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Birth of a Major Strike-Slip Fault in SW Japan

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Birth of a Major Strike-Slip Fault in SW Japan Paper 247 Disc Birth of a major strike-slip fault in SW Japan Marc-Andre Gutscher* and Serge Lallemand Laboratoire de GeÂophysique, Tectonique et Sedimentation, University of Montpellier II, Place Eugene Bataillon, 34095 Montpellier Cedex 05, France ABSTRACT A 500 km-long strike-slip fault, the North Chugoku Shear Zone Tectonic Line (MTL), near the Miocene volcanic arc, the central (NCSZ) is identified in SW Japan, responsible for four M 4 7 segment in Shikoku has shown little seismicity for the last 1000 earthquakes in the past 130 years. A new geodynamic mechanism years. Pure dextral strike-slip focal mechanisms for 15 modern of increased interplate coupling above an obliquely subducting and historical events indicate that part of the transcurrent motion flat slab is presented to explain the transfer of trench parallel is being transferred from the MTL to the NCSZ. motion 400 km inland from the Nankai Trough, to the level of the Quaternary adakitic volcanic arc. While Quaternary dextral strike- Terra Nova, 11, 203±209, 1999 slip motion is widely believed to have occurred along the Median 10 6 greater). This agrees with qualita- Takemura, 1993; Kanaori et al., 1994), Introduction tive correlations established by pre- continuing to the SW to Kyushu (Ka- There has been much debate on the vious workers (Jordan et al., 1983; mata and Kodama, 1994; Itoh et al., causes and location of slip partitioning Smalley et al., 1993). In cases of oblique 1998), where the fault trace intersects in obliquely convergent tectonic set- convergence, this increased interplate the active volcanic arc (Fig. 1). Recent tings (Fitch, 1972; Jarrard, 1986; De- coupling can detach a major forearc GPS studies also indicate modern slip Mets, 1992; McCaffrey, 1992, 1996; sliver and entrain it parallel to the along the MTL (Itoh et al., 1998; Le Pinet and Cobbold, 1992; Lallemand, trench. Seismicity along the major Pichon et al., 1998; Ozawa et al., 1999). 1999; Chemenda et al., in press). We bounding strike-slip fault can be quite However, in the last 1000 years, the present a model of strain partitioning high, with shallow M 4 7 events occur- central segment of the MTL on Shiko- near the volcanic arc based on observa- ring on average 2±3 times per century. ku has shown little to no seismicity tions from the North Andean margin This is the case in Ecuador, where (Research Group for Research Group (Gutscher et al., 1999a), and apply it to Carnegie Ridge, an oceanic plateau for Active Faults of Japan, 1980; Tsut- explain the pattern of seismicity in SW representing part of the Galapagos hot- sumi et al., 1991). In contrast to the Japan. A recent study of oblique sub- spot track, subducts beneath the MTL, western Honshu (Chugoku) has duction using analogue modelling Northern Andes (Pennington, 1981; been seismically active (Wesnousky et (Chemenda et al., in press) demon- Gutscher et al., 1999a). It produces an al., 1982; Oike and Huzita, 1988; Shen- strates interplate friction to be an im- intermediate-depth seismic gap, and Tu et al., 1995) (Fig. 2). Similar to portant factor controlling strain parti- apparently supports a 400 km-wide flat Ecuador (Gutscher et al., 1999a), shal- tioning. It also showed that interplate slab segment. Approximately 400 km low seismicity is aligned along a Qua- pressure (e.g. caused by steep or flat inland from the trench, major dextral ternary adakitic volcanic arc (Morris, subduction) is the primary factor con- strike-slip motion occurs along several 1995) far inland. trolling the tectonic regime of the upper fault splays of the Dolores±Guyaquil plate. Subduction angle, and more spe- megashear at about the level of the Flat subduction beneath SW Japan cifically the unusual subduction geome- adakitic volcanic arc (Kellogg and try known as `flat subduction' (Bara- Vega, 1995; Gutscher et al., 1999a). Along the Shikoku margin and below zangi and Isacks, 1976; Sacks, 1983; adjacent SW Honshu, flat subduction Cahill and Isacks, 1992; Gutscher et occurs (Nakanishi, 1980; Hirahara, Oblique convergence and al., 1999b), can modify interplate cou- 1981) and has been attributed to the transcurrent motion pling by up to an order of magnitude. A relatively young age (5 20 Myr) of the statistical analysis of shallow (5 70 km Plate kinematic data indicate the con- Shikoku Basin lithosphere (Sacks, 1983) depth) upper plate seismicity per- vergence vector between the Philippine as well as buoyancy effects from the formed along the length of the entire Sea Plate and SW Japan at the Nankai flanking Izu Bonin Arc to the east and Andean chain (Gutscher and Mala- trough to be N518W at 4.6 cm yr71 the Palau-Kyushu Ridge (the proto Izu- vieille, 1999; Gutscher et al., in press), (Seno et al., 1993) (Fig. 1). Geodetic Bonin arc) to the west (Hirahara, 1981). reveals that seismic energy released studies suggest a vector