A water flow model of the active crater lake at Aso volcano, Japan: fluctuations of magmatic gas and groundwater fluxes from the underlying hydrothermal system

The first crater of Nakadake, peak of Aso volcano, Japan, contains a hot water lake that shows interesting variations in water level and temperature. These variations were discovered by precise, continuous observations of the lake independent of precipitation. We developed a numerical model of a hot crater lake and compared with observational data for the period from July 2006 to January 2009. The numerical model revealed seasonal changes in mass flux (75-132 kg/s) and enthalpy (1,840-3,030 kJ/kg) for the fluid supplied to the lake. The relation between the enthalpy and mass flux indicates that the bottom input fluid is a mixture of high- and low-temperature fluids. Assuming a mixture of high-temperature steam at 800A degrees C and liquid water at 100A degrees C, we evaluated the liquid and steam fluxes. The liquid water flux shows a seasonal increase lagging behind the rainy season by 2 months, suggesting that the liquid water is predominantly groundwater. The fluctuation pattern in the flux of the high-temperature steam shows a relation with the amplitude of volcanic tremor, suggesting that heating of the hydrothermal system drives the tremor. Consequently, precise observations of a hot crater lake represent a potential method of monitoring volcanic hydrothermal systems in the shallow parts of the volcanoes.