the april 5, 1990 mariana islands earthquake and …thorne/tl.pdfs/zl_mariana...the mariana trench...

PhysicsoftheEarth andPlanetary Interiors, 72(1992)99—121 99ElsevierSciencePublishersB.V., Amsterdam

TheApril 5, 1990MarianaIslandsearthquakeandsubductionzonestresses

JiajunZhangandThorneLay

Instituteof Tectonicsand CF. RichterSeismologicalLaboratory, Universityof California, SantaCruz, CA 95064,USA

(Received30October1991; accepted1 November1991)

ABSTRACT

Zhang,J. and Lay, T., 1992. The April 5, 1990 MarianaIslandsearthquakeand subductionzone stresses.PhysicsEarthPlanet. Inter., 72: 99—121.

TheApril 5, 1990 Mariana Islandsearthquake(M~= 7.5) involved normalfaulting at shallowdepthin the Pacific plateunderthe Marianatrench,about200 km eastof the centralMariana Islands.The earthquakeis significantfor its tectonicsetting,focalmechanism,and largesize.The eventoccurredin thevicinity of a proposedslabsegmentboundaryandmayinpart owe its unusually largesize to lateralvariationin intraplatestrain,asappearsto be the casefor other large,normalfaultingeventsneartrenches.Detailed analysisof theseismic sourceparametersof the earthquakeandof the relationshipbetweenseismicity and tectonics in the Mariana Islandsis conductedto improve our understandingof the mechanicalbehaviorand seismotectonicsin islandarcswith aseismicconvergence.The 1990 Marianaearthquakeruptureinitiated withvery low levelsof energyreleasefor an intervalof about7.2 s,followed by a large,single,impulsiveeventwith a durationofabout 6 s. Body wave inversion yields a strike= 215°,dip = 490, rake= —95°, and a moment= 1.2x 1020 Nm with acentroid time of 11—12 s. The sourcemechanismis further constrainedby the long-period Rayleighand Love wave data,with inversionsyielding a strike= 184 (±5)°,dip = 67 (±)°,rake= —90 (±5)°,and moment 2.2 (±0.5)x1020 Nm. Thelong-periodcentroid depthis 23 (±)km, with a centroid time of 20 (±3)s.

1. Introduction members:the Marianatype andthe Chileantype.The Marianasubductionzoneis characterizedby

The April 5, 1990 MarianaIslandsearthquake distinctive tectonic features such as an actively(2112:35.50 UT, 15.125°N,147.596°E,M~= 7.5, opening back-arcbasin, almost purely aseismicNationalEarthquakeInformationCenter(NEIC)) plate motion at the interplate boundary,an ex-is the largestrecentevent to occur in the central traordinarily steeply dipping Wadati—BenioffMariana subductionzone(Fig. 1). This arc is the zone,and absenceof outer rise topographyandboundary between the Pacific and Philippine gravityhighs.Along theMarianasubductionzone,plates in the western Pacific ocean. The 1990 theveryold (about160 Ma) Pacificplate is steeplyeventis a normal-faultearthquake,with a larger subductingtoward the west with the dip of themoment than any known thrust event in the re- Wadati—Benioffzone increasingto almost90°atgion, reflectingthe unusualcharacterof the Mar- large depths.The convergencerate variesalongiana subduction zone. Uyeda and Kanamori the arc from 2.5 cm year1 near the Caroline(1979) classifiedsubductionzonesinto two end Ridge to 5 cm year1 near the Marcus—Necker

Ridge, and is about 4 cm year’ near the 1990

Correspondenceto: Jiajun Zhang, Institute of Tectonicsand hypocenter.Ruff and Kanamori (1980) usedtheC.F.Richter SeismologicalLaboratory,University of Califor- magnitudeof the largest earthquakein a givennia,SantaCruz,CA 95064,USA. arc to characterizethe strengthof coupling be-

0031-9201/92/$05.00© 1992 — ElsevierSciencePublishersB.V. All rights reserved

100 J. ZHANG AND T. LAY

tweenthesubductingandoverridingplates.There formed and pushed apart when the back-arcareno known greatthrustearthquakesalong the spreadingin the Philippine plate margin alongMarianasubductionzone,indicatingthat subduc- the Mariana Rift (0—4 Ma old) broke the arction is essentiallyaseismic,with the platesbeing longitudinally.effectively decoupled.Ruff and Kanamori(1980) Therearestrikingcontrastsin bothbathymetryattributethis to thecombinedeffectsof the great and frequencyof large earthquakesbetweentheage of the Pacific plate and the slow rate of northernand southernpartsof the plate bound-convergenceof the Pacificand Philippine plates. ary along the western Pacific (e.g. Kelleher and

The Mariana trenchis verydeep(up to 10 km) McCann, 1976). Along the southern part, theand lacks an extensivesedimentarywedge. The Mariana and southernIzu—Bonin trenches,nomajor topographicfeaturesof the central Man- known great earthquakeshave occurred, andanasare longitudinal structuresthat lie to the thereis a broadzoneof irregularbathymetrywithwest of the trench:the MarianaIslands,Mariana severalridgesand numerousseamounts(e.g. theRift, anda submarineridge (Fig. 1). Two ridges Marcus—Neckerridge and Magellanseamounts).are located a few hundredkilometers from the Along the northernpart, the Kurile, Japan,andtrench, with the easternmostridge extending northern Izu—Bonin trenches,greatearthquakesalong the Mariana Islands. These ridges were have occurred frequently, and the sea floor is

PACIFIC~)

PHILIP IN~\ ~ ~3 ~‘PL~TE

2 00 N ~ ~•“•~ ~3P AT E ,, U ~ ~ .~—1

WE T o~0

MAR ANA ( 0 I—. 1

~ 0 E ST

B A S N j A N i

1DC BS!N

10011 -~__

(if ~3 \~\ __

140° 150°E

Fig. 1. Bathymetricmap of the Marianasubductionzone. Star,epicenterof the April 5, 1990 earthquake.

APRIL 5, 1990MARIANA ISLANDS EARThQUAKE 101

relatively smooth and lacks rises, ridges, or The seismicity along the Mariana subductionseamounts.Kelleher and McCann (1976) inter- zone has been studied by various investigatorspreted the spatial correlation between the (e.g. Katsumataand Sykes, 1969; Samowitz andbathymetryand distribution of large earthquakes Forsyth, 1981; Burbach andFrohlich, 1986). Fig-in terms of the relative buoyancyof the under- ure 2 shows the locations of earthquakeswiththrusting and the overthrustingslabs.Under the mb > 4.9 in the central Mariana region as re-buoyancyhypothesis,the bathymetrichighsof the ported by the NEIC for the period from 1977 toocean floor delineatezonesof lithospherewith 1990. The regional seismicity pattern shown inintermediatedensitybetweenaveragevaluesfor Fig. 2 is similar to that described in previousoceanicand continentallithosphere.Thesebuoy- studies.Most shallow earthquakesaredistributedant zonesinteractwith a convergentmargin re- alongthe trenchwardslope of theislandchain,atsulting in less frequent large earthquakesthan least50 km awayfrom the trench.This patternofalong adjacentportionsof the plateboundary. shallow seismicityis commonlyobservedfor van-

I I I I\ I IO

S \+

20~ .X PacifIcx

x Ocean+ + S 09/16/1986, =6.7.D=48km(31.5kmHVD)

- Philippine ~ +

Seaop

- 00

Q

++

C

- ~ IOS

÷ Sr.

04/05/1990, M80~7.5

+

++ 00+cr o~

*1 I°~ I I I145~E 15tfE

Fig. 2. Seismicitydistribution (mb~ 4.9) in the centralMariana Islandsregion,with epicentrallocationsfrom the NEIC for theperiodfrom 1977 to 1990 for shallow (circles, depth <60 1cm), intermediate(pulses,depthbetween60 and300 km), and deepevents(x, depth > 300 1cm). Symbol sizeis proportionalto magnitude.Solid circles, shallownormal-fault events.The 1990 event(star)wasalsonormalfaulting.Thevelocity for the Pacific plateis calculatedwith thePhilippine—PacificEuler vectorfromSenoetal. (1987). Thesegmentboundary(M3) for the centralportion of the Marianasubductionzone, definedby BurbachandFrohlich(1990) is shown.

