Shear partitioning near the central Japan triple junction: the 1923 great Kanto earthquake revisited-i

Transcription

1 Geophys. J. Int. (1996) 126, Shear partitioning near the central Japan triple junction: the 1923 great Kanto earthquake revisited-i Siegfried J. Lallemant,' Xavier Le Pichon,'92 Frederic ThouC,'v2 Pierre Henry' and Saneatsu Saito2 ' Laboratoire de Gkologie-CNRS LIRA 1316, Ecole Normale Supirieure, 24 rue Lhomond, 75231, Paris CEDEX 05, France 20cean Research Institute, Unioersity of Tokyo, Minami dai, Nakano ku, Tokyo, 164, Japan Accepted 1996 May 13. Received 1996 May 8; in original form 1995 August 30 1 INTRODUCTION The Sagami trough is the plate boundary between the eastern edge of the Philippine Sea plate and the Kanto area of central Japan (Nakamura et a/. 1987). This boundary is composed of three segments with directions changing from east to west from 279" to 290" and then to 320" (see Fig. 1). Because the average direction of the plate motion with respect to Japan is about 310" (Seno, Stein & Gripp 1993), eduction (motion of the underthrust plate out of the trench) has been proposed by Nakamura, Shimazaki & Yonekura (1984) along the westernmost portion. This is the site of the 1923 great Kanto earthquake (M = 7.9), where the fault-plane solution obtained either from seismological data (Kanamori 1971) or from geodetic measurements (Ando 1971) indicates oblique thrusting which is incompatible with eduction. In this paper, we propose the existence of a large dextral strike-slip fault approximately parallel to the Sagami trough, which we call the Boso transform fault (later referred to as the Boso TF). This dextral fault is the result of shear partitioning and delimits a Boso sliver under which the Philippine Sea plate subducts with a more convergent direction, thus eliminating the eduction problem. We examine the 1923 Kanto earthquake and its aftershock SUMMARY We propose the existence of a major right-lateral transform fault which we call the Boso transform fault. It is related to the Sagami trough, a portion of the Philippine Sea plate boundary south of the Kanto area (central Japan). This Boso transform fault is the result of shear partitioning due to oblique subduction and has delimited a Boso sliver for 2 Myr. The rate of motion is estimated at 16 mm yr-' and the total offset at 30 km. The fault cuts through the Miura and Boso peninsulas onland, where it has a multiple surface expression roughly along the limit of a steeply dipping Miocene ophiolitic body. These subaerial faults have been identified as active, and their cumulated rate of slip across the Miura peninsula can be estimated to be greater than 12 mm yr-', in reasonable agreement with the above estimate. We propose that the slip on the Boso transform fault was responsible for two large (M = 7.0 and 7.5) aftershocks which occurred on the second day after the 1923 great Kanto earthquake. This explains the unusual duration of the aftershock sequence, and the large magnitudes of some of the aftershocks. Key words: earthquakes, Japan, strike slip, subduction. sequence and propose that the second part of the aftershock sequence corresponds to the rupture of the Boso TF. This hypothesis is tested in a companion paper (Pollitz, Le Pichon & Lallemant 1996; hereafter Part 11), on the basis of crustal deformation related to this rupture. 2 PHILIPPINE SEA PLATE KINEMATICS All the boundaries of the Philippine Sea plate are convergent. The relative plate motions with respect to the adjacent plates are thus computed using the slip vectors of the major subduction earthquakes such as the 1944 Nankaido and the 1946 Tonankai earthquakes in the Nankai trough (Ando 1975), and there is no direct information on the modulus of the velocities. Consequently, previous authors have used an indirect solution based on the relative motion of the Pacific and Eurasia plates (Huchon 1986; Ranken, Cardwell & Karig 1984; Seno 1977; Seno, Stein & Gripp 1993). This type of solution assumes that adjacent southern Japan belongs to the Eurasia plate. These uncertainties result in a large scatter of the magnitudes of the velocities, which vary in the Izu-Sagami area, according to the different solutions, by a factor of two. The most recent determination (Seno et al. 1993) proposes two solutions, one giving a velocity of 41 mm yr-' and the other, which is compatible Q 1996 RAS 87 1

