GEOPHYSICALKESEARCH LETTERS, VOL. 19,NO. 3, PAGES297-300, FEBRUARY 7, 1992

THELARGE NORMAL-FAULTING MAPdANA EARTHQUAKE OF APRIL 5, 1990 IN UNCOUPLED SUBDUCTiON ZONE

YasuhiroYoshida 1,Kenji Satake 2 and Katsuyuki Abe 1

Abstract.A large,Ms = 7.5, shallowearthquake occurred The epicentraldata suggest that the Marianaearthquake is beneaththe Mariana trench on April 5, 1990. From the an outerrise earthquakewhich is definedas an earthquake relocatedaftershock distribution, the fault areais estimatedto originatedwithin the subductingplate in the vicinity of the be70 x 40km 2. A tsunamiobserved on the Japanese islands trenchaxis. In theMariana region, the Pacific plate is aseismi- verifiesthat the depth of the main shockis shallow. For cally subductingbeneath the PhilippineSea plate without waveformanalysis, we use long-period surfacewaves and causinglarge underthrusting earthquakes at theplate interface. bodywaves recorded at globalnetworks of GDSN, IRIS, Thisresults from the weak coupling between the downgoing GEOSCOPEand ERIOS. The centroidmoment tensor (CMT) andoverlying plates. In suchuncoupled subduction zones, the solutionfrom surfacewaves indicates normal faulting on a outerrise earthquakesgenerally possess normal-fault type faultwhose strike is parallel to the local axisof the Mariana mechanismswith their tensional stress axes nearly perpendic- trench,with the tensionaxis perpendicular to it. The seismic ular to the trench strike [Christensenand Ruff, 1988]. It is momentis 1.4 x !020Nm (x 1027dyn.cm) which gives Mw = veryintriguing whether the Mariana earthquake is oneof such 7.3. Far-field P and SH waves from 13 stations are used to tensionalouter-rise events in an uncoupledsubducfion zone. determinethe source time function. Since the sea around the epicentralregion is about5 km deep,body waveforms are Main Shock and Aftershocks contaminated with water reverberations. The inversion results in a sourcetime functionwith a predominantlysingle event The hypocenter parameters of the 1990 Mariana witha durationof 10 sec, a seismicmoment of 2.1 x 1020 earthquakegiven by N'EIC are: origin time 21:12:35.5 on Nm,and a focalmechanism given by strike= 198ø, dip = 48ø, April 5 (UT), epicenter15.125øN, 147.596øE, depth 11 km, slip= 90ø. The shortduration indicates a small area of the and Ms 7.5. The location of the main shock is shown in rupture.The locationof the main shockwith respectto the Figure 1 with the 3-day aftershockdistribution. The main aftershockarea suggeststhat the nodalplane dipping to the shockis locatedjust beneaththe trenchaxis. An accurateesti- westis preferredfor the fault plane.The local stressdrop of mateof focaldepth from teleseismic data is generallydifficult thesingle subevent is estimatedto be 150 MPa (1.5 Kbars). andthe focal depthof the main shockwas constrained to be TheMariana earthquake is consideredto haveoccurred in an 11 km by NEIC. The followinganalysis of surfaceand body uncoupledregion, in responseto the gravitationalpull caused wavessupports this shalloworigin. The T-phasewas clearly bythe downgoing Pacific plate. recordedat ChichijimaIsland, 1400 km from the epicenter, andthis observation is consistentwith the shalloworigin. Introduction The NE!C locationsof aftershocksextend as long as 70 km alongthe trench and the depthdown to 46 km thatis de- A largeshallow earthquake occurred on April 5, 1990just terminedfrom depth phases. For morereliable locations, we beneaththe Mariana trench where the Pacific plate is subduct- applieda masterevent relocation method for arrivaltime dam ingbeneath the Philippine Sea plate. This is thelargest shal- from commonstations. The largestaftershock is selectedas a lowevent ever recorded in theregion; the last event with a similarmagnitude was on September22, 1902with Ms = 7.5 [Abeand Noguchi, 1983]. Since that event is veryold for studyingthe focal mechanism,the 1990Mariana earthquake providesus with a goodopportunity to studythe seismotec- toniccharacteristics of this region. In thispaper, we analyze waveformdata to estimatethe source process. This earthquake alsocaused a tsunami that was observed on the Japanese is- landsas well as the Pacific islands.The unusualbehavior of thetsunami and the implication for hazardassessments of tsunamipotential is describedelsewhere [Satake et al., 1992]. 147OE 148oE

1E•akeResearch Institute, University ofTokyo

2DepartmentofGeological Sciences, University ofMichigan 140'E 145'E 150'E ,-- Fig. 1. Mapof thesource area. Star symbol denotes the main shockand circles denote the 3-dayaftershocks. Data are from Copyright1992 by theAmerican Geophysical Union. NE!C. Dotted line shows the trench axis. Bathymetric Papernumber 92GL000!65 contoursare also shown. Inset shows relocated epicenters and 0094-8534/92/92GL-00165503.00 triangledenotes the master event used for relocation.

