2015 年 60 巻 3 号 p. 365-380
Akita-Komagatake volcano straddles the boundary between Akita- and Iwate-prefectures in Japan. Medake, one of its central cones, erupted in 1970-71. The condition of this volcano was monitored by geophysical observations from a few years after the end of the eruption to 2013. Geo-temperature, total magnetic intensity and gravity were repeatedly measured at fixed stations distributed mainly in and around Medake. As the temperature of the 1970-crater decreased after the end of the eruption, the geothermal activity was intensified in the surrounding areas. This geothermal activity was at its peak around the 1977-79 period and decreased gradually to about 1995, and then the geothermal zones almost diminished to a very small area. The activity, however, re-started to intensify in about 2006, and has been extending to the eastern side of Medake to date (2014). The maximum temperature at 1 m depth in the thermal zones is about 95℃, which corresponds to the boiling point of water at the elevation of the site (about 1500 m). Changes of the total magnetic intensity and the gravity were conformable with that of the thermal activity. The gravity increased and decreased with diminishing and intensifying of the geothermal activity, respectively. Model analyses suggest that these variations are due to the change of underground temperature nearly below the boiling point. The change of the magnetic field is considered to be caused by the variation of remnant magnetization, while thermal expansion affects gravity. The related behavior of pore water may also enhance the variation of gravity in some degree.