Upper crustal velocity of Lijiang-Qingzhen profile and its relationship with the seismogenic environment of the M_S6.5 Ludian earthquake

On August 3, 2014, an M_S 6.5 earthquake occurred in Ludian County, Yunnan. The Ludian earthquake ruptured along the NE-oriented Zhaotong-Lianfeng fault zone, western margin of the Yangtze block. However, there was no seismic sounding profile across the Zhaotong-Lianfeng fault zone up to now. Since the distance from Lijiang-Panzhihua-Qingzhen active source seismic sounding profile to the Ludian earthquake source region is within 50 km, we can handle the wide-angle seismic data to reveal the upper crustal velocity structure in this area, and provide the velocity model of upper crust for the earthquake locating and focal mechanism inversion of Ludian earthquake sequence, etc. The velocity profile shows that the crustal velocity around the Xiaojiang fault is lower than its periphery, thus the rheological strength of the upper crust beneath the Xiaojiang fault is speculated to be weaker than its surrounding areas. Considering the high strain rate along the Xiaojiang fault, it is possible to accumulate great strain and strong stress in the Xiaojiang fault zone and its surrounding faults. Thus we should pay attention to the potential risk of strong earthquakes on the Xiaojiang fault and its nearby Ludian-Zhaotong fault.