On the recent advances in the study of seismic electric signals (VAN method)

Abstract

Seismic electric signals (SES) are low frequency (⩽1 Hz) changes of the electric field of the earth that have been first observed in Greece [Varotsos, P., Alexopoulos, K., Nomicos, K., 1981a. Seismic electric currents. Pract. Athens Acad. 56, 277–286; Varotsos, P., Alexopoulos, K., Nomicos, K., 1981b. Seven-hour precursors to earthquakes determined from telluric currents. Pract. Athens Acad. 56, 417–433; Varotsos, P., Alexopoulos, K., 1984a. Physical properties of the variations of the electric field of the earth preceding earthquakes, I. Tectonophysics 110, 73–98; Varotsos, P., Alexopoulos, K., 1984b. Physical properties of the variations of the electric field of the earth preceding earthquakes, II. Tectonophysics 110, 99–125] to precede earthquakes, with a lead time from several hours to a couple of months. Here, we review the recent advances on the SES observation and analysis, main points of which are the following: First, at epicentral distances of the order of 100 km, the SES electric field precedes markedly the time-derivative of the magnetic field; this finds applications in the determination of the epicenter of the impending earthquake and in the distinction between true SES and noise emitted from artificial sources. Second, a detectable difference in the time evolutions of the electric field components of SES exists, which can also be used for the determination of the epicenter of the impending earthquake. Third, the analysis of SES is significantly advanced in a recently introduced [Varotsos, P., Sarlis, N., Skordas, E., 2001. Spatio-temporal complexity aspects on the interrelation between seismic electric signals and seismicity. Pract. Athens Acad. 76, 294–321; Varotsos, P., Sarlis, N., Skordas, E., 2002a. Long-range correlations in the electric signals that precede rupture. Phys. Rev. E 66, 011902] new time-domain, termed as natural time domain. This has been inspired from the theory of critical phenomena, which has been suggested long ago by our group. The natural time-domain, beyond other applications in diverse fields, enables the distinction of similar looking electric signals that are emitted from systems of different dynamics as well as provides a better estimation for the time window of an impending mainshock. The spectral content of the seismic activity in natural time, evolves consecutively in time upon the occurrence of every new event, and finally coincides to that of the SES a few hours to a few days before the mainshock, thus allowing the estimation of the occurrence time of the impending mainshock with an accuracy that was not hitherto available.