Detailed subduction structure across the eastern Nankai Trough obtained from ocean bottom seismographic profiles

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. t03, NO. Bll, PAGES 27,151-27,168, NOVEMBER 10, 1998 Detailed subduction structure across the eastern Nankai Trough obtained from ocean bottom seismographic profiles AyaKo ^,T..:o.: 2 Hajime S ' ' ' " 1,3 4 5,6 Toshihiko Kanazawa, 7,8 and Hideki Shimamura Abstract. To investigate the deep crustal structure of the Philippine Sea Plate at its northern margin, we performed a seismic refraction and wide-angle reflection survey around the eastern Nankai Trough by using an ocean bottom seismograph array with an air gun source. We derived a crustal structure model across the trough, from the Shikoku Basin to the continental slope, that explains not only our seismic data but also previously published gravity data. The origin of the Zenisu Ridge, a conspicuous topographic high along the oceanward slope of the Nankai Trough, is still a matter of argument. There has been some controversy as to whether the igneous activity of the Izu-Ogasawara Arc or the seafloor spreading of the Shikoku Basin is responsible for the formation of the ridge. Although the crustal thickness beneath the ridge is between 8 and 11 km, slightly thicker than typical oceanic crust, our structure model clearly indicates that the Zenisu Ridge has an oceanic crust and its structure is very similar to that of the Shikoku Basin. Beneath the south flank of the Zenisu Ridge, the Moho shows an offset of 5 km in depth. This offset may correlate with the recently proposed nascent subduction boundary and subduction-collision tectonics of this area. The velocity structure beneath the continental slope appears characteristic for a well-developed accretionary wedge bounded by the continental upper crust of the Japan Island Arc to the northwest, and subducting oceanic crust which can be traced beneath the accretionary wedge and the continental upper crust. 1. Introduction ferences in topographic and tectonic features between the Nankai Trough and other typical trenches. First, the Zenisu The Nankai Trough is one of the convergent boundaries Ridge, a conspicuous topographic high rising 2000 m from around Japan, and the eastern part of the trough is located the bottom of the Nankai Trough, appears along the oceanoff the Tokai district (Figure 1). The length of the Nankai ward slope of the trough and disappears south off the Kii Trough is about 600 km; its water depth is m and Peninsula. There is no agreement as to which is responsible is thus shallower than other trenches around Japan. Although for the formation of the Zenisu Ridge: the igneous activity the Nankai Trough forms the convergent boundary between of the Izu-Ogasawara Arc [e.g., Bandy and Hilde, 1983] or the Philippine Sea Plate and the Eurasian Plate, there are difthe seafloor spreading of the Shikoku Basin [e.g., Lallemant et al., 1989]. On the basis of previous geological and crustal studies there are three possible ways to explain the origin of Laboratory for Ocean Bottom Seismology, Faculty of Science, the Zenisu Ridge. Hokkaido University, Sapporo, Japan. 1. Because of its topographic features the Zenisu Ridge 2Now at Frontier Research Program for Subduction Dynamics, may be one of the enechelon ridges belonging to the Izu- Japan Marine Science and Technology Center, Yokosuka. 3Now at Department of Earth Science, Faculty of Science, Toya- Ogasawara Arc [e.g., Bandy and Hilde, 1983]. ma University, Toyama, Japan. 2. There are several remnant arcs of the Izu-Ogasawara 4Research Center for Prediction of Earthquakes and Volcanic Arc, where volcanism ceased when the spreading of the Eruptions, Faculty of Science, Tohoku University, Sendai, Japan. Shikoku Basin started [Klein and Kobayashi, 1980], for exs Research Center for Earthquake Prediction, Faculty of Science, ample, the Kyushu-Palau Ridge and the Shichito Ridge (Fig- Hokkaido University, Sapporo, Japan. ure 1 ). These remnant arcs are known to be similar to oceanic ønow at Deep Sea Research Department, Japan Marine Science and Technology Center, Yokosuka. crust, but their crustal thickness is greater than that of normal 7Laboratory for Earthquake Chemistry, Faculty of Science, Uni- oceani crust. The Zenisu Ridge thus can be classified as a versity of Tokyo, Tokyo, Japan. kind of remnant arc. SNow at Earthquake Research Institute, University of Tokyo, To- 3. The crust beneath the Zenisu Ridge may be of oceanic kyo, Japan. origin, as suggested by gravity analyses [Ishihara, 1989]. Similarly, Lallemant et al. [ 1989] deduced, mostly from the Copyright 1998 by the American Geophysical Union. magnetic anomalies 6B and 6C, that the crust beneath the Paper number 98JB Zenisu Ridge is of oceanic origin and was formed in the early /98/98JB Miocene, about 23 Ma, and presumably uplifted at a later 27,151

2 ß ß. 27,152 NAKANISHI ET AL.' STRUCTURE AROUND THE EASTERN NANKAI TROUGH 130øE 140øE 150øE 40øN 30øN 35øN EUR ß.. Tokai district ':' i 1000km ß :ill:.. :ii :..i!::i:..:i ::..-:iii:'":'..:. : '[i ::.i" Kii Pen, 34øN / / " "-,.C) ) 33øN,r 3, /. I PsPI 136'E 137øE 138'E Figure 1. Tectonic map of the northeastern Nankai Trough. Water depths are in kilometers. Solid lines with triangleshow convergent boundaries (triangles are on the ovrriding plate side). The northern margin of the Izu Peninsula (Izu Pen.) is located at the collision zone between the Izu-Ogasawara Arc and Japan Island Arc. Sa, Sagami Trough; Su, Suruga Trough; Kii Pen., Kii Peninsula; PSP, Philippine Sea Plate; EUR, Eurasian plate. Shaded part indicates a rupture area for the future Tokai Earthquakexpected by Ishibashi [ 1981 ]. stage. Aoki et al. [ 1982] suggested that this uplift was caused has been subducting beneath the Japan Island Arc. Third, by the great stress associated with convergence. Moreover, the Izu-Ogasawara Arc has been colliding with the Japan Is- Chamot-Rooke andle Pichon [ 1989] presented a mechanical land Arc since the middle Pleistocene at the northern end of model of the formation of the Zenisu Ridge, which predicts the Izu Peninsula [Huchon and Angelier, 1987], where the that the uplift occurred after 4 Ma. These studies also sup- landward continuation of the Suruga Trough and the Sagami por the hypothesis that the crust beneath the Zenisu Ridge is Trough is bent to the north and forms a collision zone [e.g., of oceanic origin. Matsuda, 1978]. Second, the Shikoku Basin has opened behind the Izu- Further, based on the remarkable temporal and spatial reg- Ogasawararc-trench system as its back arc basin, which ularity of a series of magnitude (M) 8 interplate earthquakes

