Received November 4, 1980 Revised version accepted March 10, 1981

Transcription

1 Earth and Planetary Science Letters, 55 (1981) Elsevier Scientific Publishing mpany, Amsterdam - Printed in he etherlands [5] A triple-planed structure f seismicity and earthquake mechanisms at the subductin zne ff Miyagi Prefecture, nrthern Hnshu, Japan etsuz Sen * and Bubpha Pngsawat ** Internatinal Institute f Seismlgy and Earthquake Engineerin~ Building Research Institute, Ministry f nstructin, sukuba, Ibaraki Prefecture, 305 (Japan) Received vember 4, 1980 Revised versin accepted March 10, 1981 A detailed crss-sectin f seismicity cmbined with types f earthquake mechanisms was cnstructed fr the subductin zne ff Miyagi Prefecture, nrthern Hnshu, Japan, where a large M s = 7.5 earthquake ccurred n June 12, dal-plane slutins were determined fr furteen earthquakes with bdy wave magnitudes I> 5.4 using data recrded at WWSS and Japanese statins. Fr smaller-magnitude events, the types f fcal mechanism were analysed using P-wave first-mtin data recrded at reginal statins. he results btained are: (1) Fr the area km landward frm the trench axis, the crss-sectin suggests a triple-planed seismic zne; that is, the zne f thrusting verlaps the s-called duble-planed seismic zne. he upper and lwer planes f the duble-planed seismic zne are characterized by dwn-dip cmpressin and dwn-dip tensin, respectively. (2) hrusting terminates at a depth f 60 km, where the aseismic frnt in the area is lcated, and it merges int the zne f dwn-dip cmpressin in the deeper part, in a dwn-dip directin. In cntrast, the depth f the zne f dwn-dip cmpressin appears t shift by 10 km at the aseismic frnt. (3) he extensin f the zne f dwn-dip cmpressin seaward beynd the aseismic frnt and the shift f the depth f the zne may be caused by lading within the descending slab thrugh a mechanical cupling between the plates at the fcal area f the 1978 earthquake prir t its ccurrence. 1. Intrductin he spatial distributin f earthquakes beneath trench-island arc systems prvides sme basic infrmatin n the kinematic and dynamic prcesses ccurring there. Studies f the gemetry f the Wadati-Beniff zne, cmbined with fcalmechanism slutins, cntribute t ur understanding f the state f the stress and the rhelgical prperties within descending slabs beneath island arcs [1-8]. rthern Hnshu, Japan, is a * Present address: Department f Gephysics, Stanfrd University, Stanfrd, A 94305, U.S.A. ** Present address: Meterlgical Department, Bangkk 11, hailand. unique island arc under which a duble-planed seismic zne has been unambiguusly demnstrated nt nly by the determinatin f hypcenters using a large, lcal netwrk ver hku [5,9] but als by the selectin frm published determinatins f events with well cnstrained hypcentral parameters [6,10]. In the present study, we cnstruct a detailed crss-sectin f seismicity and earthquake mechanisms alng the subductin zne ff and beneath Miyagi Prefecture, situated in the sutheastern part f nrthern Hnshu (see Fig. 1). he fcal mechanisms f earthquakes alng the seismic zne under nrthern Hnshu have been studied n the basis f cmpsite fcal mechanisms fr small and micr-earthquakes [5,9,11]; " IX/81/ /$ Elsevier Scientific Publishing cmpany

