JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 88, NO. B4, PAGES , APRIL 10, 1983 AN EXPERIMENT IN SYSTEMATIC STUDY OF GLOBAL SEISMICITY:

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1 JOURNAL OF GEOPHYSCAL RESEARCH, VOL. 88, NO. B4, PAGES , APRL 10, 1983 AN EXPERMENT N SYSTEMATC STUDY OF GLOBAL SESMCTY: CENTROD-MOMENT TENSOR SOLUTONS FOR 201 MODERATE AND LARGE EARTHQUAKES OF 1981 Adam M. Dzewonsk and John H. Woodhouse Department of Geoogca Scences, Harvard Unversty, Cambrdge, Massachusetts Abstract. Data from the Goba Dgta hypocenters and the estmaton of magntudes. n Sesmograph Network were used to obtan January 1981 the Natona Earthquake nformaton 'centrod-moment tensor' soutons usng the Servce (NES) began pubshng faut pane method of Dzewonsk et a. (1981). Resuts were soutons for earthquakes wth magntude 6.5 and obtaned for 201 earthquakes rangng n sesmc greater. Kanamor and Gven [1981, 1982] have moment from 7 x 1023 to 3 x 1027 dyne-cm. The shown that wth some restrctons on the dp sp wde dynamc range of the SRO/ASRO statons components for shaow earthquakes, moment tensor aows us to nvestgate, usng the same soutons can be routney obtaned usng data agorthm, seres of events among whch the from the nternatona Depoyment of Acceerosmaest and the argest may dffer n moment by meters (DA; see Agnew et a. [1976]) network for a factor as arge as Among the events events wth moments greater than 1026 dyne-cm, studed s a partcuary nterestng seres, n and sometmes somewhat smaer. January 1981, off the sand of Honshu. The man There are, of course, numerous faut pane shock was preceded by three foreshocks and soutons and estmates of sesmc moment for foowed by four aftershocks a fang wthn smaer events, but these are.usuay derved for the range of our anayss. There s an ndcaton earthquakes of speca nterest. Avaabty of that the source mechansm changes wth the sze the hgh quaty dgta data of the GDSN of the event. Statstca anayss of the resuts (prmary ts SRO and ASRO components) makes t for a earthquakes reveas that the haf possbe to extend the range of goba studes of duraton of shaow and ntermedate events s source parameters to earthquakes wth moments as approxmated by 1.6 x 10-8 MO 1/3. The shfts n sma as 1-2 x 1024 dyne-cm. Systematc studes orgn tme for deep-focus earthquakes are of such earthquakes woud contrbute sgnfndcatve of the genera compexty of these canty to our understandng of the process of events, whch depends ony weaky on the sesmc stress accumuaton and reease n the earth. The moment. The extreme departures from a doube arge dynamc range of the GDSN nstruments coupe mechansm seem to depend both on the aows appcaton of the same agorthm to moment and on the foca depth. Whe for shaow earthquakes dfferng by 4 orders of magntude n earthquakes these departures decrease, from a sesmc moment: somethng that cannot be done maxmum at about 1025 dyne-cm, wth ncreasng usng anaog records. moment, the reverse appears to be true wth Determnaton of the sesmc moment tensor, respect to deep-focus earthquakes. Shaow unke methods that mpcty or expcty earthquakes north of New Gunea and aong the nvove the assumpton of pane shear faure Soomon sands show systematc and substanta (doube coupe mechansm), aows us to departures from the doube coupe mechansm. nvestgate n a more genera way forces actng at the source. Athough t appears that 1. ntroducton resouton of the sotropc component s very dffcut, nonvanshng ntermedate egenvaues The 'centrod-moment tensor' (CMT) souton s are occasonay arge enough to contrbute a resut of smutaneous nverson of the sgnfcanty to the overa pattern of waveform data for the hypocentra parameters of exctaton. From numerous experments t s cear the best pont source (epcentra coordnates, that the exstence of atera heterogenetes n depth and orgn tme) and for the sx the earth may affect the souton for the moment ndependent eements of the sesmc moment tensor and the apparent poston of ts centrod. tensor. Dzewonsk, Chou and Woodhouse [1981; Athough new and exctng dscoveres have been herenafter referred to as DCW], predcted that recenty made n the area of studyng the atera wth the senstvty and the dynamc range of the heterogenety of the earth [cf. Masters et a., Goba Dgta Sesmc Network (GDSN) the method 1982] and the requred theoretca deveopments coud be used to nvestgate hundreds of events are takng pace [Woodhouse and Grnus, 1982; per year. ndeed, n the frst attempt to utze Woodhouse, 1983], the progress, judgng from past t n a systematc manner, we have obtaned CMT experence, s key to be sow. Thus, even soutons for 201 events that occurred durng though t s possbe to perform the usua Even though the procedure s hghy estmaton of the standard errors assumng that automated, processng ths number of earthquakes the dfferences between the data and synthetcs represented a major effort. Why dd we fee that are uncorreated, the standard errors obtaned n t was worth undertakng? ths way are key to be underestmated. For The routne process of quantfcaton of ths reason we cannot estabsh categorcay the earthquakes nvoves the determnaton of the sgnfcance of the derved parameters for ndvdua soutons. However, f two earthquakes Copyrght 1983 by the Amercan Geophysca Unon. n the same source regon, havng smar mechansms and comparabe network coverage, yed Paper number 2B1613. soutons that markedy dffer n, for exampe, /83/002B ther devaton from the doube coupe mechansm, 3247

2 3248 Dzewonsk and Woodhouse: Systematc Study of Goba Sesmcty dfferences then nference are rea. coud be made that these H (t s) = dt d3x{t-ts) j{x t) (3) t s for studes of ths type that J ' nvestgaton of a arge body of data s needed. J Devatons from the doube coupe mechansm may - ¾S not ony depend on the source regon but aso be f (_ s,ts) are the hypocentra coordnates of a functon of tme and of the sze of an the source determned from the frst arrvas of earthquake. the P waves and the source had a consderabe Tempora changes n the source geometry may be spatotempora extent, the eements of the frst of great nterest n probems reated to momentensor can be qute arge, and the earthquake predcton or, n genera, to the eements Mj derved from nverson of sesmc dynamcs of the source regon. Reatvey few data may not be the best estmate of the foreshocks and aftershocks are arge enough that parameters of the 'best pont source.' Foowng mante waves can be used to study ther Backus [1977], we defne the centrod ocaton_ c mechansm; extenson of the dynamc range that and the centrod tme t c to be those for whch our method aows shoud provde much more A and H are mnmzed n the east squares sense. nformaton n ths fed. A good exampe s the A a ebraca detas are avaabe n the Honshu seres of January 1981, consstng of appendx to DCW. three foreshocks, the man shock, and four The CMT nverson s a nonnear east squares aftershocks; the sesmc moments range from procedure. The startng souton s obtaned by 2.4 x 1024 to 4.6 x 1026 dyne-cm, a factor of sovng 200. We woud ke to pont out that even though 6 the voume of the data processed n ths study s Uk(,t ) = k(,,t ) ß f(t) (4) very arge, we determne here ony the = frst-order parameters of the source. There s much to be earned about the compextes of where u k s the kth record n the set of sesmc sources from the detaed examnaton of sesmograms wth the recever at poston x and the waveforms of body waves foowng, for the source at (determned, for exampe, by the exampe, the approach of Kkuch and Kanamor NES). Functons k are the exctaton kernes [1982]. Attempts to ncorporate the effect of and represent synthetc sesmograms correspondng atera heterogenety nto nverson for the to the ndvdua components of the moment tensor sesmc moment tensor are ony begnnng of unt amptude. Functons f represent the sx [Nakansh and Kanamor, 1982; Woodhouse, 1983]. ndependent components of the moment tensor; we The effects of the fnte spatotempora preserve the notaton of Gbert and Dzewonsk dmensons of the source have been studed [1975]. theoretcay [Doornbos, 1981, 1982; Woodhouse, Havng found the nta estmate of the 1982], but the effect of atera heterogenetes sesmc momentensor, f(o), we begn an has not aowed us so far to appy these teratve procedure based on the representaton deveopments n practce. n the foowng sectons we sha brefy outne the CMT method, dscuss certan aspects Uk-Uk(O) = bkars +ckaos +dkaq s of the data sources and methods of data (5) processng, present the resuts for the anayzed earthquakes, and dscuss the mpcatons of these resuts. Wth the excepton of a few partcuary nterestng events and seres of 6 (0) + ekats +>- k.af = where uk(o) s the theoretca dspacement events, we sha concentrate on the statstca cacuated for the startng source coordnates mpcatons of our resuts. and the nta estmate of the moment tensor and bk, Ck, d k and e k are the parta dervatves 2. The Centrod Moment Tensor Method wth respect to perturbatons n the hypocentra coordnates. The method, whch s genera and need not be Essentay, we are determnng the ocaton mted to our partcuar choce of data, has of the pont source whch best expans the been descrbed n fu deta n DCW, ncudng observed sesmograms. f the source s ndeed a the appendx. We defne as the zeroth sesmc moment: Mj = j dt d3x j(x,t) -m V s pont source, then A and H of (2) and (3) become dentcay zero. Tabe 1 of DCW gves an exampe of a synthetc test n whch the startng coordnates were removed by some 100 km from the (1) true coordnates; the nta estmate of the moment tensor was substantay dfferent from the one for whch the synthetc sesmograms were actuay computed. where J s the gut rate dstrbuton [Backus So far the approach s genera. t may be and Mucahy, 1976]. nterestng to notce that except for the ast The frst spata and tempora moments are term, the rght-hand sde of (5) s dentca to defned as the equaton of condton n the probem of earthquake ocaton. The data and exctaton p dt d3x(xp-xs j(x t) (2) kernes coud be those approprate for the P Akj : p), J j waves, SH waves, or fundamenta mode surface - V s waves.,