3 NAKANISHI ET AL.: STRUCTURE AROUND THE EASTERN NANKAI TROUGH 27,153 that recurred along the Nankai Trough, the possibility of an precise knowledge of the crustal structure around the eastern earthquake of M 8 in the Tokai district has been suggested Nankai Trough. since 1970 [Ando, 1975]. However, the area has not experi- In 1992 an extensive OBS experiment off the Tokai disenced a major earthquake since the Ansei Tokai earthquake trict was performed. Nakanishi et al. [1994] gave a prelimin inary crustal model obtained from these data. In this paper Despite the importance of the eastern Nankai Trough off we present final crustal structures, which explain not only the Tokai district for seismology and geology, few geophys- the OBS data but also the gravity data [Sandwell and Smith, ical investigations of this area have been undertaken, and its 1997]. We then use these models to discuss the features of crustal structure remains unclear. Seismic experiments by the crustal structure related to the subduction and collision Murauchi et al. [1968] and Asano et al. [1985] outlined tectonics, the origin of the Zenisu Ridge, and the presence of the structure of the ocean-continent transition zones off the the nascent subduction boundary at the south of the Zenisu Kii Peninsuland off the Izu Peninsula, respectively. How- Ridge. ever, these two studies could not resolve the deep structure related to subduction. An ocean bottom seismograph (OBS) 2. Data Acquisition and Modeling Procedure study performed in the southwestern Nankai Trough revealed a large accretionary wedge above the subducting oceanic Figure 2 shows the locations of our seismic refraction and crust [Nishizawa and Suyehiro, 1989]; however, the precise wide-angle reflection profiles. Twenty OBSs were deployed geometry of the subducting oceani crust and the variation in along the profiles with a spacing of 20 to 30 km. OBS 1 to structure toward the continental slope still remained ambigu- 5 were additionally deployed for monitoring the seismic acous, because their study was focused on the toe of the wedge. tivity during about 1 month after the survey [Shiobara et al., Le Pichon et al. [ 1987], on the ba s of a compilation of geo- 1996]. Four profiles (P1 to P4) were acquired parallel to the physical and geological data around the Zenisu Ridge, sug- strike of the Nankai Trough; they are on the continental slope gested the possibility of a nascent subduction boundary at the (P1, P2), the trough axis (P3), and the Zenisu Ridge (P4). southern margin of the Zenisu Ridge. We thus believe that a Another profile, P5, was extended from the Shikoku Basin knowledge of the detailed and deep seismic structure around to the Japan Island Arc, crossing the Zenisu Ridge and the the eastern Nankai Trough is a key to understanding the ori- Nankai Trough. Twenty-two OBSs were successfully recovgin of the Zenisu Ridge and the complicated tectonics around ered; three (3, 5, and 8) were not. As controlled sources we this area. Consequently, the aim of this study is to obtain used two air guns, each 16.7 L in volume; they were shot ev- 35øN 20 j 33øN 137øE 138øE & OBS Profiles 139øE I oo km I Figure 2. Location map of seismic refraction surveys. The airgun-obs profiling was conducted along profiles P1 to P5. Solid triangles show positions of OBSs.

4 27,154 NAKANISHI ET AL.: STRUCTURE AROUND THE EASTERN NANKAI TROUGH ery 60 s with a pressure of 10 MPa. The corresponding shot During the modeling procedure we checked whether the point interval is m, because the ship speed fluctuated calculated gravity anomaly expected from our structure from about 5 to 7 knots (1 knot = 0.5 m/s) as a result of the fast model is consistent with the free-air gravity anomaly data sea currents. A total of about 3400 shots were recorded by [Sandwell and Smith, 1997]. The gravity anomaly field was the OBSs. Simultaneously, we made a single-channel seis- calculated by the two-dimensional method of Talwani et al. mic reflection survey to resolve the fine structure of the shal- [1959]. The densities were obtained from P wave velocity low sedimentary layers. For this survey we used a PC-based values using the density-velocity curve shown in Figure 3 recording system developed by Shinohara et al. [1989]. [Ludwig et al., 1970]. Note that during our gravity modeling The OBSs used in this experiment were developed by the we made no modification to model parameters, which were University of Tokyo and Hokkaido University [e.g., Asada et well constrained by the OBS data. al., 1979; Kanazawa, 1986; Kanazawa and Shiobara, 1994]. We first determined models of P wave velocity structure The seismic data were directly recorded on an analog cassette along the strike lines, P 1 to P4. Because the dip line geometape with a tape speed of 0.06 mm/s, which enabled us to ac- try of profile P5 was expected to result in a more complicated quire continuous records for 33 days. The analog data were structure, the velocity model of this profile was determined digitized at 78 Hz by using a processing system developed at next. During this modeling the structures at the intersections Hokkaido University [Iwasaki and Shimamura, 1989; Shio- with P1 to P4 were kept fixed except for the deep structure bara et al., 1994]. beneath the intersection of P2 and P5 (OBS 19), because the The ship's positions during the OBS deployment and air deep crustal structure at the northeastern end of P2 was not gun shooting were determined with the use of the Global Po- determined from the strike line. The final velocity models sitioning System (GPS). Because of sea currents a free-fall were further constrained by amplitude modeling. The amtype OBS sometimes drifts away during descent; hence the plitude variations of the wide-angle reflection such as PmP location of the OBS sites may differ from the ship's position were especially useful to control the model of the deep strucat the time of deployment. To estimate the correct OBS loca- ture, for which refraction arrivals are not recognized. tions with respecto the ship's tracks during air gun shooting, The uncertainties in depth of these models are not easy to the seafloor OBS positions were redetermined by a nonlinear evaluate quantitatively. However, we expect them to be less inversion method using the arrival times of the direct water than 1 km for the following reason: because of the high vewave from the air gun shots [Shiobara et al., 1997]. locity of the lower crust the greatest uncertainty is the depth The structure of the shallow sedimentary layers, from of the Moho. Still, this uncertainty is less than 1 km, since which only wide-angle reflection phases were observed by the misfit in travel times is less than 0.1 s. Moreover, with OBSs, were derived mainly from the reflection seismic data. regard to the spacing of the OBSs and of the shot points in The P wave velocity within these layers was estimated by this experiment, the refracted arrivals from the layers beneath applying the r-p method [Diebold and Stoffa, 1981; Shino- topmost sedimentary layers are observed in reversed profiles. hara et al., 1994] to the OBS data. The r-p inversion gave For this reason the velocities cannot be changed by more than 0.1 km/s. us a one-dimensional velocity structure beneath each of the OBSs. We assumed that the lateral velocity variation of the sedimentary layers could be expressed by interpolating these inversion results between the OBSs. P wave velocity structures for the deeper part of the crust, from which refraction and wide-angle reflection arrivals were recorded by OBSs, were modeled by a trial and error approach. We calculated theoretical travel time curves and synthetic seismograms and modified the model parameters iteratively until all the OBS data could be explained consistently. We used a two-dimensional ray tracing technique [C erven] and Pgen l'k, 1983; Hirata and Shinjo, 1986; lwasaki, 1988] for travel time and seismogram calculations. Throughout the forward modeling the sedimentary structure derived earlier was kept fixed. The iteration was continued until the differences between observed and calculated travel times became less than 0.1 s for the observed first and the clear later ar- 9 I '_,_, sedimentary sediments, o / rocks 8 o metamorphic and igneous rocks o /, Oo ø -- thistudy øøøøv' -- ø o o 7 6 ß rivals in all OBSs. Since no clear mantle refraction phases (Pn) were observed at any OBSs, we estimated the velocity contrast of the Moho by using variation of the observed 1, I 2 [ I 3, amplitude-versus-offset the reflection phases from the Moho (PmP). Moreover, we determined the velocity of the Density (g/cm 3) uppermost mantle from earthquakes observed during 1 Figure 3. P wave velocity-density data (after Ludwig et al [1970]. month in the same OBS experiment along the extension of Copyright 1970 by Wiley-Interscience. Reprinted by permission the OBS profiles [Shiobara et al., 1996]. of John Wiley & Sons, Inc.) and thnctional curve used in the study. o:

5 NAKANISHI ET AL.: STRUCTURE AROUND THE EASTERN NANKAI TROUGH 27, Crustal Models Figure 4 shows the P wave velocity structure models of P1-P4, which are parallel to the trough axis. They are aligned along intersections with P5 (OBS 22, 19, 12, and 9). Figure 5 shows the P wave velocity structure of P5, the profile across the trough. In each profile the distance is counted from the southwestern end of the profile. We interpret layers A to E as representing the oceanic crust of the Philippine Sea Plate. These layers do not always correspond to oceanic layers 2a, 2b, 2c, 3a, and 3b [Harrison and Bonatti, 1981 ]. We will next discuss the most notable features of our models; their validity is examined by showing some examples of OBS data. (a) (b) o 5 t"h OBS16 Profile 5 SW NE OBS25 OBS24 OBS23 OBS22 OBS21 i!, i i i i [ i ' iedimentary layers 4.0 s Continental upper crust ' " La tdr 6' 3/, /. f 7 :, Layer C... ' Layer D (7.0), IProfile 1 i i i i i i i BS17 0BS18 0BS19 i i I i i i Layer B c 15 (c) 2o 0 (7.8) Profile OBS15 OBS14 OBS13 OBS12 OBS11 Sedimentary layers r ß, / X, 1.8 2,, 22. // / / / :/'/",.,-' 2"/",/6. /_/6.5//... Layer A (d) I I (8'0) I i, I,,, OBS7 OBS9 OBS Sedimentary layers, E ß c- lo Q- rofile 4 t"h 15,,,,,,,, so ß _ Layer A Layer C Figure 4. P wave velocities determined for four profiles: (a) P1, (b) P2, (c) P3, and (d) P4. The profiles are vertically aligned along cross points with P5 (OBS 22, 19, 12, and 9). Positions of OBSs are shown by triangles with their number at the top of each figure. Velocities are in kilometers per second. Solid lines show the velocity discontinuity obtained from the seismic data. Stippled lines denote boundaries across which the velocity is continuous but its gradient is discontinuous. Dashed lines show interpolated boundaries.

6 27,156 NAKANISHI ET AL.' STRUCTURE AROUND THE EASTERNANKAI TROUGH SSE Zenisu Ridge Nankai Trough Continental Slope P4 P3 P2 P1 N NW OBS6 OBS9 OBS12 OBS20 OBS19 OBS22 o '_,_ '_ '_,_,.,, xO 5 ayer E N. _ ' (7.85) -----,..,,.x,. [Profiles I (7.9) I I I I I I I I I I I I I Figure 5. Structure of P wave velocities determined for P Zenisu Ridge The crustal model of the Zenisu Ridge consists of six layers (Figures 4d and 5). The upper two layers are sedimen- (a) 6 tary layers whose structures were determined by the reflection profiling data and r-p inversion. Figure 6 shows calculated travel time curve superimposed on the observed record section, synthetic seismograms, and a ray diagram for OBS 7. The observed seismograms, which were recorded on the high gain channel of the vertical component, have been digitally band-pass filtered (5 to 10 Hz). Reduction velocity is 6 km/s. Trace amplitudes are scaled by the square root of the.-. 5 shot distance. The theoretical travel time curves and the syn- (b) thetic seismograms are calculated by using the model shown in Figure 4d. Observed first arrivals labeled as Da and Db in Figures 6a and 6b are well explained as refraction phases from layers A and B, respectively. Layer A is about 1 km thick, with a velocity of 4.0 to 4.3 krn/s, and undulates along P5. These undulations appear to correlate with the seafloor topography.,'-- 5 Along profile P4 the velocity decreases from 4.0 to 3.6 krn/s (c) northeastward. Lateral velocity changes also occur along the ridge at distances between 30 and 40 km from the southwestern end of P4, but a distinct interface could not be resolved by our seismic modeling. Layer A may be part of oceanic layer 2, but another interpretation for its slower velocity is discussed below. Layer B (4.7 to 5.6 krn/s), with an aver- (l) 10 age thickness of 1.5 km, also shows undulations along P5 beneath the Zenisu Ridge. The large amplitudes observed between 17 and 20 km are 15 due to a triplication (Rb), consisting of refraction phases from layers B and C, and a reflection phase from the B-C interface. The synthetic seismograms explain well the continuation of the large-amplitude Rb phase. We also interprethe increas- Figure 6. Observed and synthetic seismograms at OBS 7 for P4: (a) theoretical travel time curve superimposed on the ing amplitudes observed in the range between 23 and 30 km observed seismograms, (b) synthetic seismograms, and (c) as a triplication (Rc), and we divided the lower crust into two ray diagram. Layers A to D shown in Figures 4 and 5 are layers, C and D. The corresponding synthetic seismograms represented by letters A to D. Positions of OBSs along P4 are explain why the large amplitude of the Rc phase is confined shown by triangles. OBS 7