2 26 ~--'~ Sea f Okhtsk 40 Japan Sea ~ Pacific Ocean - "- Miyagi Pref ;: ; ~:~,~ ~ Ojika Pen in.,i ) ~, rench axis ;<W : /. (.,., 36' ). f,,,., 1360E Fig. 1. Map f nrtheastern Japan. Miyagi Prefecture is indicated. the purpse because they lack reslutin amng the different types f mechanism which ccur clse tgether in space. On the ther hand, the number f large events which can prduce reliable slutins is s limited [6,13] that the authrs culd nt describe every prtin f the seismic zne by characteristic types f mechanism. In this study, we use smaller-magnitude (mb~>4) events and classify their types f mechanism based n the P-wave first mtins recrded at reginal statins in nrthern Hnshu. Anther cncern f this study is t prduce basic data in rder t examine any changes f seismicity and fcal mechanism that may have ccurred in cnnectin with the Miyagi-Oki (M s = 7.5) earthquake f June 12, he earthquake ruptured the thrust zne dwn t a depth f km, with dimensins f apprximately km 2 near the cast f Miyagi prefecture [14,15] he rupture zne f this earth-quake is ' ',i; rench Axis fcal mechanisms fr larger (m b 1> 6) events have been derived frm data recrded at the Wrld Wide Standardized Seismgraph etwrk (WWSS) [1,6[. he upper and lwer planes f the duble seismic zne are characterized by dwn-dip cmpressin and dwn-dip tensin, respectively [5-7,9,11]. Yshii [6] presented a schematic representatin f the type f fcal mechanism beneath nrthern Hnshu, in which he prpsed that an aseismic frnt which is the seaward edge f a lw-q and lw-v zne in the mantle between the cntinental plate and the descending slab beneath nrthern Hnshu [ 12], may mark the psitin where the predminant types f mechanism changes frm thrusting t dwn-dip cmpressin. he present study fcusses attentin n the transitin zne f earthquake mechanisms near the aseismic frnt. Studies based n the cmpsite fcal mechanism slutin are nt apprpriate fr 39 3S' ' 142 1/.3' 144" Fig. 2. Epicentra] distributin f earthquakes ff MJyagi Prefecture during the perid Selectin f the events is based n data file EQ-l [13]; see the text fr details. Only the events landward f the dashed line are used fr the analysis f mechanism type. he rupture zne f the 1978 Miyagi-Oki earthquake [15] is indicated by the chain line.

3 27 indicated by the chain line in Fig. 2. It seems likely that the prtin f the descending slab near the rupture zne f the Miyagi-Oki earthquake had been stressed thrugh a mechanical cupling between the plates and it may be reflected in seismicity and earthquake mechanisms prir t the ccurrence f this earthquake. he seismicity and earthquake mechanisms cnnected with the Miyagi-Oki earthquake will be briefly discussed in the present study; the details f space-time patterns f seismicity and earthquake mechanisms during a perid f abut furteen years preceding the Miyagi-Oki event will be presented in anther paper [ 16]. 2. Data In rder t elucidate a fine structure f seismicity alng the subductin zne, it is essential t select events with well-cnstrained hypcentral parameters [6-8,10,17]. We fllw Yshii's [6] methd which mainly uses the events with fcal depths cntrlled by pp phases. We have used earthquake data file EQ-1 cmpiled by Yshii I13]. EQ-1 cntains the events during the perid taken frm the Bulletins f the Internatinal Seismlgical enter (IS) which satisfy the fllwing cnditin: the fcal depth is determined by pp-p time intervals, r it is 50 km r larger and its standard errr is nt greater than 5 km. Frm the list f file EQ-1, we selected the events with at least twenty bservatins f P arrival times. Fig. 2 shws the distributin f epicenters f the selected events. Only thse lcated within the area indicated by the rectangle are treated in this study. he area has dimensins f 180 km alng strike f, and 280 km perpendicular t the trench axis. Fig. 3 shws earthquake lci pltted in a vertical crss-sectin perpendicular t the trench axis. lsed circles indicate fci with pp-depths and pen circles thse withut pp-depths. Fr the ffshre events, we used pp-depths crrected fr the water depth, fllwing Yshii's [6] assumptin that the pp phases reprted in the bulletins are thse reflected frm the surface f the seawater (see Yshii [6]). he crss-sectin represent a dubleplaned seismic zne under the area mre than 160 km frm the trench axis. Beneath the sectr frm 150 t 200 kin, the activity f the upper zne is diffuse and appears t be cnstituted by tw znes f activity. In the next sectin, we will see that each f these znes is specified by a different type f fcal mechanism I 975 _ 2S, Ok~ 200 is0, rench Axis s,0? U if':~ '. ~ i@o 0 I 50 cb I1% pp-depth 100kin S.D. ~g5 km Fig. 3. A crss-sectin f earthquake fci perpendicular t the trench axis. lsed circles represent fci determined frm the pp-p time intervals and pen circles thse withut pp-depths which are 50 km and larger and whse standard errr is n greater than 5 km.

