Influence of depth, focal mechanism, and ... - Wiley Online Library

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106,NO. B6, PAGES 11,137-11,150, JUNE 10, 2001

Influence of depth, focal mechanism, and tectonic setting on the shapeand duration of earthquake sourcetime functions Heidi Houston Departmentof Earth andSpaceSciences,Universityof California,Los Angeles,California

Abstract. Sourcetime functionsof 255 moderateto greatearthquakesobtainedfrom inversionsof teleseismicbodywavesby Taniokaand Ruff[ 1997] andcoworkerswere comparedin a systematicway. They were scaledto removethe effect of momentandto allow the directcomparisonandaveragingof time functionshapeaswell as duration. Time functiondurationspickedby TaniokaandRuff[ 1997] areproportionalto the cuberoot of seismicmomentif momentsfromthe Harvardcentroidmomenttensorcatalogare used. The

averagedurationof scaledtime functionsis shorterandthe averageshapehasa moreabrupt terminationfor deepereventsthanshallowerones,with a distinctchangeoccurringat-40 km depth. The complexityof the time functions,asquantifiedby the numberof subevents, appearsto decreasebelow-40 km depth. Furthermore,amongeventsshallowerthan 40 km, the averagedurationof scaledtime functionsis shorter,andtheir averageshapehasa more abrupttermination(1) for eventswith strike-slipfocalmechanisms comparedto thrustevents and(2) for the few thrusteventsassociated with an intraplatesettingcomparedto the majority associatedwith an interplate(subduction)boundary. In eachof thesecases,events in moretectonicallyandseismicallyactivesettingshavea longerdurationanda more gradual termination.This canbe interpretedin termsof lower stressdropsand/orslowerrupture velocitiesat activeplateboundaries,suggesting that fault rheologydependson slip rate and may evolveastotal fault slip accumulates.Furthermore,differencesin averagetime function shapeanddurationassociated with differentsubduction zonessuggestthat differencesexist in the rheologyon the plateboundariesat the varioussubductionzones.

1. Introduction

scalingproceduredevelopedby Houstonet al. [1998] that removesthe effect of earthquakesize so that all the time The manner in which earthquakeruptures initiate, functions can be plotted on the same scale. The time propagateand terminateis centralto the processof seismic functionscanthen be groupedin varioussetsand averaged faulting. A primary avenue of investigationis through to investigate how their shapes may vary with depth, earthquake source time functions, which have been region,andearthquaketype. extensively studied for several decades(see Kanamori [1986] for a review). An earthquakesourcetime function is a time series that specifies the rate of the seismic 2. Catalog of Source Time Functions momentreleaseduring an earthquake. The time function Taniokaand Ruff [ 1997] (hereinafterreferredto as TR) is proportionalto slip, spatially integratedover the fault andcoworkersin the Universityof Michiganseismology plane, as a function of time; ideally, one would like to group determined time functions by inversion of know the completespatiotemporalhistory of slip at every teleseismicallyrecordedbody waves recordedby the point on the fault plane, but this is much harderto resolve Global SeismicNetwork (see TR and Ruff and Miller from seismograms. [1994]for details).The bodywaves(mostlyP waves)are Recently, a catalogcontainingsourcetime functionsof invertedtogetherfor the best fitting point sourcetime more than 250 earthquakeshas been available over the function(for a few very large events,a line sourcewas Internet. Somebasicpropertiesof the catalog(as of June assumed). The procedureinvolvesvaryingthe assumed 1996 when it contained99 events),includingthe moment depthandchoosingas the bestdepththatwhichminimizes dependenceof duration and peak moment release rate,

the misfit betweenobservedand syntheticseismograms.

were reported byTanioka and Ruff [1997]. Here I analyze TRconsider that shallower than 100km,their depths are thenowlargercatalog (255events available as of betterresolved thantheautomated inversions of the December 2000) toreexamine some oftheir conclusions Harvard centroid moment tensor (CMT) catalog, theU.S. and to extractmore informationaboutsourcescalingand the shapeof time functions. Part of the studyemploysa Copyright 2001bytheAmerican Geophysical Union. Papernumber2000JB900468. 0148-0227/01/2000JB 900468 $09.00.