3 Dzewonsk and Woodhouse: Systematc Study of Goba Sesmcty 3249 G.D.S.N. STATONS OPERATONAL DURNG S TAU 60S 9DS ',,,, 0 60E 120E W 60W 0 Fg. 1. Dstrbuton of statons of the Goba Dgta Sesmograph Network operatona durng Squares, SRO; up-tranges, ASRO; and down-tranges, DWWSSN statons. n DCW we have used the ong-perod body wave egenfunctons were cacuated for the earth tran extendng from the P wave unt the arrva mode PREM of Dzewonsk and Anderson [1981] of the fundamenta mode surface waves. The atter usng the 'mnor' method of Woodhouse [1981a]. are strongy affected n the perod range of t appears that wth the possbe excepton of about 50 s by atera heterogenety and cannot be very deep earthquakes, extenson of the anayss used to determne source parameters by waveform to perods beow 45.s s not justfed. The fttng for a sphercay symmetrc mode. effect of atera heterogenety s such that for Snce pubcaton of DCW we have extended the mutpy refected phases (SS, SSS) the coherence method to ncorporate aso the mante waves. We between the observed sesmograms and synthetcs seect from DA or SRO/ASRO records up to 4.5 s ost much earer n the wave tran wth a cut hours of data after the nstrumenta response off perod of 32 s than when the cut off perod becomes near agan (t sedom s for the R 1 s onger. Thus, even though the resouton for arrvas from shaow earthquakes wth moments the hypocentra parameters may be somewhat greater than 1026 dyne-cm). The data are ow-pass mproved, coverage of the foca sphere s ftered wth a cut off perod of 135 s. Not ony effectvey decreased. Because there s aso an the fundamenta modes but aso the overtone data ncrease n computaton tme, the cut off perod are used n nverson. Addton of 'mante wave' of 45 s appears to be a good compromse when an data s hepfu n t wo ways. Because the waves average earth mode s used. propagatng aong both mnor and major arcs are ncuded, the effectve network coverage doubes. 3. The Data Aso, the maxmum spectra energy n the ong-perod body wave data s n the range of Even though our procedure s desgned to perods from 50 to 60 s, whe for mante waves ncorporate the DA data, the tapes for 1981 are t s between 150 and 180 s. For events of not yet avaabe to us. Thus, n ths paper we consderabe duraton, t s possbe to seect use excusvey the GDSN data. Fgure 1 shows the the approprate haf duraton of the source tme dstrbuton of the network. functon such that the moments derved separatey There was a substanta ncrease n the number from both subsets of data are approxmatey of Dgta Word-Wde Standard Sesmograph equa. f the haf duraton of the source s T, Network (DWWSSN) statons durng Whe at then the spectra of a the kernes and exct- the end of 1980 there were ony 2 statons (LON aton functons are mutped by sn( T)/ T. and JAS), there were 10 at the end of 1981, and The exctaton functons and the kernes are the eeventh (COL) was nstaed n January obtaned through summaton of the norma modes. However, the DWWSSN statons produce records f the cut off perod for the ong-perod body of much ower quaty than SRO/ASRO network wave data s 45 s, the number of norma modes n [Peterson et a., 1976]; t s our estmate that the summaton s approxmatey 5000; owerng of the sgna-to-nose rato s about one order of the cut off perod to 32 s doubes the number of magntude ower, on average. Thus DWWSSN statons modes. The perods of norma modes and ther are generay not very usefu n studyng events

4 3250 Dzewonsk and Woodhouse: Systematc Study of Goba Sesmcty Afrcan, and South Amercan statons are partcuary prone to faure; there were tmes qx x eq when a statons on these contnents (except for j TATO and MAJO) were down smutaneousy. At such tmes the network ceases to be goba; many c events have no coverage n azmuth wndows exceedng 180 ø, and soutons are very unstabe.,0v The method of data processng for ong-perod body waves has been descrbed n deta n T secton 2d of DCW. For events wth NES sœ A magntudes MS 6.5 or for deep events, m b 6.0, we aso routney process the mante waves. The decson to ncorporate mante waves s often made after the premnary nverson of the body wave data, when the actua moment s approxmatey estabshed. The strategy s to m orporate the data begnnng as soon after the eart hquake as possbe. f we can use R2, then R 4 tes us nothng new about the source (R 4 = R2*T, where T s the transfer functon dependng ony on the earth's structure; see, for exampe, Dzewonsk and Stem [1982]), but t ncorporates A a the mperfectons n our knowedge of the earth's structure. The ong-perod body wave data and mante wave data are, after the premnary edtng, nverted separatey, then reedted and merged. Because of the arge dfferences n amptudes, a weghtng system s estabshed, such that an average 28 o. e 8 m o o record of ether type s treated wth the same weght. Exceptons are made for margnay sma Fg. 2. Sod areas ndcate perods of tme moments (~5 x 1025 dyne-cm) when sgna-to-nose durng whch SRO/ASRO statons were not rato n the mante wave data s poor. operatona. Numbers at the top of the fgure Numerous experments have shown that the represent the percentage of tme durng whch a sotropc component of the moment tensor s not partcuar staton was operatona durng 1980 ready resovabe by the data sets used n ths (eft) and 1981 (rght). The fracton of the study. For ths reason, we mpose a near 'down tme' ncreased threefod n 1981 (from constrant on the moment tensor souton: 'GDSNewsetter, Sprng 1982,' pubshed by the Mrr + M88 + M 0. Ths shoud not be construed Goba Sesmoogy Branch of the U.S. Geoogca as a retracton of the resut of Dzewonsk and Survey). unusuay Gbert [191f g.(m The 0 ~ 2 ana Y 2Ls X 8 dyne-cm), there nvoved very deep an event, and 65 WWSSN statons were used to obtan wth moments ess than about 1025 dyne-cm except the souton. Recenty, Sver and Jordan [1982] when a staton s cose to the epcenter. Aso, reported a statstcay sgnfcant sotropc the dynamc range of DWWSSN s ower than that of component for a deep event of March 7, SRO/ASRO; we have seen cppng at a staton at The entre process, ncudng the nta an epcentra dstance of 90 ø for an event wth a processng of a network day tape, requres moment of 5 x 1026 dyne-cm (staton LON for the approxmatey 1 hour of the operator's tme; ths Ageran earthquake of October 10, 1980). Thus tme roughy doubes f mante waves are the effectve dynamc range of these statons s ncuded. We nvestgate a events wth NES not very arge. t mght be worthwhe to magntudes greater or equa to 5.5. Some events consder owerng the senstvty of these wth magntudes beow ths threshod, but nstruments, snce the background nose seems to reported by a arge number of statons, were aso be reachng neary 100 counts out of the tota ncuded. Nucear exposons represent an range of 32,768. excepton: they provde too tte energy at ong Unfortunatey, the performance of the perods. 'backbone' of the GDSN system (the 16 SRO/ASRO statons) has become sgnfcanty worse durng 4. Exampes of Appcaton Fgure 2, taken from the 'GDSN Newsetter' pubshed by the U.S. Geoogca Survey (USGS), Before we present the resuts for 1981 we demonstrates ths fact. Whe the average 'down woud ke to show exampes of three earthquakes tme' was ony 7.7% durng 1980, t ncreased to that ustrate certan aspects of the CMT 23.5% n 1981, wth the performance for the method. second haf of the year beng much worse. Durng n Tabe 1 are sted the nta and fna certan perods of tme n-august 1981 and agan soutons for the Eureka earthquake of November n January 1982, ess than one haf of a 8, The startng coordnates were taken from SRO/ASRO statons were provdng usabe data. the Premnary Determnaton of Epcenters (PDE) Ths deteroraton woud not have been so card, except that the orgn tme was ncreased serous f the fang statons were eveny by 10 s to account for the antcpated fnte dstrbuted, but t seems that the Asan, duraton of the event. The correspondng 'best

