2019 Volume 125 Issue 12 Pages 835-851
Coastal sediments in Uchiura Bay and along the Iburi coast, on the western part of the Pacific coast in Hokkaido, contain organic sediments intercalated with tsunami-derived sediments from a 17th-century tsunami event. The event layers show sedimentary features characteristic of a tsunami origin, including thinning of layers, sediment fining, and decreasing heavy mineral content in an inland direction. From Uchiura Bay to the western Iburi coast, the event layers are directly overlain by the 1640 Komagatake tephra (Ko-d). From the eastern part to the central part of the Iburi coast, the event layers deposit below the 1663 Usu tephra (Us-b), but there is almost no gap between them because there is only a very thin humus layer. We infer that the event layers record the tsunami caused by the 1640 Komagatake eruption, because this eruption produced both a large tsunami and a large amount of tephra (Ko-d). Our numerical simulation based on the distribution of the event layers indicates that more than 1.2 km3 of debris formed an avalanche into the sea, causing a tsunami of Mt 7.9-8.2. The estimated run-up heights showed good agreement with the observed run-up heights from Uchiura Bay to the Shiraoi area of the Iburi coast.