Geological Survey moment tensor catalog, and the Earthquake ResearchInstituteCMT catalog,becauseTR's inversionutilizes higher-frequency data and carefully examinesthe effect of varyingthe depth. However,for events deeper than 100 km, TR depths commonly underestimate the HarvardCMT depths,oftenby 50 to 75 km. This occursbecauseTR's parameterization of the P

11,137

11,138

HOUSTON: SHAPE AND DURATION OF EARTHQUAKE TIME FUNCTIONS

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Figure 1. Locationsof all earthquakes in this studygroupedby depth.Circlesrepresenteventswith depths 6.9 for 1996 and 1997. and submittingelectronicsupplements is found at http://

About half of the eventsare smallerthan Mw 6.8.

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HOUSTON: SHAPE AND DURATION OF EARTHQUAKE TIME FUNCTIONS

11,139

bodywavemoments > 8xl019N m). Figure4b uses•the 8.3

Harvard CMT moments,which are likely more reliable since they are determinedfrom longer-perioddata; here the dependenceof durationon momentis much closerto

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thetheoretically expected exponent of 1/3. Whenonlythe eventswith Mw > 6.2 are included,which are likely better

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resolved, the durations scaleasMoø'33, thetheoretically

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reasonableand conservativeapproachis to use the theoretically expected, traditionalmoment. scaling.To do this,it is necessary to associate eachtim• functionWithits

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To scale thetimefunctions to a Common moment, the

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time axis and the amplitude are simultaneouslyadjusted, so that the areaunderthe re•ultingschledtime funi:tionis

value isthereference value More3 herechosen tobelx10•9

expectedvalue (Figure 4c). Thus it seemsthat the most

Harvard CMT moment and scale the time fdnctions so that

thetotalmoment ineachequals theCMTmoment. 3.2 Moment Scaling and Duration Scaling

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reduced or increased to the same value foi' all events. This

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N m, equivalentto Mw = 6.6. The time axisis dividedby

(Moi) u3,Where Moi is the moment of the ith event. To preservethe relationbetweenmomentand area underthe

timefunction, amplitudes must bedivided by(Mo•) 2/3.This scalingprocedurecanbe generalizedin severalways,most 50

Time (s) Figure2. Comparison of source timefunctions fromthe TR catalog(thick lines) with broadbandstacksfrom Houstonet al. [1998] (thin lines) for the 11 eventsin common.Mw, depth,anddateare givenfor eachevent. The March 9, 1994, Tongaearthquake had a changein

simplyby dividingthe time axis by (Mo•) x and the amplitudes by (Mo')l'x, wherex is closeto, but not necessarilyequal to, 1/3. A slightly more complicated procedureis given by TR in a sectionon "genericsource

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focal mechanismthat is evident in the broadbandstack.

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ill

Tanioka and RuffIhmle and Jordan

- - - McGuire et al.

compareshapesof time functionsfrom earthquakes of widelyvaryingsizes,Houstonet al. [1998] developeda methodfor scalingtime functionsto removethe effectof

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moment.

3.1 Dependence of Durationon SeismicMoment The scalingprocedure requiresassuming how duration dependson seismicmoment. If rupturevelocity,stress drop,andfaultplaneaspect ratioareroughlyconstant with

moment, thenthe definition of momentimpliesthat duration scales as the cube root of moment; this has been

foundempiricallyby severalstudies[e.g.,Kanarnoriand Anderson,1975; Furumoto and Nakanishi, 1983; Vidale and Houston, 1993; Abercrombie, 1995]. However, TR

regressed durations,whichwere estimatedsubjectively from the time functions,againstthe body wave moments 0

computed fromthe time functions, andobtainedduration

,

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scaling asMoø'4•.Figure4a shows durations versus body

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, I,,, 70

wave momentsfor all eventsin this studywith depths_6.2 according to focal mechanism.Circles,squares,and trianglesrepresenteventswith thrust,normal, and strike-slipfocal mechanisms, respectively.

