Phase velocity structure and azimuthal anisotropy beneath Sichuan Basin and surrounding areas from Rayleigh wave

Based on fundamental mode Rayleigh waves from distant earthquakes recorded at 245 broadband seismic stations within Sichuan Basin and surrounding areas, we imaged phase velocity and azimuthal anisotropy of Rayleigh wave at periods from 20 to 120 s. Together with previous studies, we discussed the coupling between the crust and the upper mantle. Image of phase velocity at short periods (20 similar to 30 s) shows good coincidence with geological structures of the Sichuan Basin. Longmenshan Fault and Xianshuihe Fault, which are surficial tectonic boundaries between Chuan-Dian block, Songpan-Garze block, and Sichuan Basin, have a role controlling the velocity structure of the crust. The Songpan-Garze and the Chuan-Dian blocks show lower velocity in the middle and lower crust, which is a sign of relatively soft lithologies. Whereas, the middle and lower crust beneath the Sichuan Basin is relatively strong with higher velocity. In the southern Sichuan Basin, areas close to the boundary of north Yangzi block and south Yangzi block show higher velocity anomaly. The southern Sichuan Basin and its southeast surrounding areas demonstrated stronger anisotropy with an almost uniform fast wave direction for different periods, indicating strong coupling between different depths. In the western Sichuan Basin and its north and northeast surroundings, direction of the fast wave varies with period, indicating weak coupling. All these features agree with the understanding that the strong Yangzi block plays a role resisting the clockwise rotational flow around the east Himalaya syntaxis.