942:35UT 4.2Mt•,andNovember 25, 1995,2314:27UT 4.1 (theincrement of thegridsearch technique) fromthestrike, dip, earthquakes, usingonly SAPSEbroadbandstations.The solution andrake of the well-constrainedsolution.No mechanismin this for theMr.4.1 eventispoorlyconstrained dueto thelow signalto paperwas calculated with fewer than the abovetested noiseratio andgives the lower magnitudelimit for resolvable observations. earthquakes. Consequently we couldonlyuseoneeventfromthe We dividedthe focalmechanisms into threegroupswith datasetshownin thispaper,theJanuary24, 1996, 1744:26UT quality A, B, andC. All selected focalmechanisms havea good earthquake withMr. 4.2. correlation (correlation coefficient > 0.7 on a scalefrom0-1) between syntheticandobservedlog amplituderatiosfor thebest 4.3. Technique Comparison fittingmechanism. Mechanisms with large numbersof first motions and amplituderatioshaveA quality(28 earthquakes Thetwotechniques giveconsistent resultsfor thethreeevents examined (Figure9a) but with varyingamounts of dip slip with an average of nine first motions and six stationsfor component.In particular,the directionsof the P and T axesare amplitudes) whentheP andT axesclustertightlyaroundthebest broadlysimilar,with trendsof P andT axesvaryingby 10ø solution (Figure7a). Mechanisms thatare constrained by, on betweenthetechniques. average, five firstmotionsandfouramplitudes (25 earthquakes) TheCassmainshockmoment tensor solutionhasa slightly haveB quality when all mechanismswith allowablemisfit higherstrike-slip component thanthemechanisms derivedwith cluster tightlyaroundthebestsolutionasillustrated in Figure7c. the first motion and amplitude ratio technique [Gledhill et al., Often, despite a satisfying amount of first motions and 2000],butis withinthelimitsof mechanisms derived by NEIC amplitudes and a small misfit, mechanisms can have several and Harvard[Gledhillet al., 2000]. The Mz.5.2 aftershock possible solutions with largedifferences betweenthe P and T axes (Figures 7d and9b).We gavethesesolutions C quality(56 earthquakes). Only qualityA andB (Table1) eventscanbe used directlyfor stressinversionor be plottedas focal mechanisms. QualityC events have an ambiguity between different focal mechanisms and areonly usefulwhenothera priori information candistinguish betweenthem.Thereforewe will onlyshowthese mechanisms whentheyare of particularinterestandcaremustbe takenwith theirinterpretation. Thefocal mechanisms of our dataset are dominatedby thrust andobliquestrike-slipmechanisms (Figure 8c). The P axeslie between 90øand160ø(Figure8a and8d) andreflecttheobserved maximum horizontalstrainof .-.110 ø [Pearsonet al., 1995; Beavan et al., 1999].Southof Jackson Bay the four available focalmechanisms of largeearthquakes haveP axesof -140ø (Figure 8e).In contrast, P axesfromthesmallerearthquakes fall (Figure9b)isanexample wherethefirstmotion/amplitude ratio techniquegives two differenttypes of mechanisms and is therefore classified asC quality.TheP andT axesof themoment tensorinversiontechniqueare similarin directionto one of the clusters of P andT axes(Figure9b, bottomright,dashed nodal plane),whichisourpreferred solution. The otherpossible focal mechanism derivedby the first motionand amplituderatio method cannot fit therelatively largeamplitudes of thetangential components (Figure9b). Thisearthquake showsthatC quality mechanisms have the ambiguityof differentmechanisms and need other a priori informationto determinethe correctfocal mechanism. TheJanuary 24,1996,1744:26UT earthquake isthe onlymechanism fromthedatadiscussed in thispaper.in Figure 6 we showthesynthetics andobserved seismograms (envelope functionof waveform,!-4 Hz) of thebestfittingsolutionfor all amplituderatiosand the full waveforminversion(waveform, between 90ø and180ø (Figure8d). It is not clearif the 0.07-0.04Hz).Fortheseevents thetwotechniques areproviding discrepancy is biasedby sparsespatialsamplingof the large reasonable fitstoverydifferent partsof thefrequency spectrum. earthquakes or if indeedthe smallerearthquakes reflectmore The amplituderatio methodshowsa rangeof P and T axes heterogeneous stress.North of JacksonBay, largeand small within the given misfit. By combiningthe first motion earthquakes havesimilarmechanisms (Figure8a and8b), andP information themethodselectsa tightlyconstrained mechanism. andT axesrespond to thesamestress field. Thisvalidates ourapproach thatamplitude ratiosprovidegood 4.2. Moment Tensor Inversion constraintsbut are best used in combination with the first motion data.The comparison demonstrates the uncertainty in using Weinverttheseismograms of theSAPSEbroadband stations eithertechniquefor smallmagnitudeevents.For the SAPSE data toobtainthe seismicmomenttensor,the sourcetime function, we estimate that the directions of P and T axes are constrained andthecentroid depth(technique described byNabelekandXia, within!5ø,butthereis -30ø uncertainty in rake.M• valuesare [1995]). Theleastsquare's inversion fitssynthetic seismograms 0.2-0.45smallerthantheML calculated fromthe shortperiod calculated withthevelocitymodelobtained in thisstudy(Figure instruments for fiveearthquakes. Thisis in agreement withrecent 4)withobserved three-component seismograms. Thefrequencyobservations [Andersonand Webb, 1994] that NZNSN band isadjusted according to thesignalto noiseratioforeach magnitudes are overestimated by 0.3 M,•.owing to a localsite event, 0.07-0.03Hz for the events< Mr. 5 and0.04-0.02Hz for effect at station WEL. theearthquakes > Mj: 5. We use all availablebroadband recordings, i.e., 4-7 stationsat distances of 50-300 km andgood 5. Stress Observations azimuthal spread, whichgiveswell-constrained mechanisms. An example of thewaveform fit for theJanuary 24, 1996,1744:26 Smallto moderate sizedearthquakes donotnecessarily reflect faultsbutmayresultalsofromsliponunmapped fit earthquake is shownin Figure6a. We firstapplied the sliponmapped fracturesurfaces within the region. The technique to theCassmainshock. Forthiseventthepermanentfaultsor distributed 2206 LEITNER ET AL.' A FOCUSED LOOK AT THE ALPINE FAULT, NEW ZEALAND a) 951124.061857 951125.081617 Mw=6 2 ML=6.3 DC=95 b) 951125.094235 Mw=4.8 ML=5.2 DC=56 951125.081617 Mw=4.0 ML=4.2 DC=65 Mw:4.76 7 20-30s 6km 951125.231427 960124.174426 Mw=3.9 ML:4.1 Mw=3,9 ML=4,2 DC=95 1( s= AMP=5 R z.• i 15ksn --"•IV ., ttOk s. L. FMO+AMP Tun,: (s) ma.x•mum amphtude: 17.4 Figure 9. (a) Focal mechanismsderi',,edby momenttensorinversion(top row) and mechanismsfrom first motion andamplituderatio method(bottomrow, whereavailable).(b) Observedand syntheticseismograms for mechanism for the largestCass aftershockderivedb) waveforminversion.Z, R. and T are vertical, radial, and transverse componentsfor the stations.Station name, azimuth, and epicentraldistanceare given at the left. Seismogram amplitudesare normalizedto 100 km distanceassumingcylindricalspreading.To theright, outputof the amplitude ratio methodis given. ShadedP and T axc• of all focal mechanisms, which fit all first motions,solidP and T axes which fit amplituderatioswithin a given error arc given on the top.P and T axes,which satisfyfirst motionsand amplituderatiosplotted on the bestmechanisms(solid circlesare compressional, shadedcirclesdilatations)are given on the bottom.The first motionand amplituderatio methodgives two possiblefocal mechanisms(two clustersof P and T axes), one of which (dashednodal planes)has a focal mechanismssimilar to the solution derivedby the waveformin,,ersion.This is our preferredsolution. focalmechanisms of smallearthquakes represent ruptureon surfaces capableof brittlefailureundertheregionalstress field. We applya grid searchtechnique to solvefor the regional deviatoric stresstensor[Michael,1987a,1987b, 1991].The Thustheregional stress tensorin thevicinityof a mapped fault methodworks under the assumption that fault slip in all with few earthquakesmay bc derivedfrom thefocal mechanisms earthquakes in a regionoccurspredominantly in response toa of smallearthquakes in the region. Occasionally small-scale uniform stresstensor. The relationsbetween the stresstensorand temporalor spatialhctcrooc ' ' •'neitics in the.rc3field will re.suitin the focal mechanism are that the directionof the tangential focalmechanisms thatareinconsistent withtheregional stress tractionon thefaultplanetendsto be nearlyparallelto theslip field,soit is moreappropriatt: to ..,olvc forthebestfittingstress directionand that the magnitude of tractionis positiveand tensor than to rely on averagingP and T ,.txcsto estimate similaron all fault planes. The misfitangle13between the principalstrcs•orientations. predicted tangential tractionandtheslipangleis minimized a• LEITNERETAL.:A FOCUSED LOOKATTHEALPINEFAULT,NEW'ZEALAND Misfit 19 deg. Phi C) Misfit 17 deg. 0.5 Phi Strike $1 120deg. 168' 2207 17o' 0.7 Strike S1 119 deg. 172' -42 -42' ML 2 -4 , (• AQuality (•BQuality a) ! Fig. 16 -43' Bay ' • •'•, v • -44' -45' ' '/ • ' ' •0 , -45' • • • .