STUDY ON HYDROTHERMAL SYSTEM DEVELOPMENT, UNZEN VOLCANO, JAPAN. A TOPIC IN PHASE I OF THE UNZEN SCIENTIFIC DRILLING PROJECT

摘要

Some geothermal investigations on Unzen Volcano have been conducted since 1999 as part of the Unzen Scientific Drilling Project. The heat discharge rates from the lava dome between 1999 to 2001, obtained by infrared imagery observations, show a trend of decreasing discharge. The 1 m-depth temperatures near the summit of Mt. Fugen are lower than an extrapolated line of the temperature-altitude relationships from the stations on the flank of the volcano. The temperatures near the summit of Mt. Fugen show seasonal changes, but those near the fumaroles on the lava dome maintain their values. Some Gamma-ray intensity anomalies of ~(214)Bi and ~(208)T1 were detected at the stations near the estimated location of the conduit. The result of repeat gravity measurements shows that the gravity values near the lava dome decreased while those of the western part of Mt. Fugen increased between 1999 to 2000. Remote observations of volcanic gases by an FTIR spectrometer detected CO and CO_2, and an equilibrium temperature of about 800℃ was estimated from the CO/CO_2 ratio. The result of a simulation, using a 3-D numerical model, showed that the downflow of permeating rainwater is dominant and the upflow of gas is limited in the conduit. This result means that there is little possibility of extensive hydrothermal activity near the lava dome.