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The relation between seismic moment Mo and magnitude M of JMA (Japan Meteorological Agency) is examined for the earthquakes of M>5 in and around Japan. The data for the events in subduction zones including the earthquakes along eastern margin of the Japan sea are approximately explained by the relation as log M0=1.5M+16.2 (1) This equation was proposed by SATO (1989) based on the similarity relation of source parameters. In consideration of the difference between M and surface-wave magnitude MS, the equation (1) can be modified so as to describe more detailed trend of the data distribution of Mo and M. On the other hand, the data for inland events which occurred in the upper crust cannot be explained by the equation (1). Another relation is available for these events as follows: log M0=1.17M+17.72 (2) Two reasons are possible for the difference of the M0-M relations between the events in the subduction zone and in the inland. One is that the stress drop of the inland events is larger than that of the subduction events. The other is that the inland events show large excitation of surface-waves in the medium periods, because the focal depths of them are shallower on the average, comparing with those of the events in the subduction zone. The JMA magnitude is determined from the maximum amplitudes of seismic waves in the medium period range, which are usually given not by S-waves but by surface-waves for large events. This fact supports the latter reason.

Magnitude-Seismic Moment Relations for the Shallow Earthquakes in and around Japan