5 Dzewonsk and Woodhouse: Systematc Study of Goba Sesmcty 3251 TABLE 1. Centrod-Moment Tensor Souton for the at the free surfaceß But our resut ndcates Eureka, Caforna, Earthquake of NovemberS, 1980 that atera heterogenety compounds the probemß _. An expanaton of ths effect foowsß Startns Fna n the epcentra coordnate system, the exctaton of sesmc waves due to the moment tensor eements Mr0 and M m s governed by the gn tme 1027: :45.6 spherca harmonc Y} or Y [see Gbert and Lattude, ON Dzewonsk, 1975, equatons.v (2.1.30) and (2.1.31)]. n the asymptotc mt these are Lon$tude, øw proportona to sn [( +1/2) + /4] or cos [( +1/2) + /4]; a the remanng eements Depth, km 14 (14) of the moment tensor depend on Y, Y, or ther dervatves, whch for arge vary as Mrr COS [( +1/2) + /4] or sn [( +1/2) + /4]. Thus, ntroducton of a fnte dp sp faut moton MOO may mmc a phase shft, whch w vary as M%% cos ( + ), where determnes the dstrbuton of the effect of atera heterogenety between Mr the components Mr0 and Mr. A change n the Mr epcentra poston of the pont source, or an - - equvaent veocty anomay, aso ntroduces a M phase shft whch vares as cos ( + ). However, the atter effect does not produce the amptude Prmcpa axes varatons that arse from the dp sp T axa components. Therefore, n an nverson that aows for a shft n the source ocaton, the Moment effect of a true or an apparent shft of the Punge/azm 360/290 ø O/282 ø epcenter can be separated from the exctaton pattern due to the dp sp eements of the N axs moment tensor. On the other hand, n an nverson Moment n whch the epcentra coordnates are fxed, the effect of atera heterogenety or ms- Punge/azm 36ø/52 ø 88o/55 P axs o ocaton of the source may be ready absorbed by an artfca contrbuton to the components Mr0 and r r. Moment The ge westward shft of the Eureka earthquake s not an excepton; other events n Punge/azm 34ø/172 ø 1ø/192 ø ths regon show smar shfts. f the network coverage were stabe, a oca correcton for ths The startng (zeroth teraton) souton was effect coud be estabshed. obtaned usng the coordnates provded by NES. The next exampe ustrates the abty of Scae factor s 1026 dyne-cm. the CMT method to fnd the ocaton of the source even when the startng souton s far removed from the true one. The entry n the PDE card for doube coupe' soutons are shown n Fgure 3. an event on September 4, 1981, was 1114:15.3, The nta souton has arge contrbutons from 11 ß 045øN, 123 ß 206øE, 33 km; the magntudes were the moment tensor components Mr and Mrs. On the m b = 5.9, M s = 5.7; accordng to our seecton other hand, the fna souton, hch s crtera, ths event shoud be processed. extremey cose to that gven by Lay et a. The nta teraton gave an extremey poor [1982], s amost entrey strke sp, n ft to the data and t was cear that the agreement wth the tectonc and other sesmc msocaton of the event was too great for the evdence. However, the source has moved to the body wave nverson to converge. However, the west by 1 ø of ongtude. Ths s nconsstent amptudes were arge enough that mante waves wth the orgna ocaton and dstrbuton of coud be used. By pacng the startng depth at aftershocks. Lay et a. suggested that the faut 350 km, we obtaned convergence after four was batera; thus even the shft of the teratons, to a depth of the order of 620 km. centrod due to the fnte ength of a unatera faut coud not account for ths effect. The apparent change n the centrod poston STARTNG FNAL must be, therefore, an artfact of atera heterogenety. Yet shftng the epcenter to ths ocaton heps n obtanng the correct moment tensor. n partcuar, the anomaousy arge eements Mr0 and become zero for a practca purposes. s s mportant because these two eements yed notorousy unstabe vaues for shaow earthquakes [Kanamor and Gven, 1981]. t has been thought that the sma Fg. 3. Comparson of faut pane soutons vaues of the tangenta stran near the surface (major doube coupe) obtaned usng startng and are the cause of ths nstabty. n a way ths fna coordnates of the Eureka, Caforna, s true because the tangenta stress must vansh earthquake of November 8, 1980.

6 3252 Dzewonsk and Woodhouse: Systematc Study of Goba Sesmcty EVENT OF g/ 4/8:1,11:15;:19.4 LAT. ;87, LONG 124.! 7, DEPTH $NZO A 68.9 AZ 141 Z (4394).EVENT OF 9/ 4/81.,11:15:19.4 LAT 9. ;87, LONG! 24.1?, DEPTH 635;. 7 SCP m m 125o 6 m AZ 21 (2590) CTAO A 36.9 AZ 144 E-W (]:7839) N-S (4444) ', ', CHTO A 25.9 AZ 293 Z (19464) MAJO A t! -- N-S (9356) ' KONO A 93.5 AZ 332 Z (321 O) N-S (5648) mm,mm,m, LON A 98.8 N-$ (7]86) AF -- -_- _ b AZ 109,, Z (6508)!, m TME CMN) TME CHN) Fg. 4. Comparson of observed (top) and synthetc (bottom trace) sesmograms for the deep-focus earthquake of September 4, Ony the sectons of records deneated by the vertca broken nes are used n nverson. The response of a statons has been normazed to the standard SRO response. The maxmum amptude for each par of traces s gven n parentheses and represents the dgta SRO count. Observed and synthetc traces have been processed wth a tapered ow-pass fter wth a cut off perod of 45 s. The statons are SRO: SNZO, CHTO; ASRO: MAJO, CTAO, KONO; DWWSSN: SCP, LON, AF. The anguar dstance and azmuth of the recevers from the source are gven n degrees. Usng ths souton as the startng souton for The mante wave records shown n Fgure 5 the combned data set, the fna teraton range n ength from 2 1/2 to 4 hours; n many yeded 1115:19.5, 9.75øN, øE, km. cases, records begn just after the orgn tme, The observed traces and synthetcs are compared so that R 1 s ncuded. The amptudes, after the n Fgure 4 for the body waves and Fgure 5 for ow-pass ftraton wth a cut off perod of the mante waves. The traces were seected to 135 s, are some tmes ower than those for show the quaty of ft for varous staton the body waves; hence the necessty of weghtng. types, components, azmuths and dstances. The records serve to ustrate the mportance of Even though the moment was as arge as 1026 ncudng overtones n the anayss. At GUMO, dyne-cm, the DWWSSN staton SCP shows a hgh where the record represents predomnanty eve of nose on both vertca and horzonta transverse moton, the G waves can hardy be components (the response of DWWSSN statons has recognzed. The same s true for CTAO; the been equazed to that of SRO, therefore the maxmum amptude at ths staton s ony 115 actua dgta count s 10 tmes ess than counts and yet, except for a few gtches, the ndcted by AMAX). Otherwse the match between match of phases s neary perfect. KONO gves an the data and synthetcs s exceent even 50 mn exceent match for a three components. There after the orgn tme. At MAJO, ess than 30 ø s an nterestng msmatch of the amptude of from the epcenter, the match s neary perfect, the enveope of a strongy dspersed overtone and t s aso exceent at LON, even though the sgna observed on the N-S component. Whe the ong-perod background nose s ceary vsbe phases match, the maxmum of the enveope s there. A fary good match s obtaned at AF shfted by some 15 mn. Ths dfference must be (one of the few tmes that ths staton has reated to earth structure rather than to the produced a good quaty record), and t s nadequacy of the source representaton. exceent at SNZO, CTAO, and CHTO. t s somewhat The revsed souton gven for ths earthquake worse at KONO; n partcuar, the S wave on the by the hes n the monthy stng for September N-S component seems to be off both n phase and s 1115:13.6, 9.964øN, øE, 645 km, ony amptude. some 20 km away from that determned by fttng

7 Dzewonsk and Woodhouse: Systematc Study of Goba Sesmcty 3253 GUMO A 2O.6 AZ 78 N-S (143) T 0.76 EVENT OF g/ 4/81,11:15:1g.4, LAT g.87, LONG , DEPTH 635.7, SNZO ' ' A 68.9 AZ 141 Z (194) T 0.12 "'J' CTAO ', AZ 141 Z (150) A T œ-w (115) T 1.44 ' CHTO m A 25.9, AZ 293 E-W (1Bb) T TTME (HOURS EVENT OF 9/ 4/81,11:15:19.4, LAT 9.87, LONG , DEPTH T 1.39 Z (172) j KONO ', A AZ , ' Z (206) ', T O.11 N-S (183) T 0.11! E-W (200) T O Z TATO 351 z (108) A ] 'vvv'vv"",,",'...,v. VV.g.-,,. --v-... -,.; TTME Fg. 5. Mante waves generated by the deep focus earthquake of September 4, T s the deay n hours of the trace shown wth respect to the orgn tme. Both observed and synthetc traces have been processed wth a tapered ow-pass fter wth a cut off perod of 135 s. Statons are SRO: GUMO, SNZO, CHTO, TATO; ASRO: CTAO, KONO. For further detas see capton to Fg. 4. (HOURS) the waveforms from a sparse network of dgta parameters of our souton n the same form as statons. We woud ke to add that among they are ncuded n the Monthy Lstngs earthquakes that we have studed, ths was the pubshed by the NES; expanaton of the entres ony case of a serous msocaton of an event by s gven n the Juy 1981 ssue of the Monthy the NES n the entre year. Lstngs. The 'best doube coupe' (M 0 s the The fna exampe n ths secton s that of average of the absoute vaues of the domnant an earthquake whose sma sze approaches the egenvaues; the ntermedate egenvaue and ts mt of our method to resove the moment tensor egenvector are assumed to represent the nu and ocaton of an event. Fgure 6 shows the axs) s shown at the bottom of the fgure. Ths

