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.

Observations by magnetotelluric technique have been carried out at 15 sites along two profiles in Tianchi volcano area of Changbaishan Mountain northeast China. The impedance tensor decomposition and a new two-dimensional inversion method (RRI) are used for data analysis and interpretation. The results show that the conductivity structure beneath the volcano and its neighbouring area is of two-dimensional character and a local three-dimensional low resistivity body exists at depth of about 12km under the volcano. It is inferred that this anomaly body may be a magma chamber.

Mathematical formulas of three-dimensional(3D) Magnetotelluric(MT) responses affected by shear,twist and anisotropy are derived based on the principle of two-dimensional(2D) MT impedance tensor decomposition,which are applicable not only to complicated 3D structure,but also to 2D or 1D situations.Mohr circle analysis are also made for 3D MT responses and the properties in the cases of both weak and strong distortions.

Decomposion of the magnetotelluric impedance tensor is used in evaluation of MT data observed at sites along profile from Yanggao in Shanxi province to Rongcheng in Hebei province.The impedance tensor at some sites is influenced by strong local telluric distortion.Analysis on all data along the profile suggests that regional anomaly is two-dimensionality in general.The pseudosection of apparent resistivity and impendance phase for regional anomaly can be compared with tectonics.

Mohr circles and their application to Magnetotelluric(MT)impedance tensor analysis aredescribed in this paper,It iS illustrated that the rotational invariants of MT impedance tensors,having the advantages of providing variety of MT in a formation and stability,are frequently usedin MT data interpretation,Mohr circle,connecting the different forms of invariant impedancetensors,becomes an important graphic dignostic tool in studying MT impedance tensor properties.With the help of this powerful tool,some MT parameters,such as structural dimensional-ty,skewness,anisotropy,can be expressed visually and simply in the circle diagrams which giveimportant information for studying both vertical and lateral variation of the conductivity structures.

The magnetotelluric sounding profile from Yixian,Liaoning,to Dong-Ujimqin Nei Mongol,was carried out and the geoelectrical structure along this profile is presented in this paper. The results show that the buried depth of the conductive layers both in the crust and upper mantle is in a good correspondence with geotemperature field data i. e. The shallower the buried depth of the conductive layers,the higher the heat flow value.According to the geoeletrical structure it is supposed that the Siramulen River fault zone seems to be a tectonic suture line between Siberia plate and China plate.

The features of deep conductivity structure along Xiangshui-Mandal Geoscience Transect are described in this paper basing on newly obtained magnetotelluric data. Large resistivity contrasts can be found on both borders of North China platform. The depth of the crustal high conductivity layer changes abruptly from 21km to 34km beneath Nei Mongol fold system on the northwestern end of the transect. While it is absent beneath Subei-Jiaonan terrane on the southeastern end of the transect,where two high conductivity layers are detected in the upper mantle. The crustal high conductivity layer is well developed in the Cenozoic basins within North China Platform with a relatively shallow depth. The depth of the upper mantle high conductivity layer along the transect ranges from 60 to 127kin,with a general tendancy of decreasing in depth from northwest to southeast,forming local uplifts beneath Ho-Bao basin,Central Hebei depression and Tan-Lu fault zone. It reflects the characteristics of the regional geology and the deep tectonic movements.

Based on the data conducted from 18 MT sites in west Yunnan, the deep conductivity characterics along the longitudial and transverse directions of the geological structure are studied, The results indicate that the deep eletrical structure in this area is a multiple-layer structure with 4-5 electrical layers. There is great lateral inhomogeneity in the deep eletrical structure which are apparently controlled by the regional geological structure. There exists a widespread highly conductive layer in the crust, and two upper mantle highly conductive layer uplift zones are found, one is situated in Jianchuan-Heqing with the principal axis of NNW direction and another in Tengchong-luxi with approximately NS direction.The relationship between the electrical structure of the crust and upper mantle and the geotectonics, the formation of northwest Yunnan rift basin and the relationship between seismic activity and the deep geological structure are also discussed in this paper.