Ballistic ejecta and eruption condition of the vulcanian explosion of Shinmoedake volcano, Kyushu, Japan on 1 February, 2011

The physical condition of the 1 February, 2011, vulcanian explosion at Shinmoedake volcano, Japan, is estimated based on the size of impact craters created by ballistic ejecta, using a ballistic trajectory model and a scaling law for impact crater formation. The initial velocity, impact velocity and mass of ejecta were estimated at 240-290 m/s, 140 ± 20 m/s and 1-3 ton, respectively. The gas mass fraction at the source was calculated to be 0.04-0.1, using the initial velocity and a theoretical model of vulcanian explosion. This gas mass fraction is higher than the petrologically estimated value for pre-eruptive magma. Low-angle jets from the explosion and the estimated depth and size of a pressurized gas region suggest a shallow source inside the lava dome. The observation and results imply that segregation and accumulation of gas in a shallow conduit played a role in an increase of excess pressure immediately below the dome surface, prior to the vulcanian explosion.