•. 168' 170' 172' 2 2 • km 50 100 -46" • -44'i/ « • Misfit 18 deg. Phi 0.4 Strike S1 108 deg. Figure 10. Stereonetprojectionof the threeprincipalstressaxesof thebestfitting stresstensorwhich fits A andB qualityfocal mechanisms shownin Figure 10c. Bold numeralsindicatethe best fitting stresstensor,and small numeralsshowthe 80% confidenceregion. For mechanisms northof JacksonBay (Figure 10a), for mechanisms northof MountCook(Figure10b),andformechanisms southof Jackson Bay(Figure10d). 1- cos[3withanLI norm.Thegridsearch method changes the equivalentto theverticalstress,whichcouldbe approximatedby direction of thestress tensor in 10østeps andtheshape factor the !ithostaticload. We cannotdeterminethe magnitudeof the (co=S2-S3/S1-S3, whereS1,S2,andS3 arethethreeprincipal differential stress,S1-S3. The mean misfit is < 20ø, which shows deviatoric stress magnitudes listedfrommostcompressional to thatthe mechanismsare adequatelyfit by a uniformstresstensor. mosttensional)in 0.1 increments. It testsboth faultplanesand Michael [1991] usedsimulationsto show that for mean misfit of a uniform.,;tress field is valid. ,lectsthefaultplanewiththebestfit to thecalculated stress < 30ø theassumption Confidenceregionsare calculatedwith Michael's [1987b] tensor. Therelativemagnitudes of principal stresses cannot be bootstraptechnique.For the 80% confidenceregionthe dataset obtained frominspecting P andT axes. Wefindmaximumstressdirections of 110ø -120ø. 4) is -0.5, meaning thattheintermediate stress, S2, is approximately the is sampled500 times, and each time, local mechanismsare randomlysampledfromthegivenfault planesandthebestfitting meanstress and that the stress tensor is far from uniaxial. stress tensor calculated. To estimate the misfit for each individual In this region, S2 is vertical,sowe canestimate thatthemeanstress is mechanism, we appliedthe inversionmethodoriginallydesigned 2208 LEITNER ET AL: A FOCUSED LOOK AT THE ALPINE FAULT, NEW ZEALAND for slipdata[Michael,1984].Foreachmechanisms we assignthe fault plane selectedby the grid searchtechniqueas the best fitting one. The stressinversiontechniquecalculatesthe best fitting stresstensorand the degreeof misfit betweenthe slip vectorandtractionvectorfor eachevent(within5øof gridsearch results).A few eventshad misfits> 100"(100ø-130ø), andwe eliminated such events from the final stress inversion. The regionalstressfieldsof all selectedareas(Figure10c) havethe principalstressaxis at ---110ø-120 ø in agreement with GPS strainobservations [Pearsonet al., 1995;Beavanet al., 1999] and other stressobservationsin the north of our study region[RobinsonandMcGinty, 2000]. At the 80 % confidence level the stresstensorsfor all regionsare the same.North of Jackson Bay the directionof theprincipalstressaxisis horizontal 6. Results To interpret theseismicity, we combineall data.SAPSEdata, MountCookearthquakes and aftershocks, and qualitylocations of the NZNSN and Pukaki network(for selectioncriteria,see datadescription) are plottedin map view (Figure11). In the Pukakiregionthe depthof earthquakes recordedby the Pukaki networkagreeswell with the depthof theSAPSEdata(Figure 120. NZNSN data(depthonly resolvedwith minimumstation distanceof 25 km) have 1-2 km deeperhypocentersthanthe SAPSEdata.Bothdatasetswerelocated withthesamevelocity modelandstationdistribution, with theonlydifference beingthe sparser networkdistribution for the NZNSN. The depthof the SAPSE data is better constrained and more reliable. At the magnitudeM•,. > 3 level, all data sets are almost at 120ø,cI)is 0.5, andthemeanmisfitfor all eventsis 190(Figure 10a). The least principalstressaxis is horizontal,indicatinga stressfieldfavoringstrikeslip.If we subdivide thisregionfurther complete (Figure11a). At themagnitudes Mr.> 1.8(Figure1lb) somespatialpatterns aremorepronounced, especially withinthe and look at events north of Mount Cook, the results are essentiallythe same,but the directionof the intermediatestress Pukakinetwork,becauseof the lower magnitudecompleteness threshold.Seismicitywith Mt < 3 is probablyas high in other regions[RynnandScholz,1978;Scholzet al., 1974],butonly axisindicates an obliquethrustcomponent (Figure10b).Southof studiescanresolvethat. Jackson Bay the directionof theprincipalstressis 108ø,(I) is 0.4 microseismicity Earthquakes occur in a wide region from just west of the and the meanmisfit is 18ø (Figure 10d). A slightcounterclockwise rotation of the principal stress axis is observed, Alpinefaultto coastalOtago.In the northernthirdof thestudy is only60 kmwide,whereas farther although it is within the error limits. A counter-clockwise regionthebandof seismicity rotationfrom north to southis in agreementwith the counter- souththe band is up to 200 km wide. The zone of persistent seismicity is narrowest wheretheregionof upliftedgraywacke is clockwiserotationof therelativeplatemotionvector. 168' -41' •-- 169' 170' -"':-- ......... oo/I NZNSN MI,>3.0 O PUKAKI • SAPSE '; ML>3.0 • ,xtz'muRs P^ss& c^ssEQS O M > 5.0eqs1928-present ' ; 169' 170' 171' !72' '......... b) / / -42" NZNSN ML>I 8 O PUKAKIML>I.8 ß SAPSE • -43' -44' 168' x•,• 41' a) -42' 172' 171' iiii ARTHURS PASS&CASSEQS -43' o -43' o ~45' -45' , km •._ 0 ,q 50 -46' 169' 170' 171' o•, C•oøc• 0 -45' ' 100 -46' 168' ,, 172" ' 168' km • 0 169' 170' 171' 50 100 172' Figure 11.Maps showing quality locations ofSAPSE (solid circles), Pukaki (shaded circles) andNZNSN (open circles) earthquakes. (a)Mt > 3. Large shaded circles areMz.> 5 earthquakes since 1920.Notethatlarger earthquakes followtheseismicity patterns forthesmaller earthquakes. (b)Mz.> 1.8.Seetextfordetails. LEITNER ET AL.' A FOCUSED LOOK AT THE ALPINE FAULT, NEW ZEALAND 2209 AF a)c 2- • o Franz b) o =r• -50- Josef Glacier oE m '-'-100 c) _=_ -10 c• -20 -30 d)c o 2- • 1- • O- e) 0 = -50 m -100 - o f) -10 - -20 -30 -20 g) , , , I • 0 20 40 60 80 IO0 _ Haast _ -lOO L• 100 ' ""• -20 0 T __•--'] 20 40 60 80 Distance(kin) Figure12. Seismicity alongthreecrosssections orthogonal to theAlpinefault,shownin Figure5. Seismicity within25 kmdistance isprojected ontothecross section. Surface traceof Alpinefaultisat0 km(marked AF). SAPSElocations areindicated by opencircles andPukaki network locations aremarked bysolidcircles. Isotherms nearFranzJosefandHaastarefromShietaL's[!996]preferred modelwithnofriction.Figures 12a,12d,and12g showtopography alongthecross sections. Figures !2b, !2e,and12hshowtheBouguer anomaly in regal.Along thecross section the350øCisotherm ofShietal. [1996]ismarked asthickblacklinein Figures 12cand12j.The 350øC isotherm fromKoons [1987]isshown asdashed lineonFigure12c.Shaded triangles inFigure12fmarkthe projected locationof thePukakinetworkstations. ErrorbarswithinthePukakinetworkare~3 km andincrease to -5 kmin depthoutside of thenetwork area.Dottedlinesshowthelockedportion of theAlpinefaultasmodeled withGPSdata:thetoptwocross sections areneartheBeavan et al. [1999]transect andhavea lockingdepthof ~6.5km(meanof 5- 8 km),andthethirdcross section isparallel toPearson etal. [2000]transect nearHaastand hasa locking depthof 10km. 2210 LEITNER ET AL.: A FOCUSED LOOK AT THE ALPINE FAULT, NEW ZEALAND narrow,with almostno seismicityin thetopographically subdued CanterburyPlain, and it widensto the southwhere the schistbelt widens(Figures2 and 11). The distinctivechangesfromsouthto north in the level and spatial distributionof seismicityare discussed below in conjunctionwith focal mechanisms and the tectonicsof the Alpine fault and the southernregion of the Marlboroughfault system. 6.1. Depth of Seismicity The maximumdepthof crustalseismicityis fairly uniform over large parts of the central South Island at -12 km depth (Figures5a and 12). For estimatingseismogenic depthwe considerthe SAPSE events, Pukaki events within the network, andNZNSN eventsthathavewell-constrained depths(numberof P picks> 5, gap < 180ø, closeststation< 25 km). The deeper earthquakesare 10-14 km deep and are relatively evenly distributedthroughoutthe area that is sampledby good hypocenters; hencethe estimatedseismogenic depth is 12 -,- 2 km. The high Alps region, discussedbelow, is slightly shallower.Microseismicity studiesin Canterbury [Reyners and Cowan,1993], alongthe Alpine fault [Rynnand Scholz,1978; Scholzet al., 1974] and within the Marlboroughfault system [Reyners,1989; Reynerset al., 1983; Reynerset al., 1997; Robinsonet al., 1994], alsoshowthe maximumdepthof upper crustalseismicityat -12 km. The maximumdepthof seismicity providesan estimateof the thicknessof the seismogenic zone andtherefore relatesto thebrittle-ductile transition zone[Scholz, 1990;Yeatset al., 1997]. The crustalstructure derivedalong bothtransects hasuniformvelocitiesof 5.8-6.2 km in theupper 25-30 km thick crust[Holbrooket al., 1998;Kleffmannet al., 1998a;Sternet al., 1997] at distances of 30-100 km eastof the Alpine fault. No crustal boundary, no change in seismic character,and no change