ABSTRACT

Significant changes in temperature and water chemistry were observed in Lake Yugama, a crater lake at Kusatsu-Shirane volcano, Japan, between 1988 and 1993. Heat, water and chloride budgets were evaluated with a box model. Until 1989, heat and chloride inputs into the lake were 3∼10 MW and 1∼3.5 tons/day, respectively; but these increased to 19–25 MW and 4–12 tons/day by 1990. The strong correlation between the Cl^– concentration and lake water pH suggests input of a HCl solution or HCl vapor as the cause of the Cl^– change. These changes coupled with the high level of seismicity observed around the volcano in 1990 seem to be consistent with the formation of fractures in the surrounding area of a cooling magma.