The major intraslab earthquakes have recurrently occurred in the subducting slab of the Philippine Sea plate in the vicinity of the Bungo Channel and caused great damage. Since the intraslab earthquakes tend to generate more short period seismic waves compared to plate boundary and shallow crustal earthquakes, seismic magnitude of historical earthquakes should be carefully estimated from seismic intensity data. We found that the 1596/9/4, 1749/5/25, 1854/12/26, and 1968/8/6 earthquakes fall under major events in this region. These earthquakes except the 1596 event were identified as intra-slab events. The 1596 event was inferred as a shallow crustal earthquake occurring at a fault in Beppu Bay from the tsunami record and geological survey. At first, the seismic intensity data of the 1596, 1749, and 1854 earthquakes estimated from damage records written in old documents were digitized to analyze their source characteristics. The seismic intensity data were modified using a site correction factor obtained from a difference of observed intensity from estimated intensity with the attenuation relationship of recent earthquakes. The inversion analysis using seismic intensity data was carried out to evaluate short-period wave radiation zones on a given fault plane. It showed that the location of centroid of short-period wave radiation zone was near the corresponding epicenter [Usami (2003)] except the 1749 event. We could also obtain an optimum magnitude MI that gives the minimum estimation error. It was indicated that the 1596 earthquake was inferred as a shallow crustal event (MI=7.2) near the center of Beppu Bay, the 1749 earthquake (MI=6.6) quite resembled the 1968 event in short-period wave radiation zone and magnitude, and the 1854 earthquake (MI=7.0) was the largest event among historical intra-slab earthquakes in this region. Based on the magnitude revaluation, the empirical equation of relationship between magnitude and isoseismal area of IJMA≥5 was modified to estimate magnitude of intra-slab earthquakes in the Bungo Channel adequately.