2 872 S. J. Lallemant et al. 0 c 9 x 0 c RAS, GJI 126,

3 Shear partitioning near the central Japan triple junction-i 873 with the full NUVEL-I (DeMets et al. 1990) solution, a velocity of 50 mm yr-'. Fortunately, geodetic measurements are now available between the Philippine Sea plate and the adjacent south Honshu to Taiwan boundary (Geographical Survey Institute 1994b Kimata et al. 1993; Yu & Chen, 1996). Fig. 2 shows the northernmost set of GPS measurements between Hachijojima (HCJ), the southern tip of Izu (MIZ), Shizuoka (SZK) and mainland Japan (TKY). These measurements are broadly compatible in direction and velocity with the second solution of Sen0 et al. (1993), although their precision is difficult to evaluate at this stage. We consequently adopt a velocity of 50 mm yr-' along a 308" direction for the subduction vector below southern Honshu in the area around Tokai. However, it is seen in Fig. 2 that the motion of the Izu peninsula with respect to Japan is significantly smaller than 50mmyr-'. Actually, it is well known that the Izu area is being shortened along a NW-SE direction by conjugate strikeslip faulting, reflecting the collision of the tip of the Izu-Bonin island arc with the Japanese islands (Sommerville 1978). Hashimoto & Jackson (1993), in their modelling of the crustal deformation around Japan based on long-term triangulation, showed that the Izu block is moving at 22 7 mm yr-' along a 294" f 7" direction. This direction is close to the predicted plate direction. We adopt an average velocity of 25 mm yr-' along 308" for the Izu block, whose southern boundary, characterized by an intense seismic deformation (Geographical Survey Institute 1994a), is drawn in Fig. 2. Note that this southwestern extremity of the Izu block coincides with the Zenisu ridge, which has been shown to be an active thrust (Lallemant et al. 1989; Le Pichon et al. 1987). Another indication that the subduction velocity below Suruga is significantly smaller than that to the west is given by the length of the seismic slab, as described by Ishida (1992). This length Figure 2. Kinematic background of the Izu collision zone compared with GPS measurements. The predicted motion (grey arrows) is computed with the NUVELl compatible solution of Seno et al. (1993). The GPS measurements (solid arrows) are taken from (Kimata et al. 1993). Velocities are given in mm yr-'. Localities named in the text are identified. The location of Fig. 1 is also identified RAS, GJI 126,

4 874 S. J. Lallemant et al. progressively increases from about 250 km northwestwards of the northern extremity of the Suruga trough to 500 km opposite the Zenisu ridge. Finally, we need to consider whether there is significant differential motion between northern Honshu, which includes the area around Kanto, and southern Honshu, which includes the area around Tokai. Nakamura et al. (1994) were the first to propose that the former area belongs to the America plate, whereas the latter belongs to the Eurasia plate. Then, according to the NUVELl global model (DeMets et al. 1990), we would expect an east-west 1.1 cm yr-' shortening between these two areas. This hypothesis accounts for the east-west protosubduction along the western margin of northern Honshu (Nakamura 1983, Tamaki & Honza 1985; Tamaki et al. 1992). Sen0 et al. (1993) adopted this hypothesis in their recent kinematic model. Whether northern Honshu belongs to the North America plate or not, Hashimoto & Jackson (1993), on the basis of a model based on triangulation data, have shown that a shortening of about this magnitude exists along the western boundary of northern Honshu. However, it is known that the southern parts of southern Honshu and Shikoku, south of a right-lateral strike-slip fault called the Median tectonic line (MTL), are also moving to the west. This motion has been interpreted by Fitch (1972) to be the result of shear partitioning in an obliquely