地震电磁场的特征及其在地震预警中的应用探讨

Observations have demonstrated that earthquake can generate preseismic and coseismic electromagnetic (EM) signals. These EM signals are of great importance owing to their potential in earthquake early warning and earthquake disaster reduction. However, the mechanism for these EM signals is still unclear. One of the possible mechanisms is the electrokinetic effect resulting from the electric double layer in a fluid-saturated porous medium. It is an urgent and necessary task to make research on the EM fields generated by an earthquake due to the electrokinetic effect, because it can benefit our understanding on those observed EM signals. Therefore, in the present project, we plan to do the following works from the point of view of the electrokinetic effect. (1) We will investigate the property of the coseismic EM fields and reveal how they connect the corresponding seismic waves by making numerical experiments in a horizontally-layered geological model. We will discuss whether the coseismic EM fields have abilities to identify the seismic P or S wave and whether they could be used for earthquake early warning. (2) We plan to collect several coseismic EM signals which were recorded during actual earthquakes. We will make theoretical simulation of those recorded EM signals using the source parameters which are inversed in terms of the seismic data. We will make comparison between the simulated and actually recorded signals in order to find the differences between them. We will also make discussion on the differences and reveal the possible reasons to cause them. (3) We plan to construct a contour-integral algorithm in order to calculate the preseismic EM waves in a porous half-space model. We will investigate the properties of the preseismic EM waves and analyze the influence factors. Finally, we will examine whether these preseismic EM waves are detectable during an earthquake.

观测资料表明，地震发生后可以接收到提前和同步于地震波到达的电磁信号，研究并掌握这些电磁信号的特征对地震预警、防震减灾有重要意义，但目前对地震电磁信号产生的机制尚不清楚。流体饱和孔隙介质中的双电层动电效应是一种可能的机制，基于动电效应研究地震诱导的电磁场是一项亟待开展和十分必要的工作，对解释相关地震电磁现象有重要价值。本项目计划从动电效应角度开展如下研究：（1）基于水平分层模型，数值模拟研究同步于地震波的电磁场的特征及其与地震波场的相关性，分析同震电磁场识别P波、S波的能力及其应用于地震预警的可能性；（2）针对实际观测的同震电磁信号，先利用地震波信号反演震源参数，然后对该实测电磁信号正演模拟，将模拟结果与实测信号对比，分析两者之间的差异及其产生的原因；（3）针对半空间地质模型，建立计算提前于地震波到达的辐射电磁波的围道积分法，并研究辐射电磁波的特征和影响因素，分析其在实际地震中的可观测性。

已有大量观测资料证实，在地震发生后，震中附近及远处的台站可以接收到提前和同步于地震波到达的电磁信号，研究并掌握这些电磁信号的特征对解释相关地震电磁现象有重要价值，对地震预警、防震减灾有重要意义。目前对地震电磁信号产生的机理和特征尚不清楚。动电效应是产生地震电磁信号的可能机制，本项目研究动电效应产生的地震电磁场，主要取得了如下研究成果：（1）理论研究了水平分层介质中的同震电磁场的特征及其与地震波场的相关性，初步结果表明同震电磁场与相应的位移场波形相似，但存在相位差，相位差随着介质分层数量、各层的参数和层厚等变化；（2）基于动电效应理论模拟了2004年 Parkfield 6级地震中所观测的同震电磁信号，理论计算的同震电场在波形和振幅上很好地解释了观测信号，表明动电效应是产生此同震电场有效合理机制。（3）针对半空间模型，建立了计算提前于地震波到达的辐射电磁波的割线积分算法，并研究了辐射电磁波的构成和空间分布特征及其受震源和介质参数的影响。在本项目的资助下，我们还研究了地震发电机效应诱导的地震电磁场，分别求解出了单点力源和地震矩张量源激发的位移场、电场和磁场的全空间解析解，对一个6.4级的地震模拟表明，当地震波加速度幅度达到0.1m/(s*s)时，同震电场和磁场的幅度分别达到了1μV/m和0.1nT，与2008年5月25日青川6.4级地震中观测到的电磁场的幅度一致，表明地震发电机效应具有足够的震电转换强度产生可观测的电磁信号，是一种有效的地震电磁场生成机制。另外，我们还比较了地震发电机效应和动电效应产生的电磁场，发现在高电导率地层中地震发电机效应是产生地震电磁场的主导机制，而低电导率地层中动电效应占优势。