Hydrological and geochemical change related to volcanic activity of Usu volcano, Japan

Abstract Mt. Usu is a stratovolcano located on the southern side of the Toya caldera in Hokkaido, Japan. In historical times it emitted dacitic products associated to remarkable crustal deformations. Usu volcano erupted on March 31st 2000 after 23 yr of dormancy and this eruption was the fourth in the 20th century. Secular variations in water levels, temperature and chemical compositions of thermal water have been observed around Usu volcano. The water levels decreased since October 1999, 6 months prior to the eruption. The decrease in water levels is a precursory change and is probably due to leakage of thermal water from the shallow aquifer hosting it. No variations in temperature and chemical composition of thermal water were evident prior to the eruption. However, in a few wells, the temperature of the thermal water increased by up to 15 °C with respect to the pre-eruptive values. Moreover, the concentration of some solutes (Na, K, Cl and SO 4 ) became higher after the eruption. These thermal and chemical changes are ascribable to increased input of a deep hydrothermal liquid into the shallow groundwater aquifer.