Sedimentological records of the C.E. 1707 Hōei Nankai Trough tsunami in the Bungo Channel, southwestern Japan

Abstract

A tsunami generated by the C.E. 1707 Hōei earthquake is largely thought to be the flood event of record for southwestern Japan, yet historical documentation of the event is scarce. This is particularly true within the Bungo Channel, where significant inconsistencies exist between historical records and model-derived tsunami heights. To independently assess flooding from the Hōei tsunami in this region, we present complementary reconstructions of extreme coastal inundation from three back-barrier lakes in the northern Bungo Channel: Lake Ryuuoo, Lake Amida, and Lake Kamega. At all sites, the age of the most recent prominent marine overwash deposit is consistent with the timing of the 1707 tsunami. When combined with historical documentation of the event, these sedimentological records provide strong evidence that the 1707 tsunami is the most significant flood of recent centuries and one of the most significant floods over the last millennium in the region. At Lake Ryuuoo, modern barrier beach elevations and grain sizes in the tsunami’s resultant deposit provide ~4 m as the first physically based height constraint for the 1707 tsunami in the northern Bungo Channel. A concurrent transition in lithology that is consistent with regional geomorphic change is also observed at all three sites around 1000 years ago, although the precise timing and nature of the transition remain unclear.