8 3254 Dzewonsk and Woodhouse: Systematc Study of Goba Sesmcty HARVARD EVENT-FLE NAME B010g$2A DATA USED: GDSN L.P. BODY WAVES: 18S, 47C, T= 4S CENTROD LOCATON: ORGN TME 12:53: LAT 46.87N B.06;LON 66.41W 0.10 DEP 10.0 FX;HALF-DURATON 2.S MOMENT TENSOR; SCALE D-CM MRR= ; MTT= MFF= ; MRT= MRF=-O.SO 0.41; MTF= o10 PRNCPAL AXES: 1.(T) VAL= 2.!3;PLG=S ;AZM=!64 2. (N) -0.38; 33; 1 3. ½P) -1.7S; 8; 266 BEST DOUBLE COUPLE:MO=f.9*10##24 NP:STRKE=324;DP=47;SLP= 42 NP2: 202, 61; 129 roughy equa; for sma events, the vaues were chosen usng an emprca reatonshp between the moment and faut ength and assumng a 3.5 km/s rupture propagaton veocty. The coumn 'EX' s the exponent n the scang factor: 10 EX dyne-cm. The vaues for the ndvdua components of the moment tensor and ther standard errors shoud be mutped by ths factor. n many cases the depth remans fxed n nverson. Because of the nstabty of the souton for Mr0 and Mr for very shaow depths, the depth was not aowed to be ess than 10 km. n numerous cases, the nta perturbaton woud change the depth to ess than 10 km; n that case the startng vaue or the vaue from the prevous teraton woud be assumed. No attempt was made to perturb the depth f t was ntay sted at 10 km. After about one haf of the data were processed (not n chronoogca order), we stabzed the depth probem by aowng t to change by ony a fracton (typcay, 0.6) of the predcted vaue; usuay the depth perturbaton 'overshoots' the fna answer. Ths resuted n many more earthquakes for whch the centrod depth coud be determned (March, May, June, October, November and December). For a few earthquakes wth partcuary poor Fg. 6. An exampe of a centrod-moment tensor network coverage the epcentra coordnates and souton; the parameters shown are the same as those contrbuted by us to the Monthy Lstngs pubshed by the NES; the frst CMT soutons appeared n Juy The 'beach ba' s the faut pane souton (major doube coupe) for the anayzed earthquake: the New Brunswck event of January 9, s a souton for the New Brunswck earthquake of January 9, The compresson axs s neary horzonta and n an E-W drecton, n genera agreement wth the regona trend n ths part of North Amerca [Pu and ToksSz, 1981]. There s a sgnfcant strke sp component, athough ths may be ntroduced by the dp sp eements whch have very arge errors, a common occurrence for shaow earthquakes. Fgure 7 shows severa traces for dfferent azmuths. Whe the ft and sgna-to-nose rato at KONO, GRFO, BOCO, and ANMO are good, a these statons are at dstances between 30 ø and 50o; t s much worse at TATO but st convncng (A=108ø), even though the maxmum amptude there s ony 63 counts (the fu range s above two mon). Ths earthquake had a moment of 2 x 1024 dyne-cm. KONO A 44.7 AZ 45 Z (255) EVENT OF 1/ 9/82,12:S3:S1.8 LAT 46.98, LONG , DEPTH 10.0 GRFO A 49.6 AZ 56 E-W (338) BOCO A 42.7 AZ 191 N-S (137) ANMO A 32.0 AZ 262 E-W (345) 5. The Resuts for 201 Earthquakes n Tabe 2 are sted the prncpa resuts of our study. The orgn tme, attude, ongtude, and depth are those of the centrod-moment tensor soutons. The perturbatons are the changes wth respect to the NES soutons (from January to June wth respect to the Monthy Lstngs, from TATO A AZ 352 Z (63) [ o Juy to December wth respect to the PDE cards). Fg. 7. Comparson of the observed and synthetc The standard devatons are the forma errors, traces for the earthquake of Fg. 6. Ths whch, as dscussed above, may underestmate the TME (BN) earthquake had a moment of 2 x 1024 dyne-cm; ths rea uncertanty. For arge earthquakes, the haf approaches the ower mt for the sze of an duraton was seected n such a way that the earthquake that can be studed usng our method. moment tensor vaues obtaned separatey from the Statons are SRO: GRFO, BOCO, ANMO, TATO; ASRO: nverson of body waves and mante waves be KONO. See capton to Fg. 4.

9 Dzewonsk and Woodhouse: Systematc Study of Goba Sesmcty 3255 gon LOCATON OF EVENTS WTH H W > 6.0 FOR YEAR g81! 60N 30N 30S 6os + 90S 0 60E 120E W 60W 0 Fg. 8. Locatons of events of 1981 wth moment greater than 1.7 x 1025 dyne-cm (M W > 6.0). Crosses are shaow events (h < 50 km); squares, ntermedate (50 < h < 300 km); and stars, deep earthquakes (h > 300 km). depth were fxed, and ony the orgn tme and seectng ths vaue of sesmc moment as the the moment tensor were sought n nverson. For unt s reated to the abty of the DA and four earthquakes t was necessary to assume that SRO/ASRO networks to record ong-perod data wth Mre = Mr =0. adequate sgna-to-nose rato for mante wave n Fgure 8 we show earthquakes wth moments propagaton on mutpe orbts around the earth greater than 1.7 x 1025 dyne-cm (equvaento M W to be ready observed; t s our experence that = 6.0; Kanamor [1977]); ths represents about wthn a factor of 2 or so, earthquakes wth 35% of a anayzed events. Shaow events (h < moments above 1026 dyne-cm satsfy ths 50 km) are shown as crosses, ntermedate events condton. Once the standng wave pattern s are squares (50 < h < 300 km), and deep events dscernbe, a whoe new cass of sesmoogca are stars. As coud be expected, about 75% of a studes s possbe, and therefore ths unt s events shown are ocated n the western Pacfc, cose to a major dvson n the methods that can aong the New Gunea, Soomon sands, New be used to study earthquakes and n the ways n Hebrdes, and the Tonga-Kermadec trenches. whch earthquakes can be used to study the earth. Some derved parameters of the moment tensor The event shown n Fgures 4 and 5 had a moment soutons of Tabe 2 are sted n Tabe 3. They of 1.0 smu. ncude projecton of the moment tensor nto ts prncpa axs system, wth the argest postve 6.1. Events of speca nterest egenvaue assocated wth the tenson axs, the argest negatve wth the compresson axs, and The Honshu sequence.. A sequence of the ntermedate egenvaue wth the nu axs. earthquakes that began off the sand of Honshu Our defnton of the 'best doube coupe' was on January 18, 1981, s perhaps the most expaned n the prevous secton. t s, of remarkabe of those studed n ths report, n course, nonunque, but t gves the best startng that soutons coud be obtaned for eght souton for the nonnear probem of fttng a earthquakes n approxmatey the same ocaton 'pure' doube coupe to the data set. n and wthn a suffcenty short perod of tme determnaton of the strke, dp, and sp anges that the network coverage remaned neary of the frst (N.P.1) and second (N.P.2) noda constant. panes we have adopted the conventon of Ak and Two foreshocks (events 5 and 6; event numbers Rchards [1980, p. 106]; these anges are dentfy earthquakes n Tabes 2 and 3 as we as desgnated there as s, 6, and, respectvey. n Fgure 9) occurred at 0943 and 1146 UT, ther Fgure 9 gves a graphc representaton of a moments were 0.06 and 0.07 smu. The next 'best doube coupe soutons'; many of these foreshock and the man shock occurred at 1811 and w be dscussed n the foowng secton and, for ths reason, soutons for both events were obtaned smutaneousy. That s, the 6. Dscusson nverson procedure dscussed n secton 2 was modfed so that the same set of the waveforms Snce vaues of the sesmc moments of (ncudng the subsets of ong-perod body wave earthquakes w be used frequenty n ths and mante wave data) was matched by two sources; secton, we sha ntroduce a sesmc moment unt the startng coordnates for both sources were (smu): 1 smu = 1026 dyne-cm. The reason for those gven by the NES, and except for the foca

10 TABLE 2. Centrod-Moment Tensor Soutons for 201 Moderate and Large Earthquakes of 1981 NO. DATE ****** TME ****** *** LATTUDE *** *** LONGTUDE *** *** DEPTH **** HALF EX ****************** M 0 M E N T T E N S 0 R ****************** D M H M SEC DTO SD LAT DLAT SD LONG DLON SD H DH SD DRTN MRR MTT MFF MRT MRF ttf VAL SD VAL SD VAL SD VAL SD VAL SD VAL SD { , ' : % , ; , , , :,

11 TABLE 2. (contnued) D M H M SEC DTO SD LAT DLAT SD LONG DLON SD H DH SD DRTN MRR MTT MFF HT MRF VAL SD VAL SD VAL SD VAL SD VAL SD VAL SD ' O.O O O O ' O.O -O O.O O.O 1.59 O O.O O.O O.O 0.09 O.O , O.O O O to to , O O.O '! ' , , ! , , The headngs are expaned n the text.

12 TABLE 3. Derved Parameters: Prncpa Axs Representatons and the 'Best Doube Coupe' Soutons for the Earthquakes Lsted n Tabe 2 NO. SCALE *** P R N C P A L A X E S *** **** BEST DOUBLE COUPLE **** FACTOR T-AXS N-AXS P-AXS MO **N. P. 1'* **N. P. 2** VA PL AZM VAL PL AZM VAL PL AZM STK DP SLP STK DP SLP 1 10" " " " " "24, "25 ' , " " " " " " " " " " " " " " " " " " " '' " '' '' " '' " '' " '' '' '' '' '' " " " " " " " " " " " " " " " '' " " " " " " '' '' " '' '' '' '' " " " " " " " " " " " " " " " " " " " " " " '' " '' " " " " " " NO. SCALE *** P R N C P A L FACTOR T-AXS N-AXS '' " '' " '' '' " " '' '' o " " " " '' " '' '' " " " " " " " " " " " " " " " " " '' " '' " " " " " " " " " " " " " '' '' " " '' '' " " " " " " " " " " " '' " " " " " " " " '' '' " " " " " " " '' " " " " '' " " " " " " " " " VAL PL AZM VAL PL AZM TABLE 3. (contnued) A X g S *** **** BEST O BLE COUPLE **** P-AXS MO **N. P. 1'* **N. P. 2** VAL PL AZM STKDP SLP STK DP SLP O ot ' ,

13 Dzewonsk and Woodhouse: Systematc Study of Goba Sesmcty ' ' Fg. 9. Faut pane soutons (major doube coupe) for a events sted n Tabes 2 and 3. The crosses ndcate the drecton of the P axes; shaded areas correspond to the expected compressona frst arrvas of P waves. depth, they were perturbed n nverson. The Aso, there s a cockwse rotaton by 10ø-15 ø n foreshock (event 7) had a moment of 0.4 smu and the strkes of the noda panes. the man shock (event 8) 4 smu. Even though the The mechansm of the frst aftershock arge procedure and data set used were dfferent for enough to be anayzed (event 9, 0.09 smu) s events 5 and 6 on the one hand and events 7 and 8 essentay the same as that of the foreshocks, on the other, the geometry of the best doube but the next one (event 12) on January 22 (the coupe souton for event 7 s practcay argest aftershock, 0.5 smu) s much more ke dentca to that of the earer foreshocks: a the man shock. The foowng, smaer aftershock thrust wth the shaow pane dppng at about on January 23 (event 14, 0.1 smu) reverts to the 27 ø and a sgnfcant horzonta component of the foreshock pattern. The next argest event n ths sp vector (average sp ange s 48ø). The man regon occurred on Apr 13 (event 60, 0.02 smu), event, however, s dfferent; the dp of the some 130 km to the southeast of the man shock, shaow pane s much ess (15 ø ) and the souton and had a mechansm smar to that of the s coser to a pure thrust (sp ange = 700). foreshocks.

