HR: 1340h
AN: S13D-1090    [Abstracts]
TI: Low Velocity Oceanic Crusts at the uppermost part of the Subducting Plates Beneath Japan Arc Derived From Seismic Tomography
AU: * Matsubara, M
EM: mkmatsu@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1, Tennodai, Tsukuba-shi, Ibaraki-ken, 305-0006 Japan
AU: Sekine, S
EM: ssekine@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1, Tennodai, Tsukuba-shi, Ibaraki-ken, 305-0006 Japan
AU: Hayashi, H
EM: rin@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1, Tennodai, Tsukuba-shi, Ibaraki-ken, 305-0006 Japan
AU: Obara, K
EM: obara@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1, Tennodai, Tsukuba-shi, Ibaraki-ken, 305-0006 Japan
AU: Kasahara, K
EM: kasa@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1, Tennodai, Tsukuba-shi, Ibaraki-ken, 305-0006 Japan
AB: Aki and Lee (1976) and Aki $et$ $al$. (1977) are the pioneer studies of the body-wave seismic tomography for the local and regional scale. Aki and Lee (1976) use blocks and Thurber (1983) and Zhao $et$ $al$. (1992) use grid nodes for model parameterization. A weak point of the grid method is that the velocity must be continuous for any direction. Zhao $et$ $al$. (1992) proposed a new parameterization for 3-D velocity-discontinuity with complex shape. Assumption of the velocity discontinuity is available only if the depth of the discontinuity is correct, however, the wrong position of the discontinuity has some bad influence on the velocity structure. We place many grid nodes and introduce correlation among velocities at surrounding grid nodes into Zhao_fs method (Matsubara $et$ $al$., 2004) to obtain more fine and stable solution. Thus the obtained models might be more close to the real. We apply this modified method to 1,033,879 $P$- and 716,561 $S$-wave arrival times from 21,126 natural sources recorded by 698 stations of the NIED Hi-net with a grid spacing of $0.125\deg$ to obtain the crustal and upper mantle structure beneath the whole Japan. No velocity discontinuity such as Moho or the upper boundary of the Pacific (PAC) plate is assumed. The PAC plate beneath the northeastern Japan arc has about +5% velocity disturbance and high-velocity (high-$V$) zone at depths of 20-40 km beneath the southwestern Japan is considered as the Philippine Sea (PHS) plate. Low-velocity (low-$V$) zones considered as oceanic crust in the uppermost part of the PAC plate is obtained from south Tohoku to Kanto district at depths of 30-140 km. Low-$V$ zones considered as the oceanic crust in the uppermosts part of the PHS plate beneath from the Kanto to Kyushu district at depths of 30-60 km beneath the Kanto and southern Kinki district and at depths of 30-70 km beneath the western Shikoku district. We focus the underground structure around the Tokyo Metropolitan area, the Kanto region, central Japan, where the PAC and PHS plates subduct beneath the Eurasian plate. The same tomographic method is applied to 422,799 $P$- and 369,596 $S$-wave arrival times from 15,214 natural sources recorded by 176 stations with a grid spacing of $0.05\deg$. The high-$V$ PHS plate subducts to depths around 80 km. We clearly found low-$V$ oceanic layer in the uppermost part of the PAC and PHS plates. Low-$V$ oceanic layer of the PAC plate exists at depths of 60-120 km. The $V_P$/$V_S$ ratio of this layer is abount 1.85-1.90 and the possibility of the existence of the molten rock is small because of not so large $V_P$/$V_S$ ratio. There is garnet-granulite oceanic crust which is transferred from basalt or gabbro and not to eclogite with temperature about 400$\deg$C. Above the oceanic layer of the PHS plate, low-$V$ zones also exist in the mantle wedge at depths of 30-60 km. The low-$V$ zone in the western part of the mantle wedge about 60 km away from the Sagami Trough is composed of gabbro and that in the eastern part of the mantle wedge about 100 km away from the trough consists of about 30% serpentinized gabbro with consideration of $V_P$/$V_S$ ratio about 1.80-1.90. The PHS plate also has double seismic zone. The seismicity is high where the $V_P$/$V_S$ ratio is large.
DE: 7220 Oceanic crust
DE: 8150 Plate boundary--general (3040)
DE: 7205 Continental crust (1242)
DE: 7218 Lithosphere and upper mantle
DE: 0994 Instruments and techniques
SC: Seismology [S]
MN: 2004 AGU Fall Meeting