Induced electromagnetic field by seismic waves in Earth's magnetic field

Abstract

AbstractStudied in this article are the properties of the electromagnetic (EM) fields generated by an earthquake due to the motional induction effect, which arises from the motion of the conducting crust across the Earth's magnetic field. By solving the governing equations that couple the elastodynamic equations with Maxwell equations, we derive the seismoelectromagnetic wavefields excited by a single‐point force and a double‐couple source in a full space. Two types of EM disturbances can be generated, i.e., the coseismic EM field accompanying the seismic wave and the independently propagating EM wave which arrives much earlier than the seismic wave. Simulation of an Mw6.1 earthquake shows that at a receiving location where the seismic acceleration is on the order of 0.1 m/s2, the coseismic electric and magnetic fields are on the orders of 1 μV/m and 0.1 nT, respectively, agreeing with the EM data observed in 2008 Mw6.1 Qingchuan earthquake, China, and indicating that the motional induction effect is effective enough to generate observable EM signal. We also simulated the EM signals observed by Haines et al. () which were called the Lorentz fields and cannot be explained by the electrokinetic effect. The result shows that the EM wave generated by a horizontal force can explain the data well, suggesting that the motional induction effect is responsible for the Lorentz fields. The motional induction effect is compared with the electrokinetic effect, showing the overall conclusion that the former dominates the mechanoelectric conversion under low‐frequency and high‐conductivity conditions while the latter dominates under high‐frequency and low‐conductivity conditions.