Excitation mechanism and behaviors of co-seismic electromagnetic waves

Abstract

For clarifying the excitation mechanism of co-seismic electromagnetic (EM) waves, I have been observing earthquake-related EM waves in the deep earth and above the ground together with measurements of seismic waves, and also conducted a laboratory experiment. As the result, I have found that EM waves were easily excited by seismic P-wave oscillations in the earth's crust due to piezo-electric effect. The amplitude of the EM wave was enlarged at arrival of seismic S-wave which largely deformed the P-wave amplitude. It has been confirmed, from observed waveforms, that a large amplitude of co-seismic EM wave always appears in the wave-front of the seismic S-wave. Since the EM wave was radiated but rapidly decayed due to a large electrical conductivity of the earth's crust, we could imagine a composite wave system, in which a rapidly decaying co-seismic EM wave is antecedent to the seismic S-wave, and the system is moving with the velocity of the seismic S-wave. It has been also confirmed that a co-seismic EM wave detected above the ground showed ellipsoidal polarization although another EM wave simultaneously detected in the earth showed a linear polarization, which is a result of phase shifts of the EM wave in its penetration through a boundary of two media (from the earth medium to the air).