关键词: 地震地磁 极化法 尼泊尔Mw7.8级地震 逐日相关

Abstract: On April 25, 2015 an earthquake, with magnitude 7.8(MW) and focal mechanism of low-angle thrust, occurred in Nepal(84.71°E，28.15°), which caused heavy casualties and property losses to Nepal, Tibet(China), and neighboring countries. The earthquake occurred in Himalaya orogenic belt, which is in long-term strong tectonic activity. And the orogenic belt is the product of the collision and extrusion of the Indian plate and the Eurasian plate. In this study, the polarization method was used to analyze the magnetic disturbance characteristics before the MW7.8 earthquake in Nepal, based on the geomagnetic second data of 48stations in mainland China Polarization method (Z/H) can be used to extract anomaly information of seismo-magnetic and suppress external interference information. Research shows that the effect of the geomagnetic field variations associated with earthquakes is much greater in the vertical direction than in the horizontal direction. The high polarization anomaly may contain some magnetic field information associated with earthquakes. The calculation of geomagnetic second data can be divided into three parts: spectrum analysis, polarization calculation and abnormal signal extraction and the dominant frequency band is used for calculation and analysis. (1) Analyzed the calculation process of polarization method, compared the correlation of polarization result and magnetic changes. The polarization value of each stage has a significant negative correlation with the geomagnetic K index. It is believed that the high polarization anomaly is not related to the external space activity, and the seismic anomaly can be extracted. (2) Analyzed the polarization characteristics of stations within 1500km of the epicenter of the MW7.8 earthquake in Nepal, It is found that there was an abnormal high value of synchronization in the 99 days before the earthquake. The analysis of spatial anomalies based on the relative change of polarization value shows that more than 20% of stations has polarization anomalies that lasted more than 3 days before the earthquake, and the spatial distribution of anomalies is obviously regional. (3) A daily correlation analysis of polarization is proposed in view of the regional synchronization of magnetic changes. The study shows that the polarization correlation between stations within 1500km of the epicenter has significantly enhanced during earthquake preparation, while the earthquake happened during the turn of synchronization. The analysis suggests that the synchronization may be caused by the additional effect of the source field on the regional magnetic field. The generation and distribution of polarization anomalies are closely related to the dynamic evolution of geological structures. Stress accumulation caused by geological activities is the main reason for the magnetic field changes. (4) The study shows that multiple synchronous polarization abnormality and daily correlation transitions are of important indicative significance for large earthquakes. The study will provide new ideas for monitoring and prediction of strong earthquakes.

Key words: Seismomagnetic, Polarization, Nepal MW7.8 earthquake, Daily correlation