Electromagnetic Field Generated by an Earthquake Source Due to Motional Induction in 3D Stratified Media, and Application to 2008 M W 6.1 Qingchuan Earthquake

Movement of the conductive subsurface medium in the Earth's magnetic field can generate electromagnetic (EM) disturbances. This phenomenon is referred to as the motional induction (MI) effect and, though previously proposed as a possible mechanism for the generation of earthquake-associated EM signals, has not yet been well understood. In this study, we present a semianalytical method to derive the EM response to an earthquake due to the MI effect in a 3D horizontally layered model. We solve the coupled elastodynamic and Maxwell's equations in the frequency-wavenumber domain through the global matrix method and then compute the seismic and EM responses in the time domain. We first verify the proposed method by comparing results to the analytical solution in a full-space model and then conduct numerical simulations to investigate the properties of the EM fields and their sensitivities to the rock conductivity. Finally, we use the proposed method to model coseismic EM data observed during the 2008 M-w 6.1 Qingchuan earthquake. The results show that the simulated coseismic electric fields can fit the observed electric signals well in both arrival time and amplitude, suggesting that the MI effect is a possible mechanism for the generation of the observed electric signals. The simulated magnetic fields are 1 order of magnitude weaker in amplitude than the observed magnetic data, implying that the observed magnetic fields are mainly generated by other mechanisms.