4 28 3. Fcal-mechanism analysis Fr earthquakes f m b and abve, we determined the fcal-mechanism slutins. he events analysed are listed in able 1 and their epicenters are shwn in Fig. 4. One event f August 12, 1969 (m b = 5.3) was added because it is the largest event which was lcated in the lwer plane f the duble-seismic zne. he fcal mechanism slutin f event E was already determined by Yshii [13], thus we refer t his result fr this event. Initial mtins f P-waves and plarizatin angles f the initial half cycles f S-waves were read frm film chips f the lng-perid recrds at WWSS statins. When the lng-perid data were nt sufficient, first mtins f shrt-perid P-waves were used as supplements. P-wave data recrded at the statins f the Japan Meterlgical Agency (JMA) and reprted in the bulletins f the IS, "were added t the WWSS data. Fcal mechanism diagrams btained are shwn in Appendix 1. he parameters f the slutins are listed in able ( IS ) MbzS.4, ~,,!)~':..? rench Axis t 39 = I 38 :'i M / 37~1 141OE / I i / I 1/., /, Fig. 4. Epicentral distributin f earthquakes f m b =5.4 and abve ff Miyagi prefecture. he rder f ccurrence f the events is dented alphabetically except fr event O. Out f the fifteen earthquakes in able 1, twelve events have shwn thrust-type mechanisms, tw events ( and O) nrmal faulting, and ne event (G) strike-slip faulting. Events and G are lcated r I t ABLE 1 Slutins f fcal mechanism fr earthquakes ff Miyagi prefecture during the perid Slutin Date Lat. Lng. H * m b A B P ( ) ( E) (km) () 8 () q~ () 8 () AZ () PL () AZ () PL () A Feb. 16, B Jan Apr. 21, D May 01, E ** Jul. 05, F Aug. 16, G Mar. 16, H Sep. 14, I Mar. 25, J Apr. 04, K Jul. 04, L v. 19, M May 05, I Apr. 08, O Aug. 12, A and B: dip directins (q~) and dip angles (8) f the tw ndal planes; P and : azimuths (AZ) and plunges (PL) f P and axes. * H is the crrected pp-depths after Yshii [13]. ** Slutin fr event E is after Yshii [13].

5 29 abut 100 km landward frm the trench axis and at a depth f abut 40 km. he -axes f these events trend nrmal t the trench axis and dip 39 and 34, respectively, which are slightly larger than the dip f the seismic zne there. Event O, lcated 170 km frm the trench axis and at a depth f 77 km, shws a dwn-dip tensin type f mechanism at the lwer plane f the duble-seismic zne. It is clear that the data frm the fcal mechanisms f large (mb I> 5.4) events are nt sufficient adequately t describe the mde f defrmatin alng and especially within, the descending slab. hus, we will nw try t extract infrmatin n the types f faulting frm smaller-magnitude events. Fr events with m b = 5.3 and less, it is usually difficult t read first mtins f lng-perid P-waves at teleseismic statins withut ambiguity. Althugh first mtins f shrt-perid P-waves are ften readable at teleseismic statins fr larger events (mb,-~ 5), they are nt reliable. We have used P-wave first-mtin data recrded at JMA statins lcated in nrthern Hnshu, fr which large amplitudes f first mtins are t be expected. Because it is impssible t determine ndal -plane slutins nly frm the first mtin data at reginal statins, we classified types f fcal mechanism based n the assumptin that a given mechanism in the regin belngs t ne f three types, viz., thrusting, the fault strike f which is rughly parallel t the trench axis in the regin, dwn-dip cmpressin, and dwn-dip tensin. he criterin f the classificatin is that the first-mtin data are cnsistent with nly ne f these three types. Fr the analysis we used nly thse events lcated mre than 140 km landwards frm the trench axis, i.e. events lcated landward f the dashed line in Fig. 2. his was because the data DE JU D OV / \ \ \, / ~ JU SER t)t D Fig. 5. Examples f the classificatin f fcal-mechanism types., and D dente the events classified int dwn-dip cmpressin, dwn-dip tensin, and thrusting types, respectively. ypical fcal mechanism diagrams f the abve three types are shwn at the bttm right.

