Reexamination of Wave Behavior of the Hokkaido-Oshima(the Japan Sea) Tsunami in 1741 : Their Comparison with the 1983 Nihonkai-Chubu Tsunami

新史料を加え,1741年渡島沖津波の波高を再検討した.また,北海道渡島沿岸を現地調査し,熊石にある津波地蔵・過去帳を紹介した.津波の高さは渡島沿岸で10m,局地的には15mに達し,津軽で5～6m,佐渡では4～5mと推定される.津波マグニチュードは,Δ-Hグラフからm=3.5と見積もれる.波高分布のピークは1983年日本海中部津波のものより北側に片寄り,波源域は渡島半島沖の水深3,000mの海域を中心に,長さ150kmの範囲にあったと考える.Adding new data on the Oshima tsunami of Aug. 29, 1741, the inundation heights along the coast of the Japan Sea are reexamined. About 2,000 persons drowned at 22 villages within the 90 km stretch (Matsumae to Kumaishi) along the west coast of Oshima Peninsula, West Hokkaido. A religious monument and an old document were newly found at a temple in Kumaishi. Considering ground level, inundation heights (above sea level) of the tsunami along the Oshima coast were estimated to be 10m with local run-ups as much as 15m. The estimated inundation heights are 5-6m at Tsugaru (Aomori Pre.), 3-4m at Noto-Shitsura (Ishikawa Pre.), 1m at Obama (Fukui Pre.) and 1-2m at Gotsu (Shimane Pre.). The distribution pattern of the heights on Sado Island is highly similar to that of the 1983 NihonkaiChubu tsunami. However, the wave-period of the 1741 Oshima tsunami is longer than that of the 1983 tsunami, because the wave-heights are large in the big bays at Aburakawa and Ryotsu. Judging from the attenuation of tsunami height (semi-amplitude) with distance, the tsunami magnitude (Imamura-Iida scale) is m=3.5. The tsunami energy is five time larger than that of the 1983 tsunami (m=2.5). Compared to the magnitude and the distribution pattern of heights for the 1983 tsunami, this suggests that the source area of the 1741 tsunami extends about 150km along the bathymetric line 3,000m off the Oshima Peninsula