Spatial Distribution of Earthquakes and Crustal Seismic Velocity in Shanxi Rift Zone ·································································· WANG Yu, LI Zihong, NING Jieyuan

WANG Yu1, LI Zihong2, NING Jieyuan1,† 1. School of Earth and Space Sciences, Peking University, Beijing 100871; 2. Shanxi Earthquake Administration, Taiyuan 030002; † Corresponding author, E-mail: njy@pku.edu.cn

Abstract To study the seismic activity and physical background of Shanxi rift zone, we use tomodd to relocate earthquakes recorded by China network from 1990 to 2008 and the data recorded by a local network run by Shanxi Earthquake Bureau from 2012 to 2016, and show three-dimensional seismic velocity structure in Shanxi rift zone. The events are concentrated around the known faults in the Shanxi rift zone, mainly on the NE and SW sides of the Taiyuan Basin. Most of the focal depths are shallower than 30 kilometers. The earthquakes in the northern part of the research area are shallow while the earthquakes deeper than 20 kilometers are mainly located to the south of the Xinding Basin. Among them, the two densely distributed seismic zones on the NE and SW sides of the Taiyuan Basin with strikes pointing NS directions form sub-vertical distributed seismic zones, which may be controlled by two strike-slip deep fracture zones. If the Taiyuan Basin is broken and the two strike-slip faults are connected, there might be strong earthquakes with magnitude 7 or higher. The results show that the velocity in Shanxi Rift Zone varies intensely. The velocity in the crust of Shanxi Rift Zone is generally low, but the velocity in the crust of the Taiyuan Basin is not the lowest when the velocity in the crust of its NE and SW is especially low. In contrast, to the NW and SE of the Taiyuan Basin as well as the west of Datong the velocity in the crust is high. These correspond well with geological features on the surface and the heat flow observations. The results show that there

might be thermal materials upwelling beneath the Shanxi Rift zones to NE and SW of the Taiyuan Basin and the hot materials might invade into the interior of the Ordos block. On the contrary, the hot materials most probably do not invade into the crust of the Taiyuan Basin, as well as its NW and SE surrounding areas. We deduce that the extension of the Taiyuan Basin might not be controlled by the upwelling of hot materials but be controlled by horizontal extensional force produced by the push of the Tibettan Plateau. Key words Shanxi rift zone; earthquake distribution; seismic velocity structure; tomodd