Nature of Electromagnetic Emission Precursory to an Earthquake.

More than 60 occurrences of electromagnetic emission data in Japan precursory to an earthquake are analyzed. It turns out that the larger the main shock magnitude (M), the larger the detectable distance (Dmax) of signal from the epicenter. The M-Dmax relation agrees with that of other geophysical precursors. No dependence of precursor time (T) on M is observed. The situation is much the same as foreshock precursor. The mean precursor time of electromagnetic radiation precursor amounts to 0.26 day, so that the precursor is essentially a short-term one.