Observation of Temperature of a Thermal Spring at NIGORIGO-ONSEN, GIFU Prefecture

Abstract

Mt. Ontake, a dormant volcano in central Japan since the dawn of the Japanese history erupted on 28th October, 1979. We set up an IC thermometer in a thermal spring well about 80m deep at Nigorigo-Onsen, 5km north-west from the summit of Mt. Ontake, 9 days after its eruption and continued the temperature measurements for a year. We found that the temperature at the well decreased exponentially with time and dropped about 1.5 deg. by the end of October, 1980. The temperature decrease must have been somehow related to the eruption of Mt. Ontake which may have affected the temperature distribution near the well source and path of the ground water. Besides the secular temperature decrease, the short period temperature variation of the well is characterized by intermittent pulse-like variations about 1 hour duration and about one-week period variations. The latter variation is well correlated with atmospheric pressure variation. These variations are reasonably well explained by a kind of geyser model with a pressure sensitiv buffer cavity.