STUDY ON THE RELATIONSHIP BETWEEN LITHOSPHERIC MAGNETIC FIELD AND GEOLOGICAL STRUCTURE AND SEISMIC ACTIVITY:TAKING THE 2021 MS6.4 YANGBI EARTHQUAKE AS AN EXAMPLE
The lithospheric magnetic field is an important part of the earth's magnetic field,which is affected by many factors,such as rock's magnetization environment,underground geological structure,rock mineralogical composition,thermal and pressure state,and the deep tectonic evolution process.Most earthquakes occur in the crust and uppermost mantle,known as the lithosphere.The preparation and occurrence of earthquakes are usually accompanied by long-term accumulation and sudden release of energy,which will lead to changes in the thermal and pressure state of rocks,causing magnetic susceptibility variation in the lithosphere.Previous studies show that seismic activities can cause abnormal changes in the geomagnetic field,and there is an obvious correlation between the lithospheric magnetic field and seismic activities.The MS6.4 Yangbi earthquake on May 21,2021,provided a unique opportunity to study the dynamic evolution of seismo-magnetic anomaly.Five-term repeat station vector geomagnetic data observed in Yangbi and surrounding areas from 2018-2021 were used in this paper,the first four terms were observed before the earthquake,and the fifth term was observed after the earthquake.After data processing and model calculation,the lithospheric magnetic fields before and after the earthquake are obtained,lithospheric magnetic field models are established using the Surface Spline method,and annual variations are calculated.Based on the analysis of lithospheric magnetic field combined with the regional geological structure,the Northwest Sichuan Subblock shows negative magnetic anomalies,which is consistent with the geological structural characteristics in the study area,altitude and crustal thickness increase sharply from Central Yunnan Subblock to Western Sichuan Plateau.Small areas of positive and negative magnetic anomalies are alternatively distributed in the Central Yunnan Subblock,which reflects the heterogeneity of deep lithosphere structure.The negative magnetic anomaly in the western boundary of the study area is also consistent with the geological characteristics of the Qingzang Plateau.There is also a correspondence between lithospheric magnetic field anomalies and faults,especially the total intensity.Negative magnetic anomaly strips are distributed along the strike of the Honghe Fault and Lijiang-Xiaojinhe Fault,while Weixi-Qiaohou-Weishan Fault appears at the junction of positive and negative magnetic anomalies.The statistical analysis of the MS6.0 and above seismic events from 1970 to 2021 shows that there is a correlation between lithospheric magnetic anomalies and seismic activities.Most earthquakes occur in the weak magnetic anomaly area,especially near zero contour.The earthquakes tend to be distributed in anomaly gradient belts,and the number of earthquakes in the negative anomaly area is higher than that in the positive anomaly area.Analyzing the characteristics of the pre-and post-seismic changes of declination and total intensity near the epicenter of Yangbi MS6.4,it is found that the epicenter of the Yangbi earthquake is located near the zero-contour-line of declination.During the preparation of the Yangbi earthquake,the total intensity gradually changed from a balanced distribution of positive and negative anomalies to the overall negative changes,and the magnetic anomalies recovered the trend of the balanced distribution of positive and negative changes after the earthquake.
Yangbi earthquakelithospheric magnetic fieldseismo-magnetic anomaly
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