Abstract:

On May 21, 2021, at 21:48(Beijing time), an earthquake with magnitude M_S6.4 occurred in Yangbi County of Yunnan Province. In order to obtain the geomagnetic field change before and after the M_S6.4 earthquake, the geomagnetic repeat measurement was conducted on geomagnetic repeat stations surrounding the epicenter. In this paper, total intensity of geomagnetic field obtained on eight geomagnetic repeat stations is used to calculate the geomagnetic field change. The measurement period before and after earthquake are April 1 to May 10 and May 23 to May 31 respectively. The continuous data recorded on Lijiang geomagnetic observatory from April 1 to May 31 is used to correct the diurnal variation. The geomagnetic repeat data measured in different time is corrected to midnight(00~03 Beijing Time)of May 6 because the external disturbance was quiet then. Finally, the geomagnetic field changes before and after the earthquake are obtained by calculating the difference of geomagnetic data before and after the earthquake. There are two factors contributing to the error. The mounting error is less than 0.2nT on all stations and the corrected error is less than 0.5nT on six stations and less than 1nT on two stations. The error analysis shows that the amplitude of geomagnetic field change is more than twice the error on five observation stations, which means that the geomagnetic field change is convincing on these five stations.

The observed result reveals the geomagnetic field with positive and negative change in the south and north side of epicenter. The amplitude of geomagnetic field change is related to the epicenter distance. The change amplitude is -2.82nT at a geomagnetic station in an epicenter distance of 20km, and there is no geomagnetic change higher than the error observed at geomagnetic stations in an epicenter distance of 100km.

The observed stable geomagnetic changes before and after earthquake are generally interpreted in terms of piezomagnetic effect in rocks, which are most probably generated by sudden stress change resulting from earthquake rupturing. In order to explain the geomagnetic field change before and after the M_S6.4 earthquake, especially the change of -2.82nT, a piezomagnetic model is constructed based on uniform slip model with a slip of 0.5m on a rectangular fault. In the piezomagnetic model, the stress sensitivity is 5×10^-3MPa^-1 and magnetization is 1A/m. The result shows that piezomagnetic field of the station closest to the epicenter is -0.3nT. It infers that the piezomagnetic effect cannot explain the observed geomagnetic field change of Yangbi earthquake because we cannot expect the stress sensitivity or magnetization are ten times the value used in our piezomagnetic model.

The electrokinetic effect which results from the fluid motion in the vicinity of fault because of the stress change can also produce the geomagnetic field change. An electrokinetic model with parameters same as the piezomagnetic model is constructed to calculate the geomagnetic change resulting from electrokinetic effect. The result shows that the electromagnetic field is smaller than the observed geomagnetic field by 1 order of magnitude in the cases without high conductivity (0.1S/m) and pore pressure change(10MPa). It infers that the electrokinetic effect cannot explain the observed geomagnetic field change of Yangbi earthquake.

Other seismo-magnetic effects may contribute to the observed geomagnetic field change before and after earthquake, such as thermomagnetic effect which results from the thermal demagnetization or remagnetization of rocks surrounding the epicenter. We can conduct the quantitative analysis of thermomagnetic effect in the future when we know how the earthquake influences the temperature change.

Key words: Yangbi M_S6.4 earthquake, geomagnetic field change, piezomagnetic effect, electrokinetic effect

摘要：

漾濞M_S6.4地震发生后, 在震中周边100km内的地磁重复测点开展了原位观测工作。 结合地震前60d内的观测数据, 使用丽江地磁台连续观测资料, 消除了地磁场的日变化和长期变化成分, 获得了地震前后的区域地磁场变化。 实测结果表明, 震中S侧区域地磁场以负变化为主, 震中N侧区域地磁场以正变化为主, 距离震中20km测点处的地磁场变化量为-2.82nT, 在距离震中100km处未观测到明显的、 高于误差水平的磁场变化。 进一步利用均匀的断层滑动模型计算了漾濞地震的压磁效应, 当选择较大的岩石磁性参数时(应力敏感系数为5×10^-3MPa^-1), 距离震中最近测点的压磁场为-0.3nT。 动电效应的计算结果表明, 若不存在较大的电导率(0.1S/m)和较高的孔隙压力变化(10MPa), 由孔隙流体流动产生的磁场变化将比实测的地磁变化小1个量级。 因此, 压磁效应和动电效应仅能定量解释部分地磁场变化, 还有其他震磁效应影响了地磁场的变化。

关键词: 漾濞M_S6.4地震, 地磁场变化, 压磁效应, 动电效应