strike,dip, and rake, which are not orthogonal,becauseit slip (8.6 s) and 34 thrustevents(10.8 s)); even for events allows a symmetrictreatmentof the three types of focal with Mw _>7.2, the averagescaleddurationof the 10 strikemechanisms[e.g., see Frohlich and Apperson, 1992; slip events (9.5 s) remains shorter than that of the 23 Kaverina et al., 1996]. Here thrust events are defined as correspondingly largethrustevents(10.9 s). thosefor which the T axis plungesmore steeplythan 45ø; It shouldbe noted that scalingof strike-slipruptures normal eventsthose for which the P axis plunges>45ø; doesnot appearbe entirelywell understood; for example, and strike-slipeventsare thosewith the N axis plunging the strike-sliprupturelengthsversusmomentsin Figure lb >45 ø. It is possible for focal mechanismsto fall into of Peglet and Das [1996], are betterfit by a slopeof 1/3 neither the thrust, normal, nor strike-slip category if all ratherthan the expectedslopeof 1/2 or 1, suggesting that three axes plunge more shallowly than 45ø (e.g., they fault width may continueto grow somewhatwith moment. could all have the sameplungeof 35.26ø); sucheventsare My preferredexplanationof the durationsin Figure 12 consideredoblique. Only one of the 143 shallow events is that strike-slipeventsmay tend to have larger stress was oblique. drops and/or rupture velocities than thrust events; this The shapesand durations of the average scaledtime would lead to shorter durations. Globally, most functionsof thrust and normal earthquakesappearto be earthquakesare thrust events and occur in subduction somewhatdifferent from that of the strike-slip events zones. Thus thrustearthquakestend to be closelyrelated (Figure 12). The reasons for this difference are not to the most active plate boundaries. Shallow thrust entirely clear; severalcompetingfactorsmay be at work. earthquakesoccurpreferentiallyon the more active, wellScalingof the lengthof strike-slipversusdip-slipruptures developed large-offset faults associatedwith subduction has been a topic of continuing discussion, as yet zones. This may promote longer duration ruptures. It unresolved [e.g., Scholz, 1982; Scholz et al., 1986; seemsplausiblethat the large relative plate motionsand Shirnazaki, 1986; Rornanowicz, 1992; Scholz, 1994a; abundantwater in subductionzonescould give rise to a Rornanowicz,1994; Scholz, 1994b;Pegler and Das, 1996]. weaker rheology, resulting in lower stress drop Strike-sliprupturesare believedto be limited in width by earthquakes,which would have longer durations for a the thicknessof the crustandpresumablycangrow only in given moment. Rupturevelocitiesmight also be reduced length and (disputably)slip as their moment increases; in sucha setting. Thus a plausiblereasonfor the longer thus, if other factorssuch as rupturevelocity and stress scaledtime functionsfor thrust-typemechanismsversus drop are similar for strike-slipand dip-slip ruptures,one strike-slip-type mechanismsis the close associationof would expect strike-slip eventswith M•v above-6.6 to thrusteventswith the mostactive(and submerged)seismic have longerdurationsfor a given moment. However, for plate boundaries.This explanationwould not accountfor this dataset (Figure 12) the strike-slipeventshave shorter the similar long durationof the normalmechanismevents, durationsfor a givenmoment;the averagescaledduration whichconstitute,however,the smallestgroup. of the 46 strike-slipevents(9.2 s) is 2.4 s shorterthanthat It is also possiblethat large strike-slipeventsare more of the 80 thrustevents(11.6 s). The situationis similarfor likely than dip-slip eventsto rupturebilaterally;ruptureof the subset of eventswithMw _>7.0 (containing 14 strike- a long, narrow strip may be more easily impededthan

11,146

HOUSTON: SHAPE AND DURATION OF EARTHQUAKE TIME FUNCTIONS

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pulses of moment release. This again points up the difficultyof definingthe terminationof rupture.

