Religious Monuments of the Oshima Tsunami in 1741, West Hokkaido

The Oshima tsunami of Aug. 29, 1741 (Kampo 1) is the largest tsunami (Imamura-Iida scale m=3) generated in the Japan Sea. According to old documents, the wide area (length 500 km) from west Hokkaido, Tsugaru to Sado Is. suffered severe damage by this tsunami. 1,467 persons were drowned at various villages within 120 km (Matsumae to Kumaishi) along the west coast of the Oshima Peninsula. In the temples of Matsumae and Esashi, there are old monuments which were built just after the tsunami to pray for the repose of the tsunami victims. From the present field investigation, these tsunami monuments are illustrated. Considering ground level, inundation heights of the tsunami along the west coast of the Oshima Peninsula seem to have reached 6 to 8 m with the localized run-up maximum of about 15 meters. The eruptions of the Oshima volcano began 11 days before the tsunami generation and the volcanic activity continued for three months, but earthquakes were not recorded. Judging from the tsunami magnitude and the distribution pattern of wave-heights, it seems that the source area of tsunami extends 100 km or more along a bathymetric line of 3,000 m off the Oshima Peninsula