Change in the Shallow Resistivity Structure After 1995 Eruption at Kuju Volcano, Japan

Abstract

We have been conducting several geophysical observations at Kuju volcano, in the central part of Kyushu Island, Japan, since the 1980's. After the phreatic eruption in 1995, we carried out gravity measurements and magnetic surveys to monitor the internal thermal state of the volcanic body. We also drilled two boreholes in 1991 and 2001 to obtain volcanic steam. Within ten years, the temperature of the shallow steam reservoir decreased more than 130 degrees. Prior to the drilling in 1991, we carried out an electrical prospect to determine the drilling point. In 2001, another electrical survey was carried out to investigate the cause of this rapid temperature change. Two inverted resistivity models revealed that the superheated reservoir was cooled and liquefied. From the combined interpretation of the geophysical observations and the repeat electrical survey, it was concluded that the cold meteoric water is cooling the whole volcanic body.