HR: 0830h
AN: S11C-0312    [PDF]
TI: Nation-wide Upper Crustal Anisotropy Distribution Beneath the Japan Island\\ -Shear-wave Splitting Analysis of Hi-net Data-
AU: * Sakakibara, Y
EM: yousuke@eps.nagoya-u.ac.jp
AF: NAGOYA UNIVERCITY, GRADUATE SCHOOL ENVIROMENTAL STUDIES, NAGOYA, 464-8602 Japan
AU: Tadokoro, K
EM: tad@seis.nagoya-u.ac.jp
AF: NAGOYA UNIVERCITY, GRADUATE SCHOOL ENVIROMENTAL STUDIES, NAGOYA, 464-8602 Japan
AU: Hirahara, K
EM: hirahara@eps.nagoya-u.ac.jp
AF: NAGOYA UNIVERCITY, GRADUATE SCHOOL ENVIROMENTAL STUDIES, NAGOYA, 464-8602 Japan
AB: There have been several regional shear-wave splitting studies of crustal anisotropy in Japan (e.g. Kaneshima, 1990). Recently, a high-sensitivity short-period seismic network, called Hi-net, has been installed, which realizes the high-density station spacing of 20 km over the Japan Islands. Then, we have executed shear-wave splitting analyses using seismic waveforms observed at such Hi-net stations to reveal the upper crustal anisotropy structure beneath the Japan Islands with higher spatial resolutions than those in previous regional studies. \\ We used two horizontal components of seismograms recorded for earthquakes with depths of 0-15 km. Since Hi-net stations have been installed in the ground, there is little influence of free surface. At first, seismograms were low-pass filtered at 10 Hz to remove the high frequency noises, and to distinguish each shear wave easier. Then, the leading shear wave polarization direction (LSPD) and the travel time differences (DT) between leading and slower split shear waves have been obtained from the horizontal components of the splitting shear waves by the cross correlation method (Shih and Meyer, 1990). The horizontal component seismograms are rotated clockwise from north to east with an incremental step of 5 degrees. For each step the cross correlation coefficient (coff) between two rotated waveforms is computed for a lag time from +0.4 to -0.4 s with increments of 0.01 s. We adopt the values of LSPD and DT, respectively, for the rotated waveforms giving the maximum coff. \\ Finally, we have obtained about 5000 LSPD and DT observed at about 300 stations during a period from 2000 to 2003. We found that the polarized directions of the faster shear waves at most of stations are generally constant, and independent of arrival directions and incident angles less than the critical angle. The polarized directions agree with the axes of the maximum horizontal compression obtained from fault mechanism analyses. These results suggest that the crack induced anisotropy is present in the upper crust beneath Japan. A larger number of measured LSPD and DT and used stations than those in previous studies would improve our understanding of the upper crustal anisotropy and provide a possibility of shear-splitting tomography.
DE: 1734 Seismology
SC: Seismology [S]
MN: 2003 Fall Meeting