7 NAKANISHI ET AL.: STRUCTURE AROUND THE EASTERN NANKAI TROUGH 27,157 to this particularange. Layer C, with an average thickness of layer 3 [Purdy and Ewing, 1986; White et al., 1992]. The ve- I km, has a velocity of 6.3 to 6.5 km/s. The velocities of lay- locity gradient of layer D also explains the very weak ampliers B and C are comparable to those of oceanic layer 2 [e.g., tudes of those arrivals. Purdy and Ewing, 1986]. Thus we interprethe assemblage On the other hand, the high velocity contrast at the Moho of these two layers as the oceanic layer 2 of the Philippine explains the large amplitudes of the reflections from this in- Sea Plate. terface; these reflections are indicated as PmP in Figure 6 The calculated travel time curve for the Dd refraction arand are also present in synthetic seismograms (Figure 6b). rivals from layer D agree with the observations in the range Similar arrivals, reflected from the Moho beneath the Zenisu of 25 to 30 kin. The velocity (6.7 to 6.9 km/s) and thickness Ridge, are also present in the record section obtained at OBS (less than 6 km) of layer D are comparable to those of oceanic 6 for P5 (Figure 7a). (a) (b) (c) (d) Figure 7. Observed and synthetic seismograms at OBS 6 on P5: (a) observed record section, (b) theoretical travel time curves superimposed on the observed seismograms, (c) synthetic seismograms, and (d) ray diagram. Layers B to E shown in Figures 4 and 5 are represented by letters B to E. Positions of OBSs along P5 are shown by triangles.

8 27,158 NAKANISHI ET AL.: STRUCTURE AROUND THE EASTERN NANKAI TROUGH 3.2. Offset of the Moho South of Zenisu Ridge The Moho is constrained to be at a depth of about 10 to 11 km at a distance of 20 to 25 km from the southwestern end of P5 by the arrival of PmP at OBS 6 (Figure 7) aqd 9 ( Figure 8); it become shallower southward beneath the outh flank of the Zenisu Ridge. If the depth to the Moho did not change farther to the south, oceanic layer 3 of the Shikoku Basin would be anomalously thin (about 1 to 2.5 km). How- (a) 7 (b) (c) 5 OBS 6 OBS 9 SSE P50BS 9 NNW ever, the observed gravity does not allow such anomalous structure: instead it requires the existence of a crust with a normal thickness of oceanic layer 3 (about 4 km) south of the Zenisu Ridge. Thus we conclude that the depth of the Moho changes quite abruptly near the southern end of the ridge, at a distance of about 20 km from the southwestern end of P5. The corresponding effect on the gravity field is negligible. This finding is further discussed below. Further evidence in support of the abrupt change in Moho depth is a coherent and strong phase (Re in Figure 7a) following the PmP reflected from the bottom of layer D beneath the Zenisu Ridge. To explain the observed amplitude of Re, a reflector with a large velocity contrast such as the Moho appears to be required. Modeling this phase as a reflection from the Moho located at a depth of about 12 to 16 km and at a distance of 12 to 19 km from the southwestern end of P5 can explain both observed travel times and amplitudes. Since there are no refractions observed from the lower crustalayer (layer E), w assumed that its velocity is the same as that beneath the Zenisu Ridge (layers C and D). Furthermore, the Moho should dip northward in order to generate the wideangle reflections in the range of 70 to 130 km. In our best fit model the dip angle of the Moho is 23 ø. A careful analysis has been performed to investigate whether Re might be a refraction from the uppermost mantle (Pn), assuming that the oceanic crust of the Shikoku Basin has a normal thickness. The depth of the Moho was not changed during this analysis, because it was determined by PmP at OBS 6 and 9 beneath the Zenisu Ridge northward at a distance of 20 km from the southwestern end of P5. How- ever, in models with a flat Moho south of the Zenisu Ridge, Pn arrivals are earlier than the observed Re phase (Figure 9). Furthermore, the observed amplitude of Re cannot result from a small-velocity gradient similar to that in the uppermost mantle. The phase De, shown in Figure 8a, can be explained as a refracted phase from layer E based on the theoretical travel time curves and the synthetic seismograms (Figure 8). The rays of this phase (see Figure 8c) pass through the uppermost mantle, emerge from the Moho at a distance of 21 km from the southwestern end of P5, and enter into layer E from the northern interface I I I Figure 8. (a) Theoretical travel time curve superimposed on observed seismograms for the south-southeast (SSE) of OBS 9 on P5. (b) Synthetic seismograms. (c) Ray diagram for PmP and De phase. Layer E is same as that shown in Figure 5. Positions of OBS 6 and 9 along P5 are shown by triangles Area Near the Nankai Trough As Figure 5 indicates, the crustal structure of the Zenisu Ridge extends to the Nankai Trough continuously. A detailed crustal model beneath the Nankai Trough is shown in Figure 4c. The oceanic crust (layer D) becomes thinner to the northeast of OBS 12 adjacento the Izu-Ogasawara Arc. Highvelocity material (7.1 to 7.2 km/s) at the bottom of the crust appears at a distance of 83 km from the southwestern end of P3 and thickens to the northeast. The velocity of layer D is higher than that of the normal oceanic crust in the Pacific [White et al., 1992]. Similar high-velocity layers have been observed on the western margin of the Izu-Ogasawara Arc [e.g., Suyehiro et al., 1996]. Figure 10 shows the record section, the calculated travel time curves, and the synthetic

