雲仙火山の溶岩の噴出率と地盤の変動速度から推定されるマグマの供給率について

Abstract

The current eruptive activity at the Fugendake, one of the central cones of Mt. Unzen started on 17 November in 1990, after the 198 years of dormant period.Dacitic lava has been extruded at a rate of 10 5m3/day from one of the summit craters since late-May of 1991.The cumulative volume was 0.12 km3 as of November, 1992.Geodetic survey detected gradual inflation prior to the extrusion of lava and subsequent deflation after the onset of lava extrusion around the western foot of the volcano.Tha analysis of the ground-deformation data suggested that a magma storage is located several kilometers deep, a few kilometers west of the eruption center.Quantitative relationship between integrated deformation volume of the ground surface and the amount of discharged lava suggests continuous supply of magma from a lower-crustal magma reservoir into the shallow storage.The supply rate during th past two years was estimated to be (1-2) x l0 7 m3/year.Some andesitic or dacitic volcanoes which have erupted over a few decades commonly indicate a similar discharge rate of magma (and its supply rate) as the rates at Mt. Unzen, and the estimated supply rate of magma also satisfies the condition of constant magma ascent deduced theoretically by Fedotov.Therefore, the volcanic activity at Mt. Unzen may presumably persist during a few decades.