6 30 frm events lcated remter frm the cast cver nly a small slid angle f the fcal sphere and thus the type f mechanism difficult t identify. Anther reasn was that the uncertainty f a fcus with respect t the Mh r nrad discntinuity affects the cmputed take-ff angles mre seriusly fr shallwer events. In the cmputatin f take-ff angles, we assumed a spherically, symmetrically layered Earth and used the crustal structure revealed by seismic refractin studies ff the Pacific cast f nrthern Hnshu [6,18]. he structure used is a representative ne fr the ffshre regin. Generally, in nrthern Hnshu, the crustal structure and Pn velcity vary frm surces t statins (see Yshii [6]); hwever, this des nt cause much difference in the value f take-ff angles frm thse cmputed in this study because their fci are mstly mre than 15 km belw the Mh depth. he effects f the descending lithspheric plate n the ray paths frm the intermediate events in nrthern Hnshu are nt serius [10,19]. Fig. 5 shws examples f the classificatin. hrusting,dwn-dip cmpressin, and dwn-dip tensin type are represented by the symbls "D", "", and "", respectively. ypical mechanism diagrams f the three specific types are shwn in the inset at bttm right. We referred t as many mechanism diagrams belnging t the specific faulting types as pssible, in additin t the fcal mechanisms derived frm previus studies [5,6,9,11-13,16,20] in the regin. We knw frm these studies that, in the regin mre than 140 km frm the trench axis, the majr types f fcal mechanism are nly thrusting (which represents underthrusting f the Pacific plate beneath nrthern Hnshu), dwn-dip cmpressin, and dwndip tensin; thus we feel that the basic assumptin that we have made fr the classificatin is, at least partly, justified. Because f the scarcity f P-wave first mtin data, there remain events which culd nt bc classified definitely int ne f the three types. Fr these events, a further weak classificatin was made as fllws: ype A represents events cnsistent with thrusting r dwn-dip cmpressin and nt with dwn-dip tensin. ype B represents events cnsistent with dwn-dip cmpressin r dwn-dip tensin but nt with thrusting. After this, a few events remained unclassified int any grup. 4. rss-sectin f fcal-mechanism type Figs. 6 and 7 shw the epicentral distributin and the crss-sectin f the types f fcal mechanism btained, respectively. Symbls "D", "", and "" are the same as in Fig. 5. he pen and clsed circles represent types A and B, respectively. rsses are the events unclassified int any grup r which were nt analysed. he results fr the larger events supprted by the WWSS data are represented by the larger symbls. w events ( and G in Fig.4), 100 km frm the trench axis, were classified int type fr cnvenience. he crss-sectin f types f faulting is almst cnsistent with previus studies f fcal mechanisms f earthquakes n duble-planed seismic znes under island arcs [3,5,6,8,9,11]; that is, the upper plane is characterized by dwn-dip cmpressin () type and the lwer plane by dwn-dip tensin () type. he present study has given a better reslutin fr the upper seismic zne under '/ / : rench i I Axis "~t 39 ~ c.?; 38" r.i 37 lo0km is, ', /, 141"E 142" 143" 144" Fig. 6. Epicentral distributin f the types f fcal mechanism.,, and D dente the events classified int dwn-dip cmpressin, dwn-dip tensin and thrusting types, respectively. Open and clsed circles dente the events f type A and B, respectively. rsses dente the events which are nt analysed r cannt be classified int any type. Larger symbls represent the results fr larger (m b >~ 5.4) events.

7 km , l 5bO rench Axis A.F c r~ c 0 X X x * D D 50 I, cc I 100 krn O ype A ype B Fig. 7. A crss-sectin f types f fcal mechanism, pp-depths and well-cnstrained depths are used fr this sectin. he symbl ntatin is the same as in Fig. 6. A.F. dentes the aseismic frnt [13]. the ffshre area frm 150 t 200 km landwards frm the trench axis. Beneath this area, the seismic activity presents a triple-planed structure; the upper seismic zne is decmpsed int the layer f D-type events and that f -type events, althugh the number f -type events is limited. he distance between the znes f D and types is km and that between the znes f and types is km. he lcatin f the aseismic frnt in the regin [13] is indicated by the brken line in Fig. 7. D-type events are distributed dwn t a depth f 60 km, that is, apprximately t the lcatin f the aseismic frnt. Yshii [12] prpsed that thrusting ccurs nly seaward f the aseismic frnt; the present study supprts this. he zne f -type events appears t shift upward at the aseismic frnt frm seaward t landward. Under the area landward f the aseismic frnt, the distance between the znes f - and -type events is km. he distributin f type A and B events is cnsistent with the distributin f thse f D, and type. 5. Discussin and cnclusin he triple-planed structure seaward f the aseismic frnt is easily understd if a thrust zne alng the plate interface verlaps the dubleplaned seismic zne which represents fractures within the descending slab. his feature is nt s prminent in Fig. 7 because f the sparsity f -type events under the zne f D type; hwever, it hecmes very distinct in the study including the mre recent perid frm 1976 thrugh June 1978 [16], during which many -type events ccurred seaward f the aseismic frnt (see Appendix 2). he lcatin f events relative t the zne f D-type events will give an estimate fr the lcatin f events relative t the plate interface, because D events are mst likely t ccur at the plate interface. Shimamura [21] estimated the lcatins f earthquakes within the descending slab relative t the plate interface in the Kuril arc by interpreting the secndary phase between P and S arrivals bserved at a statin in Hkkaid as the P wave which is reflected and cnverted frm the S wave at the upper bundary f the descending plate. He relcated the hypcenters 10 km belw the plate interface in the depth range f km. he distance between the znes f D and types is cncrdant with his results. Anther interesting feature in the crss-sectin f Fig. 7 is the apparent shift f the zne f events at the aseismic frnt. In cntrast, the zne f D events appears t merge int the zne f