9 NAKANISHI ET AL.' STRUCTURE AROUND THE EASTERN NANKAI TROUGH 27,159 (a) 7 SSE. P ; O. BS 6,,.,. 6 Flat Moho... Best fit model NNW!- 2 1 (b) 6!- 2 1 (c) 0 C3 15 2O O Figure 9. (a) Calculated travel time curves of the final model (dashed line) and a flat Moho model (solid line) at OBS 6 for P5. (b) Synthetic seismograms of the flat Moho model. (c) Ray diagram of the flat Moho model for PmP and Pn phases. Parameters of the amplitude scaling are equal to those used in Figure 6. seismograms at OBS 11. The observed phase Rd, which is and the velocity gradient change northwestward in this l ayer. close in time to the first arrivals, can be identified between This layer disappears at a distance of 115 km from the south- Rc and PmP. The calculated travel time curves (Figure 10b) western end of P5. The existence of the fifth layer (4.0.to for the reflections from the interface between layer D and the 5.3 km/s), beneath the continental slope only, is constrained high-velocity lower crust (7.1 to 7.2 km/s) fit well with the by the computed travel times and amplitudes at the OBSs on arrivals of the Rd phase observed at a distance 75 and 85 km the continental slope along P5. The appearance of this layer from the southwestern end of P3. The synthetic seismograms at a distance of 80 km is supported by our gravity analysis. (Figure 10c) show that Rc and Rd are reflections from the C- The calculated travel time curves match the difference in ap- D interface and from the interface between layer D and the parent velocities between both sides of OBS 20 (Figure 11). high-velocity lower crust, respectively. However, some parts of the interface between the fourth and fifth layer have a small velocity contrast, which does not ex Crustal Structure Beneath the Continental Slope ist along P2. Amplitude modeling shows that later phases, The crustal structure beneath the continental slope shows which have propagated from the deep crust, are weak in this an accretionary wedge and the subducting oceanic crust. The distance range. Therefore the deep crustal structure around accretionary wedge consists of five layers having velocities OBS 20 cannot be closely determined by seismic modeling of 1.8, 1.9 to 2.7, 2.8 to 3.5, 3.8 to 4.6, and 4.6 to 5.3 km/s. only. However, our model along P5 is consistent with both The wedge reaches a maximum thickness of about 10 km at the seismic and the gravity data. 45 km landward from the trough axis. The model indicates A layer with a velocity of 5.5 to 5.6 km/s appears at a disthat the third sedimentary layer with a velocity of 2.8 to 3.5 tance of 115 km from the southwestern end of P5 and bekm/s exists only between distances of about 70 to 130 km comes thicker toward the Japan Island Arc (Figure 5). A along P5 (Figure 5). The fourth layer (3.8 to 4.6 km/s) seems to be continuation from layer A found along P4 and P3; however, the thickness weak phase leads us to infer the presence of a continental upper crust, as indicated by the existence of materials with a small velocity gradient. This phase, Du (Figure 12), is de-

10 ß 27,160 NAKANISHI ET AL.' STRUCTURE AROUND THE EASTERN NANKAI TROUGH OBS 15 OBS 14 OBS 13 OBS 12 OBS 11 ß ß ß ß V (a) 8 7 E (b) (c) 8! 5 ' 4!- 3 2 (d) O Figure 10. Observed and synthetic seismograms at OBS 11 on P3: (a) observed seismograms, (b) theoretical travel time curves superimposed on observed seismograms, (c) synthetic seismograms, and (d) ray diagram. Layers C and D are same as those shown in Figures 4 and 5. Positions of OBSs along P3 are shown by triangles. tected between distances of 15 km and 45 km recorded by OBS 22 along P1. The subducting oceanic layers 2 and 3 underlie the accretionary wedge and the continental upper crust. As Figure 5 illustrates, layers B, C, and D, found beneath the Zenisu Ridge and the Nankai Trough, deepen toward the Japan Island Arc and underlie the continental upper crust at the northwestern end of P5. The structure of the subducting oceanic crust was determined from the travel time and amplitude information of P 1 and P2 and from the gravity data [Sandwell and Smith, 1997]. Figures 12 and 13 show the observed record sections and the results of the modeling of OBS 22 for P 1 and OBS 18 for P2, respectively. The observed phases Rb, Rc, and PmP are interpreted as reflections from the B-C and C-D interfaces and from the interface with Moho, respectively. The phases Rb and Rc, observed at OBS 22, appear at a distance between about 25 and 30 km and about 32 and 42 km, respectively. They are recognized at OBS 18 at a distance between about 23 and 28 km and about 33 and 39 km. The depth and geometry of the interfaces were determined by modeling the travel

11 NAKANISHI ET AL.: STRUCTURE AROUND THE EASTERN NANKAI TROUGH 27,161 (a) phases. Further, the subducting oceanic crust and the intracrustal velocity interfaces were not detected in the southwest part of P Gravity Data (b) lo 15 Figure 14 shows the observed free-air gravity anomaly and the calculated anomaly obtained by using a density model constructed from the final model of the velocity structure. The maximum difference between the observed and calcu- lated anomalies is about 27 mgal at a distance of 110 km from the southwestern end of P5. Between 100 and 120 km the difference in shape may be due to the limited resolution of the gravity data, which cannot resolve features less than 20 km in wavelength [Sandwell and Smith, 1997]. The difference between the observed and calculated anomalies may also be caused by an incorrect choice of density for the sedimentary layers. Though the density of the sediments and the sedimentary rocks varies widely (see Figure 3), the densities used in the model were obtained directly from P wave velocities as explained above. Therefore the misfit may arise where the sedimentary layers are relatively thick. 4. Discussion Origin of the Zenisu Ridge As we mentioned in the introduction, there are three possi- Distance (krn) ble ways to explain the origin of the Zenisu Ridge: an oceanic crust, an enechelon ridge of the Izu-Ogasawara Arc, Figure 11. Observed and theoretical travel time curves at or a remnant arc. In the following we compare our crustal OBS 20 on PS: (a) theoretical travel time curves superimmodel of the Zenisu Ridge to crustal models derived from posed on observed seismograms and (b) ray diagram. Layers A to D are same as those shown in Figures 4 and 5. Positions other OBS surveys around the ridge in order to discuss its oriof OBSs 12, 20, 19 and 22 along P5 are shown by triangles. gin. Although this study shows that the crust beneath the Zenisu Ridge is a little thicker than typical oceanic crust [e.g., Spudich and Orcutt, 1980], the velocities there are similar to times of these phases, assuming that the velocities in the sub- those of the normal oceanic crust, for example, those beneath ducting oceanic crust, layers B, C, and D, are the same as the Shikoku Basin [Nishizawa and Suyehiro, 1989] those in our models for P3 and P4. In the process of mod- (Figure 15). eling, the existence of a subducting layer A was considered The crustal structure of the north Izu-Ogasawara Arc, by assuming that the velocities in this layer were the same which is an active island arc near one of the enechelon ridges, as those in our models of P3 and P4. However, the imposed indicates that its origin is obviously different from that of thickness of the subducting layer A was forced to be less than the Zenisu Ridge. The thick middle and lower crustal lay- 0.2 km. Moreover, a reflection from the A-B interface cannot ers (velocities, 6 and 7 km/s), which characterize the crustal be detected in the data. It was impossible to distinguish the structure of the north and middle Izu-Ogasawara Arc [Suyesubducting layer A from the accretionary wedge because of hiro et al., 1996], do not exist beneath the Zenisu Ridge (Figthe similarity of velocities. Consequently, we adopted a sim- ure 15). The difference in crustal thickness between the Izupler model with no subducting layer A, but its existence as a Ogasawara Arc and the Zenisu Ridge is about 10 km [Suyethin layer remains a possibility. As we mentioned above, the hiro et al., 1996]. resolution of the velocity model in the deep crust is generally On the other hand, a crust without the 6 and 7 km/s layers, not as good as that in the shallow part, because the subducting only 15 to 18 km thick, and thus thinner than that of the isoceanic crust becomes a low-velocity zone beneath the con- land arc was found beneath the Shichito Ridge [Hino et al., tinental slope. However, the models presented are consistent 1991]. This ridge is considered to be a remnant arc and is with our seismic data and with gravity analyses for P5. located along the eastern margin of the Izu-Ogasawara Arc The velocity contrast at the B-C interface may be smaller [Hino, 1991b]. Although the velocity of the upper crust of along P1, because the synthetic seismograms for OBS 22 the Shichito Ridge is close to that of oceanic layer 2 beneath show that the relative intensity of the reflected phases from the Zenisu Ridge, the velocity gradients there are clearly difthe B-C interface are stronger than those of the observed Rb ferent (Figure 15). Moreover, the thickness of the upper crust