102 J.ZHANG AND T. LAY

ous subductionzones,with the trenchwardedge seismicityat large depthsin the slab and occur-of the seismicitybeing referredto as the seismic rence of shallow normal-fault events near thefront. The April 5, 1990 eventis a typical in being trench.Segmentboundariesfor the northernandlocatedright below the trench. southernportionsof the Mariana arc were pro-

Harvardcentroid—momenttensor(CMT) solu- posedbutare lesswell definedthan thosefor thetions (e.g. Dziewonski and Woodhouse, 1983; centralManianas.In the centralregion thereareDziewonski et al., 1991) for the eventsin Fig. 2 prominent gaps in the deep and intermediatealong the central Mariana subductionzonemdi- seismicity and abrupt changesin the strike andcatethat shallow normal-faulteventsoccur both dip of the Wadati—Benioffzone. The disruptionnearthe trenchandalongthe slopeof the island of the trench(Fig. 1) andlack of deepseismicitychain,while shallowthrustearthquakesoccuronly in this region(Fig. 2) havebeenattributedto thebeneaththe trenchwardslope of the island arc. subductionof seamountsor aseismicridges(WangRecentlargenormal-faulteventsincludetheApril et al., 1979).The presenceof a zoneof weakness5, 1990 earthquakeand the September16, 1986 (theseamountsor ridges)or the lubricating effectearthquake(19.3°N,146.3°E,mb = 6.5, depth: 48 of the down-goingsurfaceroughnessmay causekm (NEIC), 31.5 km (Harvard CMT)), both of the platemovementto occur with reducedinter-which are larger than any of the recent thrust plate seismic activity (Kelleher and McCann,events. The latter event is located below the 1976).We proposein this paperthat lateral gra-trenchwardslope at least 100 km away from the dients in the slab strain regime may also betrench,probablyin the downgoingslab. This dis- responsiblefor enhancedintraplateseismicitybe-tribution of recentnormal-faulteventsappearsto low the trenchnearthe segmentboundary.be slightly differentfrom that in previousstudies The April 5, 1990 ManianaIslandsearthquakeof the Izu—Bonin—Marianasarc region, which andseveralsmallereventshaveoccurrednearthefound that normal-fault events tendedto occur segmentboundaryin the centralregion.The shal-either near the deepestparts of the trench or low depthsand proximity to the trenchof thesenear the volcanic axis of the island chain eventssuggestthat theyoccurredin the subduct-(Katsumataand Sykes,1969). ing plate.The occurrenceof the largeeventsug-

There are large lateral variations in deeper gests that near the segmentboundary the ten-seismic activity in the subductedslab along the sional stressin the subductingplate is enhancedcentral Manianasubductionzone(Wang et al., relative to other regions,perhapsby lateral dis-1979; Samowitz and Forsyth, 1981; Burbachand tortion associatedwith the segmentation.LargeFrohlich, 1986). In the northern portion of the intraplatenormal faulting eventsin otherregionssubductionzone(18—20°N)historically activevol- also tend to occur in regions of strong lateralcanoesare present,and intermediateand deep gradientsin the slab strain environment,as wefocus eventsare concentratedin a very narrow, will discusslater. The decoupling of the inter-nearlyvertical zoneextendingto a depthof more plate zoneby bathymetryor zonesof weaknessthan 650 km. There are no deep events (with may allow slab pull stressesto locally extendtodepths greaterthan 300 km) in the central and shallow depthsnearthe slabsegmentation,result-southernportionof the subductionzone. ing in the large shallow normal-faultevent. We

The central portionof the subductionhaslow will first characterizethe April 5, 1990 eventseismicity at all depths,while the southernpor- usingseismicinversionmethodsand thendiscusstion of the subductionzoneis characterizedby the stress environment near the slab segmenthigh seismic activity at shallow and intermediate boundary.depths.The central portionof the Mariana sub-duction zone(14.5—16.5°N)wasdefinedas a seg-

2. The April 5, 1990 earthquakement boundaryby Burbach and Frohlich (1986)(M3 in Fig. 2). This region is characterizedby a Seismic source parametersof recent largedisruption in the continuity of the trench, low earthquakesare routinely determinedusingvan-

APRIL 5, 1990 MARIANA ISLANDS EARTHQUAKE 103

ous methods and published in the preliminary parameterdeterminations.The source parame-determinationof epicenters(PDE) listings of the tens for the April 5, 1990 MarianaIslandsearth-NEIC. For some earthquakesthere are large quake reported by the NEIC generally corre-differencesbetweenvarious publishedsourcepa- spondto a normal-faultmechanismwith a west-rameters,which complicatesinterpretationof the erlydippingplanebeingthe preferredfault plane.events. This may be owing to incompletenessof However,thefault geometryandseismicmomentdataor errors in the modelsused in the source arepoorly constrainedby the variousfirst motion

KMl (Delta 43, Az 291) (A) LZH (Delta44, Az 307) (B)

(a) Short-Period (a) Short-Period

(b) Broadband i”. (b) BroadbandIl

— / I

,0 ~ /

(c) Broadband Displacement (C) Broadband Displacement /

5 8.83 48.33 / * 5 6.70 20 A

~ Lw - . :‘ ~

-10 40 50 •60 -10 50 60Time (s) Time (s)

Fig. 3(A). The P-wavedata recordedat KMI for the Marianaearthquakeof April 5, 1990. (a)The short-periodP wave. (b) ThebroadbandP wave with instrumentresponse,which is almost linear with velocity in this passband.(c) The bradbandgrounddisplacementvertical componentobtainedby deconvolvingthe instrumentresponse.(e) The short-period(solid line, countsper6 x iO~),broadband(dottedline, countsper 8 x i0~),andbroadbanddisplacementverticalcomponent(dashedline, micronsper5 x 10—2).Thereferencetime for eachtrace is 2120:00UT. The first arrowindicatestheP-wavearrival time that is taken from theshort-perioddata.Thesecondarrowindicatestheonsetof the largesubeventthat arrives9.5 s later.

Fig. 3(B). TheP-wavedatarecordedat LZH with the sameformat as Fig. 3(A). Thebottompanel superimposestheshort-period(solid line, countsper 5 x 106), broadband(dotted line, countsper 4x 10~),and broadbanddispacementvertical component(dashedline, micronsper 4x10

2).


and moment tensor solutions reported in the earthquake.Our data include P-wave first mo-PDE; the dip rangesfrom 310°to 680°for the tions, the distribution of aftershocks,teleseismicwesterly dipping fault plane, the strike varies broadbandP andSH waveforms,and long-periodfrom 30°to 840°for the easterlydipping nodal RayleighandLove wave spectra.plane,andseismicmomentrangesfrom 6.5 x 1019

to 2.6x 1020 Nm. An explanationfor theselarge 2.1. P-wavefirst motionsvariations is not immediately obvious, thereforewe usevarious seismologicaldata to bettercon- The ruptureof the 1990 Marianaeventbeganstrain the sourceparametersof the 1990 Mariana with a low energyreleaseratefor about7.2 to 9.5

WMQ (Delta58, Az 312) (C) HIA (Delta41, Az 332) (D)

(a) Short-Period (a) Short-Period

(b) Broadband .~ (b) Broadband

‘I, -. 0.J6~, ~[* ‘1~~!~

(c) Broadband Displacement (c) Broadband Displacement

,I, ,-- ~ ‘~‘

(d) Broadband Velocity (d) BroadbandVelocity

_ L25~680~*,~~

.1~o . 20 •40

Time (s) Time (s)Fig. 3(C). TheP-wavedatarecordedat WMQ with the same format as in Fig. 3A.Thebottompanel superimposes theshort-period(solid line, countsper 2x i0~), bradband(dotted line, countsper 4x 10~),and bradbanddisplacementvertical component(dashedline, micronsper4 X 10—2)

Fig. 3(D). TheP-wavedatarecordedat HIA with the sameformat as in Fig. 3A. Thebottompanel superimposesthe short-period(solid line, countsper 3 x i0~),broadband(dotted line, countsper lx iO~),and broadbanddisplacementvertical component(dashedline, micronsper 5 x 10_2.The first arrowinidicatesthe initial P arrivals,while thesecondarrowshowsthe main P-arrivalbeginning7.2 s later and thethird arrowshowsthe impulsive event9.5 s after thefirst arrival.


s, complicatingfirst-motion analysis.The P-wave al., 1984) stations.The P-wavearrival times werefirst motions reportedby the NEIC for stations identified from short-periodvertical componentnearthe westerlydipping nodal planeshow sub- seismogramsat each station, and our picks arestantialinconsistency,preventingconfidentdeter- generallyconsistentwith the onsettimesreportedminationof the fault geometry.In order to obtain by the NEIC. The initial arrivalsareveryweakonadditional data, we examinedthe P-wave first the seismograms;their polaritiesarenot reportedmotions on seismogramsfrom the short-period by theNEIC for moststationsandaredifficult toand broadbandchannelsof Global SeismicNet- determineevenon the digital data.Figures3(A)—work (GSN) and GEOSCOPE(Romanowicz et (E) show P-wavedata recordedat stationsKMI,

CCL (Delta66, Az 47) (E) CCL (Delta 66, Az 47) (F)

(ort.Perio~4\~,~~

)BrOadba /\,/‘~ Helta41z332~,,,,~,[\\~~

(c) BroadbandDispl~ment - -

7 WMO (Delta58, Az 312)

---“S’s I ,1,

(d) BroadbandVelocity

LZDe307~f~

Kr~elta~29~~I1\.

\

.1c2*o ~° 4° 0 10 20Time (s) Time (s)

Fig. 3(E). TheP-wavedata recordedatCOL with the same format asin Fig. 3A. Thebottompanel superimposesthe short-period(solid line, countsper 2 x 10—3), broadband(dotted line, countsper 4 x l0~),and broadbanddisplacementvertical component(dashedline, microns5 x 10—2). The first arrow inidicatesthe initial P arrivals,while the secondarrow showsthe main P arrivalbeginning 7.2 s later and the third arrowshowsthe impulsiveevent 9.5 s after the first arrival.

Fig. 3(F). Comparisonof thebroadbanddisplacementverticalcomponentsat severalstationsnearthe N—S trendingnodal planefor the Mariana earthquake.The first arrow inidicates the initial P arrivals,while the secondarrow showsthe main P arrivalbeginning7.2 s laterand the third arrow showsthe impulsiveevent 9.5 s after the first arrival.


