HR: 0800h
AN: T51C-0682    [Abstracts]
TI: Temporal Changes of Shear Wave Velocity and Anisotropy in the Shallow Crust Induced by the 10/22/1999 M6.4, and M6.0, Chia-Yi, Taiwan Earthquake
AU: * Chao, K
EM: kevinchao@gatech.edu
AF: School of Earth and Atmospheric Sciences, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, GA 30332, United States
AU: Peng, Z
EM: zhigang.peng@eas.gatech.edu
AF: School of Earth and Atmospheric Sciences, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, GA 30332, United States
AB: We analyze temporal changes of seismic velocity and anisotropy in the shallow crust from similar earthquake clusters that are recorded at a 200-m-deep borehole station CHY of the Taiwan Central Weather Bureau Seismic Network. This station is located directly above the hypocenter of the 10/22/1999, M6.4 and M6.0, Chia-Yi, Taiwan earthquakes, and is immediately south of the rupture zone of the 09/20/1999 Mw7.6 Chi-Chi earthquake. We perform waveform cross correlation on three-component seismograms generated by 2571 local earthquakes with straight-line incident angle to station CHY less than 45 degrees. The similar earthquake clusters are identified with inter-hypocentral distance less than 5 km and the medium correlation coefficient (CC) greater than 0.90. The three-component seismograms show clear shear wave splitting (~0.16 s time delay and a near E-W fast polarization direction), and surface-reflected downgoing shear waves ~1 s after the direct upgoing waves [Liu et al. 2004, 2005]. We apply a 1-Hz high-pass filter, and compute the autocorrelation function for each horizontal component in a time window 2 s before and 3 s after the handpicked S arrival. The secondary peak in the autocorrelation function is used as a measure of the time delay between the upgoing and downgoing shear waves for the horizontal components of each event. The time delays for both E-W and N-S components measured from similar earthquake clusters increase by ~3 percent at the time of the Chia-Yi mainshock, and followed by a logarithmic recovery. However, the difference of time delay between the N-S and E-W components, which represents the shear-wave anisotropy in the top 200 m of the crust, remains essentially unchanged. We propose that opening of microcracks in the shallow crust with no preferred orientation due to the strong shaking of the Chia-Yi mainshock, followed by gradual closure, would explain the temoral changes of the seismic velocity for both horizontal components, but virtually no observable changes in shear wave anisotropy. We also use different median CC values to identify similar clusters, and separate earthquakes into different groups based on their hypocentral locations. The obtained results are esentially the same, suggesting that the observed temporal changes are not sensitive to the raypath between station CHY and the epicenter of each event, but are mostly controlled by variation in material property in the top 200 m of the crust.
DE: 7200 SEISMOLOGY
SC: Tectonophysics [T]
MN: 2007 Fall Meeting