convergent margin. The velocity of this strike-slip motion has been estimated to be about 5 to 10 mm yr-' (Okada 1980; Research Group for Active Faults in Japan 1991; Tsutsumi et al. 1991). Elsewhere (in a paper in preparation), we argue on the basis of geodetic measurements in Kyushu that this velocity lies between 10 and 13 mm yr-'. Thus, it is unlikely that there is a relative motion larger than 5 mm yr-' between the areas around Kanto and Tokai. This is confirmed by the absence of any large present deformation along their proposed tectonic boundary: the southern part of the Itoigawa-Shizuoka tectonic line or ISTL (Research Group for Active Faults in Japan 1991). Consequently, in this paper, we neglect any differential motion between the Kanto and Tokai areas (Fig. 3), and do not consider the proposal by Sen0 et al. (1993) to be valid, i.e. that the more northerly direction of slip of the great Kanto earthquake can be explained by such differential motion. 3 ACTIVE FAULTING ALONG THE BOSO TRANSFORM FAULT (MIURA AND BOSO PENINSULAS, OISO HILL) The active fault map of Japan (Research Group for Active Faults in Japan 1991) shows that, in the Kanto area west of the Oiso Hill area, the only series of well-defined active faults are roughly colinear along a 290" trend starting from the northern edge of Oiso Hill (Kaneko 1971) and cutting through the Miura and Boso peninsulas (Figs 1 and 4). Within the peninsulas, they have been studied by several authors (e.g. Kaneko 1969,1972; Research Group for Active Faults in Japan 1991), who identified them as right-lateral strike-slip faults. Fig. 4 shows a map of the active faults cutting the Miura peninsula. According to unpublished results of recent trenching on the Kitatake fault (A. Taka, personal communication, 1995), the average slip has been 5 mm yr-' for the last 800 years. Kaneko (1969) compared the average offsets of streams along different faults. We use the relative average offset on each fault to calibrate the rates of motion along the faults. We obtain a rate of 4 mm yr-' for the Kinugasa fault to the north, 3mmyr-' for the Takeyama fault to the south of Kitatake and 1 mm yr-' or less for the two southernmost faults. Summing these rates, we obtain a total rate of 12 to 14mmyr-'. Note that the three largest faults to the north correspond to the same strike-slip fault at depth, as shown by their geometry and by the short distance of only three km across this set. Saito (1991) pointed out the existence of rightlateral strike-slip movement within the central Boso tectonic zone during Miocene times, but no estimate of significant active motion has been made for the Boso faults. However, the obvious continuity between the Miura and Boso systems leads us to use the Miura rates as representative of the whole system. This strike-slip system coincides with a major tectonic structure in both the Miura and Boso peninsulas (see Fig. 4). The oldest rocks of the peninsulas crop out along a series of elongated hills in the middle of the actively deformed strip. They correspond to the Mineoka and Hota groups on the Boso peninsula, and the Hayama group in the Miura peninsula (Huchon 1985; Ogawa & Taniguchi 1988; Soh et al. 1991). These groups comprise pillow basalts, turbiditic and pelagic sediments as well as serpentinite blocks within a severely deformed tectonic melange. They have been interpreted as belonging to an ophiolitic body (Uchida & Arai 1978) emplaced during a seaward jump of the plate boundary in Miocene times (Huchon 1985; Ogawa & Taniguchi 1988; Soh et al. 1991), similar to the jump which seems to be occurring now in the Zenisu ridge area (Lallemant et al. 1989). From gravity and magnetic modelling in the Boso peninsula, the ophiolitic body has been estimated to dip steeply towards the north (Morijiri, Kineoshita & Nag0 1987; Soh et al. 1991). The Mineoka group is Eocene to Oligocene, whereas the Hayama-Hota group is lower to middle Miocene (e.g. Soh et al. 1991). These sequences are overlain unconfonnably by the Miura group (middle Miocene to late Pliocene) volcaniclastic and turbiditic sequences. Soh et al. (1991) show that the Miura group has