桜島火山近傍の垂直地盤変動と山頂噴火活動の関係について

Abstract

The data of levelling surveys, tidal observations and distance measurements are analyzed toexamine the surface deformation associated with the eruptive activity which has begun in October1972 at the summit crater of Sakurajima Volcano.The results obtained are as follows;(1) Surface deformation in the vicinity of Sakurajima Volcano shows the characteristicpatterns closely related to the process of the eruptive activity at the summit crater. (I) In thequiet period, the vertical displacement of uplift becomes maximum at the center of Aira Caldera, on the southern rim of which Sakurajima Volcano is situated. (II) Upheaval movement of theground of Sakurajima Volcano had begun early in 1971, preceding the beginning of the eruptiveactivity at the summit crater in October 1972, and continued till 1974 when the frequency ofexplosive eruptions becomes maximum, about 400 times per year. (III) After 1974, the eruptiveactivity is gradually decayed, and the ground of the whole caldera stops its upheaval movementand is partly depressed.(2) Distribution of vertical displacements around Sakurajima Volcano in the period de-scribed in (III), can be explained by assuming the change of hydrostatic pressure within a sphericalsource at the depth of about 3 km beneath the summit crater. This result, which is derived from thedata of levelling surveys, is compatible with the results of distance measurements.