A RESISTIVITY CROSS-SECTION OF USU VOLCANO, HOKKAIDO, JAPAN, BY AUDIOMAGNETOTELLURIC SOUNDINGS

We collected audio-magnetotelluric (AMT) data across Usu volcano, Hokk aido, Japan, which erupted in 1977 and is still active. We had a profi le of 17 sites perpendicular to the regional tectonic strike, which cr ossed the 1977 cryptodome, Usu-Shinzan. Tenser-decomposed data were in terpreted by a two-dimensional inversion. Outside the crater rim, the resistivity structure is simple. The resistive somma lava is underlain by a conductive substratum, implying altered Tertiary or Quaternary r ocks. In the crater, there are two resistive bodies bisected by a vert ical conductor, which corresponds to Usu-Shinzan fault, located at the foot of the uplift. The vertical conductor was not detected in the AM T sounding in 1985. One of the possible causes of the development of t he vertical conductor is a cold water supply from the surface into the vapor dominant fracture zone. One of the resistive bodies is located beneath Usu-Shinzan and implies an intrusive magma body which caused t he 1977 uplift.