a different sedimentological signature on either side of the Takeyama fault 'even in age-equivalent strata', thus defining a northern basin and a southern basin (see Fig.4). They suggest that the two basins had different sedimentological characteristics because they were located on different sides of a major basement thrust along which the Mineoka ophiolitic belt was emplaced about 10 Ma ago. The existence of these two basins as well as the ophiolitic nature of the Mineoka- Hayama belt suggest that the Boso TF is reactivating a major crustal structure of the southern Kanto area. We conclude that the Boso and Miura peninsulas are cut by a large-scale right-lateral strike-slip fault with an average motion of about 10 to 15mmyr-', which has a complex surface expression. We call it the Boso TF. It reactivates a Miocene crustal thrust system dipping steeply towards the north. Because there are several surface fault traces for a single deep fault, it is probable that a given earthquake activates only one of them. This is apparently what happened in 1923 for the great Kanto earthquake, where surface ruptures have been found only on the eastern part of the Takeyama fault (Fig.4) and on the southern Boso fault. We now need to consider whether there is an extension of this strike-slip fault east of the Boso peninsula RAS, GJI 126,

5 Shear partitioning near the central Japan triple junction-i 875 ILE PICHON et al. (in prep) I Figure 3. Three kinematic models of plate interactions around the Japanese islands. (a) Three-plate model of Seno (1977). (b) Recent model by Sen0 et a[. (1993) taking into account the nascent subduction zone along the east coast of the Japan Sea. (c) Our preferred model where the Nankai sliver (portion southwest of the MTL and Okinawa back-arc system) is moving with about the same velocity as northern Honshu (Le Pichon et al. in preparation). 4 SEAWARD EXTENSION OF THE BOSO TRANSFORM FAULT The shelf break is offset right laterally 30 km along the eastward prolongation of the Boso TF (Fig. 1). It is interesting to reconstruct the morphology prior to a postulated 30 km fault offset. This is done in Fig. 5. Two remarkable features are apparent. First, the eastward continental margin of the Boso peninsula is essentially rectilinear on this reconstruction. Second, a rectilinear continuous canyon is made out of what are presently two parallel portions of different canyons (the lower part of the Katsuura canyon and the upper part of the Katakai canyon). At present, the Katsuura canyon is made of two approximately perpendicular segments, the upper one along the seaward portion of the Boso TF and the lower one parallel to the Katakai canyon (Fig. 1). Note also that the present Katakai canyon, south of the Boso TF, has a poorly expressed morphology. A comparison of Figs 1 and 5 shows that the general canyon drainage pattern is simpler in the restored map. It seems unlikely to us that these observations result from a series of coincidences, and we conclude that there has been a 30 km right-lateral offset on a geological timescale sufficiently short to almost preserve the erosional morphology of the pre-fault area. Using the rate estimates made in the previous sections, the age of the fault system is 2 to 3 Myr. Thus, the Boso TF is the northern boundary of a Boso sliver RAS, GJI 126,

6 876 S. J. Lallemunt et al. Figure 4. (a) Map of the active faults in the Miura peninsula adapted from Kaneko ( 1969,1972) superimposed on a simplified geological map of the peninsula. 1: southern basin sequences (Miocene-Pliocene); 2: northern basin sequences (Eocene to Pliocene); 3: Quaternary deposits; 4 major active faults; 5: minor active faults. KN: Kinugasa fault; KT: Kitatake fault; TA: Takeyama fault; MI: Minami-Shitaura fault; HI: Hikihashi fault. (b) Schematic geological map of the Miura Peninsula, modified from Soh et al. (1991). 1: Mizaki formation; 2: Hasse formation; 3: Hayama group (including serpentinite blocks); 4: Zushi formation 5: Ikego formation; 6: Quaternary deposits; 7: serpentinite blocks; 8: major faults (old and active). (1, 2,4 and 5 belong to the Miura group; the fine-grained middle Miocene to early Pliocene sequence is called the Zushi formation in the northern basin, whereas it corresponds to the Misaki formation to the south. Similarly, the coarser early-to-late Pliocene sequences form the Ikego formation to the north and the Hasse formation in the southern basin.) Figure 5. Restoration of the bathymetry prior to the occurrence of the 30 km right-lateral displacement along the Boso TF. Bathymetry is from Kato et nl. (1985). Contours are in metres RAS, GJI 126,

