岩石圈磁场与地质构造和地震活动性之间的关系--以漾濞地震为例

STUDY ON THE RELATIONSHIP BETWEEN LITHOSPHERIC MAGNETIC FIELD AND GEOLOGICAL STRUCTURE AND SEISMIC ACTIVITY:TAKING THE 2021 M_(S)6.4 YANGBI EARTHQUAKE AS AN EXAMPLE

文中利用2018-2021年漾濞及周边地区的地磁矢量数据,构建了5期岩石圈磁场曲面样条模型,并计算得到相邻期的岩石圈磁场变化。在此基础上,分析了岩石圈磁场的空间分布与区域地质构造和历史地震震中的对应关系,并针对漾濞M_(S)6.4地震,研究了磁偏角和地磁总强度在震前、震后的磁场变化特征。研究发现:岩石圈磁场的空间展布能够反映地壳块体构造特征和主要断裂走向,同时也与地震活动的分布存在相关性。通过对比分析历史地震的震中分布发现,地震多发生在弱磁区,尤其是磁异常零值区附近,且地震震中分布偏向于高梯度带上。漾濞M_(S)6.4地震发生前后,震中附近的岩石圈磁场发生显著变化,震前、震中持续位于磁偏角负变化区,震后则位于正变化区,且震中位于震前一年磁偏角变化的零变线附近;震前研究区内相邻期地磁总强度的正负变化大致均衡,随后逐渐演变为整个研究区均呈负值,而地震后研究区又恢复了正负变化大致均衡的分布。

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 M_(S)6.4 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 M_(S)6.0 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 M_(S)6.4,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.