Drastic Change in the Thermal State beneath the Active Fumarolic Field of Kuju Volcano, Japan after the 1995 Phreatic Eruption, Deduced from Repeat Resistivity Measurement

Abstract

Kuju Volcano in central Kyushu, Japan has an active fumarolic field in the central part. Resistivity measurement was applied in order to detect the steam reservoir beneath the fumarolic field. As the result, an extremely resistive zone higher than 1000Ωm was detected at a depth of 10 to 50 m. A short directional hole (27 m long and 18m deep below the surface) was dug toward the resistive zone in May 1991. We obtained superheated steam from the margin of the resistive zone and then used it for material testing. The temperature of the steam was 233°C at the well head and was estimated to be 278°C at the bottom of the hole. In November 2001, we planned to get much hotter steam in order to conduct another experiment. Then a longer directional hole (47 m long and 30 m deep below the surface) was dug toward the center of the above-mentioned resistive zone. However, we obtained only saturated steam and the temperature at the well head was about 98°C. The saturated steam was discharged for about one month but stopped discharging because of deposition of sulfur inside the pipe. Repeat resistivity measurement was conducted in order to clarify the change in the thermal state in September 2002. The resistivity model showed that the resistivity of the above mentioned resistive zone decreased to several tens Ω m, that is, the resistive zone has disappeared. Such a drastic change would be explained if we assume that the superheated steam reservoir turned to be the liquid dominated reservoir, that is, the change in resistivity means cooling of the reservoir. The cold meteoric groundwater around the fumarolic field may be supplied to the steam reservoir. Phreatic eruptions occurred at about 300 m south of the active fumarolic field in October and December 1995. Geophysical monitoring after the eruption shows quick cooling of the central part of the volcano and also indicates a large amount of meteoric water supply to the central part of the volcano. The change in resistivity observed in this study may be one of the phenomena associated with the 1995 Phreatic eruptions.