An ash ejection on 14 August at 1050 was the fifth since a series of explosions began on 16 July (14:07). Ash emission continued for about 90 minutes, sending a blackish-gray plume to ~1,000 m above the crater rim. Amplitude of continuous tremor during the eruption was about 0.6-0.7 µm (on a seismograph 0.8 km W of the crater), compared to an average amplitude of 0.3-0.4 µm. Intermittent ash ejection continued almost daily through August, with ashfall often observed around Crater 1. During the month, a total of 787 g/m² of ash accumulated at AWS about 1 km SW of the crater. Ash emitted 20-24 August reached villages ~40 km NW of the crater, the most distant ash deposition since 1979. On 4 September, the amplitude of continuous tremor decreased for a few minutes, followed by ash ejections at 0905 (1,000-m plume), 1300 (700 m), 1540 (2500 m) and 1725 (2,500 m). A 1-km area around the crater has been closed to tourists by the Aso Disaster Authority since 9 August.

Burning gas from the crater floor's 891 vent was observed almost daily during August. The flame was 30 m high on the night of the 23rd, the highest since burning gases were first seen in June, and a 15-m flame was seen the next day.

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.