Electromagnetic responses to an earthquake source due to the motional induction effect in a 2-D layered model

Movement of the conductive earth medium in the ambient geomagnetic field can generate an electromotive force and a motional induction current, which further cause the disturbances of the electromagnetic (EM) fields. Such a mechanoelectric coupling is known as the motional induction (MI) effect and has been proposed to be a possible mechanism for the generation of the observed EM signals during earthquakes. In this paper, we study the EM responses to an earthquake source due to such a MI effect in a 2-D horizontally layered model. First we transform the governing equations that couple the elastodynamic equations and Maxwell equations into a set of first-order ordinary depth-dependent differential equations. Then we solve the seismic and EM responses to a moment tensor source. Finally, we transform the 2-D seismic and EM responses to 3-D responses using a simple amplitude correction method. We conduct several numerical examples to investigate the properties of the EM signals generated by the earthquake source. The results show that two types of EM signals can be observed. The first one is the coseismic electric/magnetic field that accompanies the seismic P and S waves as well as the Rayleigh wave. The second one is the early EM signal which arrives before the P wave. The numerical results show that the EM signals change with the inclination angle of the geomagnetic field, the azimuth angle between the wave propagation plane and the geomagnetic vertical plane, and the medium conductivity. Increase in the conductivity can enhance the coseismic electric and magnetic signals. Our simulation also shows that an EM wave can be generated by a seismic wave at the interface separating two different media. The radiation pattern of the interface EM wave generated by a P wave is similar to that of a horizontal electric dipole located on the interface.