Electrokinetic Effect Combined With Surface-Charge Assumption: A Possible Generation Mechanism of Coseismic EM Signals

usry, milton. f.;

According to field observations, electromagnetic (EM) signals accompanying seismic waves can be recorded. The orders of magnitude of observed cose is mic electric and magnetic signals associated with earthquakes are usually around 1 to 101 mVkm−1 and 10−2 to 1nT, respectively. In this paper, we carry out numerical simulation of coseismic EM signals associated with seismic waves due to electrokinetic effect and compare with field observations. The seismic source is represented by a finite fault measuring 15 × 9km2 with a max slip displacement 1.5m, corresponding to a Mw 5.9 earthquake. While using the EM surface boundary condition of continuous horizontal EM components, the magnetic signals only accompany the latearriving S waves at receiver near the ground surface. This is obviously different from field observations. Thus, we adopt another EM surface boundary condition which assumes the ground surface carries surface charge. For the used half-space model, a surface-chargedensity magnitude|Qsc|in excess of 10−4 Cm−2 is sufficient to make horizontal magnetic components clearly show up at the whole time duration of seismic waves. When |Qsc| increases, the contribution of surface-charge density to coseismic EM signals becomes more and more dominant comparing with that of the seismically induced streaming-current. We estimate the Qsc expected at the Earth’s surface might be a value between −5 × 10−4 and −0.1 C m−2 by the comparison between numerical results and field observations. The vertical magnetic signals only accompany the late-arriving seismic waves, because they are theoretically only induced by SH wave. The field observation results of vertical magnetic signals may be resulted from the scattering effect or the seismic dynamo effect. We conclude that electrokinetic effect combined with surface-charge assumption is one possible generation mechanism of the observed coseismic EM signals.