14 3260 Dzewonsk and Woodhouse: Systematc Study of Goba Sesmcty Fg. 9. (contnued) Thus, even though a precursory change n the moments were 1.3, 0.4, and 0.3 smu. A events source mechansm dd not occur, we observe the were of the norma faut type, but the strke of dependence of the mechansm on the sze of the the noda panes for the two aftershocks were event. The fact that a smutaneous souton for rotated by ø wth respect to the man shock two sources yeded a resut for the foreshock (these events were studed n deta by Kng et mmedatey precedng the man shock that s a. [1982]). Because of the proxmty of the essentay dentca to that obtaned for the events both n space and tme, t s nterestng two earer, soated foreshocks, speaks we for to examne the resut of the reocaton the stabty of the method and nearty of the procedure. nstruments; the rato of the moments of the Fgure 10 s a comparson of the PDE ocatons argest and smaest events of the seres s (crosses) wth the reocaton resuts (sod about 200. crces). There s an overa shft of ocatons The Guf of Cornth earthquakes of toward the southwest; ths may we be a February-March A seres of three arge arge-scae effect of atera heterogenety; the earthquakes n the Guf of Cornth (events 35, YugosavJan earthquake of Apr 15, , and 39) caused consderabe damage; ther [Dzewonsk, 1981], and the rpna (tay) event

15 Dzewonsk and Woodhouse: Systematc Study of Goba Sesmcty : ' õ õ g g Fg. 9. (contnued) of November 23, 1980 [Bosch et a., 1981], azmutha range. The best doube coupe souton showed westward and southwestward shfts, ndcates ether neary horzonta moton on a respectvey. Moreover, the reatve postons of shaow dppng pane or a combnaton of thrust ndvdua events have changed after nverson. and strke sp f the faut pane s neary Whe the PDE ocatons do not show any vertca. Both events show substanta devaton consstent pattern wth respect to the strke of from the doube coupe mechansm: the rato the faut, the events of February 25 and March 4 z (for defnton see secton 6.2.3) s 0.26 for are ocated aong the strke axs of the nta event 91 and 0.21 for event 105. However, n the earthquake. Uness ths s a concdence, the frst case the domnant egenvaue s postve concuson may be drawn that qute precse (tenson), whe t s negatve for the arger reatve ocatons can be obtaned by matchng event (compresson). Ths mght aow one to the waveforms of GDSN recordngs: data from 14 nfer that the departures from the doube coupe SR0, ASR0 and DWWSSN statons were used n mechansm are rea. f these devatbns were nverson. caused by atera heterogenety, one woud expect Two earthquakes n ran: June 11 and the bas n both soutons to be smar because Juy 28. Both of these events were arge enough the ocaton and network coverage are neary to generate mante waves (event 91, 1 smu; event dentca for both events. 105, 7 smu); wthout the addtona coverage The shft n the orgn tme for the frst provded by the major arc arrvas, these event s 8.8 s. n the absence of other earthquakes mght be dffcut to nvestgate, as compcatons, the shft n the orgn tme, or there were no statons n about 180 ø of the the centtod tme, s a measure of the haf

16 Dzewonsk and Woodhouse: Systematc Study of Goba Sesmcty :38.5 : MARCH x 24 FEBRUARY x / FEBRUARY :3.0 2:3.5 LONGTUDE (øe) Fg. 10. Reocaton of epcenters of three earthquakes n the Guf of Cornth. Crosses are NES ocatons; sod crces are centrod ocatons. Note that after reocaton the earthquakes e on an axs parae to the strke drecton (see events 35, 36 and 39 n Fg. 9). duraton of an event. For an event wth a moment of 1 smu, 8.8 s s cose to the expected vaue. However, for the second event the shft s horzonta components s, much too arge for an earthquake wth a moment of ony 7 smu. The nference s that ths was a compex event. ndeed, experments wth expresson of a physca process n whch the orentaton of the nta pane of faure and sp drecton may dffer from those approprate for the entre process of fautng averaged n space and tme. The earthquake of Juy 6 n the Loyaty sands regon (event 100) had a very smar sesmc moment (26 smu) and a smar shft n the orgn tme: 27.6 s, agan an ndcaton of compexty. The centrod depth s determned to be 58 km, a rare exampe of an ncrease from the NES determnaton. At ths depth the Mre and Mr eements are as we determned as the other eements of the moment tensor. n neary the same ocaton but at a greater depth (125 km, NES), there was a much smaer earthquake on August 23 (event 116); ts moment was 0.02 smu, more than 1000 tmes ess than the precedng one, yet the geometry of the best doube coupe soutons are neary dentca for both earthquakes (see Fgure 9). Ths woud ndcate that a consstent faure process n the subducted sab operates over a sgnfcant vertca range. The ast of the three argest earthquakes occurred on September 1 (event 121) and had a moment of 19 smu. t was preceded by 2 hours by a foreshock (event 120) wth a moment of 0.06 smu. Both the foreshock and the man shock are best represented by norma fautng; however, the foreshock had a arge strke sp component, whe the nu axs was neary horzonta for the man shock. t may be a concdence, but a smar effect has been observed (and dscussed above) for the Honshu seres. t appears that n zones where the stress axes e n a vertca pane, the sp vectors for sma events have arger 6.2. Statstca ropertes of the CMT soutons nverson for mutpe sources ndcate that a Hypocentra parameters. t s mportant horzonta rotaton of the prncpa axes took to bear n mnd the dstncton between the pace durng the earthquake. t s nterestng to sgnfcance of the perturbatons n the note that despte the apparent dfference n the geographca coordnates of the source on the one compexty of the events the best doube coupe hand and n the orgn tme and the centrod soutons are smar (see Fgure 9). depth on the other Three events wth moments greater than The average epcentra dstance s about 1027 dyne-cm. Ony three events durng 1981 were 10,000 km, and a shft of 50 km represents a determned to have moments greater than 1027 reatve change of ony 0.5%, whch can easy be dyne-cm (events 78, 100, and 121). caused by atera heterogenety. The foca depth, The frst of the three occurred on May 25 off however, s determned by matchng phases the west coast of the South sand, New Zeaand, propagatng downward from the source wth those and had a moment of 27 smu. The type of fautng refected from the surface; for a 600-km-deep was predomnanty strke sp; f the noda pane earthquake an error n the veocty dstrbuton strkng n the azmuth of N31øE and dppng at of the upper mante of 2% woud resut n an 66 ø to SE s the faut pane, then the sp woud error n depth estmate of ony 12 km. Resouton be rght atera, n agreement wth the sense of of the CMT method wth respect to the hypocentra the reatve moton of the Pacfc and ndan depth s good because a the surface refectons pates at ths pont. The nferred shft n the are automatcay ncuded. orgn tme (26.6 s) s rather arge for an event The shft n the orgn tme s affected by of ths sze and may be an expresson of the perturbatons n the epcentra coordnates and compexty of ths event [Ruff et a., 1982] (the the centrod depth. Thus, n the case of very moment determned by these authors was arger, 45 arge perturbatons n these parameters the tme smu, but the geometry of fautng s dentca). shft w not be a reabe estmator of source t may be worth notng that the NES determned haf duraton. However, f these perturbatons ths event to be a reverse faut on the bass of are sma or f correctons for a change n the the sgns of frst arrvas of P waves. t woud source coordnates can be made, then t 0 can be be nterestng to nvestgate whether some of the used as a measure of the compexty or duraton dscrepances between our resuts and those of the source. The nomna resouton wth pubshed by the NES are an artfact of respect to a perturbaton n the orgn tme s nterpretaton (ether ours or thers) or an good; assumng that the domnant frequency s 0.1

17 Dzewonsk and Woodhouse: Systematc Study of Goba Sesmcty 3263 RELOCATON VECTORS FOR SHALLOW EVENTS ( H < SO ) eon ' ' ' ' ' ' ' ' ' ' ' 6ON SON sos 6os 9os, 0 60E! 20E! 80! 201,/ 601,/ 0 Fg. 11a. Shft of the centrod ocatons wth respect to NES ocatons for shaow earthquakes (h < 100 km). The ength of the arrow s proportona to the magntude of the shft; 1 ø s- ft corresponds to the ength of one dvson on the ongtude scae (30o). The events potted had a moment of at east 1.7 x 1025 dyne-cm. rad/s (perod of 62.8 s), a 1 s shft n orgn 1 ø shft corresponds to the dstance between the tme eads to a 10% rms error. Some very arge dvsons on the ongtude scae (30o). The shfts n the orgn tme, such as neary 30 s characterstc feature of ths fgure s the for the ranan earthquake of Juy 28, cannot be regona consstency of the reocaton vectors; a expaned by a trade-off wth a change n the specfc exampe was dscussed n reaton the epcentra coordnates but must be an expresson the Guf of Cornth earthquakes (secton 6.1.2). of the compexty of the source. Regonay consstent shfts are observed for the Epcentra coordnates.: Fgure 11a shows Medterranean, Japan, the Atantc, South 'reocaton vectors' for shaow (h > 50 km) Amerca, South Pacfc and Tonga-Kermadec. earthquakes wth M smu (M W 6.0). The Undoubtedy, the consstency woud have been even arrows pont n the drecton of the shft of the better f the network coverage had not been so CMT soutons wth respect to NES ocatons; a varabe. Utmatey, assumng that the atter 90N RELOCATON VECTORS FOR DEEP EVENTS ( H > 300 [ 6ON e:::> 30N ß o 30S 60S 90S 0 60E 120E ,/ 601,/ 0 Fg. 11b. Same as Fgure 11a but for deep-focus earthquakes (h > 300 km).

