During 12-14 July, JMA recorded seismic signals at Aso that were associated with five small phreatic eruptions. Around 1400 on 27 July continuous volcanic tremor started. Such activity had not been recorded at Aso since November 1995. As of 28 July ~10 earthquakes occurred per day, and around 100 isolated tremor events had occurred since 23 July. On the 28th the crater lake in Crater 1 was gray, 76 °C, and bubbling in the center.

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)