Systematic Analysis of Shear-Wave Splitting in the Aftershock Zone of the 1999 Chi-Chi, Taiwan, Earthquake: Shallow Crustal Anisotropy and Lack of Precursory Variations

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 20 September 1999 Mw 7.6 Chi-Chi, Taiwan, earthquake sequence. The data set was generated by events in a 5year period before, during, and after the mainshock. The purpose is to investigate the depth extent of the 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 that matches well the local Global Positioning System (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 of 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. A likely depth distribution inferred from these two types of measurements and an S-velocity model indicates that the crustal anisotropy in the region is dominated by the top 2 to 3 km. The measured polarization directions and time delays give essentially constant values over the 2.7-year premainshock and 2.3-year postmainshock periods in the region adjacent to the Chi-Chi rupture and within 10 km from 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 precursory temporal variations of crustal anisotropy in the immediate neighborhood of the Chi-Chi earthquake sequence. The results raise doubts on the general usefulness of SWS measurements for earthquake forecasting. An apparent coseismic increase in anisotropy time delay of approximately 10% is observed for depths >0.2 km; however, this value is clearly affected by spatial changes associated with different event locations before and after the Chi-Chi mainshock.