HR: 1330h
AN: T52B-0256    [PDF]
TI: Low-velocity Oceanic Crusts at the top of Subducting Plates Beneath Japan Islands Imaged by Tomographic Method Using the NIED Hi-net Data
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
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1, Tennodai, Tsukuba-shi, Ibaraki-ken, 305-0006 Japan
AU: Obara, K
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1, Tennodai, Tsukuba-shi, Ibaraki-ken, 305-0006 Japan
AU: Kasahara, K
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1, Tennodai, Tsukuba-shi, Ibaraki-ken, 305-0006 Japan
AB: In the original grid-type tomographic method (Zhao $et$ $al$., 1992), the resolution of the images is equal to the grid spacing. It is more realistic to use as many grid nodes as possible for representing heterogeneous velocity distribution. However, the increasing of grid nodes introduces the instability in tomographic image having an artificially rough structure. We introduce correlation among velocities at surrounding grid nodes (Matsubara $et$ $al$., 2001) to stable the solution. Thus the obtained structure might be more close to the real. We apply this modified method to 1,125,272 $P$- and 854,717 $S$-wave arrival times from 21,656 earthquakes recorded by 664 stations of the high-sensitivity seismograph network of Japan, NIED Hi-net, during 2000 to 2002. In the inversion process, the configuration of the Moho discontinuity is fixed according to Yoshii $et$ $al$. (2002) without considering the upper boundary of the Pacific plate. A high-velocity (high-$V$) zone is imaged as the Pacific plate in the northeastern (NE) Japan arc. A low-velocity (low-$V$) layer above the high-$V$ zone with about -(4-10)% velocity anomalies, considered as oceanic crust, coincides with the upper part of the double-seismic zone in the upper part of the slab. A high-$V$ zone at depths of 20-60 km beneath the southwestern (SW) Japan is imaged as the Philippine Sea plate. Low-$V$ zones, as oceanic crust, are found beneath the central and SW Japan except around the collision zone of Izu Peninsula at the northern edge of the Izu-Bonin arc. In the Kanto region, the central part of Japan, the Pacific and Philippine Sea plates subduct beneath the North-American plate and the velocity structure is complicate. We also find that the low-$V$ zone of the Pacific plate is harmonious with the existence of the $S$-wave reflector found by Obara and Sato (1988). The low-$V$ oceanic crust of the Philippine Sea plate is consistent with the results obtained from the block-type tomographic method (Ohmi and Hurukawa, 1996). The $V_P/V_S$ ratio above the Philippine Sea plate is about 1.90 corresponding to the serpentined wedge mantle by Kamiya and Kobayashi (2000). Beneath the Tohoku region, NE Japan, low-$V$ zones of the Pacific plate coincide with the oceanic crust with velocity 6% lower than that in the mantle wedge by the analysis of $P$-to-$S$ converted waves (Matsuzawa $et$ $al$, 1986). Low-$V$ oceanic crusts of the Philippine Sea plate beneath Chubu, southern Kinki, and western Shikoku region, the central and SW Japan, are consistent with the result of the analysis by channel waves (Hori, 1990). The low-$V$ zone beneath the Shukoku region is harmonious with the model derived from receiver function analysis (Shiomi, 2002). Low-$V$ zones are also continuously subducted beneath the volcanic front at a depth of 40 km in the shallower part of the mantle wedge with -(3-7)% anomalies in the NE Japan.
DE: 7218 Lithosphere and upper mantle
DE: 7220 Oceanic crust
DE: 7294 Instruments and techniques
DE: 8150 Plate boundary--general (3040)
DE: 8180 Tomography
SC: Tectonophysics [T]
MN: 2003 Fall Meeting