April 5, 1990 MarianaLZH, WMQ, HIA, andCOL, whichareat similardistancesfrom the sourceand at azimuthscloseto the north—southtrending nodal planeof the - -- -

fault planesolution reportedby the NEIC. The ‘s.

emergentfirst arrivals at the nodal stationsarefollowed by a stronger, more impulsive arrival -

about 9.5 s later, and the polarities for these • I ~‘0COL

arrivalswerereportedby the NEIC. For stations • TA J4~

KM,’ 0away from the nodal plane (e.g. COL, lilA)

* ‘

0AGD *strongerarrivalsbegin about7.2 s after the first * +

arrival, with the samepolarity as the pulsewhich ; o RER 0~

arrives 9.5 s after rupture initiation. Figure 3(F) ,“ **

shows a comparisonof the broadbandvertical / o NWAO 0SNZO

componentdisplacementsat five stations. All of / 0CTAO

the signals show an emergentdilatation com- j 7mencingabout 7.2 s after the first arrival. Themore impulsive arrivals 9.5 s after the first mo-tionsarecompressionalat stationsKMI andLZH, - - - - - -

dilatational at lilA and COL, and nodal forWMQ. We believethat small fault plane irregu- Fig. 4. P-wavefirst-motion polarities for theimpulsive arrivals

about9.5 s after the first arrivals,on short-periodandinter-lanities accountfor the complexity of the nodal mediate-periodchannelsof vertical GSN and GEOSCOPE

stations. The broadbandvelocity records also recordings(lower hemisphereequal-areaprojection:opencir-

show increasinghigh frequencycomplexities for cle, dilatation; solid circle, compression;x, nodal). Nodalstationscloseto the nodal plane(Fig. 3(A)—(E)), planesof thebestdouble-coupleof the surfacewave-momenta common observationusually attributedto mul- tensorsolution obtained in this study using default Earth

models(solid lines) andof the first-motion solution reportedtipathingeffectsor smallfault irregularities, by the NEIC (dashedlines) are alsoshown(pluses,P axes;

Figure 4 shows first motion data for the ar- stars,T axes).rivals about9.5 s after the first arrivalsat digitallyrecorded stations, together with our preferredfault planesolution from long-periodsurfacewave 1990 Mariana Islandsearthquakehasa predomi-inversionandthefirst-motion fault planesolution nantly normal-fault mechanism with a north—givenby the NEIC. The long-periodsurfacewave southstriking nodal planedipping 50—70°to thesolution is completelyconsistentwith thesefirst west.motions,while the NEIC solution violates a few.A 5° shallower dip of the westward plunging 2.2. Aftershockdistributionplanewould makemostof the nodal observationsdilatational, consistent with the emergentmo- The distribution of aftershocksof the April 5,tions 7.2 s after the first arrivals. Allowing for 1990 earthquakeindicatesthat the north—southuncertaintyin take-off anglesand the apparent striking and westward dipping plane is actuallysubeventmechanismvariation observed at the the fault plane. Figure 5 shows locations of thenodes,this indicatesa 5—10°uncertaintyin dip of aftershocksreportedby the NEIC, for the firstthe westwarddipping planealongwith an indica- week following themainshock.Therewereabouttion of minor fault complexity. Our short-period 60 aftershocksduring thefirst day afterthe main-dataare adequateto reducethe ambiguities in shock, and the aftershockarea did not expandthe first motion polarities for stationsnear the significantly thereafter.Theseaftershocksaredis-westward dipping nodal plane, but not to con- tnibutedin a zoneabout60—80 km longextendingstrain the eastwarddipping plane. The P-wave in a north—southdirectionwith the epicenteroffirst motion data do indicate that the April 5, the main shocklocatednearthe southernendof


the aftershockarea(Fig. 5). The main shockhas downdip direction; however,the depthcontrol isa depth of ii km reportedby the NEIC. How- verypoor.ever,the depthsof mostaftershocksarenot welldetermined,and a default value of 33 km was 2.3. Broadbandbodywavein versionassignedto many events;for the aftershocksdur-ing the first day only 17 events have assigned The ground displacementsrecorded on thedepthsdifferent than 33 km, 11 of which have broadband channels of the GSN and GEO-depths between34 and 46 km. These events SCOPEstationsindicate that the main subeventinclude an aftershock with 46 km depth that of the Manianaearthquakeinitiated about7.2 soccurredwithin 2 hoursof the mainshock.These after the origin time reportedby the NEIC withaftershocksthus appearto have depths signifi- additional impulsive complexity 9.5 s into thecantly larger than the hypocentraldepth of the rupture. In the following we use the P and SHmainshockand are located to the north of the waveformsrecordedby broadbandinstrumentsinmainshockepicenter.The distribution of after- a momenttensorinversionto betterconstraintheshocksis consistentwith the P-wavefirst motion sourceparameters.The recordsaredeconvolveddata for the event and indicates that the fault by their instrument response to recover theplane is steeply dipping and striking in the ground displacement(Fig. 6). The ground dis-north—southdirection. The occurrenceof after- placementsat various stations indicate that theshocksto the north of the mainshockand their major moment releasefor the Mariana earth-relatively deep depths may indicate that the quakeoccurredin a singledominantpulsefollow-mainshock ruptured in the northward and ing 7 s of very small groundmotion. It shouldbe

IrN14SE 146~E l4rE 148~E

I-C(3

Philippine /~Sea

‘Cice

16~N - ‘ /3 16~N

CMT~b d~4~ ci

C

Pacific-~ Ocean15~N . —~ , 15~N

04/05/1990, M87.5 NEIC

5~E 1~~6~E l~8~E 1

Fig. 5. Distribution of aftershocksof the April 5, 1990 Mariana Islandsearthquake(large cross), reportedby the NEIC, thatoccurredin the first day (triangles) andfirst week(crosses)following the mainshock.The epicentrallocation from the HarvardCMT solution is shownby thesquarelabeledCMT.


April 5, 1990 Mariana

;T;~~\:O::a)Hj:J5\;.O7)COLPlOS(l.2a)

KEV SH 114 (0.63) COR P 125 (1.07)

-~ .‘................ ~ ~.,,,... ....

SI-I 113 (113(1 13) °~ C,,,/\SF~,3(084)

T: P122(1 25)

~

/\sH70i.i4K~J\)~~

Fig. 6. Observed(solid lines) and synthetic(dashedlines) displacementwaveforms for broadbandP and SH waveswith themaximum amplitude of the observedwaveformindicatedin microns.The focalmechanismandtime function areobtainedfrom asingle eventfault mechanisminversionof the displacementswith a sourcedepthof 21 km. The tracesstartabout1 s aheadof thefirst arrivalat eachstation, which is too small to see.The P-wavefirst-motion polarity for the arrival about9.5 s afterthe start ofeach P-wave trace is also inidicated. Thesingle subevent inversion matches mostwaveforms but can not fit all detailsof the nodalstations or the SH waves.Theamplitude ratio of synthetic to observed seismogramsis shown in parentheses.

noted that deconvolutioncausesslight baseline The inversionswere performedusing a seismicshift, particularly for stationsof the ChineseDigi- velocity structurewith 3 layersover a half-spacetal SeismicNetwork. for the sourceandonelayerovera half-spacefor

We applied a simultaneous deconvolution the receivers.The P-wavevelocity (km ~_1), 5~method for P and SH waves (Kikuchi and wave velocity (km s’), and density(g cm3) forKanamori,1991) to invert for the sourceparame- the sourcestructureare: 1.5, 0, 1 for awaterlayerters. Since the aftershockareafor the Maniana (6 km thick); 3.4, 2, 2.8 for the shallow oceanicearthquakeis small, suggestingthe directivity ef- crust (1 km thick), 6.5, 3.75, 2.9 for the mainfects will be minor, we focus our analysis on oceaniccrust (9 km thick), and8, 4.6, 3.4 for theresolving the mechanismand temporalvariation mantlehalf-space.For the receiverstructuretheof theseismicmomentratefunctionfor the event, sameparametersare: 6.0, 3.5, 2.7 for the crust


(30 km thick) and 7.8, 4.5, 3.3 for the mantle, seismic moment of 1.2x 1020 Nm. The sourcerespectively.The source time function for each duration and depth trade-off as is usually ob-subeventis assumedto bea taperedtrianglewith served,andthemomentvariesaswell. Triangular3 s rise time andfall time. sourcesfrom 3 to 5 s in halfwidth canfit thedata.