18 Dzewonsk and Woodhouse: Systematc Study of Goba Sesmcty COMPARSON OF NES AND CENTROD DEPTHS O0 m m m 80- ß / O OO azmutha term of the staton correctons reported by Dzewonsk and Anderson [1982]. Fgure 11b shows the reocaton vectors for deep events (h > 300 km), aso wth M 0 > 0.17 smu. The average ength of the reocaton vectors s much ess than n the prevous fgure, and from ths we concude that the atera heterogenety n the outermost km of the earth s prncpay responsbe for the msocaton probem. Centrod depth: Fgure 12a s a comparson of the centrod and NES'depths. Ponts are potted ony for the events for whch we have acheved convergence n nvertng for depth. Cases for whch the centrod depth became ess than 10 km and was fxed are nqt ncuded; the mpcaton s that for these sources the centrod depth s ess than ndcated by the NES. Ponts above the dagona ndcate that the centrod depth s ess than that gven by NES. Ths dfference s very common n a depth range from 10 to 50 km. Ths s probaby reated to the fact that many pp readngs are actuay refectons from the ocean surface; f the ocean s some 6-7 km deep, whch s common for the trench events, an error of up to 30 km may be easy ntroduced. The centrod depths of the events that were assgned by hes the standard determnatons was greater than 100 km. There seems to be no systematc dfference between 100 and 450 km, but beow ths depth, the centrod vaues seem to be systematcay greater. For the events deeper than 600 km the average dfference s about 10 km. Ths may be reated to the fact that the upper mante of the PREM mode s faster than that n the mode of Jeffreys [1939]; for surface focus, the teesesmc trave tmes for PREM are 2.5 s faster than the Jeffreys-Buen (J-B) tabes. The one star n the ower rght corner of the fgure corresponds to the event of September 4, whch was dscussed n secton 5. Shft n the orgn tme: Fgure 13a shows the shft n orgn tme for shaow events dspayed as a functon of the ogarthm of sesmc moment. There s an obvous ncrease n the shft wth ncreasng moment, but there s aso much scatter. The scatter may, n part, be reated to the perturbatons n epcentra coordnates and centrod depth, as was dscussed CENTROD DEPTH (KM) above. For a arge number of events, the perturbatons n the orgn tme due to changes Fg. 12a. Comparson of the NES and centrod n the epcentra coordnates may be expected to depths for events for whch both depths are ess be random. The fact that the centrod depths for than 100 km. shaow earthquakes are generay ess than NES depths shoud resut n a negatve contrbuton to 6t 0. Some of the scatter must be rea, probem can be resoved, t s possbe to thnk refectng dfferences n source tme functons. of deveopng a system of regona correctons, For two earthquakes of the same sze a symmetrc or master events; the remanng perturbatons n batera faut shoud have a vaue of 6t 0 one the epcentra coordnates coud then be reated haf that for a unatera faut. to the spata dmensons of the source. There Proceedng on the assumpton that the faut are some smartes between the drecton of ength s proportona to the cube root of the reocaton vectors obtaned from the CMT sesmc moment [Kanamor and Anderson, 1975], we soutons and the 'sow' drectons of the frst ft to the ensembe of ponts a curve t 0 = a + b M01/3 COMPARSON OF NES AND CENTROD DEPTHS m m m m j CENTROD DEPTH (KM) vaue of 33 km range from 10 to 70 km, wth the Fg. 12b. Same as Fgure 12a but for depths majorty beng shaower than 33 km. There greater than 100 km. Comment: n order to appears to be a group of events wth depths ncrease the number of observatons, we have between 50 and 100 km for whch the NES and ncuded n ths fgure the resuts for January centrod depth determnatons are very cose and 27 soutons for ntermedate and deep Fgure 12b s a comparson of the NES and focus events obtaned by D. Gardn (persona centrod depths for whch at east one of the communcaton, 1982).

19 . Dzewonsk and Woodhouse: Systematc Study of Goba Sesmcty O 3O SHFT N ORGN TME FOR SHALLOW EVENTS ( H < 50 ) T =. 3 -,-.55,,10-8M /3 (R.M.S. 3.7 SEC) 3O SHFT N ORGN TME FOR DEEP EVENTS ( H > 300 ) T = 5.6 * 4.õ#10-9 /$ (R.M.S. 3.0 SEC) "" 20 z 20- o o O , S LOGoM 0 LOGoM 0 Fg. 13a. Shft n the orgn tme (centrod Fg. 13c. Same as Fgure 13a but for deep-focus tme or haf duraton of the source) as a earthquakes (h > 300 km). See comment to Fg. functon of sesmc moment for shaow (h < 12b. 50 km) earthquakes. where t s attrbuted to M. Furumoto and. where a and b are unknown. The resutng vaues Nakansh (n preparaton, 1982); T represents nset n the fgure are reasonabe. The constant source duraton; we show haf duraton. a s sma; ts postve vaue may refect the For deep earthquakes (Fgure 13c) the pcture fact that the basene for the S wave trave s entrey dfferent. There seems to be tte tmes for PREM shoud be decreased by a second or dependence of the shft n the orgn tme on so. The constant b s somewhat ess than the one earthquake moment. The very arge constant a, predcted from a modfed equaton (31) of 5.6 s, can be ony sghty (~1 s) reduced by a Kanamor and Anderson [1975] and assumng a correcton for an average ncrease n the foca rupture veocty of 3.3 km/s (1.9 x 10-8, f M 0 depth of some 10 km. The mpcaton of ths s measured n dyne-cm). Ths coud be expaned fgure s that deep events are commony compex by the fact that some of the fauts are (or mutpe) and that ths compexty, or the batera. deay between the prmary and secondary events, For ntermedate depth earthquakes the s not a strong functon of ther moment. ncrease of t 0 wth M 0 s very smar. The For smpe events the centrod tme may be a scatter of ponts n Fgure 13b s actuay a good measure of the haf duraton of an event. tte ess (rms of the ft s 3.3 s versus 3.7 s However, for mutpe events the reatonshp for shaow earthquakes). The vaues of the breaks down. f two events of very short duraton parameter b obtaned for shaow and ntermedate and equa moment occur 10 s apart, the shft n earthquakes n ths study compare very favoraby the orgn tme, or the apparent haf duraton, wth the vaue of 1.71 x 10-8, whch can be w be 5 s. Ths mght greaty affect resuts obtaned from an expresson: M 0 = 2.5 x 1022 T 3 such as estmated vaues of the stress drop from n Fgure $ of Nakansh and Kanamor [1982], the corner frequences Magntude as a functon of the sesmc moment Magntude s by far the most commony SHFT N ORGN TME FOR NTERMEDATE EVENTS (50<H<$00) used parameter characterzng the sze of an earthquake. For shaow earthquakes both the body wave magntude (mb) and surface wave magntude (Ms) are reported. For earthquakes deeper than km ony m b s estmated by the NES. t T = O. 7 -,-. 74#10-8M1 /3 (R.M.S. 3.3 Fgure 14a shows vaues of m b reported by the NES as a functon of the ogarthm of the z 20 sesmc moment; shaow earthquakes are squares (h < 100 km), and deep and ntermedate earthquakes are shown as stars. o - 10 t s cear that m b s a very poor measure of the sze of an event. For exampe, a vaue m b = 5.9 was assgned to an event wth a moment of 0.01 smu as we as to an event of 4 smu, a factor of 400. The spread for deep events s ess, but st arge: for the same m b of 5.9 there s an event wth a moment of smu and -O 2a z4 2s 2s 27 2e another one of 1 smu, a factor of 40. Lookng at Lø OMO t n a dfferent way, there are severa Fg. 13b. Same as Fgure 13a but for nter- nstances of earthquakes of the same moment medate depth earthquakes (50 < h < 300 km). See dfferng by one unt n magntude. Obvousy, comment to Fg. 12b. statstca studes or cataog searches performed

