The stratigraphy of tsunami deposits along the Japan Sea, southwest Hokkaido, northern Japan, reveals tsunami recurrences in this particular area. Sandy tsunami deposits are preserved in small valley plains, whereas gravelly deposits of possible tsunami origin are identified in surficial soils covering a Holocene marine terrace and a slope talus. At least five horizons of tsunami events can be defined in the Okushiri Island, the youngest of which immediately overlies the Ko-d tephra layer (1640 AD) and was likely formed by the historical Oshima-Ohshima tsunami in 1741 AD. The four older tsunami deposits, dated using accelerator mass spectrometry 14 C, were formed at around the 12th century, 1.5-1.6, 2.4-2.6, and 2.8-3.1 ka, respectively. Tsunami sand beds of the 1741 AD and circa 12th century events are recognized in the Hiyama District of Hokkaido Island, but the older tsunami deposits are missing. The deposits of these two tsunamis are found together at the same sites and distributed in regions where wave heights of the 1993 tsunami (Hokkaido Nansei-oki earthquake, Mw = 7.7) were less than 3 m.Thus, the 12th century tsunami waves were possibly generated near the south of Okushiri

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北海道日高海岸北部において津波堆積物の調査を行った．その結果，クロスナや泥炭層中に 17 世紀の砂質イベント層が挟在することが明らかになった．イベント層は内陸へ薄層化および細 粒化する傾向を示し，現在の海岸線から内陸方向へ約 150 m, 標高 3.86 m の地点まで分布する．珪 藻分析の結果，イベント層では下位の泥炭に比べて外洋指標種の Thalassionema nitzschioides が 多産する傾向を示した．これらの特徴は，このイベント層が津波によって形成されたことを示し ている．このイベント層はごく薄い泥炭層やクロスナ層を挟んで Us-b 火山灰 (AD1663) の直下 にあり，17 世紀前期～中期に堆積したと推定される．

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.