in electric conductivity(P.E. Wannamaker et al., Fluid generationandpathwaysbeneathan activecompressional orogen,the New ZealandSouthernAlps, inferredfrom magnetotelluric data, submittedto Journal of GeophysicalResearch, 2000) (hereinafterreferred to as Wannamaker et al., submitted manuscript, 2000)areimagednear thebaseof theseismogenic zoneobservedin thisstudy. BeneaththehighAlpstheLake Pukakinetworkdatashowan -3-4 km shallowerbaseto the seismicity. While extensive seismicity wasrecorded by thatnetworkin thehighAlps,only a smallportionhasadequate depthresolutionowingto the station distribution.The sparser SAPSEdataalsohavedepths< 10 km in an areaextendingto thesouthwest of MountCook. We thus estimate thata 10-20km wideregionof thehighSouthern Alps stableblockwestof theAlpine fault makesthe elevatedisotherm lessdramatic andcentered > 5 km eastof the fault [Shiet al., 1996]. The seismicitysupportsa model where elevated isothermsare centeredeastof the Alpine fault and are elevated by -3-4 km. NearHaast,SAPSEearthquakes areconsistent with thermalmodelsthatpredictvery slightelevationof isotherms due to lowerupliftandconvergence rates. Shi et al. [1996] incorporatedcrustalthickeningin their uplift models,which depressesthe brittle-ductiletransitionzone in regionsof thickenedcrust(Figures12c and 12j). The seismicitydoesnot support a 10 km deepening asimpliedbytheir350øisotherm and wouldthusfavora lowerproportion of activecrustalthickening. Southof Haasta regionof shallowermaximumearthquake depthof 5 km is locatedat theAlpinefaultnearJackson Bay (Figure5, shadedellipsoid).It appears to be a localizedfeature. This sectionof theAlpinefault is characterized by almostpure strike-slipmotion with a small amountof extensionobserved near Cascade River and offshore south of Milford Sound. It is possible thatthe changeof the plateboundaryfrom theAlpine fault to subductionbelow Fiordlandis causingextensionand associated higherheatflow. Heat flow measurements nearHaast arehigherthannormalat 60øC/km[FunnellandAllis,submitted manuscript, 2000], anda hotspringnearCascadeRiverpointsto elevatedtemperatures in theregion. North of Franz JosefGlacierseismicity(Mr. > 2.0) is almost absentin a 10-20 km wide band(Figure5, shadedtriangleand Figure11). In thesameregion,resultsfrom modelingactiveand passiveseismicand magnetotelluricdata along the transects (markedT1 andT2, Figure3a) indicatea low-velocityzoneand high-conductivity zoneat 25 km depthreachingfrom theAlpine fault to 30 km eastof it [Bannisteret al., 1998;Holbrooket al., 1998;Kleffmannet al., 1998b;Sternet al., 1997; Wannamaker et al., submittedmanuscript, 2000]. The along-strikeextentof the low-velocityzone is much greater than the extent of the lowseismicityregion,so it is unlikely that the lack of seismicityis directly related to the low velocity zone. The low-seismicity regionis northof theregionof the maximumdip-sliprateon the Alpinefaultandhencecannotbe simplyrelatedto themaximum uplift region or thermaleffects. Thus we interpretthe lowseismicityregion as a result of the initial transitionto the Marlboroughtectonicregime of multiple crustalfaults better alignedfor dextralslip(discussed in section6.3.2). Deepearthquakes (80-100 km) are only observedin a small regionnearMilford Sound(Figure5) andare likely to occurat the northerntip of the subducting Australianplate.Northof Jackson Bay, only oneeventin the studyis deeperthan15 kin. of 30 km eastfrom hasa shallower seismogenic depthof-8 km (Figure5, shaded Thiseventisat 30 km depthandat a distance 5 and12f). Givenan estimated dipof rectangle). The adjacent sectionof theAlpinefaulthasverylow theAlpinefault(Figures on theAlpine seismicity, butthefewearthquakes recorded by SAPSEextendto -45ø of theAlpinefault,it couldhaveoccurred below the Lake 10-14km depth,andthewell-constrained NZNSN hypocenters fault. We do not observedeep earthquakes Pukakiregion.asReyners[1987] did.Theseearthquakes appear extendto 14 km depth. andEberhart-Phillips [ 1995]didnot Isotherms are expected to be elevateddueto the highuplift to occurveryinfrequently, themin her3.5 yearstudy.ThePukakinetwork recorded rate (Figures!2c and 12j). Given large differences between observe thermalmodels(Figure12c)[Koons, 1987b;Shiet al. 1996]and only 12 earthquakes deeperthan50 km compared to 2800 estimateddepthsto thebrittle-ductiletransition zonefromfission earthquakes in the shallowcrustbetween0-15 km depth.The of thesefew deepearthquakes marksa change in trackandfluidinclusion data(between 250øCat !0 km [Karnp occurrence at thisdepth,buttherearetoofewto indicateanactive andTippert,1993]and400øCat 5 km [Craw,1988;Crawet al., rheology deformation surface. 1994]), only the shapeandextentof the thermalanomaliescan be comparedwith the earthquakedistribution.The locationof the peak isothermelevationalso varies between the models. Island,12km is the bestestimate of the seismogenic depth, Advection wouldbe mostpronounced closeto theAlpinefault. Theeffectof conductive coolingdueto lateralheatflow intothe exceptfor theareasurrounding andsouthof MountCookwhere theestimated seismogenic depthis 8 km. In easternOtagothe For regionalseismichazard modelsof the centralSouth LEITNER ET AL.: A FOCUSEDLOOKAT THE ALPINE FAULT, NEW ZEALAND JB •) o i lOO BB 2211 WR 2OO (km) 300 •,,..•-10 '• -20 -30 .... b) -43" -43" -44" -44" "km 0 168 ø 169 ø 170 ø 50 171' Figure 13. Alpinefaultseismicity andfocalmechanisms. (a) SAPSEearthquakes within5 km to the NW and 15 km to theSE of thefaultsurfacetraceprojected on a crosssectionparallelto theAlpinefaultfromMilford Sound toArthur'sPass.JB,Jackson Bay;BB, BruceBay;WR, Wanganui River.(b) Mapviewof SAPSEseismicity with lowerhemisphere projections of A quality(compressional quadrant black)andB quality(compressional quadrant middlegray)focalmechanisms. Focalmechanisms with lightgraycompressional quadrants arelowerhemisphere projections of the focalmechanisms for Mw> 5.5 earthquakes derivedby bodywaveformmodeling[Anderson et al., 1993;HarvardCMT catalog]. theseearthquakes occurredon or close to the fault, therefore outliningitsseismogenic zone.Seismicityis highestjust northof Milford Sound(Figure3a). North of Bruce Bay the seismicity ratedropsabruptly.BetweenBruceBay andtheWanganuiRiver, extensive sensitivenetworkwith 10 km stationspacingfor a seismicityof the Alpinefault was low both duringthe SAPSE decade, additional regions withsmallvariations in seismogenicandthe 8 yearsof NZNSN recordings.The occurrenceof a few up to 10 km depth(Figure 13a) suggeststhat the depth wouldbeapparent. However, thebaseof seismicity that earthquakes wehaveobservedis relativelyuniform,and variationsare crustin the vicinity of the Alpine fault is capableof releasing elastic strain. unlikely to be> 4 km. fewwell-located NZNSNhypocen[ers extend to 13-15kmdepth andmaysuggest a slightlydeeperseismogenic depth,but the dataare too few to justify subdivisionof the region. Undoubtedly, if thecentralSouthIslandwasmonitored by an Whensimilarmagnitude ranges areconsidered, theseismicity rateis comparable withseismicity ratesalonglockedsections of the San Andreas fault (Figures 1 la and 14), where large historic SAPSE observed 60 earthquakes in a bandfrom5 km havebeenrecorded. BoththeCarrizoPlainandthe northwest to 15 km southeastof the surfacefault trace of the earthquakes of the San Andreasfault experienced no Alpine fault (Figure 13a).Given anapproximate faultdipof45ø, Mojavesections 6.2.AlpineFault 2212 LEITNER ET AL.: A FOCUSED LOOK AT THE ALPINE FAULT, NEW ZEALAND -120 ø .121 ø -119" 36 ø o o oO o -117 ø 36 ø -118" o o o o oo o o 0 o '%, o% 35 ø ß o. 35 ø o o 0 ¸ 0 0 o krn 34" o 34" - -121" - 120" -119" -118" -117" Figure14. Seismicity withM•,> 2.5(similar magnitude rangeto SAPSEandNZNSN)in southern California between 1990and1997.Opencircles haveM•,of 2.5-3.0;shaded circles haveM•,of 3.0-3.5;solidcircleshaveM•, of 3.5 andhigher.The CarrizoplainandMojavesegments of theSanAndreas faulthavelow seismicity rates during thisobservation timebuthaveruptured in twogreatearthquakes. Thehypocenters of these earthquakes are notexactlyknown,but theyruptured theSanAndreas faultbetween thetwo solidbars(datacourtesy of the Southern California Earthquake Center (SCEC)athttp://www.scec.edu). MS, Mojavesegment; CPS,Carrizoplain segment: creep,creeping segment of theSanAndreas faultthatextends to thenorthwest. earthquakes largerthanMr, 3.5 over the time period 1990-1997 (sametimespanas theNZNSN datasetshownin thisstudy)and showonly a few eventslargerthanM•, 2.5 (Figure 14), which is the thresholdof our study. Despite the relatively low frequency of magnitudeM•, > 2.5 earthquakes on this lockedsectionof the San Andreasfault, two historicearthquakesof approximatelyM•,. 7.9 (Fort Tejon, 1857) and M,,. 