Fluid flow processes in the BEPPU geothermal system, Japan

Abstract

The Beppu geothermal system is centred beneath the late Quaternary volcanoes of Tsurumi and Garandake at the northern end of the Ryukyu volcanic arc. The deep fluid has a temperature of at least 250–300°C, and an inferred chloride concentration of 1400–1600 mg/kg. Apart from fumarolic areas near the summits of the two volcanoes, most thermal activity occurs at low elevation along the two main outflow paths towards the coast. The hot spring waters of downtown Beppu have originated from outflow along the Asamigawa Fault, with their chemistry indicating predominantly dilution of the deep fluid by groundwater. The second outflow zone towards the hot spring area of downtown Kamegawa coincides with a ridge of lavas. Here boiling, steam loss, and subsequent mixing with steam-heated groundwaters have significantly modified both the deep fluid and host rocks. The area of the geothermal system above 200°C is at least 15 km 2 at sea level, and the total natural heat output is inferred to be at least 250 MW. Most of this heat output occurs as subsurface hot water outflows towards the coast due to the 1300 m of topographic relief across the system.