COMPARISON OF MAGNITUDES ESTIMATED BY THE JAPAN METEOROLOGICAL AGENCYWITH MOMENT MAGNITUDES FOR INTERMEDIATE AND DEEP EARTHQUAKES

The magnitude, M(JMA), estimated by the Japan Meteorological Agency (J MA) is generally referred to in Japan for the regional seismicity in t he area. M(JMA) is determined from maximum displacement amplitudes of the total seismic wave traces. For earthquakes shallower than 60 km, M (JMA) is determined by Tsuboi's formula, and for earthquakes deeper th an 60 km, by Katsumata's formula. These relations were designed to giv e almost the same magnitude value as that of Gutenberg and Richter. We compared M(JMA) with moment magnitude, M(W) which can be calculated f rom the centroid moment tensor (CMT) solutions. It was found that the average difference between M(JMA) and M(W) is not significant for shal low earthquakes in the magnitude range from 5 to 7, but it is signific ant at a low level for the earthquakes of deeper foci. The averaged di fference reaches about 0.4 magnitude units for the focal depth of 600 km. We derived an attenuation function for the maximum displacement am plitude assuming the validity of the moment magnitude. This relation b etween epicentral distance and amplitude for shallow earthquakes is al most identical to the one calculated from Tsuboi's formula. It is sugg ested that the estimated attenuation function for deep-focus earthquak es reflects the specific Q and velocity structure that is peculiar to the subduction zone.