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and Allen, 1986; Scholz et al., 1986; Houston, 1990; Green and Houston, 1995]. Further,the interplatesubuctionzone eventsstudiedhere occur on or near plate boundariesthat of water.

c)

-

theywereconsidered plateboundaryevents,and 11 others, considerednon-plate-boundaryevents (Figures 13a and 13b). The plate boundary events were required to lie within 100 km of an active subductionzone (with the downgoingplate oceanic)and to possessa CMT focal mechanismconsistentwith underthrustingon that zone; they are not necessarilyexactly on the plate interface, which is difficult to determinepreciselyin thesetypesof studies. The plate boundaryeventstypicallyhave longer scaled durations than the non-plate-boundary events (Figures13a- 13d). A plausiblereasonfor the differenceinvolvesthe notion discussedabove that interplate eventshave lower stress dropsand longerdurationsthan intraplateevents. Several studieshave presentedevidencein supportof a distinction betweeninterplateand intraplatefaulting [e.g.,Kanamori

are deformingrapidly and in the presenceof largeamounts

Time (s)

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The 80 shallow thrust events(with 5 km 5 depth 5 40 km and Mw >_6.2) were further divided into 69 events

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4.4 Variations Between Interplate and lntraplate

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The plate boundary events were further divided into specificsubductionzones,includingCentralAmerica, the Aleutians, the Kuriles (including Kamchatka), Vanuatu, and Tonga-Kermadec.Althoughthe numbersof eventsin each subductionzone are small, there appearsto be a differencein the shapeand durationof the averagescaled time functionsin variouszones(Figures13e and 13f). It is interestingto comparethe Central American and Tongan zones. Althoughthe averageCentralAmericandurationis much longer,the averageCentral Americantime function has lower skewnessthan that of the Tongantime function. If the secondversionof momentscalingis applied(scaling

timeby M0ø'41andamplitude by M0ø'59usingthe TR

moments), then the average Central American time function is no longer so distinct from the others,but the Figure 12. Averagescaledtime functionsof shallow Tonga time function is little changed and remains the

CMT Moment (le19 Nm)

events(depth< 40 km) according to focalmechanism.(a)

shortest.

Moment-scaled time functions of events with Mw > 6.2.

Time functions from the Vanuatu

The relative

skewnesses remain

about the same.

zone contrast with those

(b) Duration-scaled time functionswith Mw > 6.2. (c) from CentralAmerica in a differentway and appearto be Opencircles,crosses,and solidtrianglesrepresentevents the most complex. Although not shown,time functions

with thrust, normal, and strike-slip focal mechanisms,

respectively.Smallersymbolsrepresent eventswith Mw 6.2. Scaleddurationsdecreasewith depthby a factorof about 1.5, in contrastto a largerchangeseenby Bilek and Lay [1999]. (a) Scaleddurationsversusdepth. (b) Average moment-scaledtime functions grouped by depth. (c) Averageduration-scaled timefunctions grouped by depth.

samplingrate is once per secondso someof their smaller events are said to terminate after only two or three samples. Figure 15 comparesthe TR time functionswith the durationsdeterminedby Bilek and Lay [1998, 1999] for all eventsin common. In general,the TR durations

(vertical bars in Figure 15) last significantlylonger; furthermore, for thetwo largestearthquakes, it appears that differencebetweenthe averagetime functionsfor the two Bilek and Lay entirely missedthe main momentrelease. zones(compareto Figure 14c), althoughthis is in itself Even the TR durations appear to have been picked indicativeof a major differencebetweenthe zones. somewhat early in the time functions (Figure 15), Regionaldifferencesbetweenthe numberof aftershocks illustratingagain the inherentambiguityin pickingthe of large earthquakesin subductionzoneshave beenfound terminationof rupturefrom a typicaltime function. by Singh and Suarez [1988]. Atler correctingfor the mainshock size, they found variations of more than a 5. Conclusions factor of 10 in numbers of aftershocks with mb >_5.0; in

particular,there were many more aftershocksof TongaKermadec

mainshocks

than

of

Central

American

A systematicanalysisof TR's time function catalog revealsan abrupt decreasein the scaleddurationof time