7 Shear partitioning near the central Japan triple junction-i 877 which is limited to the south by the Sagami trough. We relate its origin to shear partitioning due to the high obliquity of the subduction vector on the Sagami trough. 5 FAULT-PLANE MECHANISM OF THE 1923 KANTO EARTHQUAKE We next consider the kinematics of the great 1923 Kanto earthquake to obtain the direction of motion between the Izu block and the Boso sliver. Fig. 1 shows the preferred location for the epicentre of the 1923 Kanto earthquake main shock according to Kanamori ( 1971). This earthquake ruptured the whole westernmost segment, as demonstrated by the modelling of the vertical and horizontal deformation (Ando 1971; see Part I1 for a complete discussion). Fig. 6 shows the fault-plane mechanism proposed by Kanamori on the basis of first arrivals of P waves and assuming a depth of 10 km for the hypocentre. As mentioned earlier, the slip direction of 305" is such as to produce motion of the Philippine plate out of the trench along the 320" westernmost portion of the Sagami trough (see Fig. 1). In addition, the direction of the fault plane proposed is not compatible with the average direction of the ruptured portion of the Sagami trough. Ishibashi (1981) used the same data set with a more detailed crustal structure to deduce a different fault-plane mechanism which is more consistent with the direction of the western portion of the Sagami trough (slip vector oriented 325" and fault-plane direction 312"). We show in Fig. 6 that another fault-plane solution is compatible both with the data given by Kanamori (1971) and with the geometry of the ruptured portion of the Sagami trough. With this solution, the slip vector is oriented 338" and the fault is trending 321", not significantly different from the trend of this portion of the trench. Although the fit proposed here is not better than the one proposed by Kanamori, we adopt it because of its excellent agreement with the geometry of the subduction zone. 6 KINEMATICS OF THE BOSO SLIVER We now estimate the velocities along the boundaries of the Boso sliver using the velocity diagram of Fig. 7. The assumptions used for this diagram have been discussed previously and are now summarized. The motion of the Philippine Sea plate with respect to Kanto/Tokai is 50mmyr-' in a direction of 308" assuming coherent motion of all the Izu block. The motion of the Izu block is 25mmyr-' in the same 308" direction. The direction of the Boso sliver motion with respect to Kanto is 290", and, finally, the direction of the Boso sliver with respect to Izu is 338" as given by the adopted Kanto fault-plane solution. The resulting velocity of 16 mm yr-' for the Boso TF is compatible with the 12 to 14 mm yr-' previously obtained for the Miura peninsula faults since both types of estimates have significant error bars (probably as large as 5mmyr-'). The Izu/Boso rate of motion is 12 mm yr-', which means that the 5 m of motion during the 1923 earthquake (Ando 1972; see Part I1 for discussion) correspond to 420 years of elastic loading. Finally, the motion of the subduction of the Philippine Sea plate below the Boso sliver occurs at a rate of 35 mm yr-' along a direction of 316". Figure 6. Focal mechanism of the 1923 Great Kanto earthquake mainshock. (a) P-wave first-motion data for a 10 km deep hypocentre (Kanamori 1971) and Kanamori's solution; (b) our preferred solution. In both cases, the A plane is the fault plane. Note that the A plane of our solution trends parallel to the Sagami trough. Data are represented in an equal-angle projection (Wulff type) of the lower hemisphere RAS, GJZ 126,