297 298 Yoshidaet al.'The Mariana Earthquake of 1990 masterevent. It occurredat 14:57:20.1(LIT) on April 6 (rob= TABLE 1. Centroid moment tensor solution 5.9) near the southernend of the aftershockarea. The results ' Momenttensor elements (10'19 Nm) were almost same as the NEIC locations(Figure 1). The lengthof the aftershockarea is a little shorter,about 65 km...... ñ 0"10 The aftershocks are distributed in the N-S direction with a Moo 1.38 ñ 0.08 sparseregion in thecentral part. The main shock is locatedin M• 11.45 ñ 0.12 the southerncluster. Almost the same distribution is obtained Mro -2.73 ñ 0.59 for a masterevent near the northernend. This patternof the Mr• -6.86 -+ 0.56 aftershockactivity suggests an inhomogeneous distribution of MO• 0.09 stressdrop or strengthon thefault plane. Bestdouble couple ThepP phases were reported on NEIC for 14aftershocks. (!0z9Nm). . . '14A ThepP-P times range from !0 to 15 sec.Since the sea depth Plane a Plane b in theepicentral area is 5 km or so,it is probablethat pwP '(•(strike) ' "16.0ø ...... i'86"iø phaseswere misread to pP phases.We consultedseismo- 15(dip) 60.4 ø 29.9 ø gramsrecorded at Dodaira Micro-earthquake Observatory and •, (slip) ,, -85.1 ø -98.6 ø its satellitestations at Kantoarea in Japan.The epicentraldis- tancesare about25 ø. Clearphases are observed 10 to 15 sec after the P wave arrival. For example,the event of the will get a steeperdip from the followinganalysis of body 22:52:59.9(UT) on April 5 showsclear phases 4 and 13 sec waves. after the P wave arrival. If we assume that the thickness of the waterlayer as 5 km andthe sourcedepth as 25 km, thephases BodyWave Analysis arriving4 and 13 secafter P wavecorrespond to pP andpwP phase,respectively. This suggeststhat the pP phasesreported For body wave analysis, we used P and SH waves by NEIC arepwP phases. This misreadingresults in overes- recordedat 13 stationsat epicentraldistances ranging from 300 timationof the focal depth.We calculatedthe sourcedepth to 100ø. The arrivaltimes were read directly on the broadband usingthe pP-P times of NEIC by assumingthat the phases are channel.All therecords were deconvolved to displacements. pwP and got the depthrange of 12 to 34 km for 12 after- We applieda multipledeconvolution method developed by shockswith an averageof 23 km anda standarddeviation of 6 Kikuchi andKanamori [ 1991]. By minimizingthe difference km. The fault width is estimated to be 40 km when the fault betweenthe observedand syntheticwaveforms, we deter- dip is 42ø which is derivedfrom following body wave minedthe mechanismsof subevents.For the computationof analysis. Green's functions, we assumeda structure consistingof a waterlayer 5 km thick and an oceaniccrust 10 km thicknear SurfaceWave Analysis the sourceregion. We computedGreen's functions by assumingsource We performed a Centroid Moment Tensor (CMT) depths of 6, 10, 14, 16, 20, 25 and 30 km, and derived inversion[Dziewonski eta!., 198!; Kawakatsu,1989] using sourcemechanism for eachdepth with a sourcetime function long-periodsurface waveforms (mainly R1 andG 1) recorded of triangularshape. The residualerror was minimized for the at 19 stationsof the IRIS, GDSN GEOSCOPE [Romanowicz caseof 16 km. Fixingthe depthat 16 km andusing a point eta!., !984] and ERIOS [Takano et al., !990] networks.The source with the same mechanism as that obtained from the stations used were COL, HRV, BCAO, CAY, COR, SCZ, aboveanalysis, we frrstmade only oneiteration. The seismic PAS, KIP, PPT, CTAO, SLR, CMB, KEV, KMI, SSB, moment is obtainedto be 2.1 x 1020 Nm. The sourcetime INU, MAJO, KONO and TSK. The data were deconvolvedto functionis shownin Figure2 (a) andthe synthetic waveforms displacementsand bandpassfiltered betweenthe frequency areshown in Figure3. They showwater reverberation similar range from 3 to 7 mHz. Combining the data from these to the observation,although the detailsare different. The dif- stations,we havea goodazimuthal coverage. ferencemay be dueto our simplifiedwater layer (uniform The final solutionfor the momenttensor elements is given depth)or dueto realcomplexity of the sourceprocess. If the in Table 1. The centroid location is 15.288øN and 147.397øE lattercase is true,more iterations should reveal the complex for a depthof 10 km. The inversionswere performed for the sourceprocess. We made five iterations.The seismicmoment selectivedepths of 10, 20 and33 km. The smallestvariance of is obtainedto be 3.0 x 1020Nm. Figure2 (b) showsthe thesolution was obtained for the depthof 10-20kin. The non- double-couplecomponent that is expressedas the ratio of the a) b) minimumeigenvalue to themaximum is only 8 % andnegli- gibly small.The momentof the bestdouble couple is 1.4 x (x10• Nm/s) (x10•' Nm/s) 1020Nm which gives Mw = 7.3.The best double couple indi- catesalmost pure normal-faulting with the strike almost paral- -4 •4 lel tothe local trench axis. The tensional axis is perpendicular E E to the trenchand the compressionalaxis is almostvertical. Thesestresses are typicalof outerrise normalfault events. Thedip of thewestward dipping plane is 30ø . Thoughthe so- o lO o o lO 20 lution is very similar to that obtainedby Harvard and time(sec} time(sec) GEOSCOPEgroups, the uncertainty is not smallbecause of Fig.2. Sourcetime function for singleevent model (a) and difficultyin determiningthe dip angle for shallowevents. We five subeventsmodel (b). Yoshidaetal.' The Mariana Earthquake of 1990 299

