Seismic shear wave splitting in upper crust characterized by Taiwan tectonic convergence

SUMMARY This study conducts a comprehensive investigation of crustal seismic anisotropy over varied geological regimes of Taiwan. With a large amount of earthquake data, the lateral variation of seismic shear wave splitting (SWS) is fully examined in terms of crustal deformation process. As the well‐known vigorous orogeny subjected to the Eurasian–Philippine plate collision, tectonic convergence of Taiwan is presumably propagating from east to west. The acquired SWS waveform data cover areas from the slightly deformed Western Plains to the intermediate‐to‐high metamorphic Western Foothills and central mountain ranges. By means of waveform cross‐correlation, the SWS parameters—the fast‐wave polarization orientation and delay time—infer that the mechanism of lithologic deformation of Taiwan convergence can be classified into two domains: the convergence‐parallel laminating west of the Deformation Front and the convergence‐perpendicular striking east of the Deformation Front. The convergence‐parallel SWS measurement presents the internal fabrics consisting of microfractures subject to lateral compression before the yielding of the lithologic strength, whereas the convergence‐perpendicular measurements reveal the lateral accommodation of deformation as the stress/strain surpass the yielding strength of rock, where the predominant SWS polarization is in the NE–SW direction similar to the general trend of Taiwan's mountain ranges. It is remarkable that there is no correlation between metamorphic degrees with SWS parameters. The geological province which corresponds to higher metamorphism is not consistent with large SWS parameters. This may be because of anisotropic weakness caused by multiple tectonic processes at considerable metamorphic zone. Furthermore, comparison of the SWS delay times with corresponding focal depths suggests that seismic anisotropy in the upper crust may come from multiple layers, and the fabric lamination causing the anisotropy may be confined only within the shallow crust.