When a relatively small perceptible earthquake occurred near a tripartite net of high sensitivity in central Japan, a substantial difference was found between its 25 foreshocks and 173 aftershocks in the relation of frequency of occurrence and magnitude. For that study the coefficient “b” in the magnitude versus frequency equation is 0.35 for the former and 0.76 for the latter.
A similar investigation has been carried out on the great Chilean earthquake of 1960, also accompanied by many foreshocks and aftershocks. Using four sensitive and suitably located U.S.C.G.S. stations, Eureka, Tucson, South Pole, and Byrd, foreshocks and aftershocks were located in addition to those reported by U.S.C.G.S. or B.C.I.S. Forty-five foreshocks and 250 aftershocks were found in a period of 33 hours before and 33 hours after the main shock. The same characteristic found for the Japanese earthquake was also found for the Chilean earthquake; i.e. the foreshocks showed a different picture from the aftershocks for the frequency of occurrence, and an appreciably smaller value seems to be valid for “b” of the foreshocks.