7.5 (Wrightwoodearthquake, 1812), demonstratethat largeearthquakesoccur. In contrast,the creepingsegmentwhere significantaseismicslip occursis the only place where the San Andreas fault is well-defined by microearthquakes. The stress tensor results differ from the San Andreas fault releasedin brittle deformation. Assuming that parts of the Alpine faultareboundedby thermallyweakenedcrustto theeast and strongerAustralianplatecrustto the west,we expectthatit is possibleto storeelasticstrainin thisregion.Elsewherefield evidenceand numericalmodelsof earthquakeruptureshowthat slip often propagates along the interfacebetweenweak and strongmaterial[Harris et al., 1994]. While this is usually considered for shallowdepths,it may alsoapply to the Alpine fault at depthsof 5-12 kin. An Mw5.4 earthquake southof BruceBay possiblyoccurred on the Alpine fault on 20 October1998. The depthis poorl:, constrained for theNZNSN hypocenter. Thisearthquake caused minordamageand waswidely felt alongthe West Coastand region. In the Alpine fault region, the principalstresstensoris fairly uniformthroughoutthe regionand is orientedappropriately throughoutcentralOtago.The Harvardcentroidmomenttensor (Figure 13, latitude43.8ø) is an oblique for plate motion with the only spatial variation being a small (CMT) mechanism with a 20ø clockwiserotationof strike rotation consistentwith varying plate motion. In southern strike-slipearthquake relativeto theAlpinefault.The strikeof the northeast trending rotatesover35ø alongthelengthof theSanAndreasfault[Jones, nodalplanehas an errorof -10ø-20ø, so its strikecouldbe 1988], althoughthe plate motion orientationdoes not vary. parallelto the Alpine fault. We observea few normalfaultingmechanisms at 15-30km Jones [1988] shows that the maximum horizontal stress California, the orientation of the maximum horizontal stress orientation is constant relative to the orientation of the San Andreas fault and interpretsthis as evidencethat it is a weak fault. There is no suchevidenceto suggestthat the Alpine fault distance eastof theAlpinefault.TheP axesarealignedto the regionalstressfield. They couldindicatelocalextension dueto gravitational collapse. Bearartet al. [1999] observenegatbe strainin thisregion,whichis consistent with the mechanisms. The estimatedseismogenicdepth of the Alpine fault is 10-12 6.3. Northern Transition Zone km. This is similar to the geodeticestimatedlocking depth of 6.3.1. From the Hope fault to the WanganuiRiver. The 10-12 km at Haast [PearJot:et aL, 2000] but is slightlydeeper of theHopeandAlpinefaults(Figure15) is a complex than the 5-8 km estimatedlocking depth in the central region junction is a weak fault. [Bearartet al., 1999] (Figures12 c, 12f, and 12j). Both the seismicand geodeticresultsconfirm that the upper crust in the vicinity of the,Alpine fault is storingelasticstrain that will be zoneof deformation ratherthanajunctionof distinctfaults.From the Hopefaultjunctionto the WanganuiRiver,earthquake activity is high and concentratedin the 20 km wide zone LEITNER ET AL.' A FOCUSED LOOK AT THE ALPINE FAULT, NEW ZEALAND 172' 171' 2213 [Gledhillet al.. 2000]havethrustcomponents in a regionwhere previously onlystrikeslipearthquakes havebeenobserx ed.The Arthur'sPassearthquake v,as primarilya reversefaultingevent o• with NE-SW strike.This faultdirectionis parallelto the strikeof strikeslip faultsin theregionbut is not favorablyorientedfor reverseslip in thisstressregime.Aftershocks follow a NNWSSEtrending zone,havevariedmechanisms. andrespond to the regionalstressfield [Robinson andMcGinO',2000].The Cass earthquakewas probably triggeredby the Arthur's Pass earthquake. It hadan obliquestrikeslip mechanism and.based on the 330'• trendingaftershocks. rupturedalonga NNW-SSE trendingwestwarddipping fault. The NNW-SSE trend of aftershocks of bothearthquakes andthethrustcomponent of their mechanismssuggestthat they comprisepart of the plate convergence normalto theMarlborough faultsystem[Robinson andMcGino,,2000].It appears thatearthquake slipin thisregion is partitioned into components fault paralleland normalto the platemargin[Gledhillet al., 2000:.Robinsonand McGino,, ' km ,J,, 171ø 0 10 20 172' 20001. 6.3.2. From the Wanganui River to Mount Cook. Seismicity attributed to theMarlborough faultzonereaches asfar south as Mount Cook. South of the Wanganui River the seismicity stepsfartherawayfrom the Alpine fault (Figure5) untila west-east strikingbandof seismicit)intersects theAlpine Figure15. Seismicity andfocalmechanisms fromtheHope faultto theWanganui River:lowerhemisphere projections of A quality(compressional quadrantblack) and B quality (compressional quadrantmiddlegray) focal mechanisms. -43' 170' 171' -43' Mechanisms with lightgraycompressional quadrants are lower hemisphere projections of thefocalmechanisms forM,,,> 5.5 earthquakes derived by bodywaveform modeling [Anderson et al.,1993;Doseret al., 1999'HarvardCM T catalog].Arthur's Pass tarthquake ismarked withA. TheCassearthquake (marked withC) and threeaftershock mechanisms were derivedby broadband waveformmodelingin thisstudyandhavedarkgray compressional quadrants. W is Wilberforce Riverearthquake. Qualit3 locations (seealsoFigurel lb) forearthquakes withMt >3 recorded bytheNZNSN(opencircles) showtheNNW-SSE trendof theArthur'sPassandCassearthquakeaftershockzones. Onlya few SAPSEearthquakes wereselected in thisregion. Dashed line in the northmarksthe boundaryof thestudyregion where seismicity is artificiallytruncated. PPFZis Porters Pass o fault zone. T2 immediately eastof theAlpinefault,withonlya fewearthquakes located directlyon the fault (Figure 11). The earthquakes are distributed at all depthsdownto -9 km, with theexception of oneeventat 13 km depth.StrikeslipeventsneartheHopefault 170' 171' coincide with the strikeof the Marlboroughfault systemandare Figure 16. Seismicityand focalmechanisms from Wanganui steeply dippingto thewestif we selectthe faultplanethathas right-lateral slip (Figure 15). The Arthur's Pass,Cass,and Wilberforce Riverearthquake sequences occurredin thiszone v•here theHopeandAlpinefaultsystems join. We haveonlya fewearthquakes fromSAPSEin theregionsincewe excluded (compressional quadrantblack)and B quality (compressional quadrantdark gray) focal mechanisms. Mechanismwith light •ay compressional quadrant is a lowerhemisphere projection of theaftershocks of theCassandArthur'sPassearthquakes. North ofLakePukaki, earthquake mechanisms varyconsiderably, butP axesarewithinthe samerange.The high seismicactivityis concentrated at the obliquejunctionof the dextralHopeand Alpinefaults,wherea highmaximumstrainrate is observed v•ithin theGPSnetwork [Beavan andHaities,2000]. Boththe M,. 6.7 1994 Arthur'sPass[Abercrombie eta!., 2000;Arnadottiret al., 1995;Robinson and McGino',2000; River to Mount Cook; lower hemisphereprojectionsof A quality the M,,. 6.2 Godley River [Andersonet al., 1993] earthquake mechanism.Normal faulting mechanismseast of Mount Cook are composite solutions(markedwith HT) and are possibly hydrologically triggered. Qualitylocations (seealsoFigure1lb) for earthquakes with blt. > 3 recordedby the NZNSN (open circles),SAPSE (solidcircles),and Pukaki (shadedcircles)are shown.Solid starmark• Mount Cook (MC); open starsare shot pointsof thetwotransects TI andT2. MountCookearthquakes are directly east of Mount Cook and had a thrust faulting mechanism. Note theregionof decreased seismicitybetweenthe extension of LakeTckapo. Robinson et al., 1994]and 1995M,,. 6.2 CassearthquakeAlpinefaultandthenorthern 2214 LEITNER ET AL.: A FOCUSED LOOK AT THE ALPINE FAULT, NEW ZEALAND faultobliquely (Figure11).Thisbandof seismicity is especially the classicoblique continentalcollision illustratedby twopronounced whenlowermagnitude events areincluded andisthe dimensionalnumericalmodelsof deformingorogens. southwest extension of the Porters Pass fault zone. It includes the Mw6.2 GodleyRiverearthquake [Anderson et al., 1993]. This 6.4. HydrologicallyTriggeredEarthquakes earthquake occurred in a remoteregionwith no mappedfaults East of Mount Cook, two sequencesof eight and four and hadno observedsurfacerupture.Andersonet al. [1993] earthquakes occurredon December13 and22, 1995 (composite suggested thatit occurred onthecontinuation of thePorters Pass mechanisms markedHT, Figure 16). The earthquakesoccurred faultzone,whichis similarin striketo theNE-SW trendingplane with rightlateralstrikeslip.Six earthquakes locatedjust to the southeast of the GodleyRiverearthquake (Figure16) havevery similarfocal mechanisms and possiblyoccurredon the same fault. Seismicityconnected to the PortersPassfault zone is not surprising sincegeological evidenceshowsthatit is an active, developing fault zone,•theyoungestand southernmost of the Marlborough system [Cowan et al., 1996]. Geodetic measurements showa high maximumstrainrate throughoutthe regionborderedby the PortersPassfault zoneand its southwest extensiontoward the Alpine fault zone [Beavanand Haines, 2000; Pearsonet al., 1995]. The September1997 M•, 5.0 earthquakes and aftershocks occurredin thisregion(Figure16) east of the Alpine fault. Two