of N568W The flat slab beneath SW Japan is not 250±800 km from the trench is on aver- (Shen-Tu et al., 1995) to N718W (Ha- well constrained by hypocentre data age 3±5 times higher above flat slab shimoto and Jackson, 1993). Conver- (Fig. 2), since the Philippine Sea Plate segments than for adjacent steep slab gence is thus 20±408 oblique to the slab beneath Shikoku and Chugoku is segments (in some extreme cases over margin normal direction. Geological essentially aseismic below 60±80 km data suggest that dextral strike-slip mo- depth. However, seismicity cross-sec- Correspondence and present address: tion has occurred throughout the Qua- tions beneath Kyushu and Shikoku, GEOMAR, Marine Geodynamics, Wisch- ternary along the Median Tectonic reveal dramatic differences (Fig. 2). hofstr. 1±3, D-24148 Kiel, Germany. E- Line (MTL) in Shikoku (Kanaori, While the oceanic plate beneath Kyushu mail: [email protected] 1990; Tsutsumi et al., 1991; Itoh and reaches depths of 200 km at a distance of *C 1999 Blackwell Science Ltd 203 Paper 247 Disc Birth of a major strike-slip fault, Japan . M.-A. Gutscher and S. Lallemand Terra Nova, Vol 11, No. 5, 203±209 ............................................................................................................................................................................................................................................. Flat Slab 126ûE 130ûE (aseismic) 140ûE Japan Sea Honshu S. Korea Arc NCSZH Volc. Y N 35ûN Ch A Adakitic MTL Izu - Bonin Arc Sh Unz.V Ky Nankai Trough Palau -Philippine Kyushu Ridge Sea Plate 30ûN Okinawa Trough (backarc basin) Ryukyu Arc Ryukyu Trench Fig. 1 Tectonic setting of the SW Japan region showing oblique, NW-directed subduction of the Philippine Sea Plate and active volcanoes (shaded triangles) and the zone of flat subduction (shaded light grey). Dextral strike-slip motion is partitioned along the North Chugoku Shear Zone (NCSZ), parallel to the adakitic arc, as well as along the Median Tectonic Line (MTL), the former Miocene arc. Sh = Shikoku Island, Ky = Kyushu Island, Ch = Chugoku peninsula, A = Arima-Takatsuki Tectonic Line, H = Hanaore Fault, Y = Yoro Fault, N = Neodani Fault., Unz.V = Unzen Volcano. & 300 km from the trench, the slab 1995) also place the Philippine Sea coast (along the southern margin of the beneath Shikoku has only reached 60± Plate at a shallow depth consistent with Japan Sea) with four large M 5 7earth- 80 km depth at the same distance. The low-angle subduction. These observa- quakes in the last 130 years (Fig. 3). arc-trench gap is also much greater (400 tions, combined with the adakitic vol- Another 8 strong earthquakes (6.0 4 M km) for the Shikoku section. canic arc, parallel to, but 400 km dis- 4 6.9) have occurred along this zone since Tomographic data image the higher tant from the Nankai trough (Morris, 1710. There are historical records of M velocity Philippine Sea Plate slab des- 1995), provide strong evidence for a 600 4 7 earthquakes along this part of the cending steeply beneath Kyushu (Fig. km-wide flat slab region beneath SW Japan Sea coast dating back to AD700 2, Sect. A±A') and lying horizontally Japan in agreement with previous inter- (Oike and Huzita, 1988). The global, between 70 km and 100 km depth be- pretations (Nakanishi, 1980; Hirahara, modern (1964±95) ISC teleseismic dataset neath Shikoku and Chugoku (Fig. 2, 1981; Sacks, 1983; Oda et al., 1990; of Mb 4 4.0 earthquakes relocated by Sect. B±B') (Hirahara, 1981). The latter Morris, 1995). Engdahl et al. (1998), images this trend agrees with interpretations of S-wave (Fig. 2). Extensive regional catalogues phase data attributed to a flat-lying (Wesnousky et al., 1982; Shen-Tu et al., Seismicity in SW Japan slab at 50±70 km depth beneath 1995; Japan Meteorological Agency± Shikoku and Chugoku (Nakanishi, Historical and modern catalogues reveal a JMA data set 1926±96) and modern mi- 1980; Oda et al., 1990). Seismic data from 30 km-wide 600 km-long band of high croseismicity (M 5 1) surveys, spanning the eastern Nankai trough (Ishida, seismicity parallel to the north Chugoku July 1965±June 1984 (Oike and Huzita, 204 *C 1999 Blackwell Science Ltd Paper 247 Disc Terra Nova, Vol 11, No. 5, 203±209 Birth of a major strike-slip fault, Japan . M.-A. Gutscher and S. Lallemand ............................................................................................................................................................................................................................................. 36ûN B' 500 flat slab ad. arc 300 Shikuku Chugoku 200 34ûN high p-wave velocities 100 trench A' 0 BB' 0 -20 -40 -60 -80 -100 -120 -140 -160 -180 -200 500 Unzen 32ûN steep slab 400 Kyushu A/B c.-a. arc 200 100 trench 30ûN high p-wave velocities 0 130ûE 132ûE134ûE 136ûE AA' surface historical seismicity M modern seismicity Mb -6000 -4000 -2000 0 300 1000 3000 elevation [m] earthquake 7.0 6.5 5.0 lvto nm in elevation 8.0 0 50 100 150 200 depth [km] 6.0 6.0 4.0 Fig.
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