2013 Volume 58 Issue 1 Pages 77-90
The 1914 eruption of Sakurajima volcano was one of the largest ones in its history and first observed with scientific methods of those days. The eruption flowed out a large volume of lavas, and was accompanied with remarkable deformations on and around Aira caldera. The deformations consist of three stages: pre-eruptive, co-eruptive and post-eruptive ones: The first one is determined by temporal tide-gauge observations at Kagoshima Port but, its origin is still unsettled. The second one is a quasi instant and elastic deformation, and therefore interpretable by a simple model. However, the location of center of depression is ambiguous due to the insufficient number of observations. Prior to the interpretation, it is re-examined considering the results of triangulations in the wider area, and the location changed about 5 km S, on Sakurajima Island. The revised location of depression center can well interpret the observed depression and compromises with seismological and petrological knowledge. The last one is a secular recovery stage, and it has remained highly problematic whether its origin is endogenous or exogenous. The present author interprets that the recovery changes are composed of two factors: In the period before around 1940, viscoelasticity of the crust acted predominantly (endogenous) and later magma pressure accumulated in the reservoir gradually joined to act (exogenous) and its contribution had become noticeable after 1955. The recovery of the depression is interpreted as viscoelastic with retardation time of 16.6 years. Hence, viscosity of the earth crust around Aira caldera is determined at about 3 × 1019 Pa・s on the assumption that the crust is Kelvin-type. The value is comparable to those obtained with Miyake volcano and by laboratory measurement of a granite piece. The other cases of viscoelastic deformations accompanied with volcanic eruptions are searched: the 1779 eruption of Sakurajima volcano and the 1983 eruption of Miyake volcano are exemplified.
