利用电磁台站观测数据推算汶川MS8.0地震等效电偶矩

摘要/Abstract

摘要： 本文利用地下电偶极子产生的电磁场模型, 尝试模拟和推算地震前地面电磁台站观测数据所需震源深度处等效电偶矩。结果显示, 在观测频率为1 Hz, 地下平均电导率取7.0×10^-4 S/m 时, 2008年汶川M_S8.0地震前电磁异常高潮期, 距离震中1440 km 的高碑店台NS测向观测到1.3 mV/m的电场强度所需震源深度处等效电偶矩量级为10¹³A·m; 若考虑地震主破裂长度150 km, 则产生的地电流量级为10⁸ A。同时, 这一等效电偶源在地面产生电场的2D分布图显示出很强的方向性(或不均匀性); 信号强度和电流源方向存在着密切的关系, 且电磁场值随观测距离增大而衰减迅速。这部分地解释了地震相关电磁异常信号的方向性(或不均匀性)的观测事实; 如果观测点位于非强信号区, 即使在有效的探测范围内也可能记录不到异常信息, 这对地震监测台网仪器选址和布置提出了要求, 具有现实的指导意义。

关键词: 地下电偶极子模型, 汶川M_S8.0地震, 等效电偶矩, 电场2D分布特征

Abstract: We use a relatively simple model of an underground current source co-located with the earthquake hypocenter to estimate the magnitude of the equivalent dipole required to produce observable ground signatures. The Wenchuan M_S8.0 earthquake on May 12, 2008, is used as an example of a large typical inland event. Results show that for an observed 1.3 mV/m pulse at 1 Hz at Hebei Gaobeidian observing station, 1440 km away from the epicenter, during the abnormity climax, and a typical conductivity of 7.0×10^-4 S/m, the expected equivalent dipole order is 10¹³ A·m, and that of the seismotelluric current is 10⁸ A if the main fracture length 150 km is considered. The electromagnetic field 2D distribution on the ground of this dipole is of the characteristics of orientation or selectivity. There is near relationship between the orientation of the electric element and the electromagnetic field amplitude. At the same time, the electromagnetic fields attenuate rapidly as the observing distance increases. All these can give part evidence of the fact that electromagnetic abnormities observed related with earthquakes are of orientation or selectivity. The instalment may not observe a signal if it locates in weak field regions even though it is well within the detectable range. This result put some requests on how to select locations and layout when an earthquake observing network is in preparation.

Key words: Underground dipole model, The 2008 Wenchuan M_S8.0 earthquake, Equivalent dipole, 2D distribution of electromagnetic field

中图分类号:

P315.7

李美, 王中平, 卢军, 谭捍东, 张小涛. 利用电磁台站观测数据推算汶川M_S8.0地震等效电偶矩[J]. 地震, 2018, 38(1): 49-60.

LI Mei, WANG Zhong-ping, LU Jun, TAN Han-dong, ZHANG Xiao-tao. Estimating the Equivalent Dipole of the 2008 Wenchuan M_S8.0 Earthquake using Observed Electromagnetic Ground Signals[J]. EARTHQUAKE, 2018, 38(1): 49-60.

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