1996 Volume 72 Issue 6 Pages 124-128
Deformations around volcanoes have been detected by geodetic methods since the beginning of the 20th century, especially in Japan. Generally speaking, the deformations are usually expanding or contracting due to magmatic pressure and are simple compared with the crustal deformations accompanied by earthquakes. Co-eruptive deformations are interpretable by various elastic models which are defined mainly by types of the pressure sources, their depths and pressure increments. It is not always easy to discuss pre- and post-eruptive deformations because we need accumulation of observational results of precise geodetic surveys on and around volcanoes for a long term. The author found just appropriate examples in the 1914 and 1946 eruptions of Sakurajima volcano and in the 1983 eruption of Miyakejima volcano. By use of the data, the author shows that post-eruptive deformations of the two volcanoes are interpreted by rheological recovery of the medium beneath the volcanoes. On the assumption that the recovery stages can be approximated by the Kelvin models, the viscosities of the upper crust in these areas are estimated at 1019Pa•s in order of magnitude. The value is nearly the same as the viscosity of granite beams obtained through the laboratory experiments for more than 30 years by N. Kumagai and his group.
