Seismic anisotropy beneath the Chinese Mainland: Constraints from shear wave splitting analyses

We use earthquakes recorded by the China National Seismic Network from 2015 to 2019 and measure shear wave splitting parameters of SKS to study the anisotropic characteristics beneath the mainland of China. In general, the fast directions change from nearly E-W in western China (northwest China and Qinghai-Tibetan Plateau) to nearly N–S in central China (Ordos and Sichuan-Yunnan), and then turn to approximately E-W in eastern China (North and South China). The delay times of slow wave in eastern China are about 1.0–1.7 ​s, larger than those in central and western China (about 0.6–1.0 ​s). In addition, the fast directions in eastern China are highly consistent with the plate motion direction and horizontal GPS velocities with respect to Eurasia, indicating that the observed anisotropy is mainly from the asthenosphere which is strongly coupled to the overlying lithosphere. However, the fast directions in western China are mostly in accord with the strike of the surface structures (such as faults), possibly due to the directional arrangement of crystal lattices caused by shear deformation under tectonic activities.