HOUSTON: SHAPE AND DURATION OF EARTHQUAKE TIME FUNCTIONS

11,149

Comparisonof TR durationswith Bilek and Lay durations

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Figure 15. Comparison of TR durations with durationsfromBilek and Lay [1998, 1999]. (top) TR source time functionsfor the 17 eventsin commonbetweenthis studyand Bilek and Lay [1998, 1999]. Vertical linesshowTR durations;verticalarrowsshowBilek and Lay's durations.Event date and name,Mw, and

depth(from TR) are given. (bottom)The differencebetweenTR durationsand Bilek andLay'sdurations versusMw and depth. Note that Bilek and Lay's durationsare generallysignificantlyshorterthan TR durations;two eventshave a differenceof morethan40 s and six more eventshave a difference>_5 s.

functionsat about 40 km, againsta backgroundof more gradualdecreasesaboveand below that depth,which is believed to be the maximum depth of interplate seismic coupling. Additionally,eventsin the depthrange350 to

andintraplatethrusteventsis similarto thatbetweenall

550 km, consistentwith Houston et al. [1998], do not

earthquakesmay be explained if earthquakesin

eventsaboveand below 40 km depth,as well as to that betweenthrustandstrike-slipearthquakes.Thesechanges in the average scaled time functions of shallow

conformto expectationsof steadilyshorteningdurations tectonicallyactivesettingstendto be longerin duration longertailsthanintraplate eventsor thosein with increasing depth. The relationship betweeninterplate andpossess

11,150

HOUSTON: SHAPE AND DURATION OF EARTHQUAKE TIME FUNCTIONS

lessactive tectonicregimes. Regionalvariationsbetween Kaverina,A.N., A.V. Lander, and A.G. Prozorov,Global creepex distributionand its relationto earthquakesourcegeometryand subductionzones in time function shape and duration tectonicorigin,Geophys. J. Int., 125,249-265,1996. imply that physicaldifferencesbetweeninterplatesurfaces McGuire,J.J.,D.A. Wiens,P.J.Shore,andM.G. Bevis,The March 9, in subduction zones lead to differences in rupture 1994 (Mw 7.6) deep Tonga earthquake:Ruptureoutsidethe seismicallyactive slab, J. Geophys.Res., 102, 15,163-15,182, processes. The average time function shapesmay be 1997. useful for a variety of studies,includingthosemodeling Pegler, G., andS. Das,Analysisof therelationship betweenseismic dynamicrupture. momentandfault lengthfor largecrustalstrike-slipearthquakes between1977-1992,Geophys. Res.Lett.,23, 905-908,1996. Acknowledgments. I thank L. Ruff, Y. Tanioka, and graduate Pollard,J.H., A Handbookof Numericaland StatisticalTechniques, studentsat the University of Michigan for making their catalogof Cambridge Univ. Press,New York, 1977. time functions so readily available. Helpful reviews of the Romanowicz, B., Strike-slip earthquakes on quasi-vertical manuscript were provided by AE S. Schwartz, L. Ruff, an transcurrentfaults: Inferences for general scaling relations, anonymousreferee, and S. Persh. The Harvard CMT catalogwas Geophys. Res.Lett.,19, 481-484,1992. used extensively. This work was supportedin part by NSF grant Romanowicz, B., A reappraisal of large earthquakescaling EAR-9601979 to H.H. Comment,Bull. Seismol.Soc.Am., 84, 1675-1676,1994. Ruff, L.J., Asperitydistributions andlargeearthquake occurrence in subduction zones,Tectonophysics, 211, 61-83, 1992. References Ruff, L.J., Dynamic stressdrop of recent earthquakes:Variations within subductionzones, Pure Appl. Geophys.,15 4, 409-43 l, Abercrombie,R.E., Earthquakesourcescalingrelationships from -1 1999. to 5 ML usingseismograms recordedat 2.5 km depth,•. Geophys. Ruff, L.J., andA.D. Miller, Ruptureprocessof large earthquakes in Res., 100, 24,015-24,036, 1995. the northernMexico subductionzone, Pure Appl. Geophys.,142, Bilek, S.L., and T. Lay, Variation of interplatefault zone properties 101-172, 1994. with depth in the Japan subductionzone, Science, 281, 1175Scholz,C.H., Scalinglaws for large earthquakes: Consequences for 1178, 1998. physicalmodels,Bull. Seismol.Soc.Am., 72, 1-14, 1982. Bilek, S.L., and T. Lay, Rigidity variations with depth along interplate megathrustfaults in subductionzones,Nature, 400, Scholz,C.H., A reappraisalof large earthquakescaling,Bull. Seism. 443-446, 1999.

