Electromagnetic Anomalies Prior to Large Earthquakes in Greece: A Study of Their Behavior Through Laboratory Experiments, Fault Models and the Geotectonic Structure of Each Focal Area

Clear precursory electromagnetic (EM) anomalies were recorded at Zante station (37.76^oN-20.76^oE) in W. Greece, at MHz and kHz frequency bands, prior to the three destructive earthquakes occurred in Greece, on May 13 1995 with M_s=6.6 at Kozani-Grevena area, on June 15,1995 with M_s=6.5 at Egio-Eratini area and on September 7, 1999 with M_s=5.9 at Athens area. The field detected EM signals and their wavelet analysis showed the following remarkable and compatible with the international experience, features: (i) a significant increase of the EM activity in both frequency bands (MHz and kHz) before the earthquakes; (ii) a dramatic shift of their frequency content from MHz to kHz as the earthquake approaches; (iii) an electromagnetic quiescence just before the main shock and (iv) a total absence of EM signals during and after the main shock. By means also of laboratory experiments on acoustic (AE) and (EM) emissions we attempted to investigate similarities in the time evolution of the frequency content between laboratory emissions and the field detected EM anomalies prior to the earthquakes. We found that: (i) both field EM anomalies and laboratory emissions showed a significant acceleration in their event rate activity before the earthquake and rock failure respectively and (ii) the reported by Ohnaka and Mogi frequency shift (from MHz to kHz) of the AE activity during the last deformation stage of rock samples, was in full agreement with the observed shift (from MHz to kHz.) of the EM anomalies before the earthquakes. Features (iii) and (iv) are also compatible with laboratory experiments All the above interpretation and results yield to the suspicion that the study of precursory VLF and VHF electromagnetic anomalies might be possible to illuminate from another viewpoint the last stages of the earthquake preparation process. Moreover these EM anomalies have revealed interesting correlations between the structure of pre-seismic sequence of electromagnetic anomalies and the results from the geotectonic, seismological and geodetic studies on the corresponding earthquakes. These electromagnetic anomalies preceding each individual earthquake generally differ, in the frequency bands where they appear, the number of distinct electromagnetic anomalies in each frequency band, their duration and their lead-time. In this paper, we show that these observed differences are strongly correlated with, the fault model characteristics and the focal geotectonic structure of the associated earthquake. The simultaneous presence/absence of VHF anomalies with the VLF recordings are related to the presence/absence of surface faults; the number of distinct VLF signals is related to the number of faults that will be activated; the duration and lead-time differences are related to the geotectonic structure heterogeneities. These findings strongly support the hypothesis that these EM anomalies seem to be specific signs of the preparation process for earthquakes.