Study On The Geomagnetic Short Period Variations Of The Northwestern Yunnan

The Northwest Yunnan is located in the interaction area between the Eurasian plate and the India plate., and because of its tectonic environment and frequent seismic activity, this area has been the ideal place for the research of continental dynamics, continental strong earthquake environment and the prediction for risk region of strong earthquake. Due to the differences in academic background and research ideas, there are always disputes and differences, which involve the origin of the low velocity high conductive layer beneath the lower crust and upper mantle, the cause of the earthquake and the geodynamic mechanism in the northwestern Yunnan.Electrical conductivity is highly sensitive to the temperature, the presence of water, degree of partial melting and chemical characteristics of the environment, which is the one of important physical parameters to reveal the state of the earth interior. Therefore the research of deep resistivity structure in the Northwest Yunnan is of great significance for in the exploration of Sichuan Yunnan block of deep structure, analyzes the coupling relationship between the different spheres, especially for the explanation of plate movement, soft flow convection, seismic origin and deep geodynamics.This paper is based on the geomagnetic data from the magnetometer array with 8 sites built in the northwestern Yunnan to explore the deep electrical structure by the method of geomagnetic depth sounding. We selected a total of 183 geomagnetic short period events in different apparent period, which range from 3min to 60min. And we found a north northwest dividing line, of which both sides has the opposite value in the ratio of vertical component, indicating the obvious resistivity anomaly underground. The contour maps of the ratio of vertical component and horizontal component variation amplitude △Z/△H in different periods reflects the changes of the high conductivity belt’s direction and position. On the other hand, the induction vectors within the period of 2-256min were obtained through the calculation of the transfer function. The results shows that the induction vectors pointing to deviate from each other on the both sides of the dividing line, the amplitude and direction of which varies with periods regularly. According to the results of magnetotelluric sounding, we can get the period and apparent equivalent depth curve and infer that there would be a high conductivity belt, which stretches from the crust to uppermost mantle and changes its position and shape with depth constantly in the study region. The staggered grid finite difference method is used to model the three-dimensional high conductivity anomaly, and the result shows magnetic field distributions are consistent with the observed geomagnetic short period variations characteristics in different periods, that confirms the exist of the high conductivity belt.According to the characteristics of short period geomagnetic variation and distribution of high conductivity anomaly body above, in combination with the results of previous studies, in this paper we try to give the explanation of the origin of the high conductivity anomaly body, which is the synthetic action of the combined effects of partial melting and fluid. And according to the tectonics, geophysical data, the collision of India plate and Eurasian plate, the upper mantle in lung and thermal substance invasion may be the main sources of power of the modern tectonic movement and seismic activity.