8 878 S. J. Lallemant et al. Figure 7. (a) Velocity diagram of the Boso sliver boundaries with adjacent plates and blocks. (b) Schematic map of the plates and blocks around the Boso sliver showing the effective plate boundaries. 7 A TWO-FAULT MODEL FOR THE 1923 EARTHQUAKE We noted earlier that surface ruptures occurred during the 1923 Kanto earthquake in both the Miura and Boso peninsulas along portions of faults related to the Boso TF. This suggests the possibility that the Boso TF was activated during this earthquake. In order to investigate this possibility, we now look at the aftershock sequence of this earthquake. A peculiarity of the Kanto earthquake is the high level of aftershock seismicity, running over more than 24 hours with 34 events having a magnitude exceeding 5 (Takemura 1994; Utsu 1981). Among these, Takemura (1994) reported 16 events with magnitudes >6 and five events with magnitudes 2 7, which is definitely unusual for a magnitude 8 mainshock. From a careful re-evaluation of the seismological records at Gifu station (location in Fig. 8 top), Takemura (1994) proposed an aftershock time sequence with two distinct trends. During the first hour after the main shock (12h00 to 13h00), the epicentral distances increased progressively from 200 to about 290 km. Then a new sequence started at 13h00, showing a similar increase in distance from 200 up to 330 km RAS, GI1 126,

9 Shear partitioning near the central Japan triple junction-i 879 DISTANCE from Gifu Station (Km) Figure 8. (a) Location map of the Gifu seismological station with respect to the area studied. (b) Time sequence of the aftershock events of the 1923 great Kanto earthquake (slightly modified from Takemura 1994). L.F.: low-frequency event. V.L.F.: very low-frequency event. with the largest events [M: 7.1 and 7.3 according to Utsu (1981); or M: 7.0 and 7.5 according to Takemura (1994)l being the most distant. Note that the preferred epicentral location of the main shock according to Kanamori (1971) is located close to the junction between the westernmost part of the Sagami trough and the Boso TF (see location in Fig. 1). It is, then, logical to explain this unusual sequence of aftershocks as two earthquake sequences, as shown in Fig. 8. The first sequence ruptured the whole westernmost segment of RAS, GJI 126,

10 880 S. J. Lallemant et al. the Sagami trough from Oiso Hill southeastwards. The second sequence ruptured the western part of the Boso TF between Oiso Hill and the eastern shelf break of Boso, which is the part with a sizeable crustal thickness. This hypothesis will be explored in Part DISCUSSION As discussed earlier, the total offset of the Boso TF is 30 km. As the velocity we obtained is about 16 mm yr-, the initiation of this feature probably occurred about 1.8 Myr ago. Le Pichon et a/. (in preparation) have noted that this age coincides with the initiation of the present pattern of deformation of the Japanese islands. This is because the rapid underthrusting of the Japan Sea floor below the western margin of northern Honshu started 1.8 Myr ago as shown by deep drilling in the Okushiri ridge (Tamaki et al. 1992). In the same way, the rapid opening of the central Kyushu rift (also known as Beppu- Shimabara graben) in southwest Japan began 1.8Myr ago (Fig. 8 of Tamanyu 1993). Le Pichon et al. (in preparation) show that these two events correspond to the onset of both the motion of northern Honshu towards the west and of the southern parts of southern Honshu and Shikoku also towards the west. Thus, the formation of the Boso TF coincides with the initiation of the new Japanese islands kinematic pattern. Angelier & Huchon (1986) have noted a counterclockwise rotation of the stress field around the Izu peninsula at about this time. The possibility remains that this major change is related to the effect of the collision of the Izu peninsula with mainland Japan which occurred some time between 5 and 2 Myr ago. 9 CONCLUSION We have shown that the Sagami trough boundary of the Philippine plate is characterized by a major transform fault that we call the Boso TF. This transform fault is due to shear partitioning starting about 2 Myr ago. At this time, an