Analysis On Crustal Structure Characteristics Of Southern Sichuan-Yunnan By Regional Double-Difference Seismic Tomography

The regional double-difference seismic tomography (tomoFDD) was applied to 317366 first Pg absolute arrival times and 2674110 high quality relative Pg arrival times of 49130 earthquakes recorded by 85 seismic stations of Yunnan, Sichuan and Guizhou Seismic Networks from January 2008 to December 2017. Detailed crustal 3D P-wave velocity structure and the hypocenter parameters of 39621 earthquakes were simultaneously determined in southern Sichuan-Yunnan (SSY). The 3D velocity structure provides distribution characteristics for the middle-lower crustal flow and magma of Tengchong volcanic area. The results show velocity anomalies of the upper crust in SSY are closely related to the surface topography. A low velocity anomaly extends more than 200 km in the middle and lower crust of the Xiaojiang fault zone. The southernmost end of the low velocity anomalies is blocked by the Red River fault and ends at the north side of the southern segment of the fault zone. An obvious low-velocity anomaly is visible in the middle and lower crust beneath SSY area, which may reflect the weak material flow in the middle and lower crust in the southeastern margin of Tibetan Plateau. The low velocity anomalies beneath the southern segment of the Red River fault zone turn to southeastward, which further shows that the Red River fault may be the southwestern boundary of Sichuan-Yunnan rhombic block. There are obvious differences in the velocity anomalies of each segment of the Red River fault. The focal distribution after relocation shows that the activity in the middle and southern segments of the Red River fault zone is not as strong as that in the northern segment. However, the focal depth is deeper than that of the northern segment. An obvious low-velocity anomaly in the depth range of 10 similar to 20 km in the west and north of Tengchong volcanic area may be the magma from middle to upper crust through magma channel formed by Nujiang fault. The low-velocity anomalies may represent the intra-crustal magmatic activity with olivine basalt and andesite as the main lithology since the Pleistocene. The continuous magmatic activity provides the main driving force for surface thermal activity.