Physical properties of the variations of the electric field of the earth preceding earthquakes. II. determination of epicenter and magnitude
As reported in the preceding paper, a transient change of the electric field of the earth (seismic electric signal), hereafter called SES, appears many hours before an earthquake (EQ). By measuring this change in a given direction and dividing it with a suitable relative effective resistivity one obtains a quantity that reflects the current density in this direction. Measurements in two directions (E-W and N-S) give the relative signal intensity Jrel at the station under consideration. By measuring Jrel at a number of stations and considering that it attenuates according to a 1/ r-law, the epicenter can be determined with an accuracy usually around 100 km. Once the epicenter has been determined, the product Jrel · r can be evaluated so that the magnitude M can be estimated by resorting to an empirical log( Jrel · r) versus M plot. The uncertainty of M is around 0.5 units. Following Sobolev (1975) and for the statistics to be beyond any doubt, predictions were officially documented before the EQ-occurrence. For 23 earthquakes with a magnitude equal or greater than Ms = 5.0 two events were missed. The present method is compared to other electrical methods used in China, Japan and Soviet Union. A number of problems concerning the origin of the effect, its directivity and the attenuation with distance remain open for further studies.