Vargemezis, G.

Two stations were installed in the Southeastern Thessaly basin (Central Greece), recording the geomagnetic and telluric fields from 1993 to 1996. The aim was to detect long-term abnormal changes of the telluric field, which were possibly related to imbedding earthquakes. Between January 1993 and October 1996, 213 and185 (Neraida and Mavrolofos stations respectively) abnormal changes of the telluric field were observed in association with the seismic activity. The duration of these changes varied from several minutes to 24 days and the maximum amplitude was 3.8 mV/m. Data recording detected 625 and 917 seismic events for Neraida and Mavrolofos station respectively. The percentage of the earthquakes associated with the telluric anomalies is 27% and 16% respectively for each station. Both percentages are considered to be very low. Telluric activity was followed by a burst of seismic activity in areas spreading to different directions from the stations. A correlation of the characteristics of the telluric field with the earthquake magnitude was attempted, but no reliable relationship was obtained.

Two magnetotelluric stations have been installed in the South-Eastern Thessaly basin (Central Greece), which have recorded the geomagnetic and geoelectric fields since 1993. The aim is to detect long lasting abnormal changes of the geoelectric field which may be due to impending earthquakes. The geoelectric recordings were checked against the climatic changes such as temperature changes and precipitation and no correlation was observed. Ten anomalies were observed with characteristics similar to seismoelectric signals which have been reported in the literature and thus we can assume that these changes constitute precursory phenomena. The duration of these signals varies from several days to a few weeks. Some of them keep on developing until the occurrence of an earthquake, and others appear like transient changes several days before. The high seismicity of the area where the stations are located creates difficulties in the correlation of the signals with certain shocks.

Many attempts have been made to disclose anomalous changes of the electromagnetic field in relation with tectonic earthquakes. We tentatively develop a new approach based on the energy and polarity of the electric field, and apply this method to the seismicity in Greece. The study of the parameters of the horizontal electric field is realized in a time interval of five years. The data set allows the study of long-term variations of the field. Further, we examined the possible relation of the geoelectric activity with long distance seismicity (up to 500 km). The energy of the electric signal was estimated and correlated with the logarithm of the seismic moment (M 0 ). The values of the seismic moment estimated for each earthquake were summed for daily intervals, and the logarithm of the sum was computed. The same process was applied to the energy of the geoelectric field. Then, a correlation was attempted between the energy of the geoelectric field and the seismic moment referring to daily intervals. In two cases, changes in the energy of the horizontal geoelectric field were observed before the burst of the seismic activity. The energy of the telluric field increased several months before the burst of seismic activity and recovered right after the occurrence of the mainshocks. The hodograms of the horizontal geoelectric field show polarization changes regardless of the magnetic field. This is possibly attributed to the process of generation of electric currents before major earthquakes. Due to high and continuous regional seismicity in Greece, it was impossible to attribute the response of the polarization to the activation of specific seismic areas. It seems that the long-term energy variations of the horizontal geoelectric field as well as the polarization are related to forthcoming seismic activity. Therefore, long-term energy variations of the horizontal geoelectric field as well as the polarization could be used in tandem with other possible precursors in order to contribute to earthquake prediction studies.