地球物理学的観測による桜島火山のマグマ溜りおよび火道の推定

Abstract

Recent geophysical studies and available data confirmed the existence of an active magmaconduit and a live reservoir beneath Sakurajima Volcano during its recent eruptive period.Analysis of levelling data indicated that a pressure source which induced the ground de-formation of the volcano had been located in the range of depth from 2 to 5 km beneath the sum-mit of the volcano. Seismological studies revealed that seismic shear waves were stronglyattenuated in a zone which was located between 3 km and 6 km in depth with a radius of about 1km beneath the three central cones of the volcano. In addition, no earthquakes have occurred ata deeper part than 4 km beneath the central cones during the period from 1975 to 1986, and volca-nic earthquakes with deeper focuses originated out of this region. Thus, the pressure source, thezone of anomalous wave propagation and the aseismic zone occupy almost the same location atthe volcano, especially in the range of depth between 4 km and 5 km beneath the summit. Thesegeodetic and seimological studies suggest that a live magma reservoir is located at a deeper partthan 3-4 km below sea level beneath the summit of the volcano.The spatial distributions of explosion-quakes and A-type volcanic earthquakes illustrated thelocation of the active magma conduit and its size. These two events are quite different from eachother in their focal mechanims and in the relationship between volcanic explosions, and a recentstudy suggests that explosion-quakes originate in a magma conduit. The hypocenters of explo-sion-quakes located in a narrow zone with a lateral extent of 0.3 km X 0.6 km from sea level to 2km beneath the active summit crater. The direction of the longer axis coincided that of line con-necting two active vents at the summit crater. The hypocenters of A-type volcanic earthquakessurrounded those of explosion-quakes in the range of depth from 0 to 2 km, and encircled anaseismic zone at the deeper part than 2 km. Assuming that the boundary between the hypocen-ters of A-type volcanic earthquakes and those of explosion-quakes coincides with the wall of mag-ma conduits, it is inferred that there exist two magma conduits with the diameter of 200-300 mbeneath the active crater and the two conduits are contacted with each other. Furthemore, theanalysis of tilt and strain records indicated that pressure changes occurred for individual eruptionsat the range of depth from 2 to 6 km beneath the summit. These results suggests that the mag-ma conduits are connected with presumed magma reservoir at a depth of 3 to 4 km.