Three Dimensional P-Wave Velocity Structure of the Crust of North China

To research the velocity structure of crust and upper mantle, China Earthquake Administration has finished more than thirty deep seismic sounding (DSS) profiles, altogether about twenty thousand kilometers long in North China since the 1968 Xingtai earthquake. But these researches have not given out a 3D velocity structure model of the crust of North China and can not provide seismic evidence for researching deep structure features of the crust of North China. So based on the published research papers on DSS profiles, a 3D velocity structure model of North China is obtained by using vectorization function of GIS soft (Arc/Info) and Kriging regularization. With this velocity structure model, we have come to the following conclusions: (1) P-wave velocity of the shallow crust of North China changes rapidly, and the velocity structure is complex at the same depth. In general, it can be divided into three velocity zones, mainly tending to North West. In the research area, the lowest velocity zone is the Haihe plain and Bohai bay low-velocity zone. Geological structure of upper crust is inherited by deposit shell partially, but there are also lots of differences between them. (2) Generally, P-wave velocity increases with depth in the research zone, but it is inversed in local zones. Haihe plain low-velocity anomalous zone in the east disappeared, and Shanxi valley in the west is characterized by low velocity anomaly. The crust is divided into east part and west part by Taihang mountain. The strike of the strata above crystal floor is not completely consistent between the east part and the west one. The strike of Huanghai block and Huaihai block in the east mainly tends towards north-east, while the strike of Shanxi block and the eastern edge of Ordos block in the west mainly tends towards north-west. (3) According to the feature of Moho, the crust of the research zone can be divided into six parts: in the Shanxi block, Moho looks like a nearly north-south sunk belt, and the crust is thick; at the southern edge of the Inner Mongolia block and in the south of Yanshan block, Moho presents the structural features of fold belt, tending nearly towards east-west; at the eastern edge of Ordos block, the structure of Moho is relatively complex with nearly northwest folds.