抄録
The phreatomagmatic eruptions of Usu Volcano began from vents of the western flank of Mount Nishiyama on March 31, 2000. In the next day, April l, another phreatomagmatic eruptions began from vents of the western flank of Mount Konpira. In order to assess the risk of eruption-induced sediment disasters, the Team of Experts for the Sediment Disaster has surveyed the state of the volcanic ash deposits, the devastation of torrents, and the damage of erosion control dams. The results of the surveys from April 1 to 11 are as follows. 1) Volcanic ash deposits near the vents was observed as 50 cm-1m thick, but that at the outside of the evacuation ordered area was very thin(thinner than 1mm) 2) Hot mudflows occurred mainly firom the vents of the western flank of Mount Konpira. The repeated hot mudflows fulfilled the erosion control dams of the Nishiyama River and then flooded into the Toya Spa Town. Two bridges were washed away. The depth of the hot mudflow deposits was observed very thin, but locally 1.5-2.5m thick at maximum. 3) Thick ash deposits and severe land deformation yield a large amount of unstable sediments in the upper reach of the Itaya River.
