HR: 0830h
AN: S31C-0779    [PDF]
TI: Shear-wave polarization anisotropy in the mantle wedge beneath the southern part of Tohoku, Japan
AU: * Shimizu, J
EM: shimizu@aob.geophys.tohoku.ac.jp
AF: RCPEV, Graduate School of Science, Tohoku University, 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, 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, Aramaki-aza-Aoba, Aoba-ku, Sendai, 980-8578 Japan
AB: We investigated shear-wave polarization anisotropy in the mantle wedge beneath the southern part of Tohoku, Japan, by using waveform data of intermediate depth earthquakes with M$>$2.5 recorded by the seismic networks of Tohoku University and Japan Meteorological Agency (JMA). We selected waveform data with ray paths whose incident angles to the surface are 35 degrees or less to avoid contamination of particle motions by converted phases. All the seismograms thus selected were filtered with bandpassed ranges of 2-8 Hz. Cross-correlation method [Ando et al., 1983] was used for determining delay time between the leading and following shear-waves (delay time) and the leading shear-wave polarization direction (fast direction). Two horizontal components of observed seismograms were rotated with the direction from 0 to 180 degrees with an interval of 5 degrees, and shifted one horizontal component by a time lag. The time lag varied from 0 to 1 s with an interval of 0.01 s. The length of time window used to calculate correlation coefficient was set to be nearly equal to one cycle of the shear-wave. We do not use the data whose maximum correlation coefficient is less than 0.8. Obtained results show that most of the fast directions at stations in the back-arc side are nearly E-W, whereas those at stations in the fore-arc side are N-S. We infer that the anisotropy caused by lattice-preferred orientation of olivine, which is probably produced by flow in the mantle wedge, is a likely candidate for the observed shear-wave splitting with E-W trend fast directions in the back-arc side. Although it is not certain what causes the N-S trend fast directions in the for-arc side, the same trend is seen in the previous studies of other areas in Tohoku [Okada et al.,1995; Nakajima, 2002]. Observed delay times are mostly 0.1-0.3 s, which is consistent with the results of Okada et al. [1995] and Nakajima [2002]. Acknowledgments: We are grateful to the staff of the JMA for allowing us to use their data.
DE: 7203 Body wave propagation
DE: 7218 Lithosphere and upper mantle
DE: 8121 Dynamics, convection currents and mantle plumes
SC: Seismology [S]
MN: 2003 Fall Meeting