12 27,162 NAKANISHI ET AL.: STRUCTURE AROUND THE EASTERN NANKAI TROUGH (a) OBS 25 OBS 24 OBS 23 OBS 22 OBS 21 ß ß ß ß ß sw P1 0BS22 NE Du...,,... -,:. t!:,',' ',, : '";' 2 1 ':: :-??;,-?- ;'.:?',t';:,.%-.;.,: ':;;.-.;! ;?,,.':: ;;;?..,.' ::,-' '½:::?.½,.;:: ;-:; :::,,,':,::',: :; ::::.,;: :::':: :-;,: ½:.'..'.*';:.::½:;::.;. :::: :';.,:;!": ::.':'..,.'.;, '.*: ; :. :::::..,,'. :..;;:. ;:;..:,:: : ;?:;:" ' ;.:',i ; : ;. V,,.':½.;!'½? ;<.,it-i, ;' ;:;. ::::;.:.','.. i;:;,,,,, /...':.::. ;...:.?:..::.-..4?:,;?,%;:.. t.i:!(:.,:(..?... ;..:,.,:.:,q... ::::::::::::::::::::::::::::::::.:..?v.:,..:.,..., -::..:. :.':: i:: '::!. :.5'..5..:; ½5, ( :.:-.":... ' :.:i::.:½.'-. i:' :::.:.-.,,:'::.. z...:..::...,:..:::.:: :. ;.,. ;,::.:.. ':.- ;..:.?]"...:. :...'.':: ;!.;: 5'; : :":? ;i!:.".:.- ;i'i..:.'_."..:(i:::;':..- I (b) (c) (d) o 5 Continental upper crust f 0 20 I i I i Figure 12. Observed and synthetic seismograms at OBS 22 on PI: (a) observed seismograms, (b) theoretical travel time curves superimposed on observed seismograms, (c) synthetic seismograms, and (d) ray diagram. Layers B and D are same as those shown in Figures 4 and 5. Positions of OBSs along P1 are shown by triangles. beneath the Shichito Ridge is significantly greater than that of the oceanic layer 2 beneath the Zenisu Ridge. There is thus no possibility that the Zenisu Ridge is a fragment of the rem- nant arcs. From these discussions the crustal model obtained from our study gives more direct evidence to supporthe hypothesis that the origin of the Zenisu Ridge is oceanic. However, several features of the crustal structure around the Zenisu Ridge differ from those of normal oceanic crust: a highvelocity lower crust, volcaniclastics, and a slightly thicker oceanic crust High-velocity lower crust. The crustal structure around the western margin of the Izu-Ogasawara Arc at 32øN shows a sudden thinning of the crust toward the Shikoku

13 . NAKANISHI ET AL.: STRUCTURE AROUND THE EASTER NANKAI TROUGH 27,163 OBS 16 OBS 17 OBS 18 OBS 19 ß ß ß ß (a) 8 7 sw P20BS 18 NE 2 (b) 6 (c) 6 (d) o lo E O i I Figure 13. Observed and synthetic seismograms at OBS 18 on P2' (a) observed seismograms, (b) theoretical travel time curves superimposed on observed seismograms, (c) synthetic seismograms, and (d) ray diagram. Layers B and C are same as those shown in Figures 4 and 5. Positions of OBSs along P2 are shown by triangles.

14 ß.. ß 27,164 NAKANISHI ET AL.: STRUCTURE AROUND THE EASTERN NANKAI TROUGH (a).-. 5 SSE P5 NNW 1': 9 2r 01 Ii'i il ' i '.03' Illli'' : O' (b) C ß, ' t ', ' t ' ' ' ' '. P (g/cm 3) (o 50 E v < -5o o - j'- ø ß ß. ß ß. ß J -- ß Calculated Observed, I, I t I, I, I, I, Figure 14. (a) Density model of P5 obtained from the velocity model of Figure 5 using the curve shown in Figure 3. (b) Observed and calculated free-air gravity anomalies along P5, from Sandwell and Smith [199,6] and from the density model. Basin [Suyehiro et al., 1996]. The crustal model is considered to be representative of the arc-back arc transition zone [Suyehiro et al., 1996] and is quite similar to that of the Zenisu Ridge except for the lower velocity of oceanic layer 3. Oho [1996] showed a similar thinning of the crust and described the sudden disappearance of the 7 km/s layer at the northeastern part of the Zenisu Ridge around 138ø20 N to the west. We found a similar high-velocity lower crust around Velocity (km/s) Typical oceanicrust 1. Zenisu Ridge 2. Shichito Ridge 3. Arc-backarc transition 4. North Izu-Ogasawara Arc 5. Shikoku Basin Figure 15. Velocity versus depth models obtained by recent OBS studies near the area of study: 1, from this study; 2, from Hino et al. [ 1991 ]; 3 and 4, from Suyehiro et al. [ 1996]; and 5, from Nishizawa and Suyehiro [ 1989]. Shaded zone, typical oceanic crust [e.g., Spudich and Orcutt, 1980]. the northeastern end of P3 (Figure 4c). Such a high-velocity lower crust may also exist at the northeastern part of P4, where the depth of the Moho was not determined. The lower crustal body with a velocity of more than 7 km/s has been found along active continental margins, rift zones, and active island arcs and around hotspots [e.g., Holbrook et al., 1994]. The 7 km/s material found beneath the Izu-Ogasawara Arc is interpreted as a large-scale magma intrusion responsible for the arc formation [Suyehiro et al., 1996]. Therefore we suggesthat the arc-back arc transition zone, i.e., the western margin of the Izu-Ogasawara Arc, was affected by the igneous activity of this arc and, further, that the 7 km/s layer with the thickness of 1-2 km might be widely distributed around this area. : Volcaniclastic sediments. This study shows that layer A in the southwest part of the Zenisu Ridge is continuous with oceanic layer 2 (velocities of 4.1 to 4.7 km/s) off the Kii Peninsula [Nishisaka et al., 1996]. However, layer A with a velocity of 3.6 km/s (lower than that of the oceanic layer 2) exists in the northeastern part of P4. Layers with a similar velocity are widely distributed from the arc to the back arc of the Izu-Ogasawara Arc and are interpreted to consist of volcaniclastic sediments [e.g., Suyehiro et al., 1996; Oho, 1996; Hino, 1991 a] Slightly thicker oceanic crust. Although the crustal structure of the Zenisu Ridge is basically oceanic, it may be that the crust of the ridge is influenced by the igneous activity of the Izu-Ogasawara Arc as mentioned above. The Zenisu Ridge is particularly interesting from the point of view of oceanic island arc evolution: it might be interpreted as an immature oceanic island arc created in the formation process of the Izu-Ogasawara Arc, which developed

15 NAKANISHI ET AL.: STRUCTURE AROUND THE EASTERN NANKAI TROUGH 27,165 on oceanic crust [e.g., Suyehiro et al., 1996]. The along-arc variation in the crustal structure of the Izu-Ogasawara Arc would then indicate the different stages of development of oceanic island arc [Hino, 1991 a]. On the basis of the existence of the 6 km/s layer, Hino [ 1991 a] interpreted the north and middle Izu-Ogasawara Arc as a mature island arc and the south Izu-Ogasawara Arc as an immature one. The Zenisu Ridge seems to be an oceanic island arc that is less mature than the Shichito Ridge, because the differences in the crustal structure between both ridges are limited to the thickness and velocity gradient of the upper igneous layer. An oceanic crust exists beneath the Zenisu Ridge, while the Shichito Ridge has a thick crust with a small velocity gradient and is thus quite different from an oceanic crust. That is, there is a possibility that the Zenisu Ridge is an immature oceanic island arc that is in a stage different from that of the Shichito Ridge as a remnant arc. Consequently, if the Zenisu Ridge is in an earlier stage of oceanic island arc development compared to the Shichito Ridge, four stages of the island arc formation process might exist: (1) the first stage is a normal oceanic crust, (2) a thick oceanic crust, such as the Zenisu Ridge, develops during the second stage, (3) during the third stage a thick igneous upper crust develops from an oceanic crust, as occurred in the southern part of the Izu- Ogasawara Arc, and (4) during the final stage an igneous upper and lower crust develops, as exists in the north and middle Izu-Ogasawara Arc, as the result of island arc activity Offset of' the Moho at the South Margin of' the Zenisu Ridge sists of parallel two intracrustal thrusts (offsets) dipping northward [Lattemant et at., 1989]. Le Pichon et at. [1996] also showed a density model that has a double thrust at the south of the Zenisu Ridge. From our seismic modeling, however, the existence of such a double thrust could not be identified. Furthermore, the deep crustal structure at the south of the Zenisu Ridge could not be resolved from our gravity modeling: the anomalies calculated from our model (Figure 5), our tested model with a fiat Moho (Figure 9), and the double-thrust model of Le Pichon et at. [ 1996] yield almost the same results. We believe our model to be preferable because it is derived from seismic modeling, which has a better resolving power than gravity modeling for deeper structures. Consequently, we favor a single offset of the Moho. As we mentioned above, there is a possibility that the offset of the Moho may become the location of a nascent subduction zone [Le Pichon et at., 1987]. Assuming that the new subduction has already started from the south of the Zenisu Ridge, its timing is constrained by the following considerations. We will first show that the north end of layer E does not extend farther than 17 km from the southwestern end of P5 (Figures 8 and 16). If layer E extended up to the distance of 20 km beneath layer D, it would constitute a low-velocity zone, In such a case no ray could explain a prominent phase such as De between 0 and 10 km (Figure 16). Further, be- The crustal model obtained by this study shows an offset of the Moho, although the thickness of layer E and of the entire crust near the southern end of P5 (see Figure 5) are comparable to those in the Shikoku Basin [Nishizawa and Suyehiro, 1989; Suyehiro et at., 1996; Nishisaka et at., 1996]. Such an offset does not exist in any other subduction zone. In particular, the structure between 20 and 160 km off the Kii Peninsula in the same subduction zone shows a subducting oceanic crust (near ø30'N) without any offset of the Moho ' t' i i i, i, I I I i1 '111 [Nishisaka et at., 1996]. Several geological and geophysical features appear to be related to the offset of the Moho. Using seismic reflection data, Aoki et at. [1982] showed the existence of a possible thrust fault to the south of the Zenisu Ridge. Lattemant et at. [1989] argued that the hypothesis of a major thrust cutting through most of the lithosphere was supported by gravity analysis. Moreover, Le Pichon et at. [1987], on the basis (b) o 5 lo of a compilation of geophysical and geological data around the Zenisu Ridge, suggested that such thrusts may become 15 the location of a new subduction zone when the Zenisu Ridge collides with the continental margin. If these arguments are 20 correct, the offset of the Moho found in this study may be an o 3o anomaly in the deep crustal structure related to the nascent subduction boundary. Therefore our results may supporthe hypothesis of a future jump of the subduction boundary to the Figure 16. (a) Synthetic seismograms for the SSE of OBS 9 south from the northeastern Nankai Trough. on P5 calculated from a model in which layer E extends up Another reflection survey revealed that the southeastern to a distance of 20 km. (b) Ray diagram for the PmP and De flank of the ridge is bounded by a double thrust, which con- phases. Layer E is the same as that shown in Figure 5. (a) SSE P50BS 9 NNW

