Report on Asosan (Japan) — June 1981
Scientific Event Alert Network Bulletin, vol. 6, no. 6 (June 1981)
Managing Editor: Lindsay McClelland.
Asosan (Japan) 30-minute ash and block ejection
Please cite this report as:
Global Volcanism Program, 1981. Report on Asosan (Japan). In: McClelland, L (ed.), Scientific Event Alert Network Bulletin, 6:6. Smithsonian Institution. https://dx.doi.org/10.5479/si.GVP.SEAN198106-282110.
32.884°N, 131.104°E; summit elev. 1592 m
All times are local (unless otherwise noted)
Ash and block ejection from Crater 1 of Naka-dake, the northernmost of 7 in Naka-dake, was observed at 1230-1300 on 15 June, after 9 months of quiescence. Blocks rose to 30 m, but fell within the 100 m-diameter crater. 1 µm ground shocks were recorded at 1239 and 1244, and a 3.7 µm shock at 1251 [at the Weather Station]. Activity then subsided. The explosions caused no damage. The area within 1 km of the summit, closed immediately after activity began, was reopened 17 June. Asosan Weather Station personnel [visited the crater on 15 June and] observed that the greenish water pooled in Crater 1 since October had become gray tinted [but returned to its usual green the next day]. The [level of some points on the surface of the water] rose intermittently. Naka-dake is the historically active part of the Aso volcanic complex.
Geologic Background. 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 cu km 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 AD. 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: JMA.