Frequent ash ejections resumed on 24 September and continued through late November. During October, no blocks were seen to reach the rim of Naka-dake Crater nor were incandescent blocks observed at night. By the end of October, the concentration of ash at the JMA's [Asosan Weather Station] (1 km from the active vent) had reached more than 10 kg/m², equal to about 1 cm of ash thickness. Although continuous tremor amplitude had correlated well with June-September eruptive activity, amplitudes remained low (about 0.5 µm) during October. The number of local earthquakes also remained low in October.

A characteristic decrease in the amplitude of continuous tremor began at about 0900 on 2 November, lasting until a large explosion at 1626. An eruption cloud rose 1.5 km above the crater during about an hour of ash ejection. Four mm of ash fell at the [Weather Station]. A survey by [Weather Station] personnel two days later found scoria up to 200 m from the vent, overlying 0.6 m of ash that had fallen in the summit area since the eruption began 12 June. The tremor amplitude decrease was the third since June that had preceded a sizeable explosion. An alert was issued from the Observatory one and a half hours before the explosion. No casualties occurred.

Ash emission in November was stronger than in October, causing heavy ashfalls near the volcano. Slight ashfalls occurred occasionally at Mt. Takachiho (110 km S), Kumamoto city (40 km W), and in Oita Prefecture (50 km E). Ejection of incandescent blocks was observed at night on 11 and 19 November, for the first time since 6 August. Tremor amplitude increased through most of November, but declined late in the month. The Strombolian activity of June, July, and early August occurred while tremor amplitude was high.

Further Reference. Tanaka, Y., Tsuchiya, Y., and Yamaura, Y., 1981, Detection of volcanic smoke and ashfall area at Aso from Landsat data: Papers in Meteorology & Geophysics, v. 32, no. 4, p 275-291.

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: JMA.