HR: 0830h
AN: NG41B-0065 INVITED     [PDF]
TI: Systematic Analysis of Shear-Wave Splitting in the Aftershock Zone of the 1999 Chi-Chi Earthquake: Evidence for Shallow Crustal Anisotropy and Lack of Temporal Variations
AU: * Liu, Y
EM: yunfengl@terra.usc.edu
AF: Dept. of Earth Sciences, University of Southern California, University Park, Los Angeles, CA 90089 United States
AU: Teng, T
EM: lteng@terra.usc.edu
AF: Dept. of Earth Sciences, University of Southern California, University Park, Los Angeles, CA 90089 United States
AU: Ben-Zion, Y
EM: benzion@terra.usc.edu
AF: Dept. of Earth Sciences, University of Southern California, University Park, Los Angeles, CA 90089 United States
AB: We analyze shear-wave splitting (SWS) in a high-quality waveform data set recorded at surface and downhole (0.2 km) seismometers in a region around the 1999 M$_{\rm w}$ 7.6 Chi-Chi, Taiwan, earthquake sequence. The data set was generated by events before, during and after the mainshock. 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. Results from downhole records show a stable polarization direction of the fast shear wave which matches well the local GPS velocity field. A slightly different polarization direction of the fast shear wave is obtained from surface data. This suggests a possible anisotropy change between the top 0.2 km structure and the deeper section of the crust. Measured time delays below the downhole station have an average value 0.16 sec without systematic changes for sources from about 8 km to 20 km in depth. Estimates of time delays in the top 0.2 km of the crust based on shear waves reflected from the free surface give a constant 0.04 sec. These two types of measurements and an S-velocity model indicate that the crustal anisotropy in the region is dominated by the top 2 - 3 km. The measured polarization directions and time delays give essentially constant values over the study period in the region adjacent to the Chi-Chi earthquake and within 10 km to the epicentral region of its two large M 6.0 aftershocks. Analysis of SWS in waveforms produced by earthquake multiplets confirms further the lack of temporal variations. This raises doubts on the usefulness of SWS measurements for earthquake forecasting.
DE: 7203 Body wave propagation
DE: 7205 Continental crust (1242)
DE: 7223 Seismic hazard assessment and prediction
SC: Nonlinear Geophysics [NG]
MN: 2003 Fall Meeting