地震发电机效应诱导的地震电磁场研究

Motion of the conductive earth's crust across the ambient earth's magnetic field can yield an electromotive force as well as a motional-induction electric current, which then gives rise to electromagnetic (EM) fields. Such a kind of mechanoelectric coupling is referred to as the seismic dynamo effect and has been proposed to be an important mechanism for the generation of the anomalous EM disturbances observed during earthquake events. In order to get an in-depth understanding of the earthquake-induced EM fields arising from such a mechanism, we plan to do the following works on the basis of our preliminary study in which the full-space analytical solutions of the seismoelectric wavefields have been obtained. (1) We will develop a semi-analytical algorithm to calculate the earthquake-induced EM fields in horizontally-layered media and a finite-difference method to simulate the seismoelectric wavefields in complex strata with irregular interfaces. Using the numerical simulators, we will study the properties of the coupling between the seismic and EM waves, reveal the relation between the coseismic EM fields and their corresponding seismic wavefields and investigate the generation and propagation of the EM wave which is radiated by the earthquake source and arrives earlier than the seismic wave. (2) We plan to make comparisons between the earthquake-induced EM field resulting from the seismic dynamo effect and that arising from the electrokinetic effect, and explore the conditions for each mechanism to dominate the seismoelectric conversions.(3) We will make forward simulations of the EM signals observed during actual earthquakes on the basis of the seismic dynamo effect and the electrokinetic effect，make comparisons between the simulated and observed signals in order to find the differences between them and discuss the possible mechanism for the generation of the observed data. The completion of this project can benefit the understanding of the mechniasm of the earthquake-generated EM disturbances, provide theoretical basis for the explanation of the earthquake-generated EM data, and has great importance in the application of the earthquake-generated EM fields to the earthquake early warning and disaster reduction in the future.

导电的地层做切割地球磁场运动产生动生电动势和动生电流，进而产生电磁场的现象被称为地震发电机效应，是产生地震电磁异常的一种重要机制。本项目旨深入认识这一机制诱导的地震电磁场，拟在前期获得的全空间解析解基础上进一步开展如下研究：（1）建立计算水平分层介质中地震诱导电磁场的半解析算法和含不规则界面地层震电波场的有限差分方法，研究含界面地层中地震波和电磁场的耦合特点，阐明同震电磁场与地震波场的关系,揭示提前于地震波到达的辐射电磁波的激发、传播规律；（2）开展地震发电机效应和动电效应产生的地震电磁场的对比研究，理清两种机制各自主导震电转换的条件；（3）分别基于这两种机制对实测地震电磁信号开展正演模拟，分析理论信号与实测信号之间的差异，探讨产生实测电磁信号的合理机制。项目的完成有助于认清地震电磁场的生成机理，可以为实测电磁资料的解释提供理论依据，对今后利用地震电磁场服务于地震预警和防震减灾有重要意义。

地震电磁异常是人们十分关注的现象，但对该现象的理解还很有限，弄清地震电磁场产生的机制是认识这一现象的关键。.本项目针对地震发电机效应诱导的电磁场展开研究，获得如下研究成果：发展了计算水平分层介质中地震双力偶点源激发电磁场的数值方法，并开展数值模拟分析电磁场的特征。发现地震震源可激发出两种电磁信号，一种是伴随纵波、横波和瑞利波的同震电场和磁场，另一种是提前于地震波到达的电磁波。两种电磁信号均随着地磁场的倾角、台站方位角以及地层电导率变化而变化。数值模拟还表明，地震波在遇到介质分界面时会产生电磁波。通过与动电效应产生的电磁场对比，发现两种机制产生的电磁场在垂直电场、界面电磁波以及对地磁场和地层电导率的依赖性方面有明显区别。.我们在动电效应诱导的震电耦合波场模拟和认识方面同样取得很好的进展，获得如下成果：（1）基于流体-孔隙介质模型，提出了计算海底环境中动电效应产生电磁场的方法，并对电磁信号的特征做了描述。数值计算表明，当采用的震源强度在工业气枪激发强度范围内时，电场和磁场幅度可以达到1 μV/m和0.01 nT，理论上证明了海底环境下动电效应导致的电磁信号具有可观测性。（2）研究了震电波场模拟中电磁场似稳近似所引起的计算误差，并阐明了电磁场似稳近似适用的范围。（3）发展了计算二维SHTE模式震电耦合波的频率有限差分方法，数值计算表明：SH波在传播时会引起伴随电场；当SH波遇到介质物性分界面上，会激发出独立传播的电磁波，该电磁波与一个水平电流激发的电磁波相似。（4）针对2004年美国Parkfield 6级地震中观测的同震电磁信号，基于动电效应模拟了断层破裂产生的电磁场。结果表明，理论同震电场在波形和振幅上均与观测信号有较好的吻合，为证明动电效应是产生地震电磁场的合理机制提供了直接有力的证据。.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重要的理论依据，对深入理解和认识地震电磁现象有重要意义。