Volcanic Debris-Avalanche as a Cause of a Historic Tsunami: The A.D. 1640 Eruption of the Hokkaido-Komagatake Volcano, Northern Japan

Sidescan sonar and sub-bottom reflection surveys off Shikabe town, along the Pacific coast, show the extent and volume of the submarine debris-avalanche deposit caused by the AD 1640 eruption of the Hokkaido-Komagatake volcano. The avalanche deposit extends from the subaerial part of the Shikabe lobe to as far as 20 km seaward. The subaqueous deposit, which reaches to 80 m of water depth, is 15 km wide and covers 126 km² of the seafloor. Hummocks of the subaqueous deposit decrease in height and width with distance from the source. The margin of the deposit lacks hummocks. The ratio of the collapse height to the traveled distance (H/L) is 0.06, suggesting that the debris avalanche of origin was more mobile than those of most subaerial debris-avalanche deposits. The subaqueous volume of the avalanche deposits, estimated by extrapolating the pre-eruptive topography of the surrounding area, ranges from 0.92 to 1.20 km³.