JMA reported that no eruptions at Asosan had been observed since 16 June; only white plumes rose as high as 1 km above the crater afterwards. Sulfur dioxide emissions had been low since mid-June, and volcanic-tremor amplitude decreased to low levels on 18 July. During a field inspection on 17 August, observers noted no water in the crater and a whitish area at the center of the crater floor. On 18 August the Alert Level was lowered to 1 (on a scale of 1-5).

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.

Source: Japan Meteorological Agency (JMA)