Source dynamic parameters of four Greek earthquakes and a possible correlation with the lead-times of their precursor seismic electric signals

Since 1982, eighteen telemetric stations in Greece have continuously been recording variations in the telluric field. Transient changes of the telluric field, called seismic electric signals, SES, have been observed simultaneously at some stations from several hours to several days before an earthquake. The lead-time, Δt, of the SES, which is the time difference between the occurrence of the earthquake and the SES, varies between 6 h and 4 days.Four large earthquakes (M = 5.3–6.4) which occurred in 1983 in the Cephalonia region in Greece, showed clear SES with lead-times falling into two groups (I and II) of several hours and a few days respectively.For the four events, we studied P-wave spectra of the UME Swedish seismological station, at a teleseismic distance of 26°. Both short- and long-period instruments were employed. Brune's model (1970) was used to calculate source dimensions and relative values of other source dynamic parameters.Two groups of high and low stress-drop were found. Events with lead-times of group I show high stress-drop, while events with lead-times of group II show low stress-drop. The relative values of stress-drop differ by a factor of 4 between the two groups (high and low) while within each group they exhibit only a small scatter.The largest relative seismic moment, M₀, is found for the largest event. The three other events with comparable magnitudes have similar seismic moments. The same relation was found for the radiated energy, E_s.