Thermal activity within the western-slope geothermal zone of Aso volcano, Japan: Development of a new thermal area

Since June of 2006, vigorous new fumaroles, accompanied by hydrothermal eruptions, have formed at the Yoshioka hot spring of Aso volcano in Japan. Prior to this date, the hot spring was a minor thermal area, emitting only 0.2 × 106 J/s of hot water. The steam flux increased significantly to 15–30 × 106 J/s in October 2006. After November 2006, the new fumaroles have maintained a largely constant heat discharge rate of approximately 4.6 × 106 J/s (170 tonne/day), which is almost 20 times higher than that of the previous natural thermal activity. Mudslides, earthquakes, and human-related factors did not cause the geothermal events observed at Yoshioka. On the basis of energy estimations, we infer that the hydrothermal eruptions and increased flow have resulted from an increase in the volcanic fluid flux from a deep aquifer to the steam-dominated reservoir beneath Yoshioka.