S-Wave Velocity Structure of the Sichuan-Yunnan Region, China: Implications for Extrusion of Tibet Plateau and Seismic Activities

The Sichuan-Yunnan region is located at the intersection between the South China Block, the Indian plate, and the Tibet Plateau and is crisscrossed with deep and large faults and is characterized by strong seismic activities and the extrusion of the Tibetan Plateau. Here we employ 1-year continuous waveforms of the vertical component of 89 broadband seismic stations using noise tomography in this region to evaluate the velocity structure and its implications. 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 extracted in this area. We then use the surface wave tomography method to obtain the Rayleigh wave group velocity distribution of 9-40s. Finally, the S-wave velocity structure in the depth range of 0-60 km is calculated by pure path dispersion inversion. The results show that a large-scale low-velocity anomaly in the lower crust runs through the Sichuan-Yunnan rhombus block (SYRB) to Yangtze Block from west to east, which might correspond to the 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.