20 3266 Dzewonsk and Woodhouse: Systematc Study of Goba Sesmcty m b F BODY WAVE MAGNTUDE VS. SESMC MOMENT mb = 0.23õ, LOGoMo - 0.!8 (R.M.S. 0.25) o / _. c3j. [] [] [ "1 ] dscusson of the technca aspects of ths queston may be found n DCW. f we consder the absoute vaues of the egenvaues, and Ema x s the argest and Em n the smaest egenvaue, then the rato = Emn/Emax! s a measure of the devaton of the moment tensor from the doube coupe mechansm. The vaue of z may range from 0 (doube coupe) to 0.5 (near vector-dpoe: Knopoff and Randa [1970]; or equa 'major' and 'mnor' -doube coupes: Kanamor and Gven [1981]). We now examne the dstrbuton of z as a functon of sesmc moment, foca depth, and geographc poston. Fgure 15a shows z as a functon of sesmc moment for shaow earthquakes (h < 50 km). The S O O O vaues of z range from 0 to 0.35, and there seems s to be an ndcaton of varaton of the enveope tog omo of the dstrbuton wth sesmc moment. The maxmum vaues appear to ncrease between 0.01 Fg. 14a. Body wave magntude (mb) as a functon and 0.1 smu, where they reach Ths means of sesmc moment. Squares are shaow events that devatons from the doube coupe mechansm (h < 100 km); stars, ntermedate and deep-focus cannot be an artfact of nose n the data; for earthquakes. The sod ne s the east squares moments as arge as 0.1 smu the sgna-to-nose ft to a the data. rato of the ong-perod body wave data s very good. The maxmum vaues of z seem to decrease n order to estabsh spatotempora patterns of graduay for moments greater than 0.1 smu. Wth the snge excepton of the ranan earthquake of sesmcty n a partcuaregon must be Juy 28, 10 earthquakes wth moments greater than strongy affected by ths extremey weak nk 3 smu have vaues of z at or beow 0.1. between the vaue of m b and the sze of One expanaton for fnte vaues of z s earthquakes. curvature of the pane of faure. As the faut Fgure 14b s a pot of M S aganst og10m0; area grows, the oca effects of curvature are here the correspondence s much better, athough averaged out, and the resut s consstent wth there are st cases of earthquakes wth the the regona ', usuay near, trend. same M S whch dffer by a factor of 100 n A factor contrbutng to the fnteness of sesmc moment; a factor of 10 s common. Agan, z may be reated to an mproper seecton of the one expects that much nose has crept nto foca depth or the fact that a faut may studes of regona sesmcty through the ntersect a major dscontnuty, such as the nadequacy of the magntude scae to measure Moho. Woodhouse [1981b] has shown, for exampe, propery the sze of an event. that for a doube coupe source just above a At the same tme, even though the scatter s dscontnuty there exsts an equvaent source arge, the surface wave magntude s, on average, Just beow, but that the momentensor of ths qute a good measure of earthquake sze n the equvaent source w not be of the doube range of sesmc moments covered n ths study. Wth the events denoted by stars deeted, the east squares ft of M S as a near functon of SURFACE WAVE MAGNTU VS. SESMC MOMENT og10m0 (sod ne) dffers ony by 0.1 from the 8.o theoretca vaue of M W defned by Kanamor [1977] (dashed ne). Whe the consstency of M S wth respect to M 0 coud be mproved by more carefu measurements (an earthquake on the Carsberg rdge on Juy was gven M S of 6.2 n the PDE card; ths vaue was reduced to 5.1 n the Monthy Lstng; our estmate of the moment s smu, whch shoud correspond to an M S of ), there s an nherent ack of a satsfactory premnary 6. o ß 0 0 measure of the sze of deep earthquakes. MS M s = 0.66S togom o - O.S6 (R.M.S. 0.20) O Devatons from the doube coupe mechansm. The symmetrc tensor M, obtaned nder' the constrant tr(m) - 0, has ony two. ndependent egenvaues: E 1 and E 2; the thrd s.o must be -(E 1 + E2). f an earthquake s of the s doube coupe type (pane shear stress), then togtomo there s an addtona condton E 1 = - E 2. But Fg. 14b. Surface wave magntude (Ms) dspayed ths condton s not mposed n our nverson as a functon of sesmc moment. Stars are events procedure; one of the objectves of ths study that were rejected from the estmate of the was to nvestgate the possbty that certan near reatonshp between M S and og10m0 (sod events, or groups of events, may sgnfcanty ne). The dashed ne represents the expresson dffer from the doube coupe mechansm. Further for M W of Kanamor [1977].

21 Dzewonsk and Woodhouse: Systematc Study of Goba Sesmcty DEVATON FROM DOUBLE COUPLE FOR SHALLOW EVENTS ( H < 50 ) DEVATON FROM DOUBLE COUPLE FOR DEEP EVENTS ( H > 300 ) ø4 - - /: b. z H /: z % ß. ß _ O LOGoMo ' Fg, 15a. Devaton of moment tensor soutons from the doube coupe mechansm for shaow (h! 50 kn) earthquakes, LOG omo Fg, 15c, Same as Fgure 15a but for deep-focus events (h > 300 ), See cogent to Fg, 12b. Benoff zone. Dzewonsk and Gbert [1974] found that the tenson axs for the Coumban coupe type, n genera; strke sp sources are earthquake of Juy 31, 1970, rotated durng the an excepton. event, and ther observaton has been confrmed There s a consderabe spread of vaues of by Stretz [1980] n hs anayss of the z among ntermedate depth earthquakes shown n 'subevents' of ths earthquake. Fgure 15b. n comparson wth shaow For sma events, the faure may occur aong earthquakes a much greater proporton of the a sma enough fracton of the conca surface events have z > 0.2. Even the argest event of that t can be we approxmated by a pane, and ths group (Loyaty sands, Juy 6, M 0 = 26 smu) the event has the characterstcs of a doube has = Ths may be an expresson of a coupe. For arge events, however, a much greater substanta heterogenety n the strength of a porton of the surface s nvoved, and the sab undergong a process of bendng and effect of ts curvature becomes sgnfcant. fracturng. There are, however, arge deep events for For deep events (Fgure 15c) one mght whch g s neary zero; actuay, the argest specuate that max ncreases wth sesmc event of the entre set beongs to ths group. moment; most certany the dstrbuton s There must be dfferent dmensona scaes or dfferent from that for shaow earthquakes. t perhaps even dfferent mechansms nvoved n the s characterstc of deep earthquakes that whe processes that manfest themseves as deep one of the axes of stress (usuay the earthquakes. compresson axs, for very deep events) s Fgure 16a shows the geographca dstrbuton parae to the dp of the Benoff zone, the of shaow earthquakes wthn dfferent ranges of drecton of the other axs may be hghy vaues of. Crosses denote ow vaues of (0 < varabe [sacks and Monar, 1971]. Ths coud be < 0.1), squares show wthn an ntermedat consstent wth faure on conca surfaces wth ran e (0.1 < z < 0.2), and stars are earthquakes the axs of the cone foowng the dp of a wth hgh vaues of ( > 0.2). The most strkng feature of ths fgure s that a aong the northern part of the crcum-pacfc bet from DEVATON FROM DOUBLE COUPLE FOR NTERMEDATE EVENTS (50<H<300) o.s,,,, the Phppnes to Coomba the earthquakes wth w z H 0,4 - w o - M W 6.0 are we descrbed by the doube coupe mechansm. There s a substanta concentraton of earthquakes wth moderate to arge vaues aong the northern coast of New Gunea and n the Soomon sands. But just a tte farther to the southeast there s a arge popuaton of earthquakes n the Loyaty sand regon, and most of them ha ve sma. One coud nfer that the partcuary arge devatons of the New Gunea earthquakes must be rea, as the network coverage of ths regon and the adjacent ones, for whch we obtan ow vaues of z, s not sgnfcant. We are ess certan of the sgnfcance of vaues of z for the events n the ndan Ocean because the network coverage for these events s rather poor and there s no LOG oho 'contro group' of earthquakes nearby. But the Fg. 15b. Same as Fgure 15a but for nter- same s true wth respect to a group of three medate depth earthquakes (50 < h < 300 km). See commento Fg. 12b. events n the South Pacfc, a of whch have sma vaues of.

22 ß 3268 Dzewonsk and Woodhouse: Systematc Study of Goba Sesmcty DEVATON FROM DOUBLE COUPLE FOR SHALLOW EVENTS ( H < 00 ) 90N [ [ [, [. 60N / 30N OS 6OS 90S 0 60E 120E W 80W 0 Fg. 16a. Geographca dstrbuton of shaow earthquakes (M 0 > 1.7 x 1025 dyne-cm; h < 100 km) wth dfferent vaues of z (a measure of the devaton from the doube coupe mechansm). Crosses sgnfy < 0.1; squares, 0.1 < < 0.2; stars, > 0.2. An nterestng group of events (events 49, network coverage was essentay unchanged. Aso, 153, and 167) s ocated near the coast of Che, the foca depths and the ocatons are neary the where n neary the same ocaton there are same. However, because of the dfference n foca events wth sma, ntermedate, and arge vaues mechansms, the effect of atera heterogenety of. The one wth M 0 m 3 smu and the argest on the the vaues of cannot be rued out. Ths vaue of (0.22) s aso the deepest one (65.6 exampe ustrates the compexty of the ssue km), and ts mechansm (neary vertca dp sp) and ndcates the centra roe that studes of s qute dfferent from a very cose event wth atera heterogenetes and ther effects shoud sghty arger M 0 (5 smu), whch s a neary 45 ø pay n the future. reverse faut at a depth of 40 km; the vaue of The map for ntermedate depth earthquakes for ths event s Because these two (Fgure 16b) shows a dfferent pattern from that events occurred wthn 3 weeks of each other, the for the shaow events. Whe a shaow events 90N DEVATON FROM DOUBLE COUPLE FOR NTERMEDATE EVENTS (100<H<350) 60N 30N ' S 60S 90S 0 60E 120E 180! 20W 6ow o Fg. 16b. Same as Fgure 16a but for ntermedate depth earthquakes (100 < h < 350 km). Ony 1981 events are shown.

