Double-difference tomography of crustal P- and S-wave velocity structures beneath North China

In this paper, crustal P- and S-wave velocity structures of North China （37°N−42°N, 113.5°E−118.5°E） are imaged with seismic phase data from 133 permanent seismic stations in this area by double-difference tomography, and the earthquakes used in the tomography are also relocated. The results show that the average RMS of travel time residuals decreases from 0.265 s to 0.008 s, the relocated hypocenters are mainly between 6 km and 16 km in depth, and the corresponding epicenters are mostly distributed along faults. Most of the earthquakes occur on the transitional zone between low- and high-velocity areas and on the side of the high-velocity zone. In Tangshan and Xingtai, where great earthquakes took place, low velocity bodies in middle and lower crust probably result from the upwelling of deep fluids and hot mantle material. Moreover, the velocity distribution in upper crust is very different from that in middle-lower crust. In the upper crust, it is mainly controlled by the tectonic structure and the faults; in the middle-lower crust, it is closely related to the crust-mantle interaction, the Moho uplift, and the upwelling of the asthenosphere material. Based on the tectonic pattern, evolution history and previous studies, it is deduced that the destruction of the North China Craton results in the thinning of the lithosphere and the upwelling of the asthenosphere material; hence, deep fluids and hot magma go up through faults and ruptures and intrude into the middle and lower crust; as a result, these crustal parts become weak and partly melt, which is favorable for generating earthquakes.