JMA reported that Alert Level 2 at Asosan continued during 1-8 September. A small eruption occurred on 1 September from Nakadake crater, generating an off-white plume that suggested a small amount of ash; the plume rose 1,200 m above the crater. Incandescence from the crater was detected with a camera on 2 September. Volcanic earthquakes (48-92 per day) and tremor (429-500 per day) was detected during 1-4 September.

On 6 September a small eruption occurred from Nakadake crater that generated a plume 600 m above the rim. Elevated SO2 (1,200 tons/day) was detected during a field survey (the previous measurement on 21 August was 1,000 tons/day). Volcano-tectonic earthquakes (55-129 per day) and tremor (401-463 per day) was detected during 5-7 September.

Tokyo VAAC issued advisories based on JMA reports of eruptions on 1 and 6 September, though no volcanic ash was visible in satellite images.

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.

Sources: Japan Meteorological Agency (JMA), Tokyo Volcanic Ash Advisory Center (VAAC)