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Earth Planets Space, Vol. 65 (No. 6), pp. 609-621, 2013
doi:10.5047/eps.2013.03.004

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

Fukashi Maeno1, Setsuya Nakada1, Masashi Nagai2, and Tomofumi Kozono2

1Earthquake Research Institute, The University of Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan
2National Research Institute for Earth Science and Disaster Prevention, 3-1, Tennodai, Tsukuba, Ibaraki 305-0006, Japan

(Received October 30, 2012; Revised February 21, 2013; Accepted March 9, 2013; Online published July 8, 2013)

Abstract: 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.
Key words: Ballistic ejecta, impact crater, lava dome, vulcanian, Shinmoedake.


Corresponding author E-mail: fmaeno@eri.u-tokyo.ac.jp


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