Figure6 showsthe result of inversionof theP The dip of the westwarddipping nodal plane isand SH ground displacementwaveformsassum- consistentwith that required by the emergenting a source depth of 21 km (15 km below the dilatations7.2 s after ruptureinitiation but shal-water). Inversionsassuming15 on 27 km source lower than the dip indicatedby the impulsive Pdepthsgive errors(variances)about5—10%larger arrivals 9.5 s into the rupture (Fig. 4). For sta-than that for the 21 km sourcedepth.The source tions BJI, LZH, and KMI the synthetic groundhas a time function with a centroid time of 11 s displacementsfor this mechanismhave dilata-(note the sourcecommences8 s after the origin tions slightly strongerthan the data required attime) anda normal-faultmechanismwith a strike 7.2 s and oppositein polarity to the moreimpul-of 2 15°, dip of 49°,and slip of — 95°, and a sive motion 9.5 s into rupture. While this is in

TABLE 1

Epicentraldataand phasesusedin surfacewaveinversion a

Station Net/Chan Azimuth (°) Distance(°) Phase

BCAO GSN LHZ 287.21 126.04 R123 G12

BJI GSN LHZ 318.31 37.05 R2G23CHTO GSN LHZ 281.71 46.58 R23G12CMB GSN LHZ 52.70 82.42 R12G23COL GSN LHZ 24.98 65.70 R12 G12COR GSN LHZ 46.67 78.92 R12G123CTAO GSN LHZ 182.20 34.87 R23G23GDH GSN LHZ 7.39 94.42 R2 G12HIA GSN LHZ 332.18 40.89 R2G23HON GSNLHZ 254.59 128.47 R2HRV GSN LHZ 30.23 112.22 R12KEy GSNLHZ 342.54 85.80 R12G12KMI GSNLHZ 290.70 43.44 R2G23KONO GSN LHZ 339.96 98.03 R12G2LON GSNLHZ 44.31 79.62 R23G23LZH GSNLHZ 126.54 135.98 G2MAJO GSNLHZ 340.29 22.93 R1G12NWAO GSN LHZ 210.91 55.91 R23G2PAS GSN LHZ 56.04 85.10 R12 G12SCP GSNLHZ 35.14 110.16 R23G23SLR GSN LHZ 249.12 122.69 R1 G12SNZO GSNLHZ 336.98 118.35 R2G2TATO GSNLHZ 295.95 26.65 R23WMQ GSNLHZ 312.46 58.16 R2G23ZOBO GSNLHZ 96.47 146.14 G3CAY GEO VLP 45.90 151.97 R12 G12INU GEOVLP 336.70 22.27 R12G23KIP GEO VLP 74.46 51.86 R12 G1~,PPT GEO VLP 115.32 70.02 R12 G1RER GEO VLP 250.15 97.12 R12SCZ GEOVLP 54.29 81.95 R12 G123SSB GEOVLP 332.99 111.10 R12WFM GEOVLP 30.12 112.17 R12 G123a The azimuthsanddistanceslistedhereare calculatedfrom the epicentralparametersfor theearthquakereportedby NEIC.

110 J.ZHANGANDT.LAY

part owing to the poor baselinestability for those Mariana04/05/1990(RayleighWave)stations, the model parametenizationalso does T=150.s:not havesufficient complexity to fit the detailson ~ (a)the motions at nodal stations. For most other :‘~ ‘:~

stations the single subeventinversion fits the P ~ ~ ‘~ -

dataverywell, althoughSH wavesarenot aswell ~ 2 ~ ~.H0N2~’.’aUl.3

modeled. ~ 1150 (~)a5~g~aThe seismic moment is about half that ob- 0

tamed from surfacewave inversions,which are -2 ~ SN~.

describedbelow. This may in part be attributed ~5 . ,. .. ,. .. *

to errorsin the Green’sfunctionsin theseinver- T~256s.sions and to the unmodeledcomplexitiesof the ~ ~.,..

ruptureprocessof the earthquake.Inversionsal- ‘~‘ ~“.

lowing for 5 subeventsgive a total momentof c •.. ...~..~ ,~, .

1.6 x 1020 Nm. This is still smaller than the mo- 0) 2ment of 2.2 x 1020 Nm obtained from surface ! ~waves,and mainly involves improvementsin fit- a.ting latter arrivals whichmay simply be the resut -2of poorly modeledwater and crustal reverbera- 1~=3O0tions. The multiple subevent inversions always ~ .... (e) ..

yield a mechanismfor the largestsubeventsimi- <.2 ~ . .•...

lan to that from the single subeventinversion. •. ...~• , .

This indicatesthat the major momentreleaseof 0 ~ ,..,, ~ ‘:-,- ‘

theMarianaearthquakehasa nearlypurenormal 2 (f)faulting mechanism,consistentwith the first mo- 0tions and surface waves. If we estimate static -2stressdropsassumingsourceareais equalto the _____________________________________squareof rupturevelocity timesrise time anduse 0 12Rzimth(deg2)4° 360half durations of 3 to 5 s a source rupturevelocity of 3.5 km ~ andcorrespondingoptimal Fig. 7. Amplitude (in unitsof 1027 ms) and phase(in units of

radian) spectra of observedpropagationcorrected sourcemoments,we obtaln valuesfrom 67 to 249 M Pa. spectrafor Rayleigh wavesof periodsof 150 s (a, b); 256 s (c,

Thesevaluessuggesta high stressdrop, on the d); 300 s (e, f). Dashed lines indicate theoreticalvalues

order of 1 kbar, for this intraplateevent,for the calculatedfor the moment tensorsolution obtained in thisstrong portion of the fault which radiated the study,for thedefault EarthmodelandNEIC sourcelocation.

mainbody wave pulse.

2.4. Surfacewaveinversion 8). SeveralR3 and G3 arrivals are used along

with short pathwavesR1 andG1, becauseof theFor further determinationof the long-period uncertainsignalquality of the short pathwaves.

seismic momenttensor of the Mariana Islands Many otherarrivals,mostly R3 and G3, areomit-earthquakewe use the method of Zhang and ted in our dataset becauseof inadequatesignalKanamoni (1988b) to invert the long-period qualityor uncertainpathcorrections.The spectraRayleighandLovewave spectra.Table 1 lists the arecalculatedat periodsfrom 150 to 300 s using108 Rayleighand Love wave phasesusedin this datafrom the GSN and GEOSCOPEnetworks,study, which include eighteen R1, twenty nine and correctedfor propagationphasedelay usingR2, sevenR3, fourteen G1, twenty six G2, and the laterally heterogeneousEarth model M84CfourteenG3 arrivals. The dataprovide good az- obtainedby WoodhouseandDziewonski (1984).imuthal coverageof the earthquake(Figs. 7 and In the following, we first invert the data for the

APRIL 5, 1990MARIANA ISLANDS EARTHQUAKE 111

Mariana04/05/1990(LoveWave) unilateral and bilateral models, so the rupturea. - T=150 s - . lengthsandazimutharenotwell resolved.Figure

2 - ~. ~ .~.. (a) ......‘• 9 showsthe normalizederror, o•, which measuresthe variancereductionin the first stepinversion(seeZhangandKanamori(1988a)),as a function

~ .~ ‘ ~ , of the trial azimuth,for the inversionsat various~“~‘ ~ ~ periodsfor a unilateralrupture extending70 km

O ,‘ ~ with a velocity of 2.0 km s’. For inversionsof

-2 ~ .. ~ (b) Rayleighwavesthevariancereductionis less than.4 . * 10%. The inversionsof Love waveswith the pe-

T256 ~ - . nod of 150 s have a variancereduction about

:.~I’~ :.~. 20% for a unilateralruptureto the north,but the/ “., ..“ ‘. range of the rupture azimuths obtained from

O inversions at longer periods is very large This~, 2 ... . suggeststhat the directivity effectsaresmall. The

c . _______________ small horizontaldirectivity effectsmaybe an indi-a. ~ (d) ~ cation that the ruptureof the earthquakehasa

________________________________ largecomponentin the downdip direction,which

-T=300s -

<.2 .,. -.. ~. ....,e, :~- VarianceReduction

________ ______ ~ ~j-~oPe6odH‘~ ‘:::~i/-. (a) RayleighWaves

4-225

2 (f) 2 ::

0 12Q . 240 360 ~ .3 ——~-- ~—~°°-— —

#~zimuth(deg) - -

Fig. 8. Amplitude (in units of 1027 ms) and phase(in unitsof . 4

radian) spectra of observedpropagationcorrected source _6 — — —

spectrafor Love wavesof periodsof 150 s (a, b); 256 s (c, d); .2 .. — — —

300 s (e, f). Dashedlines indicatetheoreticalvaluescalculated I ..... I ..... I ..... I

for the momenttensorsolutionobtainedin this study,for thedefaultEarth model andNEIC sourcelocation.

(b) LoveWaves

sourcefiniteness,then determinethe depth and ..~ - 2

momenttensorof the earthquake.We also exam- .4 . . . -....

me the effects of epicentral mislocationon the ... - - - - - - - - - - ~

determinationof theseismicmomenttensorfrom - - - - - - - - - - - - - -

long-periodsurfacewavesusingthe procedureof — — — — -.LZhangandLay (1990a). ~

First, we investigatethe spatial and temporal ,—~----~-.~ ~ I, .. I.

sourcefinitenessby inverting for rupturedinectiv- 0 60 120 180 240 300 360

ity and duration. We invert the long-period Azimuth (deg)Rayleigh andLove wavesfor bothunilateraland Fig. 9. Thenormalizederrorsof inversionsof variousperiods

- for Rayleighwaves(a), and Love waves(b), as a function ofbilateral rupturemodels.The variancereduction assumedrupture azimuthsfor a unilateral rupture extending

relativeto a point sourcemodel is small for both 70 kn~.

