PN Wave Velocity Structure and Anisotropy in the Central Tien Shan Region

Abstract

AbstractPn wave velocity structure and anisotropy in the central Tien Shan are inversed by using seismic arrival data from the broadband temporary network (GHENGIS) and the Kyrgyzstan seismic network (KNET). Results reveal lower average Pn velocities and a strong heterogeneity at the uppermost mantle beneath the central Tien Shan, which are often found in tectonically active zones. Particularly, very low Pn velocities are found in the south‐central Tien Shan, which is believed to be underlain by an unusual mantle with higher heat flows. There is a clear correlation between seismic activity and Pn velocity variation. Most earthquakes are concentrated in the north‐central Tien Shan with hight Pn velocities, while almost no earthquakes located in the south‐central Tien Shan with very low Pn velocities. Our result suggests that the very low Pn velocity beneath the southcentral Tien Shan is caused by high temperatures produced by heating from mantle upwelling, and that this heat has been conducted well up into the crust, inducing a ductile rheology that prevents significant seismic activity. Moreover, strong anisotropy is observed in the north‐ and south‐central Tien Shan. Fast Pn velocity anisotropy in the south‐central Tien Shan is nearly in N‐S directions, which is consistent with the SKS wave anisotropy concerning mantle flow. However, fast Pn velocity directions in the north‐central Tien Shan show a southward rotation variation. We estimate that it is probably related to the inserting of the crust of the Chu basin from the southern Kazakstan platform to the central Tien Shan, which caused a change of stress fields and lithosphere deformation.