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MS7.0地震前后的地磁场变化特征,并根据芦山MS7.0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MS7.0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。
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 MS7.0 Lushan earthquake are presented.Precise measurements of geomagnetic fields have been obtained at a continuous geomagnetic station,Chengdu(CDP),which is approximately 99 km from the epicenter of the MS7.0 earthquake.The 12th 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 1100 km and 650 km from the epicenter of MS7.0 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 theΣKp 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+0.09 nT and+0.04 nT,with WHN and LZH as reference stations.Meanwhile,the coseismic geomagnetic variation is+0.02 nT using the geomagnetic values sampled per second observed in 20 s 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 MS7.0 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 MS7.0 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 30 km because the magnetized rocks beneath 30 km 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 1×10-3 MPa-1 and magnetization is1 A/m,the calculated piezomagnetic field is about 5 nT,1 nT and+0.007 nT in the epicenter,15 km 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^-3 MPa^-1 and magnetization of 1 A/m.In this study,a uniform initial magnetization of 1 A/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^-3 MPa^-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.
作者
宋成科
张海洋
SONG Cheng-ke;ZHANG Hai-yang(The First Monitoring and Application Center,CEA,Tianjin 300180,China;Baoding Central Seismic Station of Hebei Earthquake Agency,Baoding 071000,China)
出处
《地震地质》
EI
CSCD
北大核心
2020年第6期1301-1315,共15页
Seismology and Geology
基金
国家自然科学基金(41804091)
中国地震局地震科技星火计划项目(XH19063Y)
中国地震局监测、预报、科研三结合课题(3JH-202001105)共同资助
2020年度震情跟踪定向工作任务(2020010423)。
关键词
芦山M_S7.0地震
地磁观测
压磁效应
同震磁场变化
Lushan earthquake
geomagnetic field observation
piezomagnetic effect
coseismic geomagnetic variations