focal mechanismsfrom the aftershocksindicate thrust and oblique slip mechanisms consistent with uplift of theSouthernAlps andsmall-scale strain partitioningasseenin theArthur'sPassearthquake. The southwest extension of the Porters Pass fault zone forms the southernedge of the triangularregion where seismicityis nearlyabsent(Figure11).The low-seismicity regioncorresponds to the positionof a majorchangein the geologyof the Southern Alps and a changeto relatively low topography. Moderately west dippingsequences of schistand semischist with abundant faults are dominant in the Mount Cook region, whereas equivalentsequencesto the north in the Whataroa-Wanganui Rivers area are flat lying and have relatively few faults. The changereflectsa major differencein the amountof shortening and style of uplift along the Alps (S. Cox, personal communication, 2000). The low-seismicity regionis includedwithinthe zoneof high shear-strain rate that extends from Arthur's Pass to Mount Cook [BeavanandHaines, 2000]. At the westernend of the Porters Passfault zone seismicityand the southernedge of the lowseismicitytriangle,thereis a saddlein the vorticity field, which couldbe the resultof additionalstrikeslip faultingreducingthe amountof convergence at that point (M. Henderson,personal communication, 2000), consistentwith the geology. However, smallrotationsof the strainfield neednot causelow seismicity. Reducedseismicitywould result from higher normal stress and/orfewerdistributed faultsalignedproperly,assuggested by geology,so that the regionbehavesin a moreblocklikemanner. Corresponding to the low-seismicity region,thereis a regionof NW-SE contraction nearandjust southof the WanganuiRiver thatdoesnot appearto be presentfarthersouth[Beavanet al., 1999]. The high contraction would increasethe normalstress in the samelocationat shallowdepth,were separatedby a few hours,and had magnitudesof aboutM•. 2.5-3. The waveforms andpolaritiesof all eventsare very similar,and showdistinctP andS arrivals.Thereforerockavalanches andglacierbreakup are unlikely sources.No man-madeactivity occursin this remote area of the SouthernAlps. The compositesolutionsfor these eventssuggest normalfaultingwith NW-SE orientedextensional axis. The T axesof theseearthquakeshave a large misfit to the regional stress tensor and were excluded from the stress inversion. Theoccurrence withinseveralhoursof eachotherandduring the early summersuggests that they couldbe hydrologically triggered. Severerains and flooding occurredin the Southern Alps and centralOtagoon December12 and 13, 1995. Some 102 mmof rainfell in a 32 hourperiodin Alexandra,thehighest sincerecordkeepingbeganin 1922.The communityof Wanaka experienced floodingandwascut off by slipsandrockhllsonall roads,includingthe Lindis Passto the northand the HaastPass to the west (Otago Daily Times, December14, 1999). The earthquakes beganduringthe latter part of the periodof peak rainfall and ended--12 hoursafter the rain ended. Wolf et aI. [1997]observed similarseriesof seismiceventsnearMt Ogden, Canada.They occurredduring summerand early fall months, when rain and glacial melt water levelswere high. Wolf et al. [1997] suggestthat pressurevariationsdue to increased pore fluidspossiblyinitiatedfailure. The epicentersof thesenear-surfaceearthquakesoccurin steepterrain on the northwestside of the Liebig Range,well abovethe MurchisonGlacierand River. In the Liebig Range, airphotos showsomelinearionson steepslopesthat appeartobe faultsand are also subparallelto bedding. There are numerous mapped fault segments,many of which appear to have Quaternaryactivity. The triggeredearthquakes may represent gravitational failureon poorlyorientedpreexistingfault surfaces thatareableto slipunderhighporepressure. Time-limited swarms are a feature of Southern Alps seismicityand a similar cluster occurredin this locationin January 1994[Eberhart-Phillips, 1995],whichwasalsoa period of heavy rain and flooding. While focal mechanisms are unavailablefor those earthquakes,they may also represent hydrolc•gica[ly triggeredgravitational failure. 6.5. From Mount Cook to Haast In the regionfromMountCook to Haastthe seismicity is comparatively lower and more broadlydistributed, including andfaultstrength on faultsandfractures in thatregion[Sibson, bands of seismicity 30 and80kmeastof theAlpinefault(Figure 1993],resultingin a lowerlevelof background seismicity thanin 5). Thisregionmaybemostcompatible withrelatively simple areasof decreasing normalstress [Abercrombie andMori, 1996]. models of a continent/continent collision zone[Koons, i990, The seismicactivity on the southwestextensionof the Porters Passfault zone,changesin geology,and variationsin the strain northof the Puysegur subduction zone.Numericalmodeling of field suggestthat development of secondary strikeslip fault thecollision zoneasa two-sided orogen witherosion onthesteep 1994]sinceit liessouthof the Marlborough faultsystem and zones,which will eventuallybecomesignificantfaults in the inboard wedgeanda gentleslopingsurface on theoutboard Marlboroughsystem,beginin this regionnorthof the zoneof wedge [Koons, 1990,1994]predicts thehighest strain rates (both maximumuplift. Thus this region may also be considered contractional androtational) adjacent totheplateboundary anda transitional to theMarlborough regionandnotsimplydisplaying broadhigherstrainzonein theoutboard region.At themain LEITNER ET AL.' A FOCUSEDLOOK AT THE ALPINE FAULT, NEW ZEALAND a) 221> 7'0.0 NZNSN(1990-1997) E 60.0 z .r'-I- 50.0 o 40.0 03 30.0 • 20.0 E lO.O o o.o -60 -40 -20 0 20 40 60 80 1 O0 120 140 Distanceto Alpinefault (kin) . 169" e_R' 170' - r-4k'o'- .a 171" -.' -44" ' o•' I ' , ,, ' ',,.Z./ o , i o• ",', •, / %",....-• [•, ' ' ( ¾• t• ',J ) o T2 o / ?".,.,._ )' • Oo •,/•. f • 4 •_ • o \ -- •, "• \. •\ okm o 169 ø 170' lO 2o ! 171' Figure17. Seismicity andfocalmechanisms fromMountCookto Jackson Bay.(a) Moment release peryear calculated forrectangular regions 130kmlongparallel totheAlpinefault(zonebetween arrows located onAlpine faultin Figure17b)and25 kmwidthperpendicular to it. Included arequality locations forM'L>3.0 earthquakes recorded by theNZNSNduring1990-1997 (Figure 3c).(b) Lo•,-er hemisphere projections of A andB quality (compressional quadrant black) andC quality (compressional quadrant darkgray)focalmechanisms. Mechanisms withlight•ay compressional quadrants arelower hemisphere projections ofthefocalmechanisms forthe1.998 M,, 5.4Alpine fault(R.E.Abercrombie, personal communication, 1999;Harvard CMTcatalog) andtheM,,6.ø_.. GodIcy River[Anderson ezaL,1993]earthquakes. Quality locations (seealsoFigure l lb) forearthquakes withhi/.> 3 recorded bytheNZNSN(open circles), SAPSE (solid circles), andLakePukaki (shaded circles) nc. tworks show thatseismicity clusters around some oftheNNE-SSW striking faults. Solidstarmarks Mount Cook(MC),open starsareshotlocations alongthetwotransects T1 andT2. front, havea high thrustcomponentand tend to be alignedwith parallel to the plate boundary[Koons,1990, 1994].The the NNE strike of mappedfaults (Figure l?b). The moment seismicity zoneat 30 km distance fromtheAlpinefaultis near releaserate, calculatedfrom NZNSN seismicitybetween 1990 the main divide and has oblique thrust and strike-slip and 199'7acrossthe region(Figure l?a), is highestat the Alpine mechanisms, whichmayberepresentative of thebackthrust zone. fault, decreasestoward the east,and showsanothersmall high at Thedistributed zone80-100kmfromtheAlpinefaultisnearthe 75-95 km distancefrom the Alpine fault. The relativechangeof outboard toe, whereobliquethrustingis expected.Focal moment release across the Southern Alps is similar to the divide,there is little contractionalstrain, and extensionoccurs mechanisms eastof theSouthern Alps,especially at thethrust numericalcalculatedstraincurve [Koons, 1994; Koonset al., 2216 LEITNER ET AL.: A FOCUSED LOOK AT THE ALPINE FAULT, NEW ZEALAND 168' 169' M>5 ML2-4 -44 Quality o Quality o o 4 o o œo.o •o oo ._ -45' -45' 168' 169' Figure 18. Seismicityand focalmechanisms southof JacksonBay; lower hemisphere projections of A quality (compressional quadrantblack)andB quality(compressional quadrant darkgray) focalmechanisms. Mechanisms with light gray compressional quadrants are lower hemisphere projectionsof focal mechanisms for M,, > 5.4 earthquakes derivedby body waveformmodeling[Andersoneta!., 1993; Doser et al., 1999; Harvard CMT catalog].NZNSN earthquakelocationsof ML > 3 (opencircles)areselectedusingthequalitycriteriadescribedin thetext (shownin Figure 1l a) for theregioneastof theAlpine fault.Offshore,all recordedearthquakes with ML > 3 are plotted. Thus location errors are relatively large, but the distributionshowsthat in this region seismicity occursxvestof the Alpine fault. Earthquakesrecordedby SAPSE are markedby solidcircles.MLFZ is Moonlight fault zone. Dashedline in the southmarksthe endof the studyregionwhere seismicityis artificially truncated. 