The heterogeneous characteristics of crustmantle 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 seismic 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 differences 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 energy 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 C^sub 1^ 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.[PUBLICATION ABSTRACT]