Water discharge and recharge balance of Kuju volcano, Japan, deduced from thermal and gravity measurements

Repeat thermal and gravity measurements have been conducted at Kuju volcano, an erupting volcano in central Kyushu, Japan, in order to monitor the underground geothermal fluid flow system. A very high level of heat discharge rate, over 2000 MW, had been maintained after the first phreatic eruption which created some new craters in October, 1995. However after the second eruption in December, 1995, the heat discharge rate from the new craters decreased rapidly. Nevertheless, the heat discharge rate from all fumarolic areas, in which the new craters are included, has maintained its value of 600 to 800 MW since September, 1996 to date. Gravity decreased rapidly just after the first eruption, and then decreased gradually. After some assumptions, underground water mass balance can be estimated by using the mass decrease data based on the Gauss's theorem and the steam discharge data. Estimation of mass balance of underground water shows that the recharge of ground water from the region around the new craters and the fumarolic areas was increasing after the eruption, and then, the underground water flow is gradually approaching a new equilibrium state.