Rayleigh wave azimuthal anisotropy in western Sichuan region

Based on the observation data of dense seismic array in western Sichuan, we obtain Rayleigh wave phase velocity and azimuthal anisotropy distribution images at 14−60 s period by eikonal surface wave tomography method. The results show that the low velocity of the northwestern block of the Sichuan-Yunnan diamond block is obvious and anisotropic fast wave direction of the lower crust is dominated by NS. The low velocity anomaly inside the Songpan-Garze block is slightly weaker and the anisotropic fast wave direction of the lower crust is dominated by NW-SE, indicating that there may be lower crust channel flow in the northwestern Sichuan-Yunnan diamond block and there is a relatively weak channel flow inside the Songpan-Garze block. The velocity structure and azimuthal anisotropy are significantly different on both sides of the Longmenshan fault zone and the Lijiang-Xiaojinhe fault zone. We speculate that the crustal flow inside the Tibetan Plateau is blocked by two high-velocity and high-intensity blocks in the Sichuan basin and the Central Yunnan block in the east and south, and lateral flows occur along the boundary zone. The azimuthal anisotropy of the surface wave over the period 25 s by NW-SE is similar to that of SKS splitting. It is concluded that the shear wave anisotropy in Sichuan basin may be mainly from the upper mantle. The anisotropy of the crust in the vicinity of the Longmenshan fault zone is complex and the azimuthal anisotropy of the surface wave is different from the weak anisotropy of SKS splitting by NW-SE, indicating that the shear wave anisotropy may be from the deeper mantle and needs further research.