PHYSICAL-PROPERTIES OF THE VARIATIONS OF THE ELECTRIC-FIELD OF THE EARTH PRECEDING EARTHQUAKES .1.

The electric field variations of the earth that occur before earthquake have been studied in a network of eighteen stations in Greece. These precursor seismic electric signals (SES) occur 6-115 h before the earthquake (EQ) and have a duration of 1 min to 1 one-half h. The duration and the lead-time, in contrast to other precursors, do not depend on EQ-magnitude (M). These signals appear as a transient change of the potential difference between two electrodes (up to a few millivolts for electrodes at a distance of about L equals 50 m) depending on M, the epicentral distance r and the local inhomogeneities. The components of electric field are measured in two perpendicular directions (E-W and N-S). The totality of experiments showed that the interesting quantity of each SES is the maximum value DELTA V of the potential change. The SES of an impending EQ appears simultaneously at a number of stations without being accompanied by any significant change in the magnetic field.