Reconstruction of the S-wave velocity structure of crust and mantle from seismic surface wave dispersion in Sichuan-Yunnan Region

We apply the technique of matched filter frequency time analysis to process long period surface wave data covering Sichuan and Yunnan regions,and then obtain the surface wave group velocity dispersion in the period range 10～100s.Then we use the grid inversion method to extract pure path dispersion in the grid of 1.5°×1.5°after getting the distance and time in the grid of the study area by ray tracing,at the same time,we employ the damped least-square method to determine the crust-mantle S wave velocity structure in the Sichuan and Yunnan region.The result shows that the crust of the region is thickened and shortened.In spite of some local low velocity anomalies,the crust and uppermost mantle of the rhombic Chuan-Dian block as a whole have relatively high velocity as compared with the surrounding areas.The peripheral strike-slipping faults display negative velocity anomalies down to the top of upper mantle,which facilitates the lateral extrusion of crustal blocks along the faults.The high conductivity layer in the crust and upper mantle in western Yunnan and Sichuan is connected with partial melting or detachment structure.The average depth of Moho of Sichuan basin is about 48 km.The Moho in Yunnan reaches 58 km in the north end and is about 51 km in the south end,which proves that different blocks are affected differently by tectonic movement.