抄録
The 2000 eruption of Usu volcano started at 1 : 07 PM, March 31, 2000 following 3-and-a-half days precursory activities of earthquakes and ground cracking. Initial explosion took place at the western lower slope of the volcanic edifice discharging ash plume. Ballistics were thrown westward at least 1.2 km away from the source. More than 60 explosion craters were formed on the following 3 weeks. Type of the eruption cloud was similar to geyser discharging hot water vapor and smaller amount of ash. Hot lahars were directly discharged from some of the craters. A small-scale and fiat-topped cryptodome was formed during this stage. Graben-like fault swarms were formed above the cryptodome. Explosive activity had gradually declined since the later half of April. Bursting-type explosions with air shock wave frequently occurred at the water-saturated crater floors. Geothermal activity became clear on and around the cryptodome since middle May. Geothermal activity had still continued and water vapor was being discharged from three craters in summer time of 2001.
