Event deposits recognized in the lowlands along the Tsugaru Strait, Hokkaido, Japan

Abstract

Geological records of tsunamis along the Tsugaru Strait in southwestern Hokkaido, Japan, were investigated. Event deposits were identified in lowland peat at four survey sites located 100-450 m from the present coastline and 2.2-6.6 m above the present sea level. The event deposits are several centimeters thick and consist of well-sorted fine- to medium-grained sand. Based on radiocarbon dating and tephrochronology, the sedimentary ages of the event deposits were determined to be 589-516, 734-670, 1656-1538, 1745-1639, 2401-2265, and 2771-2618 cal yBP. Comparison of the clastic composition and roundness of the event deposits with those of nearby beach and river sands revealed that most of the provenance of the event deposits are beach sands. Whereas, some of the event deposits were sourced from volcanic-ash sands, which were distinct from both beach and river sands. The chemical compositions of volcanic glasses and mineral grains indicate that the volcanic ash did not originate from any known Quaternary tephra in this region. Considering the source of the event deposits, as well as the inundation distance from the coastline and the infrequency of events, the event deposits were likely formed by tsunamis. Volcanic-ash sands may have been widespread in the seafloor beyond the foreshore and were transported by tsunamis. From the sedimentary ages and depositional rates of event deposits and peat, the youngest event deposits were formed in the 13-15th century, and may be correlated with a tsunami that reached the eastern coast of Hokkaido in the 13-14th century or the AD 1454 Kyotoku tsunami. Neither the well-known 17th century tsunami that was triggered along the Pacific coast of eastern Hokkaido nor the AD 1611 Keicho-sanriku tsunami appear to have left deposits in this area. These results can be used to determinate accurately the source area of 17th century tsunamis.