The Kamimura River, an upper tributary of the Yahagi River, flows through steep valleys in the mountainous Kamiyahagi area of Gifu Prefecture. The Tokai Gou flood of September 11-12, 2000 destroyed embankments along the river, and exposed sedimentary layers at Umi that are typical of a lacustrine depositional setting. This confirms the existence of a paleo-lake from which the name Umi originated. The lake was formed as the result of a huge landslide that dammed up the Kamimura River. To determine when the landslide occurred, radiocarbon (
14C) ages of trees buried in lacustrine sediments were determined using accelerator mass spectrometry (AMS). The
14C ages are 300±25, 340±25, 345±25 and 380±25 BP for the outermost rings of tree trunks, which include bark, and a single age of 620±25 BP. Calibrated ages of (1637), (1520, 1590, 1624), (1517, 1597, 1619) and (1482) cal AD within a range of 1455-1643 cal AD are consistent with a landslide triggered by the Tensho Earthquake of January 18, 1586, which had an estimated magnitude of 8.2-8.3 and an epicenter in Ise Bay.
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