关键词: 地壳, 上地幔, 大地电磁测深, 张家口-蓬莱构造带, 张北-尚义地震

Abstract: It is shown that the structure of the crust and upper mantle (thickness,geothermal field etc.) beneath the Zhangbei Shangyi earthquake area,which belongs to Inner Mongolia geo axes,has the nature of a transition zone as compared to its surrounding regions,Yanshan mountain fold belt,rifting basin of North China,rift system of Shanxi province and Erdos block.The Zhangbei Shangyi earthquake area is located near the Zhangjiakou Bohai Sea Penglai tectonic belt (ZPB) which is a significant tectonic boundary.Along the ZPB a relative low resistivity zone appears in the whole lithosphere.The width of ZPB is about 40km and becomes bigger with increasing depth.The structure of seismic velocity similar to electricity also occurrs along the ZPB.A NNW trending tectonic belt in the crust is newly detected beneath Zhangbei Shangyi earthquake area.The NNW trending tectonic belt and ZPB are postulated to play an important role in a combining way for pregnancy and genesis of the earthquake.The recent results of magnetotelluric observation indicate that there is a low resistivity layer at the depth of about 15km beneath earthquake area,which is deeper and thinner than that to the south of Shangyi Chongli fault,which is tectonic boundary between Inner Mongolia geo axes and other southern tectonic units.The foci of main shock and most aftershocks are located above the low resistivity layer in the crust.

Key words: Crust, Upper mantle, Magnetotelluric sounding, ZhangjiakouPenglai tectonic belt, ZhangbeiShangyi earthquake