Breakout flood from an ignimbrite-dammed valley after the 5 ka Numazawako eruption, northeast Japan

Abstract

Numazawa volcano in northeast Japan erupted most recently at ca. 5 ka, forming a 2-km-diameter caldera and emplacing at least 4 km 3 of valley-confined ignimbrite. The ignimbrite dammed the Tadami River to a depth of >100 m, temporarily impounding >1.6 km 3 of water. Overtopping of the barrier triggered breaching and catastrophic release of the dam lake. Pyroclastic material redeposited by the ensuing flood is widely distributed along the Tadami and Agano Rivers in deposits tens of meters thick as far as the coastal Niigata Plain >150 km downstream of the volcano. Evidence for damming and flooding found along the rivers includes (1) fine-grained, thinly laminated lacustrine deposits upstream of the pyroclastic blockage; (2) 30-m-thick, pumiceous debris-flow and hyperconcentrated-flow deposits that show continuous sedimentation with no major hiatus downstream of the volcano; (3) fine-grained slackwater deposits at tributary river mouths; and (4) large flood boulders that form lags on stripped ignimbrite surfaces and younger terrace surfaces, or cropping out within hyperconcentrated-flow deposits. Paleohydraulic estimation techniques indicate a peak discharge of 30,000–50,000 m 3 /s at the breach point. Burial of Neolithic Jomon settlements by flood and volcaniclastic aggradational deposits at distal locations indicates that such indirect volcanogenic hazards are significant, even where direct primary consequences of volcanic eruptions are minor or absent.