Activity at [Crater 1] has been moderate since an explosion on 20 February 1993 ejected scoriae 100 m above the vent. During the daily rim visit on 2 May 1994, mud ejection was observed for the first time since 10 June 1993. However, the crater floor has been covered by water and frequent water ejections have been observed. Continuous tremor was registered at a seismic station 800 m W of the crater. Average amplitude of continuous tremor had been 0.2 µm through May, but on 7-9 June the average amplitude suddenly increased to >6 µm.

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.