The country of Japan and its surrounding area has been divided into 12 seismogenic sources on the basis of certain seismotectonic and geomorphological criteria. In each of these sources, interevent times between successive mainshocks with magnitudes equal to or larger than certain cutoff magnitudes were considered. These interevent times as well as the magnitudes of the mainshocks have been used to determine the following relations:
where Tt is the interevent time measured in years, Mmin is the surface-wave magnitude of the smallest mainshock considered, Mp is the magnitude of the preceding mainshock, Mf is the magnitude of the following mainshock, and M˙0 is the yearly released seismic moment in each source.
On the basis of these two relations, and taking into account the time of occurrence and the magnitude of the last mainshock in each source, time-dependent conditional probabilities for the occurrence of the next large (Ms ≧ 7.5) shallow mainshocks in the 12 seismogenic sources during the next 10 yrs (1993 to 2002) and the magnitudes of the expected mainshocks are determined. For seismogenic sources 5 (Fukushima), 9 (western Honshu), 7 (southern Hokkaido), 1 (Ryukyu islands), and 12 (Okushiri island), relatively high probabilities (P10 ≧ 0.5) were assigned. It is of interest to note that in seismogenic source 12 (Okushiri), for which an expected magnitude of 7.5 was calculated, an earthquake with a magnitude of 7.6 (National Earthquake Information Center) occurred on 12 July 1993.