112 .1. ZHANGAND T. LAY

would be consistentwith the occurrenceof many model for propagationcorrection, inversionsofaftershocksat depthssignificantly larger than the the Rayleigh waves give r = 30 s and variancemain shockhypocenter. reductionssimilar to the M84C model, but inver-

In thefollowing analysiswe focuson determin- sions of the Love wavesgive scatteredestimatesing a realistic model for the temporal source of the source-processtime for various periodsfinitenessalone,recognizingthat we havelimited and less variancereductionthan usingthe M84Cspatial resolution. Figure 10 shows the normal- model. This indicates that model M84C signifi-ized error, u, for boxcar source—timefunctions cantly improvesthe inversionsof the Love wavewith a rangeof durationsfrom 0 to 100 s. For a spectrafor earthquakesin the Maniana Islandsgiven period, the duration that yields the mini- region, andwe preferthe resultsfor that model.mum error provides an estimate of the actual Now we determinethe sourcedepth andmo-source duration, T. Both the inversions of ment tensorfor a point sourcemodel with 20 sRayleighwavesandinversionsof Jovewavesgive sourcecentroid time (40 s apparentboxcarduna-T = 40 ±5 s for the M84C model. This result tion from NEIC origin time). Previous studiesexplicitly usesthe origin time of the NEIC. The indicate that the modelsof pathattenuationandlong-period waves are mainly sensitive to the source excitation functions are critical for esti-centroidof the sourcefunction, thustheyprefera matesof the centroiddepthand seismicmomentsourcecentroidabout20 s after the NEIC origin when long-periodsurfacewavesare used in thetime. If the onsetof significant momentreleaseis sourcemomenttensorinversions(ZhangandLay,delayedfrom the origin time as indicatedby the l990b; Wallaceet al., 1991). The earthquakeoc-body wavesthe inferredruptureprocesstime will curredat shallow depthin the PacificPlateunderreduceby twicethe delay, remainingcenteredon the Manianatrench,which is the boundarybe-the preferredcentroidtime. Of course,the model tween the Pacific and Philippine plates. Thecentroid time itself may be biasedby inaccurate long-periodsurfacewave excitationfor the earth-propagationcorrections.Using the PREM Earth quakemayinvolve boththeveryold Pacific(about

Mariana Earthquake(04/05/1990).9 I I I , ~- , , , , , , I I/A /

M84C / ,‘,‘, M84C / ,‘ ,“8 • No. Period (s) / / I ,‘ / /1--iSO / / ,‘. / ‘

2 --175 / ,‘.-‘ / / ,‘

3--200 / ~‘‘.‘ ,/ ‘

- (a) RayleighWave (b) Love Wave2 I I I I I I I I I I I I I

- 0 50 1000 50 100Apparent Duration (s)

Fig. 10. Theresidualerror, a-, in thefirst stepinversionof Rayleighwaves(a), andLove waves(b), vs. trial sourcedurationfor theMariana earthquakeat periods of: (1) 150; (2) 175; (3) 200; (4) 225; (5) 256; (6) 275; (7) 300 s, respectively.The durationcorrespondingto theminimum erroron eachcurve is taken asthe sourceprocesstime measuredat that period.

APRIL 5, 1990 MARLANA ISLANDS EARTHQUAKE 113

160 Ma) andthe young back-arcocean(0—4 Ma) (>100 Ma) gives a centroiddepth a few kilome-along the easternmarginof the Philippine plate. tens deeperthan the averageocean model. AnIn the determinationof the centroid depth and average ocean model is perhaps an adequatemomenttensorof the 1990 earthquake,our ‘de- model for the surface wave radiation for thisfault’ Earth model incorporatesthe M84C model earthquake.Thus we feel that the long periodof Woodhouseand Dziewonski (1984) for the centroiddepthof theManianaearthquakeis real-phase velocity propagation corrections, the istically estimated as 23±5 km. This can beDziewonskiand Steim(1982) model for attenua- comparedwith the centroiddepthsfor the earth-tion, and the averageoceanmodel of Reganand quake reported by the NEIC (USGS momentAnderson(1984) for calculation of the surface tensorsolution: 15 km; Harvardmomenttensorwave excitation functions.For the default Earth solution: 15 km fixed; GEOSCOPEmomentten-model,which assumesapointsourceat theNEIC sor solution: 35 km) and with our body waveepicenter,our seismicmomenttensorsolution is modeling result of 21 km. Satakeet al. (1990)essentiallya pure double coupleand has a cen- suggestthat the eventoccurredat a very shallowtroid depthof 23±5 km, seismicmoment22±5 depth judging from the observedT-phase at ax 10~~Nm (M~= 7.5), anda normal-faultmech- nearbystation.anism with 175°±3 strike, 670°±6 dip, and Now we considerthe effects of possibleepi-— 99°±2 slip. Figures 8 and 9 show observed central mislocationor shift of the centroid loca-amplitude and phase spectra of Rayleigh and tion on the surfacewave momenttensorsolutionLove waves, respectively,corrected for instru- using the procedureof Zhang and Lay (1990a).ment and propagationeffects, along with the Previousstudiesof seismicity in several subduc-spectrapredictedfrom this sourcemodel at pen- tion zoneshaveshown that teleseismiclocationsodsof 150, 256 and300 s. The observedRayleigh of earthquakesareoften biasedon the order ofandLove wave radiationpatternclearly indicates tens of kilometers, relative to epicentersdeter-that the faulting of the Marianaearthquakehasa mined usinglocal seismic networks.For eventsindominantnormal-faultcomponent. the ManianaIslandsregion, travel time observa-

When other standardEarth models are used tions are limited to teleseismicdistanceswith thein the inversionthe momenttensorsolutions re- exceptionof stationsat Guam,which preventsanmain about the same,but the sourcedepthsvary accurate assessmentof the uncertainty in theconsiderably.Using the attenuation model de- teleseismiclocation of the Mariana earthquake.rived by Mastersand Gilbert (1983) and excita- ZhangandLay (l990a)showthat there~is a directtion functions for the averageoceanstructureof trade-offbetweenassumedepicentrallocation(orReganandAnderson(1984), the momenttensor centroid location for a finite fault) and faultsolution hasa centroiddepthof 17±4 km anda mechanismobtained using longperiod surfaceseismic moment of 3 ±1 x 1020 Nm. For the waves. This trade-off may be used to obtain aPREM Earth model attenuationand excitation gross estimateof the best point source locationfunctions (Dziewonski and Anderson, 1981) the when the fault plane solution is independentlymomenttensorsolution hasa depthof 33 km and known.The bodywave dataandaftershockdistri-a seismicmomentof 2.0 x 1020 Nm. For various butions indicate that the fault planeof the Aprillarge earthquakesit has been shown that using 5, 1990 MarianaIslandsearthquakestrikesin thethe PREM model for excitationfunctionssystem- north—south direction and dips 50—70°to theatically gives centroid depths about 10—15 km west. Given the unknown effects of deepstruc-deeperthanothercommon Earth models,which tune and ambientnoise on the P-wavefirst mo-is often inconsistentwith body wave constraints tion andP andSHwaveformdata,the fault plane(Zhangand Kanamori, 1988b; Zhang and Lay, of the earthquakemay havea strike in the range1990b). The 17—23 km centroid depth obtained 195 ±20°with the upperlimit correspondingtofor the averageoceanmodel is reducedwhen a the strike of the fault planesolution found by theyoungeroceanmodelis used.An old oceanmodel body wave inversion.Our surfacewave moment

114 J. ZHANGANDT. LAY

tensorsolution for the NEIC location hasa 175° Zhang and Lay (1990a)). This is probably thestrike.The slight inconsistencyin the strike of the manifestationof errors in the referenceEarthfault planesolutions from body wavesand long- modelsandsourcefinitenessmodels.The effectsperiodsurfacewavesmay be attributedto mislo- of referenceEarthmodelson the momenttensorcationof the long-periodcentroidalongthe strike inversionsof long-period surfacewaves for theof the fault plane(Zhangand Lay, 1990a). ManianaIslandsearthquakeare similar to those

For anassumedpointsourcelocation 10 km to for shallow eventsin central Mexico (Zhangandthe north of the NEIC location (our preferred Lay, 1990a).Basedon the calculationsin Zhangcentroidlocation), the surfacewave inversionhas andLay (1990a),if we assumethat thefault planea better variancereduction and gives a double of the Manianaearthquakedips steeply to thecouplemechanismwith 184°strike, 67°dip, and west andstrikesin the SSWdirection(195 ±10°),— 90°slip, centnoiddepthof 23±5 km, andseis- the optimal point sourcelocation inferred frommic moment2.2 x 1020 Nm for our default Earth our momenttensorinversionsis predictedto bemodel. This mechanismis very consistentwith 20 ±20 km to the north of the NEIC location.the fault plane geometryobtained from the P- Epicentral mislocation towards the outer risewave first motionsfor the impulsiveeventat 9.5 s seemsless likely, since this results in a shallowafter rupture initiated and the aftershockdistri- westerly dipping plane with a strike in thebutions(Figs. 4 and5), andaccountsto first order south—eastdirection.for the slight componentof northwarddirectivity. Our preferredlong-periodmomenttensorso-For an assumedunilateralruptureextending70— lution for the event has a seismic moment of80 km northwardwith a velocity of 2—3 km s~ 2.2±0.5 x 1020 Nm, centroid depth of 23±5moment tensor inversions give a very oblique km, strike of 184 ±5°,dip of 67±5°,and slip ofsource mechanismwith 205 ±5°strike, 70±5° —90 ±5° The source mechanism,seismic mo-dip, and74±5°slip. The deviationof this source ment, andcentroiddepthobtainedfrom the sur-mechanismobtained for the unilateral rupture facewave inversionremainaboutthe sameif anmodel from the fault plane geometryobtained origin time 9.5 s later than that given by thefrom body wavessuggeststhat the rupturedirec- NEIC is usedto accountfor the intervalof weaktivity of the earthquakeis smaller than such a body wave radiation. Using this delayed startfinite sourcemodelpredicts,so we prefera more time in the surfacewave inversion results in aconservativemodel with a slight northwardshift source duration of about 20 s, which is longerof the pointsourcelocation, than the bodywave time function,butcompatible

