The seismic network of the Japan Meteorological Agency (JMA) registered periods of highly increased amplitude tremor at the volcano starting at noon yesterday.
At about 12:20 PM local time, a one-minute seismic signal with an amplitude of 2.5 micrometers has been detected in a N-S pattern located on the western flank of the volcano. Later on, after 03:00 PM local time, the 2.5 micrometers amplitude was being exceeded to higher values.
A gas and steam plume, rising about 300 meters above the crater, contained 800 tonnes of sulfur dioxide (SO2) emissions monitored on 19 Jan.
Given the current evolving state of the volcano, explosive eruption is not ruled out and may occur suddenly.
The warning bulletin states that ballistic impacts of volcanic bombs and pyroclastic flows could affect an area of about 1 km distance from the Nakadake crater including Minamiaso Village and Aso City in Kumamoto prefecture.
Therefore, the JMA made a decision to raise the Volcanic Alert Level to Level 2.
Source: Japan Meteorological Agency volcano activity update 31 January 2023