Semi-analytical solutions of seismo-electromagnetic signals arising from the motional induction in 3-D multi-layered media: part II—numerical investigations

Earth, Planets and Space

Ling Zeng¹  Yao-Chong Sun¹  Xiaofei Chen²  Qinghua Huang³  Ken’ichi Yamazaki⁴  Hengxin Ren⁵ 

[1] Department of Earth and Space Sciences, Southern University of Science and Technology, 518055, Shenzhen, Guangdong, China;Department of Earth and Space Sciences, Southern University of Science and Technology, 518055, Shenzhen, Guangdong, China;Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou), 511458, Guangzhou, China;Department of Geophysics, School of Earth and Space Sciences, Peking University, 100871, Beijing, China;Miyazaki Observatory, Research Center for Earthquake Prediction, Disaster Prevention Research Institute, Kyoto University, 3884, Kaeda, 889-2161, Miyazaki, Japan;Shenzhen Key Laboratory of Deep Offshore Oil and Gas Exploration Technology, Southern University of Science and Technology, 518055, Shenzhen, Guangdong, China;Department of Earth and Space Sciences, Southern University of Science and Technology, 518055, Shenzhen, Guangdong, China;

关键词: Seismo-electromagnetic signals;    Motional induction effect;    Numerical computation;    Coseismic EM signals;    Evanescent EM waves;   

DOI  :  10.1186/s40623-021-01458-5

来源: Springer

【 摘 要 】

In this paper, numerical computations are carried out to investigate the seismo-electromagnetic signals arising from the motional induction effect due to an earthquake source embedded in 3-D multi-layered media. First, our numerical computation approach that combines discrete wavenumber method, peak-trough averaging method, and point source stacking method is introduced in detail. The peak-trough averaging method helps overcome the slow convergence problem, which occurs when the source–receiver depth difference is small, allowing us to consider any focus depth. The point source stacking method is used to deal with a finite fault. Later, an excellent agreement between our method and the curvilinear grid finite-difference method for the seismic wave solutions is found, which to a certain degree verifies the validity of our method. Thereafter, numerical computation results of an air–solid two-layer model show that both a receiver below and another one above the ground surface will record electromagnetic (EM) signals showing up at the same time as seismic waves, that is, the so-called coseismic EM signals. These results suggest that the in-air coseismic magnetic signals reported previously, which were recorded by induction coils hung on trees, can be explained by the motional induction effect or maybe other seismo-electromagnetic coupling mechanisms. Further investigations of wave-field snapshots and theoretical analysis suggest that the seismic-to-EM conversion caused by the motional induction effect will give birth to evanescent EM waves when seismic waves arrive at an interface with an incident angle greater than the critical angle θc = arcsin(Vsei/Vem), where Vsei and Vem are seismic wave velocity and EM wave velocity, respectively. The computed EM signals in air are found to have an excellent agreement with the theoretically predicted amplitude decay characteristic for a single frequency and single wavenumber. The evanescent EM waves originating from a subsurface interface of conductivity contrast will contribute to the coseismic EM signals. Thus, the conductivity at depth will affect the coseismic EM signals recorded nearby the ground surface. Finally, a fault rupture spreading to the ground surface, an unexamined case in previous numerical computations of seismo-electromagnetic signals, is considered. The computation results once again indicate the motional induction effect can contribute to the coseismic EM signals.

【 授权许可】

CC BY