For assumedepicentrallocations to the north- with theruptureof the 60—80 km long aftershockeastof the NEIC epicenter,residualerrorsin the zone.long-periodsurfacewave inversionsare also re- Althoughuncertaintiesin thecentroidlocationduced. For an assumedepicenter40 km to the andin themodelsfor the surfacewaveexcitationnortheastof the NEIC location, the bestdouble and propagationpreventus from uniquelydeter-coupleof the momenttensorsolution hasa nodal mining the sourceprocess,the combinedanalysesplanesteeplydipping to the eastwith the second of the body wave data, aftershockdistribution,nodal planemoderatelydippingto thewest.This and the long-period Rayleigh and Love wavelocation is close to the CMT centroid location radiation suggestthat the April 5, 1990 Maniana(Fig. 2), and for that centroid location we obtain Islandsearthquakerupturedon the north—southa momenttensorvery similar to the CMT result striking, steepwesterly dipping fault plane. It(Dziewonski et al., 1991). However, this mecha- appearsthat the main body wave radiation wasnism is inconsistentwith the body wave dataand producedby high stressdrop failure of a limitedthe aftershockdistribution.This indicatesthat an portion of the fault surface,with slip expandinginversion for location basedsolelyon reduction beyondthis asperitycontributingprimarily to lowof surfacewave spectralvariancemay not give a frequencymomentrelease.This accountsfor thesatisfactory location or mechanism (see also differencein ruptureduration andtotal moment

APRIL 5, i990MARIANA ISLANDS EARTHQUAKE 115

from the inversion of different types of seismic pull forces probablyalso play a significant role inwaves, the shallow deformation of the oceanic litho-

spherein the MarianaIslandsregion. However, itis unclearwhy large normal-faulteventsonly oc-

3. Subduction zonestressorientation cur in localized regionsalong eacharc.The source mechanismsof subductionzone

The largenormal-faultmechanismof the 1990 earthquakesprovide information about the na-Manianaearthquakeindicates that the Pacific ture of lithospheric stressesand strains. We useplatenearthe deepestpart of the trenchhasthe the HarvardCMT solutions for the eventsin Fig.minimum stressaxis subhorizontaland perpen- 2 to examinethe strain orientationsin the centraldicular to the trench. Various hypotheseshave Marianasubductionzone.We projectedthe pnin-beenproposedfor the origin of tensionaldefor- cipal strain axesof the seismicmomenttensorformation near trenchaxes.The plate bendinghy- every earthquakeon three planes: the verticalpothesisemphasizesthe role of the plate defon- planefrom the eventhypocenterto a pole aboutmation near the trench and attributes the nor- which the trench and arc form small circlesmal-fault earthquakesbeneaththe deepestpart (cross-sectionview), theverticalplaneperpendic-of the trench or seawardof the trench to the ular to the crosssection(arcview), andthe planebendingof the lithospherejust before it plunges on the Earth surface(map view). Samowitz andbeneathan island arc(Isackset al., 1968; Hanks, Forsyth(1981)useda similar cross-sectionprojec-1970; Chapple and Forsyth, 1979; Christensen tion for the seismicity for the Maniana Islandsand Ruff, 1983). In the plate bendingmodel a arc.We usethe following parameterto representneutral bending surface separatesan upper the distribution of focal mechanismorientationsregimeof extensionalstrainand a deeperregime with respectto the Wadati—Benioffzone:of compressionalstrain. In the context of thismodel the 1990 event representssimple bending — 2 N 2~ N 1stresses,although thereare few examplesof such ~ — • 1 cos —

largenormal-faultevents(Christensenand Ruff, 1—

1983),andthe stressdrop is quite high. where ~j = ln 2/ln 1.5 = 1.71, O~ is the plungeOther interpretations of large normal-fault angleof a principal axis for an eventwith respect

eventsin oceanictrencheshavearguedthat some to a given plane of projection, and N the totalevents actually detach the entire lithosphere numberof eventsfor a segmentof the subductionratherthanjustbendingthe slab(Kanamori,1970; zone.PCross,~ and PMap areparameterscalcu-Spence,1987).In an analysisof aftershockdepths latedfor the threeprojectionplanes,respectively.and focal mechanismsfor the great 1977 Sumba Thevalue of p rangesfrom — I to 1, whichcome-earthquakein the easternSundatrench,Spence sponds to the axes of all events for the given(1986) found no evidencefor thrust faulting in segmentbeing perpendicularor parallel to thethe deeperaftershocksand proposeda slab pull plane of projection, respectively.The expectedmodel to explain the occurrenceof the Sumba value of p for isotropically distributedaxesis 0.earthquake.The slab pull hypothesisattributes The parameterp representsthe closenessofboth the occurrenceof normal-faultearthquakes tension(T) or compression(P) axeswith respectnearthe trenchandtheplatebendingto slab pull to the three orthogonalplanesdefined for theforces. The slab pull forces effectively decouple Mariana subductionzone. This representationthe interface thrust zone, provide the bending differs from that usedby Appersonand Frohlichmomentat the trench, andallow tensionalstress (1987) in that their statistic representsthe inten-to accumulatenearthe trench.The oceaniclitho- sity of clustering of the axes with respect tosphereat the Mariana subductionzone(about reference axes directed along strike, downdip,160 Ma in age) is olden than that in the eastern andnormal to the Wadati—Benioffzone.Never-Sunda trench(about 148 Ma in age), thus slab theless,our simple representationsshould yield

116 J.ZHANG AND T. LAY

Trench in severaldepthranges:0—60, 80—160, and 180—C I 220 1cm, and thereare dominantstrain orienta-

- tions for eachdepth range. Seismicitygapsneardepthsof 70, 180, and 240 km are well defined

// for this small populationof events.The distribu-101 tion of principal strain axesfor theshallow events

is morecomplexthan for intermediateand deep201 events,which reflectsthelargevariationin mech-

anismsof the shallow earthquakes.While all in-04/05/1990, M57.5termediateanddeepeventsarebelievedto occur

.c 301 /1 09/16/1986,M5.6.7 within the subductingplateandhavesimilar faulta) mechanisms,the shallow earthquakesoccur not

I only within the subductingplate but also at the401 I~ plate interfaceandin the overridingplate.

a There are a total of 40 shallow earthquakes~0,E ~ f’~5~*~~1 betweenthe trenchand islandchain (Fig. 2) with

50 50 ~ b “ 10 eventsinvolving normal faulting and most of~ C0 ~/L.i j~I the remainingeventsbeing thrusts.The normal-

-50 I C fault eventshave depths less than 35 km and—~ 0

60 / 1 j~~I’ probably occurredwithin the Pacific plate, and.....-, a they havehorizontaltensionalaxesperpendicular

-50700 800 900 1000 to the trench and vertical compressionalaxes.

Distance(km) These normal-fault earthquakesclustered nearFig. 11. Orientationof principal axesof Harvardcentroid-mo- the segmentboundarybeneaththe central Man-ment tensorsolutions for events in the centralMariana Is-

anaswith the exceptionof two eventsnear thelands subductionzone. The earthquakehypocenteris pro-jected on to a vertical cross-sectioncontaining the event northernpart of the Mariana arc. There is onlyhypocenterandthe pole 17°.5N,138°.1E.For eacheventthe one thrust earthquakenear this central segmenttension axis(bars)or compressionaxis (arrows), whichever is boundary.closerto the vertical cross-section,is plotted along the pro- The shallow thrust earthquakesin the centraljected direction of the axis. Earthquakesare plotted with Marianasubductionzoneare locatedoutsideof adepthsof Harvardcentroid-momenttensorsolutionsand epi-centrallocationsreportedby NEIC. For shallowearthquakes 200—300km wide zonecenteredon the segmenttensionalaxes(a)andcompressionalaxes(b) with unit lengths boundary.FortheseeventsPCross, PArc, and PMapare projectedonto the cross-section,and tensionalaxes (c) for the compressionaxesareequal to 0.6, — 0.8,andcompressionalaxes(d) with unit lengthsareprojectedon and0.6, respectively,andfor the tensionaxes0.6,to the planeperpendicularto the crosssection.