8 32 events deeper than 70 km in a dwn-dip directin. It seems necessary t examine whether the apparent shift f the zne is due t systematic errrs f the hypcentral determinatins. Fcal depths f many f the events seaward f the aseismic frnt are cntrlled by pp readings but, in cntrast, few are cntrlled landward f the frnt (see Fig. 3). hus, this might have prduced a systematic bias fr the depth f events between the landward and seaward parts f the aseismic frnt. Hwever, the shift f the zne can als be seen in the crss-sectin which des nt use pp-depths, but uses instead the usual rutinely prcessed hypcenters (Appendix 2). On the ther hand, in this case, the distinct change f Pn velcity frm 8.1 t 7.5 km/s at the aseismic frnt in nrthern Hnshu [6,22,23] culd be a cause f the systematic bias between hypcenters seaward and landward f the aseismic frnt. he systematic errrs in hypcenters due t the Pn velcity change at the frnt are n the rder f 10 km [24], which is cmparable with the apparent shift f the zne. It is nt a straightfrward matter t estimate the sense and extent f the systematic errrs due t Pn velcity change fr the lcatins f the events in Fig. 7, because the number and azimuthal distributin f the reginal and teleseismic statins used t lcate the events vary cnsiderably. nsidering the accuracy f the data used in the present study, it seems difficult t decide whether the apparent shift f the zne is real r nt. Using ScSp waves cnverted frm ScS waves at the bundary f a sharp velcity cntrast between the descending slab and the mantle abve it, Hasegawa et al. [19] lcated the bundary exactly n the upper plane f the duble-planed seismic zne under nrthern Hnshu, in the depth range greater than 60 kin. hus the spatial distributin f D and events shwn in Figs. 7 and A-2, if we interpret D events as lcated at the plate interface, is in agreement with the result by Hasegawa et al. [19], within the accuracy bth f the hypcentral determinatins and the estimatin f the bundary f ScSp cnversin. Yshii [6] prpsed that the aseismic frnt may mark the change in type f faulting between thrusting and dwn-dip cmpressin. In the crsssectin in the present study, the zne f events extends further seaward beynd the aseismic frnt by 50 km. he crss-sectin f seismicity, cmbined with fcal mechanisms in the regin just nrth f the area in this study ( ) [6,16], des nt present a triple-planed structure seawards f the aseismic frnt, at least during the perid frm 1964 thrugh June his suggests that the ccurrence f events seaward f the aseismic frnt may be a reginal characteristic f the area studied and pssibly be a premnitry phenmenn prir t the 1978 Miyagi-Oki earthquake. Many and D events, alng with a few events, ccurred within and near the rupture zne f the Miyagi-Oki earthquake (see Fig. 6). he cntinental plate wuld have laded this part f the descending slab, thrugh a mechanical cupling between the plates at the rupture zne f the Miyagi- Oki event, befre its ccurrence. his may have induced the activity f - and -type events within the slab in the vicinity f the rupture zne f the Miyagi-Oki earthquake. he shift f the zne at the aseismic frnt, that is, the shift f the neutral surface between the dwn-dip cmpressinal and dwn-dip tensinal stresses, may als have been caused by this lading which is likely t have put an additinal lngitudinal frce n the part f the descending slab. Gt and Hamaguchi [25,26] have explained the duble seismic zne beneath nrthern Hnshu by a thermal stress within the slab caused by heating frm the surrunding mantle as it descends. Recently they have shwn that, when the plate interface seaward f the aseismic frnt is fixed, that is, when the ceanic plate is cupled with the cntinental plate there, the znes f dwn -dip cmpressin and dwn-dip tensin becme distinct under the fixed interface and shift dwnward, in cntrast t the case when the interface is set free [26]. heir result may supprt the abve speculatin as t the cause f the extensin and shift f the zne seaward f the aseismic frnt. In rder t be certain f the uniqueness f the triple-planed structure in the vicinity f the rupture zne f the 1978 Miyagi-Oki earthquake, mre clse studies f seismicity and fcal mechanisms are necessary in ther regins alng island arcs, and in the area f the present study fr the perid after the Miyagi-Oki earthquake; they shuld in-

