Electromagnetic signals generated in the solid Earth by digital transmission of radio-waves as a plausible source for some so-called 'seismic electric signals'

Claims by the VAN group [Varotsos, P., Alexopoulos, K., 1984. Physical prop erties of the variations of the electric field of the earth preceding earth quakes, I. Tectonophysics 110, 73-98, and later works] to have developed a shea-term earthquake prediction technique in Greece continue to arouse cont entious debates [Claims of success in using geoelectrical precursors to pre dict earthquakes are criticized and defended, 1998. Letters, Phys. Today 51 (6), 15-100; Great debates in seismology: the VAN method of earthquake pre diction, 1998. Eos 79, 573-580]. This is partly because of the unknown orig in of the so-called 'seismic electric signals' (SES) precursors. Their part icular characteristics are not those of the usual electromagnetic noise (cu ltural noise) or of the natural electromagnetic field (magnetotelluric fiel d). In this paper, we show that transient electric signals looking Like SES can be generated by digital transmitters of the radio-telecommunication ne twork. Such signals have been observed in different regions of the world, i ncluding Greece and Vietnam. Their characteristics have been analyzed in a broad band of frequencies (10(-3)-10(3) Hz) in the Ioannina, Greece, site w hich is considered as the most 'sensitive area' of the VAN network. It is c oncluded that some of the signals recorded at this site and identified as S ES are probably of artificial origin, and that the criteria used by the VAN group are not sufficient to guarantee that the so-called SES are not man-m ade. Without an extended and thorough study of the ambient electromagnetic noise in a broad band of frequencies and better information about the elect rical properties of the deep structure beneath the monitoring station, eart hquake predictions issued on the basis of signals recorded by the VAN netwo rk are of dubious significance. (C) 1999 Elsevier Science B.V. All rights r eserved.