HR: 0800h
AN: S31A-0282    [Abstracts]
TI: Mapping phase velocity anomaly in Japan using Hi-net tiltmeters
AU: * Nishida, K
EM: knishida@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, 1-1-1,Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Kawakatsu, H
EM: hitosi@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, 1-1-1,Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Obara, K
EM: obara@bosai.go.jp
AF: National Res. Inst. for Earth Sci. and Disaster Prev, 3-1, Tennoudai, Tsukuba, 305-0006 Japan
AB: From 0.05 to 0.2 Hz random excited surface waves, known as microseisms, are dominant, and they mask seismic signals of earthquakes as noise. The microseisms are excited by oceanic disturbances at random. Recently using their random excitation properties, researchers (e.g. Shapiro et al., 2005) measured group velocity anomaly, which is shown by cross-correlation function between pairs of stations. Here we developed a new method to measure phase velocity anomaly, and mapped phase velocity anomaly of Rayleigh and Love waves using very dense network of Hi-net tiltmeters in Japan. We first divided the whole record in a time period from June 2004 to December 2004 into 1024 sec segments with an overlap of 512 sec, and calculated radial and transverse components of the cross-spectra of microseisms for every pair of 679 stations from 0.05 to 0.2 Hz. Next for each station we selected cross-spectra between pairs of stations within a 100 km circle of the station. For each station we calculated dispersion curve with the assumption that lateral heterogeneity is not so large within the circle. We measured phase velocity anomaly of every cross-spectra from the reference dispersion curve for each circle. Using these measurements we mapped phase velocity anomaly of Rayleigh and Love waves with 0.1 degree grids in three frequency bands(0.05-0.1 Hz, 0.1-0.15 Hz, 0.15-0.2Hz). Resultant phase velocity maps show that similar pattern: high velocity anomaly in mountain ranges and low velocity one in flat region. In particular resultant maps in Hokkaido show clear low phase velocity anomaly, which correspond to the Hidaka Collision Zone. In further study we also try to infer anisotropy.
DE: 7255 Surface waves and free oscillations
DE: 7270 Tomography (6982, 8180)
SC: Seismology [S]
MN: Fall Meeting 2005