1998] and measuredstrainratesacrossthe Alps (J. Beavan, personalcommunications, 20001but is 2-3 ordersof magnitude smaller than the predicted strain accumulation from plate convergence andGPS observations. The earthquakedataover the last 150 yearsshow that only a small fractionof the strain accumulationacrossthe Alpine fault has been releasedseismically.The Alpine fault appearsto be locked and has the potential to rupture in big earthquakesin agreementwith paleoseismicevidence for large earthquakes. Bern'anet aL [1999] modelGPSdatato showthat-60 % of plate motion is being stored as elastic strain in the vicinity of the Alpine fault. The broadregionof deformationeastof the Alpine fault is markedby earthquakes(Figure 171and is best modeled by numerousdistributedNNE trending reversefaults, scattered throughoutthe region with a locking depth of 12 kin. The distributedstrain acrossthe SouthernAlps is observedby the GPS networkas far as 80 km east from the fault (Figures l 1c, 11f, and l 1j). Bern'anet al. [1999] modelthe long wavelength displacementwith a NW-dipping shear zone slipping stably below 30 km depth, which allows the middle to lower crustto accom,qodatethe remaining-40 % non-Alpine fault plate motion as distributed•iscous deformationeast of the Alpine fault. The earthquakedepthsin this studyare most consistent with distributeddeformationbelow a 12 km thick brittle upper crust. 6.6. Southern Transition Zone Southof JacksonBay is the transitionalregion betweenthe Alpine fault and the Puysegursubductionzone. At the southern partof the Alpine fault,wherethe thrustcomponenton thefault tracedisappears, seismicity is observed on bothsidesof thefault, includingnumerousM > 5 events(FiguresI and 111. This contrasts sharplywith thecontinentalcollisionmodelof oblique slip andeasterndistributed deformation discussed in thesection 6.5. In this transitionzone, crustalshorteningis occurringon bothsidesof the Alpine fault,while the Alpine fault continues to accommodate a major portionof plate motion,evidenced b} palcoseismology [Sutherland andNorris, 1995]. With few exceptions thenodalplanesof thefocalmechanisms (Figure181 are not parallelin striketo the trendof local structures. They canbe modeledby a uniformstressfield.The deepearthquakes (Figure5) represent the northernedgeof the subducted Australian plate.Thereis a relativelyhighlevelof seismicity in thevicinityof thisedge,andthe focalmechanisms vary widely, presumablydue to small-scalecomplexities betweenthe Australianand Pacific plates. 7. Conclusions We calculated 130 focal mechanismswith a first motion and amplituderatio method;53 had well-constrained solutions. LEITNERET AL.: A FOCUSEDLOOKAT THE ALPINEFAULT,NEW ZEALAND 2217 mechanisms whichhavea strongthrustcomponent in the fault i HopeF I 100 km •rspassFZ normaldirection(Figure19). BetweenMountCook andHaast,thrustingoccurson NNE trending faultsorfoldsto a depthof upto 12 km.Southof Haast, deformationis distributedbetweenoffshoreand onshoreregions 36 mm/yr (Figure19). A cluster of hydrologicallytriggeredearthquakesthat occurred on a dayof historicrainfallandfloodingmayrepresent gravitational failureon preexisting faultsurfaces thatareableto slipunderhighporepressure. The stressfieldremainsuniformthroughout thestudyregion andthereforeindicates thatstrainpartitioningasobservedat the //f SanAndreas fault[Zobacket al., 1987]is notpresent nearthe Alpinefaultnorthof Jackson Bay. Evidencefromgeologyand GPSmeasurements indicates thatobliqueslipontheAlpinefault accommodates bothdip-slipandstrike-slipmotionof the plate motionvector(Figure19). It remainsuncertainif all large earthquake ruptures of theAlpinefaultwill demonstrate oblique Figure 19. Sketch mapsummarizing thedifferent tectonicslip. regions inthestudy area.Tothenorth isthetransition zone to theMarlborough faultsystem. Earthquakes south of theHope andthePorters Pass faultzonehavea highthrustcomponent and Acknowledgments. We thanktheparticipants of theSouthern Alps Passive SeismicExperiment andSouthIslandGeophysical Transect. Specialthanksto Tom McEvillyand RobertUhrhammer for the accommodate partof the dip-slipconvergence of the plate of theSAPSE experiment. MarkChadwick, KenGledhi!l, motion.Distributed deformation is observed on NNE-SSW organization trending thrust faults eastoftheAlpine fault,through thestrike slipGodley Riverearthquake eastof theAlpine fault,andon thrust faultsat thesouthwestern endof the Alpinefault. The andTerryWebbprovided datafromtheNewZealand Seismic Network and the Cassearthquake sequence. The manuscript benefitedfrom discussions withRichardNorris,PeterKoons,ChrisPearson, SimonCox, KelvinBerryman, andJohnBeavan. ReviewsfromMartinReyners, RussellRobinson, RobertYeats,Dick Walcott,BarryParsons, andan Alpine faultaccommodates bothfault-normal andfault-parallel platemotion. anonymous referee greatly improved thismanuscript. Thisresearch was supported by theNewZealand Foundation for Research Science and Technology andtheU.S.NSFContinental Dynamics program (EAR9418530)duringdatacollection. BeateLeitnerthanksGeoSphere Exploration Ltd.fortheirsupport during thefinalstages of thiswork. technique is fully exploitedsinceowing to relativelylow HelenAnderson wasfunded jointlyby OtagoUniversity, Dunedin, and magnitudes and averagestationdistances of 50 km, both theInstitute of Geological andNuclearSciences whileundertaking this PlotswerecreatedusingGMT software [WesselandSmith, amplitude ratiosandfirstmotions arenecessary to constrain the research. 1998].Institute ofGeologic andNuclear Sciences contribution 1930. focalmechanisms. Reliablemomenttensorsweredetermined for earthquakes with M,•,> 4 and demonstrate the magnitude thresholdof this method in the New Zealand environment. TheAlpinefault'smaximum seismogenic depthis-10-12 kin.Theseismicity rateon theAlpinefaultis comparable to locked sections of theSanAndreasfaultwith potentialfor large earthquakes on theAlpinefault. References Abercrombie, R.E., Near surfaceattenuationand site effectsfrom comparison ofsurface anddeepborehole recordings, Bull.Seismol. Soc.Am., 87, 731-744, 1997. Abercrombie, R.E.,andJ. Mori, Occurrence patterns of foreshocks to largeearthquakes inthewestern United States, Nature, 381,303-307, Themaximum depthof seismicity isfairlyuniform overlarge 1996. parts oftheSouthIsland.Locally,beneath thehighAlpsthebase Abercrombie, R.E.,T.H. Webb,R. Robinson, P.J.McGinty,P.J.Mori, oftheseismogenic zoneis possibly 3-4 km shallower. Thisis andJ.R. Beavan, The enigmaof theArthur'sPass,New Zealand, consistent withmeasured andmodeled highertemperature eastof earthquake, 1:Reconciling avariety ofdataforanunusual earthquake J. Geophys. Res.,105,16,119-16,137, 2000. theAlpinefaultsbutindicates thatthebrittle-ductile transition sequence, R.G.,andY. Shi,Newinsights totemperature andpressure beneath zone ismoreuniform thanpredicted by current thermal models. Allis, thecentral Southern Alps,NewZealand, N. Z. J. Geol.Geophys., 38, Thedecrease of seismic activitynorthof BruceBaycoincides 585-592, 1995. H., andT. Webb,NewZealandseismicity, patterns revealed withlargevariations in upflftratesat theAlpinefaultandthe Anderson, by the upgraded National Seismograph Network,N. Z. J. Geol. junction of theAlpinefaultwiththesouthwest extension of the Porters Passfaultzone. The lackof seismicityat andnearthe Geophys., 37,477-493,1994. Anderson, H., T. Webb,and J. Jackson,Focal mechanisms of large Alpine faultnorthof FranzJosefGlacieris offsetto thenorth earthquakes intheSouth Island of NewZealand: Implications forthe accommodation of Pacific-Australia plate motion, Geophys.J. Int., fromtheregionof highest upliftrateandtherefore notsimply 115, 1032-1054,1993. connected withelevated temperatures. It corresponds to a major T. McEvilly, F. Wu, and R. difference in theamount of shortening andstyleof upliftalong Anderson,H., D. Eberhart-Phillips, Uhrhammer, Southern Alpspassive seismicexperiment, Rep.97/21, theAlpsreflected bya change inthegeology andcoincides with Inst. of Geol. and Nucl. Sci., Lower Hurt, New Zealand, 1997. largefaultnormalcontraction. Amadottir,T., J. Beavan,and C. Pearson,Deformationassociated with the I8 June1994Arthur'sPassearthquake,New Zealand, N. Z. J. Earthquake activityalongthe southwest extension of the Geol.Geophys., 38, 553-558,1995. Porters Pass faultzonesuggests thatdevelopment of secondaryBannister, S.,D. Eberhart-Phillips, andB. Leitner,3-D velocitystructure, strike-slip faultzonesbeginsin thisregion~ At the southwest southernAlps region,from local earthquakeand active-source extensions of the PortersPassfault zone and the Hope fault we observe strike-slip mechanisms along thetrendof thefaultzones andcrustal deformation in theadjacent crustwithvariedfocal SAPSE/SIGHTdata,paperpresented at GeologicalSocietyof New Zealand and New Zealand GeophysicalSociety Joint Annual Conference, Geol.Soc.of N. Z., Christchurch, New Zealand,1998. 