0.6, and —0.8, respectively.This indicates thattheseearthquakesare thrust eventsand in gen-

consistentdescriptionsof strain orientationsfor enalhavehorizontalcompressionaxesperpendic-various subductionzones. The advantageof our ular to the trenchandverticaltensionaxes.Themerepresentationsis in the simplicity in characteriz- is a tendencyfor the tensionalaxesto dip steeplying the spatial distribution of the axeswith re- in the downdip direction of the slab while thespectto a given plane. For the Manianasubduc- compressionaxes dip gently toward the trenchtion zoneour resultsareconsistentwith thoseof (Fig. 11). Forthe thrusteventsat depthsless thanAppersonandFrohlich (1987). 20 km, the tensionand compressionaxes show

Figure 11 shows the direction of tension on strong clustering implying that theseeventsoc-compressionaxes (whichever is closer to the curred at the interfacethrust zonebetweentheplane)of the earthquakesprojectedon the cross- PacificandPhilippineplates.section plane. It is apparentthat most earth- The segmentboundaryin the centralManianasquakeshavedepthsless than220 km andcluster appearsto have an important influence on the


occurrenceof the large April 5, 1990 Mariana ing togetherwith one segmentmoving into theearthquake.The seismicity near the boundary mantlemorefreely thanan adjacentsegment.showssystematicdifferencesin the seismicactiv- Earthquakesin the Manianasubductionzoneity, fault mechanism, and distribution of the with depths greater than 220 km are relativelyearthquakescomparedwith other regionsalong few. Eventswith depthsgreaterthan 400 km arethe arc.Severalshallow normal-faultearthquakes all clusteredin the northernportion of the sub-occurred near the trench at the northern and duction zone.For the depth range from 220 kmsouthernparts of the Mariana arc between1954 to 580 km the seismicactivity is sparse,andthereto 1967 (KatsumataandSykes, 1969),where seg- is a lack of coherentorientationof the principalment boundarieshavebeenproposed,but these strain axes.The downdip tensionnear 300 kmfeatures are less well defined than the central changesto downdip compression,near 400 kmManianassegment(Burbachand Frohlich, 1990). and below, rather uniformly. There are twoIt seemsthat the lateralslab deformationassoci- earthquakeswith magnitudeslarger than 6 (Fig.atedwith the central Marianasegmentboundary 2) at depths greaterthan 600 km; one being aaugmentsthe slab pull forces and platebending magnitude7 earthquake(7 March 1962, 19.06°N,stressesthat exist along the entire Mariana sub- 145.17°E,683 km depth) (Katsumataand Sykes,duction zone, causing the large normal-fault 1969). The fault mechanismsfor events withearthquakenear the trench. This is discussed depthslargerthan600 km in the MarianaIslandsfurtherbelow, region shows strong clustering of vertical com-

Table 2 showsPCross,PArc and PMap valuesfor pressionandplatenormal tension(Table 2). Thethe central Mariana Islandsearthquakesin van- high seismic activity and the clustering of theousdepth rangesof seismicity.The intermediate compressionandtensionaxeswith respectto thedepth earthquakes(60—220 km) show platenor- slab indicate the subductingplate encountersmal compressionand lateral tension along the strongresistanceas the slab sinks into the mantlesubductingslab. The lateral tension can be ex- andis forcedto deformnearthe top boundaryofplained by the arc curvature model. For slabs the lower mantle. This can be explainedby awith small arc radii andsteepdips the subducted higher densityor higher viscosity for the lowerplatemustundergoconsiderablelateralextension mantle(e.g. RichterandMcKenzie, 1981;Hager,(Frank, 1968). In this model the lateral tensionis 1984).causedby the deformationof the rigid subductingplate when the slab pull acts as the primarydriving force.The lateraltensionmay be substan- 4. Discussionand conclusionstially reducedby the slab segmentation.For asubduction zone with well-defined segment The occurrenceof the April 5, 1990 centralboundaries,such as the Marianas, the lateral Mariana Islandsearthquakesuggeststhat lateraltensionalong the slab may also be createdby the slab distortion may influence the occurrenceofinteractionbetweenadjacentslab segmentsjoin- large normal-faulttrenchearthquakes.Although

TABLE 2

Stressorientationfor Marianasubductionzone

Segment D Events Map Cross-Section Arc(km) T P T P T P

I <60 41 —0.48 0.05 0.61 0.66 —0.01 —0.55II 60—220 35 —0.05 —0.10 —0.39 0.59 0.52 —0.39

III 220—400 3 —0.64 0.26 0.76 —0.31 0.11 0.09IV 400—500 4 0.73 —0.65 —0.52 0.55 —0.05 0.29V >500 6 0.84 —0.81 0.62 0.42 —0.91 0.76


the magnitudeof the 1977 Sumbaearthquakeis followed by an impulsive main event with highmuch larger than the 1990 Mamianaearthquake, stressdrop and subsequentsmooth ruptureex-therearesimilarities in theseismotectonicsof the pansion.sourceregionsof the two events.The great1977 The segmentboundary where 1977 SumbaSumbaearthquake(M~= 8.4) is a normal-fault earthquakeoccurredis the transition region be-event, which also occurred near a segment tweenthe oceanic lithospheresubductingunderboundaryin the Sunda—Bandatrench(Burbach the easternSundatrenchandthe continental-arcandFrohlich, 1990; LynnesandLay, 1988).Near collision at the western Bandaarc. Beneaththethe sourceregionsof both the 1977 Sumbaand easternSundaarc, the Wadati—Benioffzoneex-1990 Mariana earthquakesthe age of the sub- tends to about 650 km with a dip of about 60°,ductingoceaniclithospheresis veryold; thereare while at thewesternBandaarc the subductionofapparentlateral gaps in the shallow- and inter- Australian continental lithosphere has nearlymediate-depthseismicity, andthereis a historical stopped. Although the complexity of theabsenceof large interfacethrust earthquakes.It Wadati—Benioffzonebeneaththe Bandaseapre-is also interestingto note that the Sumbaevent ventsthe segmentboundaryfrom being well de-rupturedin the samefashionasthe 1990Mariana fined at largedepths,the boundarydoescoincideevent, with an initial interval of weak radiation with a major changeat the surfacein thevolcanic

60 uIIII,::i:~I~XI~•.I I

0•• - ~ -

V

-60

I I I I I I I I I I I I I I I I I I I IL? I I I I

120 180 -120 -60Fig. 12. Map of plate segmentboundaries and outer rise earthquakes.Contours of the mean depth of seismicity for theWadati—Benioffzoneof the circum-Pacificandplatesegmentboundaries(solid lines, well constrained;dashlines, poorlydefined)from Burbachand Frohlich (1986). Shadedregionsrepresentbathymetricfeaturesthat intersectsubductionzones.Twenty oneouterrise earthquakesthat have occurredsince 1962 with M

5 ~ 7.0 are shownas opencircles.


line and a discontinuity between the Sunda Figure 12 showsplatesegmentboundariesfromTrench and the Timor Trough. Transition be- BurbachandFrohlich(1986) andepicentralloca-tween the northward dipping slab under the tions of 21 largeouter-rise earthquakeswith M~Sunda Trench and the westward dipping slab > 7.0 that occurredsince 1962 with 13 tensional,under the Aru Trough occursalong the Timor 5 compressional,and3 strike-slip type events.AllTrough. outer rise earthquakes(M~~ 7.0) given by Chris-

Largenormal-faultearthquakeshavealso been tensenandRuff (1988)are included.Table3 listsfound near segmentboundariesin othersubduc- epicentral parametersof theseearthquakesandtion zones.The 1977 Tonga earthquake(M~= plateboundariesadjacentto theseevents.These8.0; Christensenand Lay, 1988) occurredin the large outer-rise earthquakestend to occur intransition region between the Tonga and Ker- proximity to the proposedplatesegmentbound-madectrenches,where the Louisville Ridge im- aries.pingeson the arc. There is a lateraltransitionin The largestnormal-faulteventis the 1933 San-dip of the slab andevidencefor lateraldeforma- niku earthquake,which occurredat the Hokkaidotion of the slab nearthe 1977 rupture(Christen- corner,thejunctionbetweentheKurile andJapansenand Lay, 1988). This suggeststhat in regions trenches.The junction correspondsto a verynearsegmentboundariesof subductionzonesthe well-defined plate segment boundary. Chris-subductingplate is morestronglydecoupledfrom tensenand Ruff (1988) characterizedthe Kumilethe overriding plate by the the slab pull forces, and northeasternJapan subduction zones asand the slab pull forces arecommunicatedupdip strongly and intermediatelycoupled subductionresultingin largetensionalstrainsnearthetrench. zones,respectively.

TABLE 3

Outer rise earthquakesand plate segmentation

Eventa Location Origin Time Latitude Longitude Magnitude Type PSB(UT) (M

5) (T/C/SS) b

P3 Philippines 3110 1975 08:28 12.47N 126.O1E 7.2 T PHIIi Japan 0203 1933 17:31 39.2N 144.5E 8.3 T HOKMl Mariana 05 04 199021:12 15.12 147.60E 7.5 T M3J2 Java 2111196902:05 1.94N 94.61E 7.7 T JV2J7 Sumba 1908 197706:08 11.16S 118.41E 7.9 T JV5S4 SolomonIslands 1708 197223:44 6.04S 152.90E 7.1 T NB2S8 SolomonIslands 29 07 1977 11:15 8.04S 155.56E 7.2 SS NBINil New Hebrides 28 11198502:25 14.04S 166.24E 7.0 T NIH2N12 New Hebrides 28 11198503:49 13.99S 166.18E 7.1 SST NH2Cii Chile 16 10 1981 03:25 33.15S 73.10W 7.2 C SA7Al Aleutian 07 03 192901:34 50.88N 169.71W 7.7 T AKA4 Aleutian 1311196009:20 51.41N 168.86W 7.0 T AKAi0 Aleutian 3003 1965 02:27 50.32N 177.93E 7.5 T AA5K13 Kurile Islands 3006 198201:57 44.56N 151.03E 7.0 SST KK2T7 Tonga—Kermadec 02 07 1974 23:26 29.22S 175.94W 7.2 C TK2T9 Tonga—Kermadec 1110 1975 14:35 24.91S 175.16W 7.8 C TK2T10 Tonga—Kermadec 0204 1977 07:15 16.79S 172.02W 7.6 C TK1Ti3 Tonga—Kennadec 10 10 1977 11:53 25.87S 175.37W 7.2 T TK2T14 Tonga—Kermadec 1706 1978 15:11 17.06S 172.28W 7.0 T TK1Ti8 Tonga—Kermadec 01 09 1981 09:29 15.08S 173.12W 7.5 T TK1T30 Tonga—Kermadec 26 09 198507:27 34.69S 178.66W 7.0 C TK3

a Event data from ChristensenandRuff (1988)with the exceptionof the April 5, 1990 Marianaearthquake.b Typeof fault: tensional(T), compressional(C), and strike slip (SS).

