Seismic Azimuthal Anisotropy for the Southeastern Tibetan Plateau Extracted by Wave Gradiometry Analysis

Wave Gradiometry (WG) is a dense array data processing method used to extract seismic velocity and other properties of the earth. In this study, we propose using WG for a plane wave field to measure azimuthal anisotropy by fitting the phase velocities from different back azimuths. We applied this new method to the Temporary Western Sichuan Array and obtained the isotropic and azimuthal anisotropy for period bands centered at T = 20, 40, and 60 s in the southeast Tibetan Plateau. Our results show that the fast orientations of anisotropy are well consistent with the strikes of the fault system and orogenic belt. The weak anisotropic magnitude in the northern part of the south Chuandian subblock and a consistence of fast orientation in boundary fault zone (BFZ) at different periods may be attributed to anisotropic relics associated with the Emeishan Large Igneous Province. Comparing our results with those of previous studies, we found that our anisotropic results are consistent with anisotropic structures measured by the Pms phase in the plateau area, while a systematical angular difference exists in the BFZ, Sichuan Basin, and south Chuandian subblock where high Rayleigh wave phase velocity was observed. The Lijiang-Xiaojin fault and Longmenshan fault appear to form a deep-rooted strong boundary at the lithospheric scale, as they serve as a boundary for anisotropic structures, isotropic velocity, and the other three WG parameters.