The relationships between eruptive activities and changes in groundwater level, temperature, and chemical composition of thermal water at Toyako Onsen are discussed in relation to the 1977-1978 and 2000 eruptions of Usu volcano, Hokkaido, Japan. The temperature and chemical composition of thermal water observed since the 1977-1978 eruption are found to have changed after the eruptions. The groundwater levels in the hot spring wells, observed before the 2000 eruption, are found to have changed approximately six months before the eruption. Changes of groundwater level, which reflect pore water pressure, are generally faster than those of the temperature and chemical composition of thermal water attributed to fluid flow. However, thermal water flow during the 2000 eruption of Usu volcano was probably triggered as fast as propagation. Because the thermal water at Toyako Onsen is affected by the eruptive activities of Usu volcano, understanding the relationship between volcanic activity of Usu volcano and changes in temperature, chemical composition and groundwater levels of the thermal water is useful for inferring volcanic eruption and activity of Usu volcano.
