MAGNITUDE-FREQUENCY RELATIONS FOR NORTHEASTERN JAPAN

Magnitude-frequency relations are investigated for the on-land area, c overed by the Tohoku University seismic network, northeastern Japan. W e combine the Japan Meteorological Agency (JMA) catalog with the Tohok u University catalog for the crustal events. The suite for the upper p lane events of the double-planed Wadati-Benioff zone, from 60 km throu gh 100 km deep, is based on the Tohoku University catalog only. Comple teness analyses of the catalog reveal that the crustal events and the upper plane events suite is complete down to magnitude 2.0 and 2.1, re spectively. Including events only with magnitude above the completenes s threshold, the magnitude-frequency relations are found to be nonline ar both for the crustal events slightly and the upper plane events str ongly. Consistency between total seismic wave duration magnitudes and velocity amplitude magnitudes suggests that the magnitude scale in the catalog is nonbiased. The completeness analyses and the non-biased ma gnitude scale suggest that the nonlinear magnitude-frequency relations are not artificial but substantial features of the earthquake occurre nce.