HR: 0800h
AN: S11A-0283    [Abstracts]
TI: Temporal changes in S-wave velocity structure at a borehole site after strong ground motion shock
AU: * Sawazaki, K
EM: sawa@zisin.geophys.tohoku.ac.jp
AF: Tohoku University, Aramaki-Aza-Aoba, Aoba-Ku, Sendai-shi, Japan, Sendai, 980-8578, Japan
AU: Sato, H
EM: sato@zisin.geophys.tohoku.ac.jp
AF: Tohoku University, Aramaki-Aza-Aoba, Aoba-Ku, Sendai-shi, Japan, Sendai, 980-8578, Japan
AU: Nakahara, H
EM: naka@zisin.geophys.tohoku.ac.jp
AF: Tohoku University, Aramaki-Aza-Aoba, Aoba-Ku, Sendai-shi, Japan, Sendai, 980-8578, Japan
AU: Nishimura, T
EM: nishi@zisin.geophys.tohoku.ac.jp
AF: Tohoku University, Aramaki-Aza-Aoba, Aoba-Ku, Sendai-shi, Japan, Sendai, 980-8578, Japan
AB: We analyzed a pair of surface and downhole accelerographs at station SMNH01 of KiK-net, which experienced strong ground motion up to 844 gal by the 2000 Western Tottori Earthquake (06/10/2000, MW6.7), Japan, to examine how the shallow subsurface structure changes with time. Station SMNH01 has a borehole of 100 m depth, where sandy gravel distributes from 0 to 11 m depths, and solid basalt from 11 m to the bottom. We calculated the average spectral ratio and the average cross-correlation function of coda waves of more than 100 local earthquakes which occurred in six periods: before the mainshock, 50 to 280s, 0 to 10 days, 10 to 100 days, 100 to 1000 days, and 1000 to 2000 days after the mainshock. Before the mainshock, the lowest peak frequency of the average spectral ratio was 4.5 Hz. In the period from 50 to 280 s after the strong ground motion shock, the lowest peak frequency decreased to 4.0 Hz, and the other peaks also decreased their frequencies. After that, the peak frequencies have continued to recover to their original values for over 1 year. The peak level of the average coda spectral ratio dropped just after the direct S-wave arrival of the mainshock to the station, but it quickly recovered within a few minutes. The average cross-correlation function of coda waves shows a clear peak at a positive lag time in each of the six periods. Before the mainshock, the peak lag time was 0.09s. The peak lag time increased to 0.105s in the period from 50 to 280s after the strong motion shock, however, it has continued to recover to the original peak lag time for over 1 year. We estimate the S-wave velocities (VS [m/s]) of shallow structure beneath the station for each period by fitting the theoretical coda spectral ratio and peak lag time to the observed ones. The theoretical coda spectral ratio is calculated by using the modified propagator matrix method, where coda waves are composed of SH and SV waves isotropically incident on the borehole sensor with random phases. We divide the shallow structure into six layers and fix P-wave velocity (VP [m/s]) and thickness of them based on the well-log data. We further suppose that attenuation parameters are expressed as QP=QS=0.008·VS·f, where f is frequency in Hz and that density is d=310·VP⁰.25 [kg/m3]. We find that the S- wave velocity changes at the shallow layers can well explain the observed change just after the strong motion shock and recovery process observed for the following 1 year. The S-wave velocity of deeper layers didn't show any significant changes compare to that of the shallowest layers.
DE: 3255 Spectral analysis (3205, 3280)
DE: 3270 Time series analysis (1872, 4277, 4475)
DE: 7212 Earthquake ground motions and engineering seismology
SC: Seismology [S]
MN: 2007 Fall Meeting