青藏高原东南缘深部地壳流与地磁日变化短时畸变源电流的联系

Relationship between deep crustal flows and source currents of short time geomagnetic diurnal-variations distortion in southeast margin of Qinghai-Tibet Plateau

在青藏高原东南缘,前人使用大地电磁探测和地震学方法得出的结果都揭示了可能存在部分熔融状态的地壳流,而这种地下熔融体与周围物质的作用可能引起了地下强电流异常,进一步导致地表地磁响应.基于连续的地磁观测,发现2018年7月31日在川滇块体周边出现大范围的地磁Z分量日变化短时畸变,畸变发生后100天内发生了4次5级以上地震.为了定量研究这一现象,本文基于Biot-Savart定理和采用SVD(Singular Value Decomposition,奇异值分解)的阻尼最小二乘法对地磁日变化短时畸变数据开展反演.结果显示:(1)以大地电磁测深给出的电性模型作为初始条件,反演得到的电流强度为3700~5000 A,有效深度为25~60 km;(2)地下畸变电流的空间分布位置和深度和地下电性高导体分布一致,与前人给出的地壳流位置吻合;(3)地壳流偶然微小运动可能引起了大范围的强电流,这种短时存在的高强度电流沿高导带分布,可能是地磁日变化短时畸变的源电流;(4)推测深部地壳流的运动具有传递应力作用,参与诱发了100天内发生多次5级以上地震.对源电流进行反演的定量化工作,以地下电流的方式佐证了可能存在地壳流.

Previous studies of magnetotelluric sounding and seismology have revealed the possible existence of a partially melting crustal flows in the southeast margin of Qinghai-Tibet Plateau.And this interaction between the underground melt and the surrounding materials may cause strong deep electrical current anomaly,resulting in surface geomagnetic distortion.Based on continuous geomagnetic observation,it was found that a large range of short-time geomagnetic Z component diurnal-variations distortion was observed around the Sichuan-Yunnan block,and four large earthquakes(M≥5)occurred in the following 100 days.In order to explain this geomagnetic distortion phenomenon,this paper used Biot-Savart law and generalized inverse method of SVD(Singular Value Decomposition)to inverse the depth and intensity size of source currents,then discussed the correlation with these four earthquakes.The results showed that:(1)Taking the electrical model of magnetotelluric sounding as an initial condition,the current intensity obtained from inversion was 3700~5000 A,and the effective depth is 25~60 km;(2)The distribution of underground source currents is consistent with the melting crustal flows,suggested by previous studies,and the location of the high conductivity zone;(3)The occasional slight movement of the crustal current may cause a wide range of strong current.This high-intensity current distributed along the high conductivity zone may be the source of surface geomagnetic distortion;(4)It is speculated that the movement of crustal flows plays a role of stress transfer,which was involved in inducing the four earthquakes(M≥5)within 100 days.The inversion of source currents have been quantified,and the possible existence of crustal current has been proved by source currents of geomagnetic distortion.