Crustal thickness and Poisson's ratio beneath the Yunnan region

A total of 1939 receiver functions were obtained from 732 teleseismic events (M > 5.0) recorded at 21 broadband portable seismic stations on the Tengchong, Baoshan and Simao blocks and Yangtze platform. These stations were installed by the Institute of Crustal Dynamics, China Earthquake Administration during 2010 and 2011. Using the H-kappa stacking and searching method, crustal thickness and velocity ratio beneath the stations are obtained. Results show that crustal thickness and Poisson's ratio inferred from the velocity ratio clearly vary, and they illustrate block features in deep structures. Except for the Tengchong block, crustal thickness increases from south to north along the same block and from west to east across different blocks. In the Yangtze platform, Poisson's ratio and crustal thickness show a consistent and significant increasing trend from south to north, possibly indicating that crustal thickening is caused mainly by lower crustal variations. In contrast, Poisson's ratio has no significant change within the Baoshan and Simao blocks. Such differences demonstrate that the Jinshajiang-Red River fault is a southern boundary of the South China block. The H-kappa results inferred from three portable stations on the Tengchong block show high Poisson's ratios, but they vary clearly with back-azimuth, implying the existence of strong anisotropy in the crustal medium beneath the stations.