| Popis: |
A tsunami deposit investigation was conducted to verify the presence and source of tsunami deposits in northern Hiyama district, southwest Hokkaido, Japan. Three (Th-2–Th-4) and one (Ku-2) possible tsunami deposits were identified in the Hirahama and Kumaishi-Ayukawa lowlands, respectively. Th-2–Th-4 were divided into sands (Th-2) and gravels (Th-3–Th-4), although they were found in almost the same sites. These differences might have been in harmony with beach clastics as their sources during the Th-2 and Th-3–Th-4 events. It was possible that strong tsunami return flow eroded and transported sands seaward, leaving only gravels behind. Ku-2 consisted of volcanic-ash sands with the overlying silts and showed three fining-upward sequences (Units A–C). The modal compositions of Units B–C were distinct from beach and river sands, although that of Unit A was similar with beach and river sands. The chemical composition of volcanic glasses of Ku-2 indicate that volcanic-ash sands originated from reworked tephra. Considering the rounded clastics, landward paleocurrent, and high proportion of marine–brackish diatoms in Unit B, volcanic-ash sands might have been widespread in the seafloor as a provenance beyond the foreshore and were transported by tsunami run-up flow. Seaward paleocurrent of Unit C was interpreted as return flow. The sedimentary process of Unit A remains unclear because of lack of sedimentary structures and diatoms. Unit A might have been formed by a series of tsunami or lahar. Based on radiocarbon dating, tephrochronology, historical records, and known paleotsunamis in the adjacent area, Th-2 and at least Units B–C, Th-3, and Th-4 could be correlated with AD1741, 12th century, and 2.4–2.6 ka tsunamis, respectively. Considering turbidites in the Shiribeshi Trough, it is reasonable that ca 2–2.6 ka event deposits were formed by the same earthquake with accompanying tsunami. Further studies are needed on whether tsunami deposits—correlated with abovementioned tsunami event—existed in central Hokkaido or southern Hiyama District, where effects of tsunamis triggered by the active faults of F14/F15 and F17/F18 were much stronger, respectively. Confirmation of existence is expected to help clarify the activity record of tsunami sources in southwest Hokkaido. |
