Study On Pre-seismic Variation Characteristics Of Underground Electrical Structure Based On The Magnetometer Array In Northwest Yunnan

Studying the spatiotemporal changes of underground electrical structures is of great significance for exploring the lithospheric material composition,structural transformation,and related regional tectonic stress fields.Because the resistivity of underground rock is sensitive to the temperature,fluid,melt and volatile composition content and volume in the earth,we can explore the lithospheric material composition,structural transformation and related regional tectonic stress field by studying the underground electrical structure.The geomagnetic transfer function can be obtained by using the geomagnetic deep sounding method.The spatiotemporal variation characteristics of the geomagnetic transfer function can be regarded as the function of the underground electrical structure.The variation of the underground electrical structure with time can be obtained by tracking the variation of the geomagnetic transfer function.Therefore,geomagnetic deep sounding can be used to study the variation of underground electrical structure at different time points in seismogenic region.The northwest Yunnan region is located in the southeast margin of the Tibet Plateau and southwest of the Sichuan-Yunnan rhombic block.Under the impact of the collision between the Indian plate and the Eurasian plate,neotectonic activities are intense and destructive earthquakes occur frequently.Based on the geomagnetic data of 8 measuring points of the northwest Yunnan geomagnetic array,this paper adopts the geomagnetic deep sounding method to study the change process of underground electrical structure before the Ms5.5 Eryuan earthquake.The main research contents and conclusions of this paper are as follows:(1)A comparative analysis of the methods for obtaining the geomagnetic transfer function is performed to verify the stability of the bounded influence estimator.The results of the real induction vector calculated from the vertical transfer function indicate that there are near-north-south low-resistance bodies in the study area.(2)The results of the vertical transfer function changing with time show that the absolute value of the real part of the transfer function of the Yunlong and Eryuan sites during the period of 1 to 2 months before the earthquake has an increase-turning-recovery process,and the anomalous change range exceeds 1 times the standard deviation,there are no obvious anomalous changes before the earthquake at the measuring points with larger epicenter distances or farther from the aftershock area.The results show that the anomaly range of the vertical transfer function accompanying the Ms5.5 Eryuan earthquake may be less than 50 km,which may be related to the scope of the aftershock area.The abnormal shape and change range of the transfer function at different periods of the same measurement point are inconsistent,which indicates that the electrical structure changes at different depths during the seismogenic process are more complicated.(3)The Mod EM three-dimensional electromagnetic inversion method is used to invert the three-dimensional electrical structure of the vertical field transfer function results in the quiet period of the earthquake,two weeks before the earthquake and three weeks after the earthquake,to obtain the underground electrical structure at three time nodes.The results show that about two weeks before the occurrence of the Ms5.5 Eryuan earthquake,the rock resistivity near the epicenter was significantly lower than that of the surrounding rock,with an amplitude of 1 to 2 orders of magnitude,similar to the experimental results of the relationship between rock stress and resistivity.The low-resistance band anomaly appeared along the middle and upper crust along the Qiaohou-Weixi fault.Combined with geophysical observations such as anomalous fluid increase in the Eryuan seismic station before the earthquake and deflection of the fast S-wave polarization direction,it is speculated that this low-resistance band may be mainly caused by fluid intrusion into the fissures arranged along the Qiaohou-Weixi fault.