雲仙火山の温泉とその地学的背景

抄録

Many hot springs are gushing from the inside of the Unzen Graben which traverses the central part of Unzen Volcano in east-west direction. Their geochemical characteristics are of salt type at the western foot, of sulfur type at the central mountain area and of bicarbonate type at the eastern foot of the volcano. They are hot in the west and warm in the east. A hydrothermal system at Unzen Volcano was presented by Ohta (1972) that the original magmatic emanations are supplying from a magma reservoir which lies beneath the Tachibana Bay assuming Chijiwa Caldera located at western side of volcano, and also that magmatic emanations have been ascending obliquely toward the east generating each type of thermal waters corresponding to the degree of differentiation. But this hydrothermal system was not accepted widely. Unzen Volcano began to erupt in November 1990 after 198 years dormancy. The forgoing earthquake swarm occurred beneath the Tachibana Bay in November 1989, and the foci migrated toward the summit of the volcano repeatedly for One year. They are arranged in semicircle at the southern part of Tachibana Bay and in a parallel two lines which run from the bay to the summit as if indicating the caldera rim and the conduit, respectively. Deformation of ground during the eruptions also showed that pressure sources were underlain beneath the inside of the parallel foci lines, and final pressure source was detected beneath the Tachibana Bay (Matsushima et al., 2003, 2005). So, the hydorothermal system presented by Ohta (1972) may be proved to be true because of these new facts. Based on these, modified supplying model of magmatic emanations were proposed here.