Open Access

A resistivity cross-section of Usu volcano, Hokkaido, Japan, by audiomagnetotelluric soundings

  • Y. Ogawa1Email author,
  • N. Matsushima1,
  • H. Oshima2,
  • S. Takakura1,
  • M. Utsugi2,
  • K. Hirano2,
  • M. Igarashi2 and
  • T. Doi2
Earth, Planets and Space201450:BF03352120

https://doi.org/10.1186/BF03352120

Received: 11 April 1997

Accepted: 17 March 1998

Published: 6 June 2014

Abstract

We collected audio-magnetotelluric (AMT) data across Usu volcano, Hokkaido, Japan, which erupted in 1977 and is still active. We had a profile of 17 sites perpendicular to the regional tectonic strike, which crossed the 1977 cryptodome, Usu-Shinzan. Tensor-decomposed data were interpreted 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 rocks. In the crater, there are two resistive bodies bisected by a vertical conductor, which corresponds to Usu-Shinzan fault, located at the foot of the uplift. The vertical conductor was not detected in the AMT sounding in 1985. One of the possible causes of the development of the 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 the 1977 uplift.