Japan Meteorological Agency (JMA) reports for July 2003 noted ash-bearing eruptions from Aso (BGVN 28:10). The 10 or 11 July 2003 eruption was followed by seismically inferred phreatic eruptions a few days later, and a mud eruption on 14 January (the end of this report interval).

Seismic signals during 12-14 July implied there had been about five small phreatic eruptions. Continuous volcanic tremor started at ~ 1400 on 27 July. The JMA report of 28 July 2003 noted that seismometers had recorded ~ 100 isolated tremors. Earthquakes also occurred, at a rate of ~ 10 per day. On 28 July, lake water in Crater 1 was gray colored with a temperature of 76°C and with boiling regions in its central area.

A later JMA report noted a mud eruption in Crater 1 at 1541on 14 January, the first such eruption since July 2003. Associated tremor also occurred. Small amounts of very fine ash from this eruption were seen in Takamori, a town ~ 10 km ESE of the crater. The report noted that thermal activity had risen since last year, causing the water level in the crater to decrease about 40% below normal. The hazard status rose from 2 to 3, and accordingly, authorities restricted tourists from the area within 1 km of the crater.

Geological Summary. The 24-km-wide Asosan caldera was formed during four major explosive eruptions from 300,000 to 90,000 years ago. These produced voluminous pyroclastic flows that covered much of Kyushu. The last of these, the Aso-4 eruption, produced more than 600 km3 of airfall tephra and pyroclastic-flow deposits. A group of 17 central cones was constructed in the middle of the caldera, one of which, Nakadake, is one of Japan's most active volcanoes. It was the location of Japan's first documented historical eruption in 553 CE. The Nakadake complex has remained active throughout the Holocene. Several other cones have been active during the Holocene, including the Kometsuka scoria cone as recently as about 210 CE. Historical eruptions have largely consisted of basaltic to basaltic-andesite ash emission with periodic strombolian and phreatomagmatic activity. The summit crater of Nakadake is accessible by toll road and cable car, and is one of Kyushu's most popular tourist destinations.

Information Contacts: Volcanological Division, Japan Meteorological Agency (JMA), 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100, Japan (URL: http://www.jma.go.jp/); Fukuoka District Meteorological Observatory, Japan Meteorological Agency (JMA), 1-2-36 Oohori, Chuo-ku, Fukuoka 810-0052, Japan.