9 33 clude precise relcatins f earthquakes relative t each ther, t the plate interface, and t the earth's surface. Acknwledgements We wuld like t thank M. Otsuka, Directr at the Internatinal Institute f Seismlgy and Earthquake Engineering (IISEE) fr his encuragement and critical review f ttie manuscript. he majr part f this study was carried ut during the individual study by ne f the authrs (B.P.) at the IISEE. We thank. Yshii fr prviding us with the data file EQ-1 and fr discussin and P. Smerville fr critical review f the manuscript. Discussins with S. Miyamura and with the staffs at the IISEE, and assistance by M. anaka in typing the manuscript were helpful. his wrk was partly supprted by the atinal Science Fundatin under grant EAR References I B. Isacks and P. Mlnar, Distributin f stresses in the descending lithsphere frm a glbal survey f fcal mechanism slutins f mantle earthquakes, Rev. Gephys. Space Phys. 9 (1971) K.F. Veith, he nature f the dual zne f seismicity in the Kuril arc, EOS 58 (1977) E.R. Engdahl and.h. Schlz, A duble Beniff zne beneath the central Aleutians: an unbending f the lithsphere, Gephys. Res. Lett. 4 (1977) B.L. Isacks and M. Barazangi, Gemetry f Beniff znes: Lateral segmentatin and dwnwards bending f the subducted lithsphere, in: Island Arcs, Deep Sea renches and Back-Arc Basins, Am. Gephys. Unin, Maurice Ewing Ser. 1 (1977) A. Hasegawa,. Umin and A. akagi, Duble-planed structure f the deep seismic zne in the nrtheastern Japan arc, ectnphysics 47 (1978) Yshii, A detailed crss-sectin f the deep seismic zne beneath nrtheastern Hnshu, Japan, ectnphysics 55 (1979) K. Fujita and H. Kanamri, duble seismic znes and stresses f intermediate depth earthquakes, preprint (1981). 8 I.R. Samwitz and D.W. Frsyth, Duble seismic zne beneath the Mariana island arc (submitted t J. Gephys. Res.). 9. Umin and A. Hasegawa, On the tw-layered structure f deep seismic plane in nrtheastern Japan arc, Zisin 27 (1975) (in Japanese). 10 M. Barazangi and B.L. Isacks, A cmparisn f the spatial distributin f mantle earthquakes determined frm data prduced by lcal and by teleseismic netwrks fr the Japan and Aleutian arc, Bull. Seisml. Sc. Am. 69 (1979) A. Hasegawa and. Umin, Spatial distributin f hypcenters and earthquake mechanisms under nrthern Japan, Abstr. Annu. Meet. Seisml. Sc. Jpn. 2 (1978) 36 (in Japanese). 12. Yshii, Prpsal f the "aseismic frnt", Zisin 28 (1975) (in Japanese). 13. Yshii, mpilatin f gephysical data arund the Japanese islands, I, Bull. Earthq. Res. Inst. 54 (1979) (in Japanese). 14 hku University, Earthquake ff Miyagi prefecture, June 12, 1978, Rep. rd. mm. Earthq. Predict. 21 (1979) (in Japanese). 15. Sen, K. Shimazaki, P. Smerville, K. Sud and. Eguchi, Rupture prcess f the Miyagi-Oki, Japan, earthquake f June 12, 1978, Phys. Earth Planet. Inter. 23 (1980) Sen, Seismicity and fcal mechanisms prir t the Miyagi-Oki, Japan, earthquake f June 12, 1978, preprint (1980). 17 W.M. happle and D.W. Frsyth, earthquakes and bending f plates at trenches, J. Gephys. Res. 84 (1979) S. Asan,. Yamada, K. Suyehir,. Yshii, Y. Misawa and S. Iizuka, rustal structure ff nrtheastern Japan as revealed frm refractin methd using cean bttm seismgraphs, Abstr. Annu. Meet. Seisml. Sc. Jpn. 2 (1979) 104 (in Japanese). 19 A. Hasegawa,. Umin and A. akagi, Duble-planed deep seismic zne and upper mantle structure in the nrtheastern Japan arc, Gephys. J. R. Astrn. Sc. 54 (1978) akajima, Spatial and sequential distributin f fcal mechanisms befre and after the kachi-oki earthquake f May 16, 1968, Gephys. Bull. Hkkald Univ. 32 (1974) (in Japanese). 21 H. Shimamura, Distributin f thickness f the Kuril plate and hypcentral distributin f earthquakes within the plate, Abstr. Annu. Meet. Seisml. Sc. Jpn. 1 (1973) 118 (in Japanese). 22 Research Grup fr Explsin Seismlgy, Reginality f the upper mantle arund nrtheastern Japan as derived frm explsin seismic bservatins and its seismlgical implicatins, ectnphysics 37 (1977) S. Suzuki, Lateral variatin f P-wave velcity in the upper mantle beneath nrthern Japan, as derived frm travel times f earthquakes, Zisin 29 (1976) (in Japanese). 24 S. Suzuki, Determinatin f earthquake hypcenters in cnsideratin f the lateral variatin f velcity in the upper mantle beneath the island arcs f Japan--n the emur-oki earthquake f 1973, Zisin 28 (1975) (in Japanese). 25 K. Gt and H. Hamaguchi, Duble-planed structure f the intermediate seismic znes--thermal stress within the