5 sec.We call At7a localstress drop to distinguishit from an HIAp • HIA37 KEV23 averagestress drop over the entire fault. The localstress drop 30 • is estimatedby therelation Ao = 2.5m / (vx)3 wherem is the momentand v is the rupturevelocity [Fukao and Kikuchi, 1987]. Using v = 3.0 km/s, as was used by Fukao and 17 14 Kikuchi [1987], we obtainAo = 150 MPa (1.5 Kbars)for • = 5 secand m = 2.1 x 1020Nm. This stressdrop is much higherthan the averagestress drop of 3.2 MPa over thefault 39 28 planeof 70 x 40 km2. Figure 4 showsthe relation betweenthe durationand

9 "•"• 15 seismicmoment of the largest subeventin large shallow earthquakesof the world.The dataare takenfrom Kikuchi andFukao [1987] for interplatethrust events and Sugiet al. 15 22 17 [ 1987]for intraplatenormal-faulting events. It is indicatedthat 30 see thelocal stress drop for intraplateearthquakes is oneorder of ! , magnitudehigher than that for interplateearthquakes. Sugi et Fig.3. Focalmechanism and body waves. The top and bot- al. [1987]interpreted this in termsof differentnature of fault tomtraces are the observedand syntheticP and SH waves, interface.The Marianaevent showsthe highestvalue among respectively.All the records show ground motion displace- the intraplate shocks.This suggeststhat the Mariana ments.Focal mechanism solution from bodywave analysis is earthquakewas caused by ruptureof almostintact material. plottedon lower hemisphere of focal sphere. Compressional quadrantsareshaded. Seismograms arelabeled with station Discussion and Conclusion namesand maximum amplitude for eachtrace in grn. In the Marianaregion, the Pacificplate is subducting northwestwardlybeneath the PhilippineSea plate. The sourcetime function. Residual reduction (49 %) is not signifi- seismicityindicates that the slab is subductingvertically. The canflydifferent from that of the singleevent (53 %) andit deepestdepth of the trench in theworld and the steepest dip of cannotbe determinedwhether the sourceprocess is complex the Wadati-Benioff zone are the characteristicfeature of the ornot. The seismicmoment from the singleevent is closerto Marianaregion. At theMariana trench, the convergence rate is thatfrom the surfacewave analysis.We prefer the single as low as 4 cm/year[Seno, 1977], and the age of the eventsolution for simplicity.The final sourceparameters are: subductingslab is veryold, about150 Ma [e.g.Ruff and strike= 197.5%dip = 47.8ø, slip= 90.2ø;su'ike = 17.2ø, dip = Kanamori,1980]. Uyeda and Kanamori [1979] categorize 42.2ø, slip =-90.2 ø.The mechanismis purenormal faulting. thesecharacteristics as an extremeend of the evolution of the subductionprocess. StressDrop Thereare two types of stresseswhich may be important in theorigin of the outer rise normal-faulting earthquakes. One is We canderive the stressdrop At5 for the largestsubevent causedby lithosphericflexure [e.g. Chapple and Forsyth, fromthe seismicmoment m and duration2z. In the present 1979]and the other is causedby gravitationalpull fromthe model,the sourcetime functionof triangularshape is as- subductinglithosphere [e.g. Kanamori, 1971]. The flexure sumed,and half theduration z correspondsto therise time of modelsuggests that the outer rise events rupture by tensile stressesin responseto theupward bending of oceanicplate nearthe trench.The gravitationalpull modelsuggests that tensile stressesare transferredfrom the subductingplate ./' directlyto theouter rise due to theuncoupled nature of the plateboundary [Christensen andRuff, 1988]. From the depth extentand the seismic moment, we cannotjudge which model is preferable.In view of thelocation and size, however, we interpretthe Mariana event as due to the gravitational pull. It is well known that the 1933 Sanriku [Kanamori, 1971], the March30, 1965 Rat Island[Mw=7.5; Abe, 1972;Beck andChristensen, 1991] and the !977 