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

Since the Xingtai (邢台) earthquake in 1966, China Earthquake Administration has carried out a survey campaign along more than thirty deep seismic sounding (DSS) profiles altogether about twenty thousand kilometers long in North China to study the velocity structure of the crust and the upper mantle in this region, and has obtained a great number of research findings. However, these researches have not provided a 3D velocity structure model of the crust of North China and cannot provide seismic evidence for the study of the deep tectonic characteristics of the crust of the whole region. Hence, based on the information from the published data of the DSS profiles, we have chosen 14 profiles to obtain a 3D velocity structure model of North China using the vectorization function of the GIS software (Arc/Info) and the Kriging data gridding method. With this velocity structure model, we have drawn the following conclusions: (1) The P-wave velocity of the uppermost crust of North China changes dramatically, exhibiting a complicated velocity structure in plane view. It can be divided into three velocity zones mainly trending towards north-west. In the research area, the lowest-velocity zones lie in the Haihe (海河) plain and Bohai (渤海) Bay. Although the geological structure of the sedimentary overburden in the study area is somewhat inherited by the upper crust, there are still several differences between them. (2) Generally, the P-wave velocity of the crust increases with depth in the study area, but there still exists local velocity reversion. In the east, low-velocity anomalies of the Haihe plain gradually disappear with increasing depth, and the Shanxi (山西) graben in the west is mainly characterized by relatively low velocity anomalies. Bounded by the Taihang (太行) Mountains, the eastern and western parts differ in structural trend of stratum above the crystalline basement. The structural trend of the Huanghuaihai (黄淮海) block in the east is mainly north-east, while that of the Shanxi block and the eastern edge of the Ordos block is mainly north-west. (3) According to the morphological features of Moho, the crust of the study area can be divided into six blocks. In the Shanxi block, Moho apppears like a nearly south-north trending depression belt with a large crustal thickness. In the southern edge of the Inner Mongolia block and the south of the Yanshan (燕山) block, the Moho exhibits a feature of fold belt, trending nearly towards east-west. In the eastern edge of the Ordos block, the structure of Moho is relatively complex, presenting a pattern of fold trending nearly towards north-west with alternating convexes and concaves. Beneath the Huanghuaihai block, the middle and northern parts of the North China rift zone, the Moho is the shallowest in the entire region, with alternating uplifts and depressions in its shape. For the anteclise zone in the west of Shandong (山东) Province, the Moho is discontinuous for the fault depression extending in the north-west direction along Zaozhuang (枣庄) -Qufu (曲阜).