2218 LEITNER ET AL.: A FOCUSED LOOK AT THE ALPINE FAULT, NEW ZEALAND Batt,G., andJ. Braun,The tectonicevolutionof theSouthernAlps,New Zealand:Insightsfromfully thermallycoupleddynamicalmodelling, Geophys. J. Int., 136, 403-420, 1999. Beavan,J., andJ. Haines,Contemporary horizontalvelocityandstrain ratefieldsof thePacific-Australian plateboundaryzonethroughNew Zealand, J. Geophys. Res.,in press,2000. Beavan,J., D. Darby, G. Blick, C. Pearson,T. Arnadottir,R.I. Walcott, B. Parsons,and P. England,GPS measurements at the transition between continental collision and subduction on the Pacific- System description andfieldtrials,Bull.Seismol. Soc.Am.,81, 13801390, 1991. Gledhill,K., R. Robinson, T. Webb,R. Abercrombie, J. Beavan, J. Cousins, andD. Eberhart-Phillips, The Mw6.2 Cass,NewZealand earthquake of24November, 1995: Reverse faulting ina strike-slip region,N. Z. J. Geol.Geophys.,43, 255-269,2000. Haines, A.J.,A localmagnitude scaleforNewZealand earthquakes, Bull. Seismol.Soc.Am., 71, 275-294, 1981. Haines, A.J.,I.M. Cahaem, andD.E.Ware,Crustal seismicity nearLake Pukaki,SouthIsland,R. Soc.N. Z. Bull., 18, 87-94, 1979. AustralianPlateboundaryin New Zealand,EosTrans.AGU, 75(44), Fall Meet. Suppl.,F669, 1994. Harris,R.A.,S. Day, M. Steven,L. Yong-Gang, andV.J. E, Numerical Beavan,J., et al., Crustaldeformation during1994-1998dueto oblique simulations of an earthquake spontaneously propagating in fault continental collisionin thecentralSouthern Alps,New Zealand,and gouge, EosTrans.AGU, 75(44),FallMeetingSuppl.,F442,1994. implications for seismicpotentialof the Alpinefault, J. Geophys. Herrmann, R.B.,Computer programs inseismology, SaintLouis Univ., Res.,104, 25,233-25,255, 1999. Berryman, K.R., S. Beanland, A.F. Cooper,H.N. Cutten,R.J.Norris,and P.R. Wood,The Alpinefault,New Zealand:Variationin Quaternary structural styleandgeomorphic expression, in Major ActiveFaultsof the World;Resultsof IGCP Project206, editedby R.C. Buckman, and P.L. Hancock,pp. !26-163, EditriceI! Sedicesimo,Florence, Italy, 1992. Bibby, H.M., A.J. Haines, and R.I. Walcott, Geodeticstrain and the presentdayplateboundary zonethroughNew Zealand,Bull. R. Soc. N. Z., 24, 427-438, 1986. Bourne,S.J.,P.C. England,andB. Parsons, The motionof crustalblocks drivenby flow of thelowerlithosphere andimplications for sliprates of continental strike-slipfaults,Nature,391,655-659, 1998a. Bourne, S. J.,T. Arnatoddir, J. Beavan,D. J. Darby,P. C. England, B. Parsons,R. I. Walcott and P. R. Wood, Crustaldeformationof the Marlboroughfault zone in the SouthIslandof New Zeland: Geodetic constraints overthe 1982-1994,J. Geophys. Res.,103, 30,147-30,166, Saint Louis, Mo., 1991. Herzer,R.H.,andJ.D. Bradshaw, The Motunaufaultandotherstructures atthesouthern edgeoftheAustralian-Pacific plateboundary, offshore NewZealand:Discussions, Tectonophysics, 115, 161-164,1985. Holbrook, W.S.,D. Okaya,T. Stern,F. Davey,S. Henrys,andH. Van Avendonk, Deepseismic profiles across thePacific-Australian plate bounday, South Island, NewZealand, EosTrans. AGU,79(44),Fall Meet. Suppl, F901, 1998. Holm,D.K., R.J.Norris,andD. Craw,Brittleandductiledeformation in a zoneof rapid uplift:CentralSouthern Alps, New Zealand, Tectonics,8, 153-168, 1989. Holt,W.E.,andH.J.Haines, Thekinematics of northern South Island, NewZealand, determined fromgeologic strain rates,J. Geophys. Res., 100, 17,991-18,010, 1995. Hull,A.G.,andK.R.Berryman, Holocene tectonism in theregion ofthe AlpineFaultat LakeMcKerrow,Fiordland, NewZealand,in Recent Crustal Movements ofthePacificRegion., edited byW.I. Reillyand B.E.Harford, pp.317-331,R. Soc.of N. Z., Wellington, 1986. Bull, W.B., andM.T. Brandon,Lichendatingof earthquake-generatedJenkins, G.R.T.,D. Craw,andA.E. Fallick,Stableisotopic andfluid regionalrockfallevents,Southern Alps,New Zealand,Geol.Soc.Am. inclusionevidencefor meteoricfluid penetration into an active Bull., 110, 60-84, 1998. mountain belt:AlpineSchist, NewZealand, J. Metamorph. Geol.,12, 1998b. Carter, R.M., and L. Carter, The Motunau fault and other structuresat the southernedge of the AustralianPacific plate boundary,offshore Marlborough, New Zealand,Tectonophysics, 88, 133-159,1982. Cooper,A.F., and R.J. Norris, Estimatesfor the timing of the late coseismic displacement ontheAlpinefault,northernFiordland, New Zealand,N. Z. J. Geol.Geophys., 33, 303-307,1990. Cooper,A.F., and R.J. Norris,Anatomy,structural evolution,andslip rate of a plate-boundary thrust:The Alpine Fault at GauntCreek, Westland,New Zealand,Geol. Soc.Am. Bull., 106, 627-633, 1994. Cowan,H.A., A. Nicol, andP. Tonkin,A comparison of historical and paleoseismicity in a newlyformedfault zoneanda maturefaultzone, NorthCanterbury, New Zealand,J. Geophys. Res.,101, 6021-6036, 1996. Cox, S.C., andR.H. Findlay,The maindividefaultzoneandits role in formationof the SouthernAlps, New Zeal.and,N. Z. J. Geol. Geophys., 38, 489-499, 1995. Craw,D., Shallow-level metamorphic fluidsin a highratemetamorphic belt;AlpineSchist,New Zealand,J. Metamorph. Geol.,6, 1-16, 1988. Craw, D., M.S. Rattenbury,and R.D. Johnstone,Structurewithin Greenschist faciesAlpine Schist,CentralSouthernAlps, N. Z. J. Geol.Geophys.,37,101-111,1994. DeMets,C., R.G. Gordon,D.F. Argus,andS. Stein,The effectof recent revisionsof the geomagnetic reversaltime scale on estimatesof currentplatemotions,Geophys. Res.Lett.,21, 2191-2194,1994. Doser,D.I., T. Webb,andD. E. Maunder,Sourceparameters of large 429-444, 1994. Jones,L.M., Focalmechanisms andthe stateof stresson the SanAndreas Faultin southern California,J. Geophys. Res.,93, 8869-8891,1988. Kamp,P.J.,andJ.M. Tippett,Dynamicsof PacificPlate crustin the SouthIsland(New Zealand)zoneof obliquecontinent-continent convergence, J. Geophys. Res.,98, 16,105-16,118,1993. Kissling,E., W.L. Ellsworth, D.Eberhart-Phillips, and U. Kradolfer, Initialreference models inseismic tomography, J. Geophys. Res., 99, 19,635-19,646, 1994. Kleffmann, S., F. Davey,A. Melhuish, D. Okaya,andT. Stern,Crustal structurein the central South Island from the Lake Pukaki seismic experiment, N. Z.J. Geol.Geophys., 41, 39-49,1998a. Kleffmann, S., T. Stern,S. Henrys,andSIGHTworkinggroup,Low seismic P wavevelocities at theplateboundary: Implications for physical properties withintheAlpinefaultzone,paperpresented at Geological Societyof NewZealand andNewZealandGeophysical SocietyJointAnnualConference, Geol.Soc.of N. Z., Christchurch, New Zealand, 1998b. Koons, P.O.,Erosional controls onthemechanics ofdeforming wedges; an example fromtheSouthern Alps,in Geological SocietyofNew Zealand AnnualConference: Programme andAbstracts., edited by R.E.Fordyce, p. 69, Geol.Soc.of N. Z., Christchurch, NewZealand, 1987a. Koons, P.O.,Somethermal andmechanical consequences ofrapiduplift: Anexample fromtheSouthern Alps,NewZealand,Earth.Planet.Sci. Lett., 86, 307-319, 1987b. historical(1918-1962) earthquakes,South Island, New Zealand, Geophys. J. Int., 139, 769-794, 1999. Koons, P.O.,Two-sided orogen: Collision anderosion fromthesandbox totheSouthern Alps,NewZealand, Geology, 18,679-682,1990. Dreger,D.S.,andD.V. Helmberger, Determination of source parameters Koons,P.O., Three-dimensional critical wedges:Tectonicsand at regionaldistances with three-component sparsenetworkdata, J topography in obliquecollisional orogens, J. Geophys. Res.,99, Geophys.Res.,98, 8107-8125, 1993. Eberhart-Phillips, D., Examination of seismicity in the centralAlpine faultregion,SouthIsland,NewZealand,N. Z. J. Geol.Geophys., 38, 571-578, 1995. Ellsworth,W.L., Three-dimensional structure of the crustandupper mantle beneaththe island of Hawaii, Ph.D. thesis, Mass. lnst. of Technol.,Cambridge,1977. Fan,G.W.,andT.C. Wallace,Thedetermination ofsource parameters for small earthquakes from a single,very broadband seismicstation, Geophys. Res.Lett., 18, 1385-1388,1991. 