复杂孔隙地层中动电效应诱导的地震电磁场数值模拟研究

It is known that electromagnetic (EM) signals can be generated by earthquakes. These earthquake-generated EM signals are of great importance due to their potential application in earthquake early warning and disaster reduction, because they contain a lot of useful information associated with the source parameters, the propagation path of seismic and EM waves as well as the property of the near-surface material. But the understanding of these EM signals is still insufficient and the mechanism for their generation is also controversial. One of the possible mechanisms is the electrokinetic effect, based on which the theoretical modeling can explain the observed EM signals to some extent. However, the techniques for the modeling of the earthquake-generated EM fields were developed mostly for regular models, e.g., the full-space model and the horizontally layered model, while the real media are usually complex. To deepen the understanding of the earthquake-generated EM fields arising from the electrokinetic effect, we plan to do the following works. (1) We will develop a three-dimensional finite-difference time-domain method to simulate the EM fields generated by an earthquake in complex porous media. (2) We will study the numerical error caused by the quasi-static approximation, which is often used in the modeling of the coupled seismoelectric wavefields. (3) We will develop a two-dimensional finite-difference frequency-domain method for the modeling of the EM fields generated by an earthquake in complex porous media. (4) After the two numerical methods are developed, we will use them to study the properties of the EM fields generated by an earthquake in complex media. The completion of this project will improve the technique for the modeling of the earthquake-generated EM fields, benefit the understanding of characteristics of the EM field generated by earthquakes in complex strata, and provide theoretical basis for the interpretation of the observed EM data as well as the application of the earthquake-generated EM fields to the earthquake early warning and disaster reduction in the future.

地震发生过程中可以产生电磁扰动信号，其携带着震源参数、传播路径以及近地表介质特性等有用信息，研究地震电磁场对地震预警、防震减灾有重要意义。但目前对地震电磁场的认识并不充分，对其产生的机制仍存有争议。动电效应是一种可能的机制，基于该机制的理论模拟在一定程度上可以解释实际观测的电磁现象，但现有的模拟和认识大都是基于规则介质模型获得的。为深入认识动电效应诱导的地震电磁场，本项目拟开展如下研究：（1）建立模拟复杂地层中地震电磁场的三维时域有限差分方法；（2）研究震电波场模拟中常采用的电磁场似稳近似可能造成的计算误差；（3）建立模拟复杂地层中地震电磁场的二维频域有限差分方法；（4）利用所建立的方法研究复杂地层中地震电磁场的特征。项目的完成将进一步完善模拟地震电磁场的数值方法，有助于揭示复杂地层中地震产生的电磁场的特征，为今后电磁观测资料的解释和利用地震电磁场服务于地震预警及防震减灾打下理论基础。

孔隙岩石的固液界面附近存在双电层，其可以导致地震波-渗流-电磁波的耦合，这种耦合现象被称为动电效应，是产生地震电磁异常的重要机制。研究动电效应产生的地震电磁场对认识地震电磁异常现象和利用地震电磁场服务于地震预警和地震减灾有重要意义。.本项目围绕复杂地层中动电效应产生的地震电磁场开展研究，获得如下研究成果：（1）发展了二维模型震电耦合波场的频率域有限差分方法。（2）初步建立了基于结构化网格的二维频率域有限元方法。（2）利用所开发的方法揭示了含起伏地表和地下界面的复杂地层中震电波场的一些重要特征。研究发现，地震波在平的界面处可以产生界面电磁波，在起伏地表和地下不规则界面处还可以激发出散射电磁波，这两种电磁波都比地震波速度快很多，会比地震波更早到底远处的台站，在地震预警方面有重要价值。我们还研究了震电和电震信号在探测地下介质结构方面的可能性，发现后者在地下界面成像方面更有优势。.我们在地震电磁场产生的其他机制以及实际地震电磁资料的解释方面也取得了较好的进展，获得如下成果：（1）基于地震发电机效应，发展了模拟二维和三维水平分层模型中地震激发电磁场的半解析方法，并开发了相应的软件。通过开展数值模拟，揭示了地震电磁场的一些重要特征。研究发现，该机制可以导致三种电磁信号，一种是震源激发的电磁波，另一种是伴随地震波（包括纵波、横波和面波）的同震电磁场，还有一种是地震波在不同介质分界面产生的电磁波信号。（2）发展了模拟磁棒旋转诱导同震磁场的半解析法，开展数值模拟研究了磁场的特征，发现P波和瑞利波主要产生径向和垂直磁场，而SH波和Love波主要产生切向磁场。（3）利用发展的数值模拟方法，基于动电效应、地震发电机效应以及磁棒旋转诱导磁场效应，很好地解释了实际地震事件中的观测的电磁信号，包括2008年青川Mw6.1级地震和2017年九寨沟Mw6.5地震以及台湾地区微地震事件中的电磁信号。.本项目的研究结果为今后地震观测电磁数据的解释提供了正演模拟方法和理论依据，对深入理解和认识地震电磁现象有重要意义。.

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