Introduction to Seismic Electric Signals

Abstract

In the early 1980s, Varotsos and Alexopoulos showed that when the pressure (stress) on an ionic solid reaches a critical value, a cooperative orientation of the electric dipoles (that anyhow exist due to lattice defects) may occur, which results in the emission of a transient electric signal. This may happen before an earthquake since the stress gradually increases in the focal region before rupture. Thus, a detailed experimentation started in Greece in 1981, which showed that actually transient variations of the electric field of the Earth, termed seismic electric signals (SES), are observed before major earthquakes. In the meantime, several other SES generation mechanisms have been proposed. The field experiments revealed the physical properties of SES including those that SES can be observed only at certain points of the Earth's surface called ``sensitive points'' and that their amplitude is interrelated with the magnitude of the impending earthquake. Each sensitive station enables the detection of SES only from a restricted number of seismic areas, a phenomenon termed ``selectivity effect'', which provides the basis for the determination of the epicenter of an impending earthquake. These physical properties can be theoretically explained on the basis of Maxwell equations, if we take into account that the earthquakes occur by slip on pre-existing faults and that the faults constitute conductive paths (electrical inhomogeneities) in the solid Earth's crust. Finally, the observed SES activities, which are series of SES observed within a short time, as well as the associated magnetic field variations, exhibit scale invariance over four orders of magnitude. This is consistent with the original model suggested by Varotsos and Alexopoulos for the SES generation according to which SES should be characterized by critical dynamics which is always accompanied by scale invariance.