ophiolitic suture, called the Mineoka belt, was reactivated as a right-lateral strike-slip fault. The rate of recent motion along this fault is about 16 mm yr- and is in a reasonable agreement with the estimate of slip of the active faults across the Miura peninsula. We have proposed further that the western portion of the Boso TF slipped during the 1923 Kanto earthquake and that this slip was responsible for the two largest aftershocks on the second day. It is obvious that, if this hypothesis is correct, the potential slip along this fault is of great significance for the large urban concentrations which surround them on the Boso and Miura peninsulas. This is tested and discussed further in Part 11. Finally, the existence of this active strike-slip fault east of the Boso peninsula is of great interest for the investigation of the fluid circulation in the accretionary wedge there, as one would expect fluid circulation to be focused along the new active vertical fault. ACKNOWLEDGMENTS This paper was initiated by the second author during a three month appointment as a visiting Professor in the newly created Center for International Cooperation in the Ocean Research Institute of the University of Tokyo. The Center provided logistic help for the visit of the other French scientists and for the field trips. K. Tamaki helped with computer work. Discussions with J. Segawa, A. Taira and K. Tamaki were helpful. We thank Freysteinn Sigmundsson and an anonymous reviewer for their comments. REFERENCES Ando, M., A fault origin model of the Great Kanto Earthquake of 1923 as deduced from geodetic data, Bull. Earthq. Res. Inst., 49, Ando, M., Source mechanisms and tectonic significance of historical earthquakes along the Nankai Trough, Japan, Tectonophysics, 27, Angelier, J. & Huchon, P., Tectonic record of convergence changes in a collision area: the Boso and Miura Peninsulas, central Japan, Earth planet. Sci. Lett.. 81, DeMets, C., Gordon, R.F., Argus, D.F., & Stein, S., Current plate motions, Geophys. J. Int., 101, Fitch, T.J., Plate convergence, transcurrent faults, and internal deformation adjacent to Southeast Asia and the western Pacific, J. geophys. Res., 77, Geographical Survey Institute, 1994a. Summary of Observations for Earthquake Prediction in Japan (Part 2)-Kanto and Chubu Areas [report], Vol. 5 Geographical Survey, Ministry of Construction, Japan. Geographical Survey Institute, 1994b. Summary of Observations for Earthquake Prediction in Japan (Part 3)-Kinki. Chugoku, Shikoku. Kpshu and Okinawa areas [report], Vol 6 Geographical Survey, Ministry of Construction, Japan. Hashimoto, M. & Jackson, D., Plate tectonics and crustal deformation around the Japanese Islands, J. geophys. Res., 98, Huchon, P., Gkodynamique de la zone de collision dizu et de la jonction triple du Japon Central, leur place dans 1 6volution de la plaque Philippine, ThCse d Etat, Universite Pierre et Marie Curie, Paris. Huchon, P., Comments on Kinematics of the Philippine Sea plate by B. Ranken, R.K. Cardwell and D.E. Karig, Tectonics, 5, Ishibashi, K., Reevaluation of the fault mechanism of the 1923 Kanto earthquake deduced from seismological data, Earth. 7, Ishida, M., Geometry and relative motion of the Philippine Sea plate and Pacific plate beneath the KanteTokai district, Japan, J. geophys. Res., 97, Kanamori, H., Faulting of the Great Kanto earthquake of 1923 as revealed by seismological data, Bull Earthq. Res. Inst., 49, Kaneko, S., Right-lateral faulting in Miura peninsula, south of Tokyo, Japan., J. geol. SOC. Japan, 75 (4), Kaneko, S., Neotectonics of Oiso hills and contiguous districts in South Kanto, Japan, J. geol. SOC. Japan, 77 (6), Kaneko, S., Some remarks on the strike-slip faulting in Miura peninsula and Sagami bay area, south Kanto, Japan, J. geol. SOC. Japan, 78 (4), Kate, s., Nagai, T., Tamaki, M., Kondo, T., Tomiyasu, Y., Kato, G., Muneda, K. & Asada, A., Submarine topography of the Eastern Sagami Trough to the Triple Junction, Report of Hydrographic Researches, 20, March 1985, 1-24 (in Japanese). Kimata, F., Satomura, M., Sasaki, Y., Murata, I. & Fuse, K., GPS measurements in the Tokai region and Izu Hachijo island, Proc. CRCM, Kobe, Lallemant, S.J., Chamot-Rooke, N., Le Pichon, X. & Rangin, C., Zenisu Ridge: a deep intraoceanic thrust related to subduction, off Southwest Japan, Tectonophysics, 1, Le Pichon, X. et al., The eastern and western ends of Nankai RAS, GJI 126,

