Velocity structure and its relation to the seismicity of Datong Window and adjacent regions in Shanxi Province, North China

Datong Window is at the junction of Datong Basin and Liulengshan Range in the northern part of Shanxi Graben, China. It is important to analyze the relationship between the frequency of small earthquakes and the deep velocity structure in this region since the earthquake swarm in 1989 with the maximum surface-wave magnitude M_s of 5.9. A high-resolution 3D P-wave velocity model of the crust in this region was obtained with double-difference tomography using seismic data from July 2009 to June 2019. The results show that the Datong-Yanggao seismic sequence occurred, in general, within the NE rupture zone of the earthquake swarm of 1989. The sequences from 1991, 1999, and 2010 were in the rupture zone of 1989. A series of earthquakes developed in this region, which is at the intersection of multiple faults and a low-velocity anomaly of the intermediate-lower crust. A gap of length of about 3 km was found at the southwest end of the NE Dawangcun Fault (near Lujialing). Several moderate earthquake ruptures failed to break through it. It was speculated that it may be a potential asperity. There is a possibility of an earthquake with magnitude about 5 from the gap. The relocation results for the earthquakes show that intensive small-earthquake events in Datong Window corresponding to moderate-to-strong earthquakes in Shanxi Graben were highly clustered horizontally and distributed vertically in a column. In contrast, when there was no moderate earthquake in Shanxi Graben after intensive small-earthquake events, the spatial distribution of these events in Datong Window was scattered and dispersive.