12,301-12,315, 1994. Koons,P.O.,R.J.Norris.A.F. Cooper,andD. Craw,Mechanics of a single structureaccommodating obliqueconvergence: The development of theAlpinefault,paperpresented at NewZealand Geological Society andGeophysical Society of NewZealand Joint AnnualConference, Geol.Soc.of N. Z., Christchurch, NewZealand, 1998. Krodolfer, U., Seismische Tomographie in derSchweizmittels1okaler Erdbeben, Eidgenoessische Tech.Hochschule, Ph.D.thesis,Zurich, Switzerland,1989. Gledhill,K.R., and M. Chadwick,The EARSSdigitalseismograph: Lahr,J.C.,Hypoellipse: A computer program for determining local LEITNER ET AL.: A FOCUSEDLOOKAT THE ALPINE FAULT, NEW ZEALAND earthquake hypocentral parameters, magnitude, and first motion pattern (Y2Kcompliant version), U.S.Geol.Surv.Open-File Rep., 99-23,116pp.,1999. Lamb, S.,andH.M. Bibby,Thelast25 Ma of rotational deformation in part oftheNewZealand plate boundary, J.Struct. Geol., ll, 473-492, 1989. Leitner, B., Plateboundary deformation of the PacificPlate:Two case studies. (1) Crustalstructure of thenorthwestern VizcainoBlockand Gorda Escarpment, offshorenorthern California,andimplications for postsubduction deformation of a paleoaccretionary margin. (2) A focused look at the Alpine fault New Zealand:Seismicity,Focal mechanisms andstress observations, Ph.D.thesis,154pp.,Collegeof Oceanic andAtmos.Sci.,Oreg.StateUniv.,Corvallis,1999. Michael, A.J.,Determination of stressfromslipdata:Faultsandfolds,J. Geophys. Res.,89, 11,517-11,526, 1984. Michael, A.J.,StressrotationduringtheCoalingaaftershock sequence,J. Geophys. Res.,92,7963-7979, 1987a. Michael, A.J., Use of focal mechanisms to determinestress:A control study, J. Geophys. Res.,92,357-368,1987b. Michael,A.J., Spatial variationsin stresswithin the 1987 Whittier Narrows, California,aftershock sequence: New techniqueandresults, J. Geophys. Res.,96, 6303-6319,1991. Moore,M.A., Crustaldeformation in the southernNew Zealandregion, Ph.D.thesis,200 pp., Fac. of Phys.Sci., Univ. of Oxford,Oxford, England, 1999. Nabelek, J., and G. Xia, Moment tensoranalysisusingregionaldata: 2219 the regionalinduced stressfields,J. Geophys. Res.,105, 16,13916,150, 2000. Robinson, R., andT. Webb, "Amprat"and "Mechtool":Programsfor determiningfocal mechanismof local earthquakes, rep. 7, Inst. of Geol. and Nucl. Sci., Lower Hutt, New Zealand, 1996. Robinson, R., M. Reyners,T. Webb,T. Anadottir,J. Beavan,J. Cousins, R.V. Dissen,andC. Pearson, The M,• 6.7 Arthur'sPassearthquake in the SouthernAlps, New Zealand,June18, 1994, Seismol.Res.Lett., 66, 11-12, 1994. Roecker,S.W., Seismicity andtectonics of the Pamir-HinduKushregion of centralAsia, Ph.D. thesis,Mass. Inst. of Technol., Cambridge, 1981. Rynn, J.M.W., and C.H. Scholz, Seismotectonics of the Arthun Pass region,SouthIsland,New Zealand,Geol. Soc.Am., 89, 1373-1388, 1978. Scholz,C.H., TheMechanicsof Earthquakes and Faulting,Cambridge Univ. Press,New York, 1990. Scholz, CM., J.M.W. Rynn, R. Weed, and C. Frohlich, Detailed seismicityof theAlpinefaultzoneandFiordland,SouthIsland,New Zealand,in Geophysics of theEarthand theOceans,part IIL pp. 199200, Elsevier Sci., New York, 1974. Schwartz, S.Y., Sourceparameters of aftershocks of the 1991CostaRica and1992CapeMendocino,California,earthquakes frominversionof localamplituderatiosandbroadband waveforms,Bull Seismol.Soc. Am., 85, 1560-1575, 1995. Shi,Y., R. Allis, andF. Davey,Thermalmodelingof the SouthernAlps Application tothe25 March,1993,ScottsMills,Oregon earthquake, New Zealand,PureAppl. Geophys., 246, 469-501,1996. Geophys. Res.Lett.,22, 13-16,1995. Sibson,R.H., Loadstrengthening versusload weakeningfaulting, J. Norris,R.J.,and A.F. Cooper,Originof small-scalesegmentation and Struct. Geol., 15, 123-128, 1993. transpressional thrusting alongtheAlpineFault,New Zealand,Geol. Simpson,G.D.H., A.F. Cooper,and R.J. Norris,Quaternary-Holocene Soc.Am. Bull,, 107, 231-240, 1995. evolutionof the Alpinefault zoneat Pafinga,SouthWestland,New Norris,R.J.,andA.F. Cooper,Structure,kinematicsandpalcoseismicity Zealand, in Geological Society of New Zealand 1993 Annual of the central and southernsectionsof the Alpine fault, paper Conference: Programme andAbstracts.,editedby P. Froggatt,p. 134, presented at Geological Societyof New ZelandandNew Zealand Geophysical Society,JointAnnualConference, Geol.Soc.of N. Z., Geol. Soc. of N. Z., Christchurch,New Zealand, 1993. Simpson,G.D.H., A.F. Cooper, and R.J. Norris, Late Quaternary Christchurch, New Zealand, 1998. evolutionof theAlpine fault zoneat Paringa,SouthWestland,New Zealand,N. Z. J. Geol.Geophys., 37, 49-58, 1994. Norris,R.J., P.O. Koons,and A.F. Cooper,The obliquely-convergent Stein, R.S., and R.S. Yeats, Hiddenearthquakes,Sci. Am., 6, 48-57, plateboundary in theSouthIslandof New Zealand:Implications for 1989. ancientcollisionzones,in Australasiantectonics,editedby J.P. Platt Stem, T.A., P.E. Wannamaker,D. Eberhart-Phillips,D. Okaya, F.J. etal.,pp.715-725,Pergamon, Tarrytown,N.Y., 1990. Davey,andSouthIslandProjectWorkingGroup,Mountainbuilding Pearson, C.F.,J. Beavan,D.J.Darby,G.H. Blick,andR.I. Walcott,Strain and active deformationstudiedin New Zealand, Eos Trans.AGU, 78 distribution across theAustralian-Pacific plateboundary in thecentral (32), 329, 335-336,1997. SouthIsland,New Zealand,from 1992 GPS and earlierterrestrial Sutherland,R., and R.J. Norris, Late Quaternarydisplacementrate, observations, J. Geophys. Res.,200,22,071-22,081, 1995. palcoseismicity,and geomorphicevolution of the Alpine fault: Pearson, C., P. Denys,andK. Hodgkinson, Geodeticconstraints on the Evidencefrom Hokuri Creek, South Westland, New Zealand, N. Z. J. kinematics of the Alpinefault in thesouthern SouthIslandof New Geol.Geophys., 38, 419-430,1995. Zealand, usingresultsfromtheHawea-Haast GPStransect, Geophys. Tippett,J.M., andP.J.J.Kamp,Fissiontrackanalysisof thelate Cenozoic Res.Lett., 27, 1329-1322, 2000. Rattenbury, M.S.,Latelow-angle thrusting andtheAlpinefault,central Westland, New Zealand,N. Z. J. Geol.Geophys., 29, 437-446,1986. Reyners, M., Subcrustal earthquakes in the centralSouthIsland,New Zealand, andtherootof the Southern Alps, Geology,25, 1168-1171, 1987. Reyners, M., Reservoir induced seismicity at LakePukaki,NewZealand, Geophys. J. Int., 93, 127-135,1988. Reyners, M., NewZealand seismicity 1964-87: Aninterpretation, N. Z.J. Geol.Geophys., 32, 307-315,!989. Reyners, M., and H. Cowan, The transitionfrom subduerionto continental collision:Crustalstructure in theNorthCanterbury region, vertical kinematics of continental Pacific crust, South Island, New Zealand,J. Geophys. Res.,98, 16,119-16,148,1993. Van Dissen,R., andR.S. Yeats,HopeFault,JordanThrust,anduplift of the SeawardKaikouraRange,New Zealand, Geology,!9, 393-396, 1991. Walcott, R.I., Presenttectonicsand Late Cenozoic evolution of New Zealand,Geophys.J. Roy.Astronom.Soc.,52, 137-164,1978. Walcott,R.I., Modesof obliquecompression: Late Cenozoictectonicsof the SouthIslandof New Zealand,Rev. Geophys.,36, 1-26, 1998. Wellman,H.W., An uplift mapof the SouthIslandof New Zealand,and a modelfor upliftof the SouthernAlps, R. Soc. of N. Zeal. Bull., 18, 13-20, 1979. NewZealand, Geophys. J. Int.,115,1124-1136, 1993. Reyners, M., andP. McGinty,Shallowsubduction tectonics in the Wessel,P., and W. H. F. Smith, New improvedversionof generic mappingtoolsreleased, Eos Trans.AGU, 79 (47), 579, 1998. Raumakura Peninsula, New Zealand,as illuminatedby earthquake Wolf, L.W., C.A. Rowe,and R.B. Homer, PeriodicseismicitynearMt. Ogden on the Alaska-BritishColumbia border: A case for focalmechanisms, J. Geophys. Res.,204,3025-3034, 1999. hydrologically triggeredearthquakes?, Bull. Seismol.Soc.Am., 87, Reyners, M.,C.E.Ingham, andB.G.Ferris, Microseismicity oftheupper Clutha Valley,SouthIsland,NewZealand,N. Z. J. Geol.Geophys., 26, 1-6, 1983. Reyners, M., R. Robinson, andP. McGinty, Platecoupling in the northern SouthIslandandsouthernmost NorthIsland,New Zealand, asilluminated by earthquake focalmechanisms, J. Geophys. Res., I02, 15,197-15,210, 1997. Ritsema, J.,andT. Lay,Rapid source mechanism determination oflarge (M•> 5) earthquakes in thewestern UnitedStates, Geophys. Res. Lett.,20,1611-1614, 1993. Robinson, R.,andP.McGinty, Theenigma oftheArthur's Pass, New Zealand, earthquake, 2:Theaftershock distribution anditsrelation to 1473-1483, 1997. Wright,C., R.J. Norris,and A.F. Cooper,Paleoseismic historyof the centralAlpine fault, paperpresented at GeologicalSocietyof New Zealand and New Zealand GeophysicalSociety Joint Annual Conference, p. 249, Geological Society of New Zealand, Christchurch,New Zealand, 1998. Yeats,R.S.,Tectonicmapof centralOtagobasedonsatelliteimagery,.hr. Z. J. Geol. Geophys., 30, 261-271, 1987. Yeats, R.S., and K.R. Berryman, South Island, New Zealand, and Transverse Ranges, California; A seismotectoniccomparison, Tectonics,6, 363-376, 1987. 2220 LEITNER ET AL.: A FOCUSEDLOOKAT THE ALPINEFAULT, NEW ZEALAND Yeats, R., K. Sieh, and C. R. Allen, EarthquakeGeology,568 pp., OxfordUniv. Press,New York, 1997. Yetton,M.D., and A. Wells, Paleoseismic historyof the cemraland northern Alpinefault,paperpresented at Geological Societyof New Zealandand New Zealand GeophysicalSociety,Joint Annual Conference, Geol.Soc.of N. Z., Christchurch, NewZealand,1998. H. Anderson, Ministryof Research, Science,andTechnology, P.O. Box 5336, Wellington,New Zealand. D. Eberhart-Phillips, Instituteof GeologicalandNuclearSciences, DunedinResearchCenter,PrivateBag 1930, Dunedin,New Zealand. (d.eberhart-phillips@gns.cri.nz) B. Leitner,GeoSphere Exploration Ltd.,P.O.Box44285,LowerHutt, NewZealand.(beate@geosphere.co.nz) J. Nabelek,Collegeof OceanicandAtmospheric Sciences, Oregon State University, Adm. Bldg. (jnabelek@oce.orst. edu) 104, Corvallis, OR (Received November 4, 1999;revised May25,2000; accepted August4, 2000.) 97331.

A focused look at the Alpine fault, New Zealand: Seismicity, focal mechanisms,

Related documents

Products

Support

A focused look at the Alpine fault, New Zealand: Seismicity, focal mechanisms,

Related documents

Add this document to collection(s)

Add this document to saved

Suggest us how to improve StudyLib