Abstract
Studies on the supply system of magma and its behavior based on seismic and geodetic observations are reviewed for recent volcanic activity of the Sakurajima volcano, where three characteristic types of eruptions occurred in the 20th century: the 1914 gigantic flank eruption; the 1946 minor flank eruption; and frequent vulcanian eruptions at the Minamidake summit crater, continuing from 1955 for more than 50 years. The magma plumbing system of Sakurajima is approximated by a deep (～10km) magma reservoir beneath the Aira caldera, a shallow reservoir (3-6km) beneath the central cones, and a conduit connecting the shallow reservoir to the summit crater. From the inflation-deflation pattern and intensity of the vertical displacement, it is estimated that magma has been supplied to the deep reservoir at a rate of 107 m3/year. The shallow reservoir is inferred from vertical displacements, a tilt vector showing the crater side up before the vulcanian eruptions, and an anomalously attenuated zone of seismic waves. The magma migrated from the deep reservoir to the shallow reservoir before the increase in vulcanian eruptivity at the Minamidake crater, as demonstrated by the relocation of the upheaval center of the ground deformation. The increase in seismicity of A-type earthquakes and the migration of their hypocenters toward the shallow part also support the migration of magma. The hypocenters of B-type earthquakes and explosion earthquakes with volumetric sources are concentrated beneath the crater. The hypocenters of A-type earthquakes generated by shear fractures surround the origins of B-type earthquakes and explosion earthquakes. The separation of the hypocenters indicates the existence of a volcanic conduit connecting the shallow reservoir to the bottom of the summit crater. The intrusion of magma into the conduit and smooth ascent up to the crater bottom induce strombolian eruptions and swarms of B-type earthquakes. The magma at the uppermost part of the conduit becomes a cap rock of the conduit against the following intrusive magma, which generates vulcanian eruptions. Upward tilt of the crater side and extensional strain are observed prior to both strombolian activities and the vulcanian activities that follow and turn into downward and contraction strain, respectively, associated with the eruptions. The volatility of the magma has an important role in vulcanian eruptions, in the formation of a gas pocket at the uppermost part of the conduit, and in sudden outgassing triggered by a pressure decrease in the conduit due to gas leakage.