23 Dzewonsk and Woodhouse: Systematc Study of Goba Sesmcty 3269 DEVATON FROH DOUBLE COUPLE FOR DEEP EVENTS C H > 350 ) 90N ] ON - 60S ' 0 60E! 20E! 80! 20W 60W 0 Fg. 16c. Same as Fgure 16a but for deep-focus earthquakes (h > 350 kn). Ony 1981 events are sho. n Japan had ow vaues of c, the ntermedate and resouton of the entre goba network. For depth earthquakes have c > 0.1; the same s true ths reason, events n Asa and the Medterranean wth respecto the events n Tonga-Kermadec. The have been partcuary dffcut to study, as regon of hghy nondoube coupe shaow events there has frequenty been an azmuth wndow of n the New Gunea-Soomon sands regon has 180ø-200 ø wth no statons at a. ntermedate depth events wth sma. The deep 3. The centrod-moment tensor method yeds events (Fgure 16c) under the Chna-USSR border reabe resuts for earthquakes rangng over and the Phppnes have sma c, whe four neary 4 orders of magntude n moment. t s events under Fj-Tonga span a wde range of thus possbe to study, usng the same agorthm, vaues. As n the case of the shaow events off sequences of earthquakes that dffer greaty n the coast of Che, the proxmty of these events sze. By nvertng smutaneousy for more than excudes atera heterogenety as a contrbutng one event, t s possbe to recover source factor. mechansms for earthquakes that are cosey Athough we cannot provde absoute proof, spaced n tme. there s a strong ndcaton that for certan 4. Of the hypocentra parameters of th e regons and depth ranges there are rea and centrod souton the depth and the shft n sgnfcant devatons of source mechansms from orgn tme are of greatest vaue. Depth that of a pane shear faure. There aso appears determnatons are reabe s they do not depend to be dependence of devaton from the doube on the often subjectve dentfcaton of the coupe mechansm on the sze on the earthquake, depth phases. Epcentra coordnates are subject but ths nference must be verfed by to regona bas caused by atera oo o,,., hn,,nk tze hetero enetes. The msocaton vectors are for ndvdua sesmc regons. There are, as consxs en -=.= w yet, nsuffcent data for such an anayss. constant. Provdng that the atter condton s satsfed, t shoud be possbe to estabsh 7. Concusons regona correctons for ths effect and begn to assess the spata dmensqns of the source, Network whch, n theory, the method shoud yed. 1. The Goba Dgta Sesmograph provdes data that aow us to determne source Shaow and ntermedate depth earthquakes parameters (centrod coordnates and sesmc have centrod tmes that depend on the sesmc moment tensor) for earthquakes wth moments of moment n a way that s consstent wth the the order of 1024 dyne-cm ocated anywhere n the reatonshps proposed by Kanamor and Anderson word. [1975]. Deep earthquakes seem to have arge 2. The 16 SRO/ASRO statons represent the centrod tmes, ndcatve of the presence of backbone of the system. The sgna-to-nose rato mutpe earthquakes. of these observatores s, at a perod of about 5. The magntude scae may be too naccurate 30 s, some 10 tmes better, on average, than that for estmaton of earthquake sze n studes that of the DWWSSN statons. The hgh-quaty nvove tempora patterns of sesmcty n a statons, whenever there s a necessty of partcuar regon. Somearthquakes assgned the choce, shoud be gven prorty n mantenance. same vau of M S dffer n moment by a factor of StatOns n South Amerca, Afrca, and Asa 100. have been noperatve for a regrettaby arge 6. t appears that there are earthquakes fracton of tme, whch mpared the performance whch devate sgnfcanty from the doube

24 3270 Dzewonsk and Woodhouse: Systematc Study of Goba Sesmcty coupe mechansm. Among the regons wth shaow dentfcaton of Sesmc Sources --- Earthquake sesmcty, the earthquakes off the north coast or Underground Exposon, edted E. S. Husebye of New Gunea seem to produce events that and S. Mykketvet, D. Rede, pp , commony depart from the doube coupe condtons. Ths effect seems to depend on the DoornbQs, J. J., Sesmc source spectra and sze of an earthquake. For shaow earthquakes momentensors (abstract), Eos Trans. AGU, 63, the arge events seem to depart ess frequenty 375, from the doube coupe mechansm; for deep Dzewonsk, A. M., Routne determnaton of earthquakes the stuaton s reversed. source parameters from the anayss of wave forms recorded by the goba dgta network, Acknowedgments. n a conversaton wth one of Semannua Technca Report, Sesmc Dscrmus (AMD), some 12 years ago, Anton Haes brought naton, 1 OctQber March 1981, pp. up the queston of deep earthquakes as one of the 61-64, Lncon Lab., Mass. nst. of Techno., geophysca probems of great nterest. To a Cambrdge, arge extent, the work done together wth Freeman Dzewonsk, A. M., and D. L. Anderson, Gbert on the Coomban earthquake and the very Premnary Reference Earth Mode (PREM), Ph s. frst momentensor anayss foowed from ths Earth. Panet. nter., 25, , suggeston. t s our peasure to dedcate ths Dzewonsk, A. M., and D. L. Anderson, Trave paper to Anton Haes and to thank hm for a hs tmes and staton correctons for P waves at work and for the thoughts that he has so teesesmc dstances, n press, J. Geophys. generousy shared and contnues to share wth Res., others. We woud ke to thank Domenco Gardn Dze onsk, A. M., and F. Gbert, Tempora for ettng us add hs unpubshed resuts to the varaton of the sesmc moment and the data shown n Fgures 12, 13, and 15. Many tmes evdence of precursve compresson for two deep n ths paper we have compared our resuts wth earthquakes, Nature, 247, , those of the Natona Earthquake nformaton Dzewonsk, A. M., nd J. M. Stem, Dsperson Servce of the U. S. Geoogca Survey. At no and attenuaton of mante waves through pont were these comparsons meant to mpy waveform nverson, Geophys. J. R. Astron. drtcsm; the NES s most certany the best Soc., 70, , organzaton of ths type n the word. Wthout Dzewonsk, A. M., T.-A. Chou, and J. H. the nformaton provded n the PDE cards, our Woodhouse, Determnaton of earthquake source work woud have been many tmes more dffcut or parameters from waveform data for studes of next to mpossbe. We woud partcuary ke to goba and regona sesmcty, J. Geophys. thank Bob Engdah for hs hep and encouragement Res., 86, , n carryng out ths project. The network day Gbert, F., and A.M. Dzewonsk, An appcaton tapes wth the GDSN data were sent to us (Harvard of norma mode theory to the retreva of s one of the three academc Regona Data structura parameters and source mechansms Centers) by the Abuquerque Sesmoogca from sesmc spectra, Phos. Trans. R. Soc. Laboratory, whch s aso responsbe for the London, Ser. A, 278, , wordwde operaton of the network. We woud ke sacks, B., and P. Monar, Dstrbuton of to congratuate the entre group, and especay stresses n the descendng thosphere from a Jon Peterson and John Hoffman, for producng data goba survey of foca mechansm souton of of unsurpassed quaty. Our remarks wth respect mante earthquakes, Rev. Geophys. Space Phys., to the performance of the network were an 9, , expresson of a concern, whch shoud be shared Jeffreys, H., The tmes of P, S, and SKS and the by the geophysca communty, that the resources veoctes of P and S, Mon. Not. R. Astron. assgned to ths operaton are nadequate. Ths Soc. Geophys. Supp1.,, , research was supported by grant EAR from Kanamor, H., The energy reease n great the Natona Scence Foundaton. earthquakes, J. Geophys. Res., 82, , References Kanamor, H., and D. L. Anderson, Theoretca bass of some emprca reatons n Agnew, D., J. Berger, R. Buand, W. Farre, and sesmoogy, Bu. Sesmo. Soc. Am., 65, F. Gbert, nternatona Depoyment of , Acceerometers: A network of very ong perod Kanamor, H., and J. W. Gven, Use of ong-perod sesmoogy, Eos Trans. AGU, 57, , Ak, K., and P. G. Rchards, Quanttatve Sesmoogy, vo., 557 pp., H. W. Freeman, San Francsco, Caf., Backus, G. E., nterpretng the sesmc gut moments of tota degree of two or ess, Geophys. J. R. Astron. Soc., 51, 1-25, Backus, G. E., and M. Mucahy, Moment tensors and other phenomenoogca descrptons of sesmc surface waves for fast determnaton of earthquake source parameters, Phys. Earth. Panet. nter., 27, 8-31, Kanamor, H., and J. W. Gven, Use of ong-perod surface waves for fast determnaton of earthquake source parameters: 2. Premnary determnaton of source mechansm of arge earthquakes (M S > 6.5) n 1980, Phys.'Earth Panet. nter., n press, sources,, Contnuous dspacements, Geophys. Kkuch, M., and H. Kanamor, nverson of J. R. Astron. Soc., 46, , compex body waves, Bu. Sesmo. Soc. Am., Bosch, E., F. Muarga, E. Mantovan, M. 72, , ' Bonafede, A.M. Dzewonsk and J. H. Woodhouse, Kng, G. C. P., J. A. Jackson, G. Houseman, C. The rpna earthquake of November 23, 1980 Soufers, J. Gagnepan, P. Papadmtrou, and (abstract), Eos Trans. AGU, 62, 330, J. Vreux, Sesmcty, norma fautng and the Doornbos, J. J., Sesmc moment tensors, n geomorphoogca deveopment of the Guf of