Dynamics of Volcanic Explosion Revealed by Geophysical Observations at Sakurajima Volcano

Abstract

Mechanical process of explosive eruptions revealed by seismic and ground deformation observations are reviewed at mainly Sakurajima and some volcanoes. From macroscopic view point, mechanical process of explosive eruption is approximated by expansive process due to intrusion and storage of magma in the volcanic body prior to the beginning of the explosion, and implosive process by ejection of magma and downward single force as a counter force against ejecting product after instantaneous removal of lid of the pressurized magma chamber. More detailed process of explosions has been estimated at Sakurajima volcano. Prior to the explosions, a pressurized gas pocket is formed under the lava dome as a lid of it. Increase in internal pressure in the expansive process of the conduit start several minutes to several hours before the explosions, and it may turn decrease in pressure by weak leakage of volcanic gas immediately before the explosions, as inferred from displacement seismograms of explosion earthquakes and visual observation at Suwanosejima volcano. The decrease in pressure induces sudden bubbling at a depth of 2km causing compression first motions of explosion earthquake. Contraction of volcanic conduit follows the expansion. Expansion of the gas pocket at the uppermost part of the conduit occurs ～1 s after the first expansion, destroying the lid of the pressurized gas pocket in the conduit. After removing the lid, sudden pressure and volume decrease occur at the top of the conduit, and the deflation of magma reservoir occurs at deeper part by ejecting magma.