Study on relationship between deep and shallow structures along north boundary fault of Yanqing-Fanshan basin

Based on the results of surface geology, shallow and deep seismic survey, features of micro-earthquake activity along the north boundary fault of Yanqing-Fanshan sub-basin and their relationship with the surface active faults and the deep-seated crustal structure are analyzed using the recordings from the high-resolution digital seismic network. The focal mechanism solutions of micro-earthquakes, whose locations are precisely determined by the seismic network, have confirmed the structural characteristics to be the rotational planar normal fault and demon-strated the surface traces of the north boundary fault of Yanqing-Fanshan sub-basin. By using the digital recordings of earthquakes with the high resolutions and analyzing the mechanism solutions, our study has revealed the rela-tionship between the geological phenomena in the shallow and deep structures in Yanqing-Huailai basin and the transition features from the brittle to ductile deformation with the crustal depth.

The heterogeneous characteristics of crust-mantle structures and the seismic activities in the northwest Beijing region

In this paper, the abnormal characteristics of the crustal structures in the seismic active region, Yanqing-Huailai and Zhangbei-Shangyi, are obtained by means of comprehensively interpreting and studying the data of deep seis- mic sounding profiles passing through the northwestern part of Zhangjiakou-Bohai seismic zone. The results show that the fluctuation of crystalline basement in the study region is obvious and that there exist considerable differ- ences in depth in different geological units. The locally abrupt variation of crystalline basement depths may be regarded as a mark of existence of crystalline basement faults. These crystalline basement faults and deep crustal faults provide a pass for the magma upwelling, resulting in the strong inhomogeneity of crustal structures. These phenomena of the complex seismic reflected waves and locally discontinuous reflection zones with different en- ergy indicate that the intensive squeeze and deformation of crust took place, which have led to the complex crustal structures and offered the dynamic source for the earthquake occurrence in this region. The low velocity bodies in different depths of crust and the local interface C1 in Zhangbei-Shangyi region may result from repeated magmatic activities. The certain stress accumulation in the brittle upper crust can cause the occurrence of earthquake under the action of local tectonic activity.