Soc.Am., 84, 215-218, 1994a.

of largeearthquakescaling- Reply,Bull. Frohlich, C., Aftershocks and temporal clustering of deep Scholz,C.H., A reappraisal Seismol.Soc.Am., 84, 1677-1678, 1994b. earthquakes, J. Geophys.Res.,92, 13,944-13,957,1987. Frohlich, C., and K.D. Apperson,Earthquakefocal mechanisms, Scholz,C.H., C.A. Aviles, and S.G. Wesnousky,Scalingdifferences betweenlarge interplateand intraplateearthquakes, Bull. Seismol. momenttensorsand the consistencyof seismicactivity near plate Soc.Am., 76, 65-70, 1986. boundaries,Tectonics,11, 279-296, 1992. Furumoto,M., andI. Nakanishi,Sourcetimesandscalingrelationsof Shimazaki, K., Small and large earthquakes:The effects of the thickness of the seismogeniclayer and the free surface, in largeearthquakes, J. Geophys.Res.,88, 2191-2198,1983. EarthquakeSourceMechanics,Geophys.Monogr. Set., vol. 37, Green,H.W., and H. Houston,The mechanicsof deepearthquakes, edited by S. Das, J. Boatwright, and C. Scholz, pp. 209-216, Annu. Rev. Earth Planet. Sci., 23, 169-213, 1995. AGU, Washington D.C., 1986. Houston, H., A comparisonof broadbandsourcespectra,seismic energies,and stressdrops of the 1989 Loma Prieta and 1988 Singh, S.K., and G. Suarez, Regional variation in the number of aftershocks (mb>_5) of large,subduction-zone earthquakes (Mw >_ Armenianearthquakes, Geophys.Res.Lett., 17, 1413-1416,1990. 7.0), Bull. Seismol.Soc.Am., 78, 230-242, 1988. Houston,H., H.M. Benz, and J.E. Vidale, Time functionsof deep earthquakes from broadbandand short-periodstacks,J. Geophys. Tanioka, Y., and L. Ruff, Sourcetime functions,Seismol.Res. Lett., 68, 386-400, 1997. Res., 103, 29,895-29,913, 1998. Ihmle, P.F., and T.H. Jordan,Sourcetime functionof the great 1994 Tichelaar, B.W., and L.J. Ruff, Depth of seismic coupling along subduction zones,J. Geophys.Res.,98, 2017-2037, 1993. Bolivia deep earthquakeby waveform and spectralinversions, Vidale, J.E., and H. Houston,The depthdependenceof earthquake Geophys.Res.Lett., 22, 2253-2256, 1995. durationand implicationsfor rupturemechanisms,Nature, 365, Kanamori, H., Rupture processof subductionzone earthquakes, 45-47, 1993. Annual Reviewsof Earth and Planetary Sciences,14, 293-322, 1986.

Kanamori,H., and C. Allen, Earthquakerepeattime and average stressdrop,in EarthquakeSourceMechanics,Geophys.Monogr. Ser., vol. 37,editedby S. Das, J. Boatwright,and C. Scholz,pp. 227-235, AGU, Washington,D.C., 1986. Kanamori, H., and D.L. Anderson, Theoretical basis for some empirical relations in seismology,Bull. Seismol.Soc. Am., 65, 1073-1095, 1975.

H. Houston,Departmentof Earth and SpaceSciences,University of California,Los Angeles,595 Young Dr. East, Los Angeles,CA 90095-1567.(hhouston•ess.ucla. edu) (ReceivedApril 19, 2000; revisedDecember12, 2000; acceptedDecember14, 2000.)