Enhancement of seismic electric signals using magnetotellurics

Abstract

During two field studies carried out in Greece in 1986 and 1987–1988, electric precursors, superimposed on the telluric field, and the time dependence of the magnetotelluric apparent resistivity, were studied using the magnetotelluric method applied in the frequency band of 0.00028–0.5 Hz. The recognition of any transient electric signal in the telluric field was improved by reducing the main part of the magnetotelluric induction by making use of a residual electric field. The residual field is defined as the difference between the measured electric field and the field which is calculated by multiplying the measured magnetic field with the magnetotelluric impedance tensor. In concordance with a previous study, this method is shown to be rather effective. One conclusion from our study is that it is practical to employ the residual field method in a continuous daily analysis within an earthquake prediction research experiment. We have demonstrated that suspected electric transients are revealed as anomalies of non-magnetic origin. After a careful analysis, in three cases significant transient electric signals were classified as suspected seismic electric signals. They were clearly resolved on the residual field and all correlated well in time with three earthquakes occurring close to the stations during the time of recording.