Usu is a stratovolcano with lava domes at its summit and on its flanks. Usu
has erupted nine times since 1626. All of these eruptions have been
explosive and most generated pyroclastic flows. Eruptions in 1663, 1822,
1910, 1944-1945, and 1977-1982 caused fatalities. Deaths were caused by
ashfall, pyroclastic flows, and mudflows. Photograph courtesy of and
copyrighted by Mike Lyvers.
On 17 April, the Mount Usu Volcano exploded yet again. Craters on the west side of the volcano were active expelling black smoke due to underground magma explosions. Residents of the Usu communities were moved to evacuation centers as seismic activity began to increase.
This information was summarized from Discovery Online.
On 3 April, Japan's Meteorological Agency discovered three new fault lines near the crater of the Mt. Usu Volcano. On 4 April, smoke plumes soared to 3,900 feet and more fault lines were discovered. The possibility of deadly pyroclastic flows remains high. On 5 April, officials said that the dome of hardening lava is growing, and the next big eruption could occur within the next two weeks. Rain triggered a large-scale mud mud and debris flow down Mt. Usu's western slope. More faults are developing on the north and northwestern sides of Mt. Usu.
This information was summarized from Discovery Online.
On 28 March, Japan's National Coordination Committee of Volcanic Eruption Prediction announced the imminent eruption of the Mount Usu Volcano. The number of volcanic earthquakes on 29 March was 1,629. The hypocenter of volcanic earthquakes was located on the northwestern slope. Approximately 10,000 people living in the city and two surrounding towns were required to evacuate by the afternoon of 29 March. On 30 March, cracks as long as 100 m were visible on the northwestern part of the caldera rim. The intial eruption lasted approximately 2 hours and came from a new vent about 1.5 km to the northwest of the summit of Usu in a small valley. No large pyroclastic flows occurred.
This information was summarized from Smithsonian Institution's Preliminary Notices of Volcanic Activity.
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A winter view of the crater of Mt. Usu. Photograph courtesy of and copyrighted by Leslie Sonnenschein.
Usu volcano and nearby lava domes. Note location of the Toya caldera wall. Based on Kuno (1962).
Toya, a Pleistocene caldera, is associated with Usu. The caldera is
partially filled by a lake. Photograph courtesy of and copyrighted by Mike
Lyvers.
Kuno, H., 1962, Part XI, Japan, Taiwan and Marianas: Catalogue of the active volcanoes of the world including solfatara fields, International Association of Volcanology, Rome, Italy, 332 p.
Newhall, C.G., and Dzurisin, D., 1988, Historical unrest at large calderas of the world: U.S. Geological Survey Bulletin 1855, 1108 p.
Simkin, T., and Siebert, L., 1994, Volcanoes of the World: Geoscience Press, Tucson, Arizona, 349 p.
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