2013年芦山MS7.0地震同震地磁变化分析

doi:10.3969/j.issn.0253-4967.2020.06.003

摘要/Abstract

摘要： 同震地壳应力变化能够引起地磁场的变化。文中利用成都地磁台连续观测资料分析了芦山M_S7.0地震前后的地磁场变化特征, 并根据芦山M_S7.0地震的同震破裂模型和岩石磁化强度-应力的线性模型, 计算了芦山M_S7.0地震同震引起的稳定地磁场变化。分均值分析结果显示成都地磁台观测的同震稳定地磁变化非常微弱, 以武汉台和兰州台为参考, 同震稳定地磁变化分别为+0.09nT和+0.04nT; 秒观测值分析结果显示同震稳定地磁场变化为+0.02nT。压磁效应计算结果显示, 断裂带下盘磁场垂直分量以向下变化为主, 断裂带上盘磁场垂直分量以向上变化为主, 水平分量的变化则比较散乱。当应力响应系数为1×10^-3MPa^-1、岩石磁化强度为1A/m时, 计算得到震中稳定地磁场总强度变化达5nT, 而距离震中约15km处为1nT, 在距离震中99km的成都地磁台处为+0.007nT。以上结果为同震地磁变化研究提供了新的数据。

关键词: 芦山M_S7.0地震, 地磁观测, 压磁效应, 同震磁场变化

Abstract: Stress perturbation related to earthquakes could cause the anomalous changes of magnetic field. Numerous observation data from high-precision magnetometers before and after earthquakes have revealed obvious coseismic changes in the geomagnetic field during earthquakes. In order to assess the ability of observing geomagnetic variations related to earthquakes with present geomagnetic stations, the coseismic geomagnetic variations corresponding to the 2013 M_S7.0 Lushan earthquake are presented. Precise measurements of geomagnetic fields have been obtained at a continuous geomagnetic station, Chengdu(CDP), which is approximately 99km from the epicenter of the M_S7.0 earthquake. The 12^th Generation International Geomagnetic Reference Field is used to reduce the secular variation of geomagnetic field. The geomagnetic station, Wuhan(WHN)and Lanzhou(LZH), which are approximately 1 100km and 650km from the epicenter of M_S7.0 earthquake respectively, are regarded as reference stations to reduce the diurnal variation of geomagnetic field. A corrected method based on observed data from CDP, WHN and LZH is also used to correct the effect of short-period geomagnetic variations of external origin. Minute average values of CDP from April 16, 2013 to April 23, 2013 is used to analyze the coseismic geomagnetic changes because the ΣKp indices in this period are less than 16 which indicates that there are no strong magnetic disturbances, solar flares, or solar storms. The result reveals that coseismic geomagnetic variation at CDP is little, which is about +0.09nT and +0.04nT, with WHN and LZH as reference stations. Meanwhile, the coseismic geomagnetic variation is +0.02nT using the geomagnetic values sampled per second observed in 20s before and after earthquake from CDP.
The observed coseismic geomagnetic variations are generally interpreted in terms of the piezomagnetic effect in rocks, which are most probably generated by sudden stress changes resulting from earthquake rupturing. According to slip model of 2013 M_S7.0 Lushan earthquake which is constrained by GPS data, the coseismic stress changes are calculated. Then, piezomagnetic magnetic field in the epicenter and surrounding area of the 2013 M_S7.0 Lushan earthquake is calculated based on linear model of magnetization and stress. In the model, we assume that the initial magnetization is consistent with the present magnetic field, which has an inclination and declination of 47.2° and -1.7°(westward). The in-situ Curie depth is chosen as 30km because the magnetized rocks beneath 30km contribute negligibly to the ground geomagnetic field. The calculated piezomagnetic effect shows that the vertical component is downward and horizontal component is mainly southward resulting from stress release in footwall of faults. The vertical component is upward and the horizontal component is mainly northward in hanging wall of faults. When the stress sensitivity is 1×10^-3MPa^-1 and magnetization is 1A/m,the calculated piezomagnetic field is about 5nT, 1nT and +0.007nT in the epicenter, 15km away from epicenter and at CDP, respectively. According to coseismic geomagnetic variations acquired from observed data and calculated data, it is obvious that stations (sites) located close enough to the epicenter could record coseismic signals above several nano Tesla. This implies that it would be difficult to observe geomagnetic signals associated with moderate earthquakes with present geomagnetic stations in this area and we need more improved observations. The piezomagnetic model tended to underestimate the observed coseismic geomagnetic variations with stress sensitivity of 1×10^-3MPa^-1 and magnetization of 1A/m. In this study,a uniform initial magnetization of 1A/m is expected to be reasonable according to aeromagnetic data and ground-based geomagnetic data, which indicates a relatively small magnetic anomaly. In this case, the stress sensitivity of 2×10^-3~3×10^-3MPa^-1 is suitable for explaining the observed coseismic geomagnetic variations. This result provides new data for coseismic geomagnetic variations. We can evaluate the initial magnetization, the stress sensitivity and slip model of earthquake if more observed data in near-field are available.

Key words: Lushan earthquake, geomagnetic field observation, piezomagnetic effect, coseismic geomagnetic variations

中图分类号:

P315.72⁺1

宋成科, 张海洋. 2013年芦山M_S7.0地震同震地磁变化分析[J]. 地震地质, 2020, 42(6): 1301-1315.

SONG Cheng-ke, ZHANG Hai-yang. ANALYSIS OF COSEISMIC VARIATIONS IN MAGNETIC FIELD OF THE LUSHAN M_S7.0 EARTHQUAKE IN 2013[J]. SEISMOLOGY AND GEOLOGY, 2020, 42(6): 1301-1315.

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