A preliminary study on crustal anisotropy in Chia-Nan area of Taiwan

Twenty nine events recorded at three stations in the Chia-Nan area of Taiwan showed split S waveforms with delay times ranging between 0.1 and 0.2 s, Aspect ratio method was applied to retrieve anisotropic properties of these phases. Data at station CHY were characterized by roughly E-W fast direction that is parallel or sub-parallel to the compressional axis in the region, as determined by other approaches, Stations ALS and SGS recorded split waveforms that showed fast directions oblique with respect to the apparent regional compressional axis, but sub-parallel to that of the orientation of the secondary faults in the vicinity. To verify the anisotropic nature of splitting, 6 events were selected to construct polarization patterns in which splitting was removed. The observed splitting-corrected polarizations agreed with those given by reliably determined focal mechanisms of these events, therefore excluding extrinsic contributions from the sources. Analyses from ALS and SGS indicated that seismic anisotropy observed at local and regional scales may not be fully accounted for by the Extensive Dilatancy Anisotropyc (EDA) theory but rather are controlled by conditions in the local fault system.