11 Shear partitioning near the central Japan triple junction-i 881 Trough: results of Box 5 and Box 7 Kaiko survey, Earth planet. Sci. Lett., 83, Morijiri, R., Kinoshita, H. & Nago, T., On the geophysical survey of the Mineoka ophiolite belt, southern part of Boso peninsula, Chiba prefecture, Japan, Ann. Rep. Inst. Mar. Ecosystem, Chiba University, 1, Nakamura, K., Possible nascent trench along the eastern Japan Sea as the convergent plate boundary between Eurasian and North American plates, Bull. Earthq. Res. Inst., 58, (in Japanese with English abstract). Nakamura, K., Shimazaki, K. & Yonekura, N., Subduction, bending and eduction. Present and Quaternary tectonics of the northern border of the Philippine Sea plate, Bull. SOC. gcol. France, 26 (2), Nakamura, K. et al Oblique and near collision subduction, Sagami and Suruga Troughs-preliminary results of the French- Japanese 1984 Kaiko cruise, Leg 2, Earth planet. Sci. Lett., 83, Ogawa, Y. & Taniguchi, H., Geology and tectonics of the Miura- Boso Peninsulas and the adjacent area, Modern Geology, 12, Okada, A., Quaternary faulting along the Median Tectonic Line of southwest Japan, in Median Tetonic Line of southwest Japan, pp , ed. Ichikawa, K., 18, Geol. SOC: Japan, Tokyo. Pollitz, F., Le Pichon, X. & Lallemant, S., Shear partitioning near the Central Japan triple junction: the 1923 Great Kanto Earthquake revisited-11, Geophys. J. Int., 126, (Part 11; this issue). Research Group for Active Faults in Japan, Active Faults in Japan, sheet maps and inventories. The University of Tokyo Press, Tokyo (in Japanese). Ranken, B., Cardwell, R.K. & Karig, D.E., Kinematics of the Philippine Sea Plate, Tectonics, 3, Saito, S., Strike-slip movement and strike-slip basin formation in the Mineoka tectonic belt in the Boso peninsula, Central Japan, J. Tectonic Research Group of Japan, 36, (in Japanese with English abstract). Seno, T., The instantaneous rotation vector of the Philippine Sea plate relative to the Eurasian plate, Tectonophysics, 42, Seno, T., Stein, S. & Gripp, A.E., A model for the motion of the Philippine Sea Plate consistent with Nuvel 1 and geological data, J. geophys. Res.. 98, Soh, W., Pickering, K.T., Taira, A. & Tokuyama, H., Basin evolution in the arc-arc Izu collision zone, Mio-Pliocene Miura Group, central Japan, J. geol. SOC. Lond., 148, Sommerville, P., The accomodation of plate collision by deformation in the Izu block, Japan., Bull. Earthq. Res. Inst. Tokyo Uniu., 53, Takemura, M., Aftershock activities of two days after the 1923 Kanto Earthquake (M = 7.9) inferred from seismograms at Gifu observatory, Zishin, 46, (in Japanese). Tamaki, K. & Honza, E., Incipient subduction and obduction along the eastern margin of the Japan Sea, Tectonophysics, 119, Tamaki, K., Pisciotto, K.A., Allan, J. & ODP Leg 127 Shipboard Scientific Party, Background, objectives and principal results, ODP Leg 127, Japan Sea, in Proc. O.D.P. Init. Repts, pp. 5-33, eds Tamaki, K., Pisciotto, K.A. & Allan, J., 127, Ocean Drilling Program, College Station, TX. Tamanyu, S., Geotectonic developing history in the Beppu-Kuju graben, central Kyushu, Japan, in The Memoirs of the Geological Society of Japan, pp , eds Hase, Y., Ono, K., Watanabe, K. & Takemura, K., 41, Geological Society of Japan, Tokyo (in Japanese). Tsutsumi, H., Okada, A., Nakata, T., Ando, M. & Tsukuda, T., Timing and displacement of Holocene faulting on the Median Tectonic Line in central Shikoku, Southwest Japan, J. struct. Geol., 13 (2), Uchida, T. & Arai, S., Petrology of ultramafic rocks from the Boso peninsula and Miura peninsula, J. geol. SOC. Japan. 84, Utsu, T., Seismicity of Central Japan from 1904 through 1925, Bull. Earthq. Res. Inst., 56, (in Japanese). Yu, S.B. & Chen, H.Y., Global Positioning System measurements of crustal deformation in the Taiwan arc-continent collision zone, Mem. geol. SOC. China, in press. Q 1996 RAS, GJI 126,