Estimation of temporal variations in the magnetic field arising from the motional induction that accompanies seismic waves at a large distance from the epicentre

摘要

Temporal variations in the electromagnetic field that accompany earthquakes are generated by various mechanisms, of which this study focuses on variations in the magnetic field arising from motionally induced electric currents that accompany seismic waves at large distances (several hundred kilometres) from the epicentre. A simple situation is considered in which seismic waves are approximated by plane waves and the electrical conductivity of the Earth's crust has a stratified structure. Solutions of Maxwell's equations corresponding to this situation have analytical expressions. Analysis of the solutions verifies that SH waves do not generate variations in the EM field above the ground surface, thereby implying that Rayleigh waves are dominant at a significant distance from earthquake epicentres. Numerical examples demonstrate that the amplitudes of the variations in the magnetic field monotonically increase with increasing conductivity, although attenuation because of the skin effect cannot be ignored. The amplitudes of the generated magnetic field can be sensitive to the conductivity of both the shallow and deep crust. Nevertheless, calculations assuming a simplified conductivity structure provide an upper limit to the possible amplitudes of variations in the magnetic field because of seismic waves. For example, the amplitudes of variations in the magnetic field arising from a Rayleigh wave with displacement amplitude of 10 cm and a period of 30 s are as large as 0.1 nT, close to the limit of detection under typical observation conditions. It is also suggested that phase differences between seismic ground motions and variations in the magnetic field are insignificantly influenced by details of conductivity structures, and they occur within a rather narrow range of values determined by the direction orientation of the ambient geomagnetic field. In the future, if a detection limit of 0.01 nT becomes available, phase difference may be used to distinguish variations arising from the motional induction, from variations arising from other mechanisms.