Plate segmentboundary(Burbachand Frohlich, 1986)closestto the event epicenter.


Both tensional and compressionalouter-rise Tonga regionassociatedwith subductionof the Louisvilleearthquakesoccurrednear the Tonga and Ker- Ridge.J. Geophys.Res.,93: 13367—13389.

Christensen,D. and Ruff, L.J., 1983. Outer-riseearthquakesmadec trenches,where the Pacific plate is sub- andseismiccoupling. Geophys.Res. Lett., 10: 697—700.ductingunderthe Australianplate.Severalevents Christensen,D. and Ruff, L.J., 1988. Seismiccoupling and

clustered near the northern end of the Tonga outer-riseearthquakes.J. Geophys.Res.,93: 13421—13444.

trench,where the Niuafo’ou fracture zone trun- Dziewonski, A.M. and Anderson, D.L., 1981. Preliminary

catesthe Tongasubductionzone. referenceEarth model. Phys. Earth Planet. Inter., 25:297—356.

The correlation between the large trench Dziewonski, A.M. and Steim, J.M., 1982. Dispersion and

eventsand plate segmentationsuggeststhat the attenuationof mantlewavesthroughwaveform inversion.

accumulationof very large extensionalstrain at Geophys.J.R.Astron. Soc., 70: 503—527.

oceanic trenchesrequiresspecial circumstances, Dziewonski,A.M. andWoodhouse,J.H., 1983. An experimentand lateral segmentsor distortions in the slab in systematicstudy of global seismicity:Centroid-moment

tensorsolutionsfor 201 moderateand largeearthquakesprovide thesecircumstances. of 1981. J. Geophys.Res., 88: 3247—3271.

Dziewonski, A.M., Ekstrom, G. Woodhouse,J.H. andZwart,G., 1991. Centroid-momenttensorsolutionsfor April—June

Acknowledgments 1990. Phys.Earth and Planet. Int., 66: 133—143.Frank, F.C., 1968. Curvatureof island arcs.Nature,220: 363.Hager,B.H., 1984. Subductedslabs andthegeoid: constraints

We thank Kenji Satakeand Tim Ahern for on mantlerheologyand flow. J. Geophys.Res., 89: 6003—providingthe datafor this study.We thank Kenji 6015.

Satakeand Katsuyuki Abe for commentson the Hanks,T.C., 1970. The Kurile trench—Hokkaidorise system:

paperand for pointing out an error in an early largeshallow earthquakesandsimplemodelsof deforma-tion. Geophys.J.R.Astron. Soc., 23: 123—189.

version of a figure. Heidi Houston for helpful Isacks,B.L., Oliver, J. andSykes, L.R., 1968. Seismologyand

commentson the manuscript.SusanSchwartzfor the new global tectonics.J. Geophys.Res.,73: 5855—5899.helpful commentsand assistingin obtaining the Kanamori, H., 1970. Seismologicalevidence for lithospheric

body-wave inversion code.The datawere made normal faulting: the Sanriku earthquakeof 1933. Phys.

availableby courtesyof the GEOSCOPEproject Earth and Planet. mt., 4: 289—300.Katsumata, M. andSykes, L.R., 1969. Seismicityandtectonicsteam at the Institut de Physiquedu Globe de of the WesternPacific: IzuMariana—Carolineand Ryu-

Paris,and the IRIS DMC. This work was sup- kyu—Taiwan regions.J. Geophys.Res., 74: 5923—5948.ported by NSF grantsEAR-8451715 and EAR- Kelleher, J. and McCann,W.R., 1976. Buoyant zones,great

9017767, and by the W.M. Heck Foundation. earthquakesand unstable boundariesof subduction.J.

Contributionnumber 133 of the Instituteof Tec- Geophys.Res., 81: 4885—4896.Kikuchi, M. and Kanamori, H., 1991. Inversion of complextonics and C.F. Richter SeismologicalLabora- body waves— III. Bull. Seismol.Soc. Am., 81: 2335—2350.

tory, University of California, SantaCruz. Lynnes,C.S. and Lay, T., 1988. Sourceprocessof the great1977 Sumba earthquake.J. Geophys. Res., 93: 13407—13420.

Masters,G. andGilbert, F., 1983. Attenuationin theearthatReferences low frequency.Philos.Trans. R. Soc. London,Ser.A, 308:

479—522Apperson, X.D. and Frohlich, C., 1987. The relationship Regan,J. and Anderson, DL., 1984. Anisotropic modelsof

betweenWadati—Benioffzone geometryand P, T, and B the uppermantle.Phys.Earth Planet. Inter., 35: 227—263.axesof intermediateand deep focusearthquakes.J. Geo- Richter, F.M. and McKenzie, D.P., 1981. On some conse-phys. Res., 92: 13821—13831. quencesandpossiblecausesof layeredmantleconvection.

Burbach,G.V. andFrohlich, C., 1986. Intermediateanddeep J. Geophys.Res., 86: 6133—6142.seismicity andlateralstructureof subductedlithospherein Romanowicz,B., Cara, M., Fels, J.F. andRouland,D., 1984.the circum-Pacific region.Rev. Geophys.,24: 833—874. GEOSCOPE:A French initiative in long-periodthree-

Chapple,W.M. and Forsyth, D.M., 1979. Earthquakesand componentglobal seismicnetworks.Eos Trans. AGU, 65:bendingof platesat trenches.J. Geophys.Res., 84: 6729— 753—756.6749. Ruff, L. and Kanamori,H., 1980. Seismicityand the subdue-

Christensen,D.H. andLay,T., 1988. Largeearthquakesin the tion process.Phys.Earth Planet.Int.,23: 240—252.


Samowitz, I.R. andForsyth,D.W., 1981. Doubleseismiczone Wang,S-C., Geller,R.J.BenAvraham,Z. andStein, S., 1979.beneaththe Mariana Island arc. J. Geophys. Res., 86: Seismicity,aseismicridgesubductionandvolcanism along7013—7021. the Izu—Bonin—Marianasarc(abstract).Eos Trans. AGU,

Satake,X., Yoshida,Y. and Abe, K., 1990. Fault parameters 10: 958.of the Marianaearthquakeof April 5, 1990: the largest Woodhouse,J.H. and Dziewonski, A.M., 1984. Mapping theevent in the region has a normal-fault mechanism.Eos uppermantle: three-dimensionalmodelingof Earthstruc-Trans.AGU, 71: 1438. ture by inversionof seismic waveforms.J. Geophys.Res.,

Seno,T., Moriyama, T., Stein, S., Woods,D.F., DeMets,C., 89: 5953—5986.Argus, D. and Gordon,R., 1987. Redeterminationof the Zhang,J. and Kanamori,H., 1988a.Sourcefinitenessof largePhilippine sea plate motion(abstract).Eos Trans. AGU, earthquakesmeasuredfrom long-periodRayleigh waves.68: 1474. Phys.EarthPlanet. Inter., 52: 56—84.

Spence,W., 1986. The 1977 Sumbaearthquakeseries:Evi- Zhang,J. and Kanamori,H., i988b. Depths of largeearth-dence for slabpull force acting at a subduction zone.J. quakesdeterminedfrom long-period Rayleigh waves. J.Geophys.Res., 91: 7225—7239. Geophys.Res.,93: 4850—4868.

Spence,W., 1987. Slab pull and the seismotectonicsof sub- Zhang,J. and Lay, T., i990a.Effects of centroid location onducting lithosphere.Rev. Geophys.,25: 55—70. determination of earthquakemechanisms using long-

Uyeda,S. and Kanamori, H., 1979. Back-arcopening and period surfacewaves.Bull. Seismol.Soc. Am., 80: 1205—themodeof subduction.J. Geophys.Res., 80: 1053—1064. 1231.

Wallace, T.C., Velasco,A. Zhang, J. and Lay, T., 1991. A Zhang,J. and Lay, T., 1990b. SourceParametersof the 1989broadbandseismologicalinvestigation of the 1989 Loma Loma Prieta earthquake determinedfrom long-periodPrieta,Californiaearthquake:evidencefor deepslow slip? Rayleighwaves.Geophys.Res.Lett., 17: 1195—1198.Bull. Seismol.Soc. Am., in press.

the april 5, 1990 mariana islands earthquake and …thorne/tl.pdfs/zl_mariana...the mariana trench...

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