Three-dimensional upper crustal seismic velocity model for the northern Xiaojiang fault and tectonic analysis for Ludian Ms6. 5 earthquake

We present P-wave velocity structure(V-P) and P-wave to S-wave velocity ratio structures(V-P/V-S) for the northern Xiaojiang fault and Ludian M(S)6.5 earthquake area derived from P and S arrival times of local earthquakes recorded by Qiaojiao Array. The results show that: the northern Xiaojiang fault present a low V-P and high V-P/V-S feature within about 6 km depth of the upper-most crust. From 9 to exceed 16 km depth, Xiaojiang fault presents a significant high-velocity anomaly which may be related to the underplating of basalt, and it is inferred that the Xiaojiang fault may be the major channel for the eruption and underplating of Permian Emeishan basalt. The intrusion of these high-velocity basalt made the crust medium more brittle and strength lower, which was conducive to the formation of the Xiaojiang fault zone. The structure of the northern of Xiaojiang fault shows a relatively segmentation feature. The eastern blocks of Qiaojia and Daduo sections show obvious high-speed anomalies, while the middle Menggu section shows high-speed velocity anomalies. On the whole, the shallow and deep structure features and the different features between the eastern and the western block of Xiaojiang fault indicate Xiaojiang fault are characterized by large strike-slip fault and plate boundaries. The block between Lianfeng fault and Zhaotong-Ludian fault where the Ludian M(S)6.5 earthquake occurred, is characterized by low to high velocity anomaly. In the process of extrusion deformation of Daliangshan secondary block, the low-velocity zone extends to the southeast with the increase of depth, and it is blocked by the high-velocity Zhaotong-Ludian fault and Baogunao-Xiaohe fault. This process led to stress accumulated and released in the transition zone of high and low velocity, and then produced the Ludian M(S)6.5 earthquake. In addition, the aftershock distribution and velocity structures of the two conjugated rupture planes show that the mainshock occurred in the transition zone between high and low velocity, and most aftershocks occurred in velocity and V-P/V-S transition zone and in the regions with high velocity and low V-P/V-S ratios.