16 27,166 NAKANISHI ET AL.: STRUCTURE AROUND THE EASTERN NANKAI TROUGH cause the Moho at a distance of 20 km is determined from the ilarly, velocities in the continental upper crust beneath the PmP arrivals at OBS 6 and 9, layer E cannot extend north- continental slope were km/s slower than those obward up to a distance of 20 km. Although various models tained around the Ryukyu Trench [Iwasaki et al., 1990] and with different geometries were tested, they could not consis- the southern Kuril Trench [Iwasaki et al., 1989]. tently explain the data at OBS 6 and 9. We thus conclude that The subducting oceanic layers 2 and 3 are found beneath the upper interface of layer E of only extends to a distance the accretionary wedge. The depth of the top of the subduct- 17 km and has a northwar dipping interface at its north end. ing oceanic crust obtained along P5 is comparable to that of This feature of layer E is also constrained by the record sec- the top of the Philippine Sea Plate, as obtained from earthtions of P3 and P4, which show no reflection phase from a quake data [e.g., Ishida, 1992]. Our model does not contain reflector deeper than the bottom of layer D (e.g., Figures 10 the subducting layer A. Such a layer, consisting of volcaniand 6). Furthermore, the overlaps of the oceani crust (layers clastic sediments, may have been scraped off together with D and E) beneath the Zenisu Ridge is only 3 to 5 km (Figures the upper sedimentary layers during the subduction process. 5 and 8). Taking historicalarge earthquakes in the Tokai Alternatively, layer A may be a thin layer subducting beneath district into consideration, we show that the subduction zone the continental slope, as we described before. If the velocalong the Nankai Trough is still active. Segawa et al. [ 1996] ity of this layer increases with depth, as it does in the crustal show the rate of relative displacement of the Zenisu Ridge model of the Cascadia subduction zone [Flueh et al., 1997], against the Japan Island Arc to be 2 crn/yr with the direc- it may be impossible to separate the subducting layer A from tion to the northwest by GPS measurements. They suggested layer B in seismic modeling. that 2 crn/yr should be absorbed in the new subduction zone 5. Conclusions at the south of the Zenisu Ridge because Seno [ 1977] estimates the rate of convergence of the Philippine Sea Plate in Our detailed crustal structur explains not only the OBS this area to be 4 crn/yr. Assuming a steady state subduction data but also previously published gravity data extending at a rate of 4 cm/yr [Seno, 1977] and 50% absorption at the from the Shikoku Basin to the continental slope off the Tokai ridge, the corresponding time is only 0.15 to 0.25 Ma. How- district. The crustal structures of the Zenisu Ridge and the ever, it was found that the horizontal crustal displacement of Nankai Trough consist of sediments (1.6 to 2.2 km/s), the Zenisu Ridge is approximately consistent with that of the oceanic layer 2 (4.0 to 4.3, 4.7 to 5.6, 6.3 to 6.5 km/s), and Philippine Sea Plate by recent GPS measurements [Tabei et oceanic layer 3 (6.7 to 6.9 km/s). Although the thickness al., 1998]. From this result the new subduction might not of the crust beneath the Zenisu Ridge, 8-11 km, is slightly be steady. Further, note that the GPS measurement point on greater than that of a normal oceanic crust, our crustal model the Zenisu Ridge is located at the northeastern margin of the clearly indicates that the ridge has features of oceanic crust. ridge (near OBS 1 shown in Figure 1). Therefore their data The Moho has an offset of 5 km in depth beneath the south might reflect not the real tectonic movement of the Zenisu margin of the Zenisu Ridge. The oceanic crust has a north- Ridge, but the movement of the Izu-Ogasawara Arc. For the ward dipping interface on the north side of the offset. In the present, although it is not evident that the new convergent eastern Nankai Trough, the Moho offset exists only off the boundary is located along the Moho offset at the south of the Tokai district and not to the southwest off the Kii Peninsula, Zenisu Ridge in our crustal model, it is certain that even if the despite the proximity of these two areas. new subduction has already started, it is still in a very early The velocity structure beneath the continental slope shows stage. both the subducting oceanic crust and the accretionary wedge. The accretionary wedge beneath the continental slope 4.3. Subducting Oceanic Crust and Accretionary Wedge consists of five layers with velocities of 1.8, 1.9 to 2.7, 2.8 to A well-developed accretionary wedge has been delineated 3.5, 3.8 to 4.6, and 4.6 to 5.3 km/s. The maximum thickness with a thickness reaching 10 km at 45 km landward from the of the wedge is about 10 km, 45 km landward from the trough trough axis. This wedge is thus as large as that of the middle axis. The accretionary wedge is underlain to the northwest Ryukyu Trench located about 700 to 800 km southwest of our by the upper crustal layer with a velocity of 5.5 to 5.6 km/s, surveyed area [Kodaira et al., 1996]. The maximum veloc- which can be interpreted as the continental upper crust of the ity at the bottom of the wedge is 5.3 km/s, which is similar Japan Island Arc. The subducting oceanic crust consisting to that found in the middle Ryukyu Trench but higher than of oceanic layers 2 and 3 is continuous beneath the accrethat found at the Nankai Trough off the Kii Peninsula, i.e., tionary wedge and the continental upper crust of the Japan 4.2 km/s [Nishisaka et al., 1996]. Le?ichon et al. [1996] Island Arc. suggested, mainly from the magnetic data, that the higher- Acknowledgments. Particular contributions to the OBS survelocity layer of the wedge corresponds to the body of the vey were made by scientists and students from the Russian Acade- Paleo-Zenisu Ridge, but this suggestion could not be con- my of Science (RAS), from the Laboratory for Ocean Bottom Seisfirmed by our seismic modeling. mology in the Hokkaido University, and from the Kobe University. The accretionary wedge is underlain by a km/s A. Gorbatikov (RAS) is especially acknowledged for his support during the experiment. The skilled ship maneuvering and help by layer. This layer probably corresponds to a continental upthe captain and the crew of the Shinyo-maru, Shin Nihon Kaiji Co. per crust, although the velocity is slightly slower than 6.0 Ltd., are gratefully acknowledged. We are grateful to J. C. De Brekm/s obtained for the Japan Island Arc [Ikami, 1978]. Sim- meacker of Rice University for his help in correcting the English

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