10 34 Appendix I Fcal mechanism slutins f the earthquakes listed in able 1 are presented as equal-area prjectins f the lwer hemisphere f the fcal sphere in Fig. A-I. Large circles indicate WWSS lng-perid P-wave first-mtin data and small circles WWSS shrt-perid and JMA data. Plarizatin angles f lng-perid S-waves are indicated by the arrws. dal lines are indicated by thin lines in this figure and their parameters are listed in able 1. Fr thrust-type mechanisms, it is ften difficult t cnstrain the shallwer dipping ndal planes. Fr such cases, we assumed that the strike f the shallwer dipping plane is parallel t the trend f the trench axis in the regin, i.e., ~I5 W, fllwing Sen et al. [15]. A ( B l) F 6 H 14

11 35 I J K i... L M 0 Fig. A-I. Fcal mechanism slutins represented as equal-area prjectin f the lwer hemisphere f the fcal sphere, lsed and pen circles represent P-wave first mtin cmpressin and dilatatin, respectively. Arrws indicate S-wave plarizatin directin. he letters beside each subfigure dente earthquake events in Fig. 4 and able I.

12 36 Appendix 2. Sen [16] analysed fcal mechanisms f earthquakes frm 1976 t June 11, 1978, that is, during a tw and a half year perid just prir t the Miyagi-Oki earthquake f June 12, 1978, in the same regin and using the same methd as in this study. Fig. A-2 shws the crss-sectin f fcal mechanism types which incrprate the results fr this perid. he ppdepths are nt used in this figure in rder t examine the systematic bias f the fcal depth distributin due t pp-depths (see discussin in the last sectin). he zne f -type events seaward f the aseismic frnt becmes mre distinct and the shift f the zne f events at the aseismic frnt can als be seen in this figure JU.11,1978 ;~Owrn 2q0,s, rench Axis! A.F. D x D 0 x 50 e ~ ~. ~.c,.. Q c ccc 100km ype A ype B Fig. A-2. A crss-sectin f the fcal-mechanism types incrprating the results during the perid frm 1976 thrugh June 11, 1978 [16]. he fcal depths frm the rutine determinatins f the IS are used instead f pp-depths. he symbl ntatin is the same as in Fig. 7. lithsphere, Abstr. Annu. Meet. Seisml. Sc. Jpn. 2 (1978) 20 (in Japanese). 26 K. Gt and H. Hamaguchi, Distributin f thermal stress within the descending lithspheric slab, Abstr. Annu. Meet. Seisml. Sc. Jpn. 2 (1980) 48 (in Japanese).