ABSTRACT
The Atotsugawa fault system (AFS), right-lateral sense and 60 km long, is one of the most active faults in Japan. A 480 m-long observation tunnel was excavated across the Mozumi-Sukenobu fault, a subsidiary fault of AFS. Different kinds of observations have been made inside and outside the tunnel: high-precision continuous monitoring of seismic velocity, electromagnetic resistivity and strain measurements, ground water level changes, geochemical surveys of ground water, geological examinations of fault zone materials, GPS, seismic and geological surveys. The new installation of high-gain short-period seismic stations was significantly improved the hypocenter accuracy in and around the Atotsugawa fault, revealing a clear seismic gap, which corresponds to the creep detected from geodetic surveys. This comprehensive study of an active fault reveals a variety of fault widths, 500 m by geochemical analysis to 10–20 m by structural geological survey, which gives a clue to understand physical and chemical processes of the fault.
