Hydrothermal System Beneath Usu Volcano Inferred from Self-potential Observation and Numerical Simulation

Abstract

Usu volcano is located at southwestern part of Hokkaido, which is one of the most active volcanoes in Japan. We conducted self-potential (SP) surveys on the volcano, since July to December 2006. The compiled SP map reveals positive anomalies around Gin-numa crater and northwestern part of Ko-Usu, and a negative anomaly on the top of O-Usu lava dome on the summit caldera. The locations of the positive anomalies are corresponding to the fumarolric area, which are likely to be affected by hydrothermal system beneath the volcano. To confirm the consideration, we conducted numerical simulations of groundwater flow and SP of the electrokinetic origin within the volcano. The simulation results indicated the existences of, (a) hydrothermal upwelling driven by the intruded conduit beneath Usu-shinzan as a heat source, (b) a caprock layer at the upper margin of the hydrothermal and (c) low-resistivity channels from the upper margin of the hydrothermal area to the surface.