HR: 0800h
AN: T21B-0518    [Abstracts]
TI: Shear-wave Polarization Anisotropy in the Mantle Wedge Beneath Hokkaido and Tohoku, NE Japan
AU: * Shimizu, J
EM: shimizu@aob.geophys.tohoku.ac.jp
AF: RCPEV, Graduate School of Science, Tohoku University, 6-6, Aramaki-aza Aoba, Aoba-ku, Sendai, 980-8578 Japan
AU: Nakajima, J
EM: nakajima@aob.geophys.tohoku.ac.jp
AF: RCPEV, Graduate School of Science, Tohoku University, 6-6, Aramaki-aza Aoba, Aoba-ku, Sendai, 980-8578 Japan
AU: Hasegawa, A
EM: hasegawa@aob.geophys.tohoku.ac.jp
AF: RCPEV, Graduate School of Science, Tohoku University, 6-6, Aramaki-aza Aoba, Aoba-ku, Sendai, 980-8578 Japan
AU: Obara, K
EM: obara@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1,Tennodai, Tsukuba, 305-0006 Japan
AB: We investigated shear-wave polarization anisotropy in the mantle wedge beneath northeastern (NE) Japan. Waveforms of intermediate-depth earthquakes recorded by numerous seismic stations were analyzed by the cross-correlation method [e.g., Ando et al., 1983], and 2528 splitting parameters were obtained. In Hokkaido, obtained results show that most of the leading shear-wave polarization directions (fast directions) at stations in the back-arc side are nearly perpendicular to the trench-axis, whereas those at stations in the fore-arc side are sub-parallel to it. A similar pattern of the shear-wave splitting has been also observed in Tohoku. We infer that the anisotropy caused by lattice-preferred orientation of olivine, which is probably attributable to the upwelling flow potion of the mechanically-induced convection in the mantle wedge [Hasegawa and Nakajima, 2004], is a likely candidate for the shear-wave splitting in the back-arc mantle wedge. If this is the case, the present results for Hokkaido may indicate that the upwelling flow direction is sub-parallel to the maximum dip-direction of the subducted slab, oblique to the relative plate motion direction. Although it is not clear what causes the anisotropy in the for-arc side, where the trench-parallel fast directions are observed, similar features have been observed in other areas of Tohoku [Okada et al., 1995; Nakajima and Hasegawa, 2004]. Average delay times between leading and following shear-waves observed at stations in the back-arc side are 0.1 _E0.4 s, and those at stations in the fore-arc side are smaller (0.05 _E0.15 s). The distribution of average delay times observed in Hokkaido dose not show such a clear spatial pattern as that observed in Tohoku, where delay times apparantly increase toward the back-arc side.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 7218 Lithosphere and upper mantle
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting