Systematic analysis of crustal anisotropy and attenuation using seismic data associated with the 1999 Chi-Chi, Taiwan, earthquake

We analyze shear-wave splitting (SWS) in a high-quality waveform data set recorded at surface and downhole (0.2 km) seismometers in Taiwan. The data set was generated by events in the time period 1993--2000, which includes the September 20, 1999 Mw 7.6 Chi-Chi, Taiwan, earthquake sequence. The purpose is to investigate the spatial distribution of stress-induced crustal anisotropy and its possible temporal evolution in relation to the occurrence of large earthquakes.; The SWS analysis employs both the aspect ratio and cross-correlation methods to obtain robust measurements. Measured results from strong motion stations that cover well the study region show that velocity anisotropy seems to be mainly controlled by local tectonic stress. Some measurements from stations nearby active faults display a polarization direction parallel or sub-parallel to the fault strike. Measured fast polarization directions and time delays of SWS vary significantly with location. No dependence of time delay on depth has been found in various areas over depth range 5--18 km. Seismic Anisotropy in study region appears to be distributed in shallow crust. Analysis results based on recordings from a borehole station further indicate that observed anisotropy is confined primarily to the top 2--3 km of the crust and is dominated by the top few hundred meters. The observed SWS parameters and the great similarity of waveforms generated by clusters of earthquake multiplets, show no appreciable systematic temporal changes over the 2.7 year period before the 1999 Mw 7.6 Chi-Chi mainshock and the 2.3 year period after. The observed lack of temporal change of SWS parameters is obtained from a data set recorded in the vicinity of large rupture zones that experienced stress changes that are as big as expected to occur in the brittle crust during large earthquake cycles.; We have also measured the seismic attenuation in the top 0.2 km of the crust by analyzing surface-reflected waves in the borehole recordings. The results reveal a substantial difference of attenuation between the fast and slow shear wave components and show a clear evidence of attenuation anisotropy in the near-surface structure. An attenuation-associated dispersion is clearly observed and it has a significant effect on the shapes of waveforms.