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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