含起伏地形地层中点源激发的震电响应研究

Study on earthquake-induced electromagnetic (EM) phenomena is an important subject in geosciences. Previous understanding of the seismoelectric coupled wavefields is mainly based on the simulations for regular model. In order to study the excitation and propagation characteristics of the coupled seismic and EM wave field in the media with undulating topography, we use the finite element software COMSOL Multiphysics to simulate the electromagnetic field excited by a point source. Firstly, we present the two-dimensional SHTE mode equations in frequency domain. Then the COMSOL software is used to establish the computational model and solve the frequency-domain response of the seismoelectric wave fields. Finally, the time-domain waveforms of the seismic and electromagnetic fields are obtained by applying the fast Fourier transformation. The simulations show that there are three types of EM signals. The first one is the direct EM wave excited by the source; the second is the EM wave excited by the seismic wave at interfaces (including the free surface, the underground interfaces separating different media); The third one is a coseismic EM signal accompanying a seismic wave. The former two kinds of EM waves reach the distant observation station earlier than the seismic wave, and thus they are important for earthquake early warning. The results also show that when the seismic waves propagate along the surface, they also cause EM fields in the air layer. We observe the same EM signals at the receivers in the air and underground medium near the surface. Such a phenomenon agrees with some previous observations. The results in this study provide theoretical evidence for interpreting the earthquake-associated EM signals in the future.