Sumba[Mw=8.2-8.3; Spence,1986; Spence, 1987; Lynnes and Lay, 1988] earthquakesoccurred in the brittle part of theoceanic litho- I 5 lO sphereinresponse tothe pull of the subducting slab. Both the Seismic Moment (x10TM Nm) 1933Sanriku and the March, 1965 Rat islandearthquakes oc- Fig.4. Relationbetween d•adon •d seismicmoment of •e curredin subductionzones where the seismiccoupling is in- l•gestsubevent of majorsh•ks of •e world.Open ckcles termediateto strongand underthrusting events with similar representinte•late e•hqu•es [•uchi •d F•ao, 1977], sizeor larger occur. These earthquakes took place in a spatial closedckcles rapresent incaplate no•-fauldng e•hqu•es ortemporal transition stage of thestress regime in thenearby [Sugiet •., 1987] andstar symbol represents the 1990 plateboundaries [Christensen andRuff, 1988]. On the other ••a e•qu•e. DottedUnes eyes •e relationof const•t hand,the Mariana region is a typicaluncoupled subduction s•ess•ops. zone.According to Ruff andKanamori [1980] and Peterson 300 Yoshidaet al.:The Mariana Earthquake of !990 andSeno [ 1984], the subducting and overlying plates in the Kawakatsu,H., Centroidsingle force inversion of seismic Marianaregion are purely uncoupled, that is, the Pacific plate wavesgenerated by landslides,J. Geophys. Res., 94, is subductingaseismically without causing large underthrust- 12363-12374, 1989. ing earthquakes.There is no evidenceof a similar size or KikuchiM. andY. Fukao,Inversion of long-periodP waves largerunderthrusting event during historic times. The 1977 from greatearthquakes along subduction zones, Sumbaearthquake is another example of anouter rise normal Tectonophysics,144, 231-247, 1987. faultingevent in an uncoupledregion. It is the largest Kikuchi,M. andH. Kanamori,Inversion ofcomplex body earthquakein the Sundaarc ever recorded.These show that waves- III, Bull. Seisin.Soc. Am., in press,199!. thelargest earthquakes in the uncoupled zone are outer rise normal fault events. Lynnes,C. S. andT. Lay,Source process of thegreat 1977 Sumbaearthquake, J. Geophys. Res., 93, 13407-13420, The shallowMariana earthquake of 1990occurred beneath 1988. theMariana trench where the Pacific plate is aseismicallysub- Peterson,E. T. andT. Seno,Factors affecting seismic mo- ducting.Waveform analyses and aftershock distribution reveal mentrelease rates in subductionzones, J. Geophys.Res., thatthis event represents pure normal faulting on a faultwith 89, 10233-10248, 1984. strike=197.5ø, dip---47.8 ø and slip=90.2 ø. The seismicmoment Romanowicz,B., M. Cara,J. F. Felsand D. Rouland, is 2.1x 1020Nm. The duration of therupture is 10sec and GEOSCOPE:A Frenchinitiative in long-periodthree- the local stressdrop is as high as 150 MPa. The Mariana componentglobal seismic networks, EOS Trans. AGU, 65, earthquakeis consideredto haveoccurred in theuncoupled 753-754, 1984. region,in responseto the gravitationalpull causedby the Ruff, L. andH. Kanamori,Seismicity and the subduction downgoingPacific plate. process,Phys. Earth Planet. Inter., 23, 240-252,1980. Satake,K., Y. Yoshida and K. Abe, Tsunamisfrom the Acknowledgments.We thank BarbaraRomanowicz for Mariana earthquakeof April 5, 1990: its abnormal sendingus the GEOSCOPE data, staff in IRIS DMC for propagationand implication to tsunami potential from outer- sendingus the IRIS data,staff in JMA for Chichijimaseismo- riseearthquakes, Geophys. Res. Lett., thisissue, 1992. gram and Taku Urabe for showingus the data of Dodaira Seno,T. Theinstantaneous rotation vector of thePhilippine Micro-earthquakeObservatory. We alsothank Larry Ruff, Seaplate relative to theEurasian plate, Tectonophys., 42, Doug Christensenand Roland LaForgefor readingthe 209-226, 1977. manuscriptand givingus valuablecomments. Spence,W., The1977 Sumba earthquake series: Evidence for

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