Three-dimensional P-wave velocity structure around the sources of Ludian and Qiaojia earthquakes

Using travel times of earthquakes from January 2010 to June 2020 in the region of Qiaojia and Ludian, we obtained three-dimensional P-wave seismic velocity structures around the source areas of Qiaojia and Ludian earthquakes by double-difference tomography. The result show that the velocity structure in this area is strongly heterogeneous, which affects the distribution of active faults. In the source region of the Ludian earthquake, there is a significant P wave high velocity anomaly with a maximum velocity up to 6.4 km/s, and an obvious P wave low velocity anomaly with a minimum velocity of about 5.8 km/s below the depth of 10 km. The occurrence of Ludian earthquake may be related to the difference of mechanical properties between deep and shallow rocks and the presence of fluid in mid-upper crustal rocks. In the source areas of the 2010 Qiaojia M_S4.8 earthquake and the 2020 Qiaojia M_S5.0 earthquake, although there are no significant high-velocity anomalies in the shallow crust, there are obvious local low-velocity anomalies beneath the initial rupture point of the main shock. In addition, there are obvious low velocity anomalies in the middle and upper crust of northern Qiaojia.