The amplitude of volcanic tremor signals at Asosan increased in December 2022, and then further intensified on 30 January, prompting JMA to raise the Alert Level to 2 (on a scale of 1-5) and warn the public to stay at least 1 km away from the crater. The amplitude fluctuated at high levels for a few weeks, and then decreased on 19 February and again on 14 March. Daily sulfur dioxide emissions had exceeded 1,000 tons per day starting in December 2022; emissions declined to below that threshold by mid-January and remained at lower levels. At 1100 on 23 March the Alert Level was lowered to 1.

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)