On the Magnitudes of Earthquakes

Abstract

This is the synopsis of one of the opening lectures at the symposium “Magnitudes of Earthquakes” which was held on April 15, 1957, at the Annual Meeting of the Seismological Society of Japan. First, history of the magnitude scale was briefly sketched and special attention was called to the assumption originally made by C. F. RICHTER that the ratio of maximum amplitude due to an earthquake to that due to the standard (M=0) earthquake depends only on the magnitude of the former and does not depend on the epicentral distance. Then followed the discussions of the relation between the maximum amplitude A and epicentral distance Δ, in particular reference to Japanese earthquakes. Various formulas by which to derive M from Δ and A were compared. The number N of earthquakes in terms of their magnitudes was the next subject of talk. It was pointed out that earthquakes in and near Japan are contributing well over 10% to the total number of earthquakes in the whole world. Then the amount of energy released by earthquakes was surveyed. Japan's contribution was estimated to be 0.5×10²⁴erg/year in total or 1.5×10³erg/cm² year. This value 0.5×10²⁴erg/year is again more than 10% of the total energy for the whole world which was estimated by the present writer to be 3.6×10²⁴erg/year (one third of GUTENBERG's value.) Finally, the relations among M, E, V (earthquake volume), A (aftershock area), and x (ultimate strain of the crustal material) were discussed from the writer's own view point. The lecture closed with suggestions for future problems.