[Abstract] The 1 000 km-long Marqin-Lanzhou-Jingbian comprehensive geophysical profile was deployed for the purpose of studying the interaction between the Qinghai-Xizang Plateau block and the Ordos block as well as for studying the deep driving mechanism of tectonic deformation. This paper presents only the preliminary results obtained with deep seismic sounding (DSS) observation data. The crust is clearly stratified. On the whole, it can be divided into two parts, i.e., the upper crust and the lower crust, with interface C as the boundary surface. Each of the two parts contains a number of secondary interfaces. The general trend of lateral variation is that the crust becomes gradually thicker from northeast to southwest, which is mainly due to the variation in the thickness of the lower crust. The average crustal velocity tends to become gradually lower from northeast to southwest-the velocity is obviously low west of Zeku and in Haiyuan region. A number of crustal low velocity layers exist west of Zeku and another low velocity layer exists near Haiyuan. The number of reflecting horizons increases gradually from northeast to southwest. As seen in seismic reflection waveforms, Pc wave is very strong at Haiyuan and it also lasts long. Besides, the complexity coefficients of Pm wave in Haiyuan and west of Zeku are extremely large, much larger than those in other parts. The above results clearly show that anomalous structure exists in the crust and upper mantle west of Zeku and in Haiyuan region, which reflects the interactions between Bayankar block and Chaidamu block, and between Qilian block and Ordos block.

In order to carry out a unified analysis and interpretation of the 2‐D data from multiple profiles, constraints at the intersecting points of the profiles are taken into account, and solution of simultaneous equations with integrated data from all the profiles is accomplished by constructing a unified matrix of partial derivatives. The method is used to reprocess and reinterpret data from 6 wide‐angle reflection/refraction Deep Seismic Sounding (DSS) profiles conducted in the Capital Area by the Research Center of Exploration Geophysics, China Seismological Bureau (CSB). As a result, a 3‐D velocity structure and tectonic characteristics of the crust in this area are revealed. On this basis, and in the light of results by other geophysical techniques, the spatial scope of 4 crustal lowv elocity layers (bodies) and fault strikes are determined.

In this paper, the deep crustal structures in Wutai mountain area are studied by using deep seismic sounding (DSS) data obtained from wide-angle reflection/refraction profiles which pass through Shanxi downfaulted zone and Wutai mountain area. The results show that the crust in this area presents obviously layered structures. The boundary interface between upper and lower crust is determined by means of reliable intra-crust reflected waves P3 that can be traced continuously. The thickness of upper crust is 23∼28km, and the depth of Moho discontinuity is 39∼43 km. The Moho discontinuity in Wutai mountain area is the deepest in this region. In Wutai mountain and its adjacent areas, the crust structures have strong inhomogeneous characteristics. The large-scale low velocity bodies in upper crust and strong intra-crust reflected wave groups can be regarded as a sign of magma activity in deep crust. The breaking of intra-crust interfaces and the local discontinuation of seismic wave group Pm as well as existence of deep crustal faults and so on indicate that the crust in this region suffered violent squeeze, deformation, and tectonic activities. Therefore the complex topography and anomalous structural characteristics in deep and shallow crust have been formed in this region.

A comparatively complete layout chart is built by using small shot interval and receiver spacing in Tianjin-Beijing-Chicheng Deep Seismic Sounding profile. Based on the DSS data of the profile, the crustal structure and the tectonic coupling are studied in the rift-depression basin of eastern North China and Yanshan mountain uplift. Based on the existing researches in the region, the seismic structure environment is further analyzed in the eastern segment of Zhangjiakou-Bohai seismic belt. The result shows that the tectonic feature and interface structure of the crystalline basement in the rift-depression basin of eastern North China Craton are correlated with the geotectonic movement since Cenozoic era. The entirely different crustal structures between the stable Yanshan uplift with high velocity in the north and the incompact rift basin with low velocity in the south make the intergradational zone between the tectonic zoning line, Baodi-Tongbai and Ninghe-Changli faults, and Yanshan uplift provide a favorable structural environment for generation and occurrence of earthquakes in the eastern plain region of Zhangjiakou-Bohai seismic belt.

In this paper, the data from deep seismic reflection and wide angle reflection profiling completed in Sanhe‐Pinggu strong earthquake (M=8.0) region are comprehensively studied and interpreted. The result shows that the layered crustal structures obtained by the two investigation methods are basically consistent. In Sanhe‐Pinggu strong earthquake region, the fluctuation of basement structures is considerable, shallow faults well developed, and the thickness of upper crust and the depth of Moho discontinuity are 21–23km and 36–37km, respectively. Among several faults determined by this study, the Xiadian fault is a tectonic zone with obvious characteristic and coexistence of both shallow and deep faults. Surrounding the focus, anomalous velocity structures and complex geological tectonic environment mean that these positions are favorable for strong earthquake developing and occurring. The phenomena of Moho discontinuity fluctuation and rather thick overlapping reflection layers and existence of locally complex cuneiform reflection zones indicate that the strong compression and deformation of the crustal structures had taken place in this region, and these phenomena also reflect that magmatic activity put strong reformation on material and structures in lower crust to result in complicated deep crustal structures in the region which may be considered as deep background for the development and occurrence of Sanhe‐Pinggu strong earthquake (M=8.0).