Abstract: We determined a 3D P-wave velocity model of the crust under the northern part of North China (37.5°~41.5°N,111.5°~119.5°E) by using double-difference seismic tomography and jointly inverting 183909 absolute arrival times and 495753 differential arrival times from 10461 local events recorded by permanent seismic network and North China Seismic Array. Our results show considerable differences in the velocity structure of the upper crust between the main tectonic units in the study area. The trends of velocity anomalies are consistent with the trends of geological structures. The velocity images of the upper crust reflect well the surface geological and lithology features. The majority of hypocenters is located between 0~20 km depth and located in the low velocity anomalies or in the conjunctional areas of low and high velocity anomalies. Significant low velocity anomalies are found in the source areas of the 1976 Tangshan earthquake (M7.8) and the 1679 Sanhe-Pinggu earthquake (M8.0), which are considered to be associated with fluids. Combining previous studies with our present model, we think that the deep subduction and stagnancy of the Pacific slab have resulted in deep slab dehydration and upwelling of hot asthenospheric materials, and the asthenospheric materials have reached the top of uppermantle and broken into crust along the deep faults which may cut through the crust, then the hydrous minerals in the top of uppermantle and lower crust are dehydrated to produce fluids, and these fluids further cause the weakening of the seismogenic layer in the middle and upper crust. Hence, we believe that the crustal heterogeneity and the seismotectonics under the northern part of North China are closely related to a series of deep dynamic processes associated with the deep subduction and stagnancy of the Pacific slab under East China.

Key words: 3D P-wave velocity structure, Double-difference seismic tomography, Subduction of slab, Northern part of North China