Crustal S-wave Velocity Structure In The Sichuan-yunnan Region Inferred From An Ambient Noise Tomography

The Sichuan-Yunnan region is located at an area where multiple plates assemble,which recoded the long-distance effect of the continental collision and subduction.The deep and large faults are widely distributed in the study region.It belongs to the south part of the North-South seismic zone with strong seismic activities and is a key region of the eastward extrusion of the Tibetan Plateau,which is focused by geoscientists.To reveal the model for the extrusion of the Tibetan Plateau and earthquake mechanism,a number of geophysical studies have been carried out in this area.However,the size,the direction and the channel of the eastward extrusion of the crustal material flow in the Tibet Plateau and the earthquake mechanism are still unclear.Therefore,we employ the noise tomography technique and carried out a study on the crustal S-wave velocity structure in the Sichuan-Yunnan region.In this study,one-year continuous waveforms of the vertical component of 89 broadband seismic stations are collected.Through single station data preprocessing,cross-correlation calculation,stacking,group velocity dispersion measurement and quality evaluation,the group velocity dispersion curves of Rayleigh waves for the different periods were obtained.We then use the surface wave tomography method to obtain the Rayleigh wave group velocity distribution of 9-40 s.Finally,the S-wave velocity structure in the depths of 0-60 km is obtained by pure path dispersion inversion.The results show that the velocity structure of different geological units is obviously different.The Longmenshan fault zone is the boundary of the abrupt change of S-wave velocity structure between the Songpan-Ganzi terrane and the crust of the Sichuan Basin,and the velocity structure of both sides of the Honghe fault is obviously different.There is a good correspondence between the short-period group velocity and the sedimentary thickness in the Sichuan Basin,and the crustal velocity in the Sichuan Basin has obvious lateral heterogeneity.The low velocity anomalies in the middle and lower crust of the study area pass through the Sichuan-Yunnan rhombus-shaped Block and the Yangtze block from west to east,which might correspond to the eastward extrusion of the Tibetan Plateau or channel flow.On the other hand,the seismic activities are mainly concentrated at the western part of the region,with the earthquakes distributed at the boundary between the low-and high-velocity structures,as well as the adjacent region,which we correlate with the extrusion of the Tibet Plateau.