We carried out magnetotelluric (MT) soundings across Usu volcano, Hokkaido, Japan, which has been active since 1663. The objective was to image an intrusion expected below the formation of 180m-high cryptodomes formed during the 1977-78 eruption.
A NE-SW two-dimensional resistivity model has been obtained from the inversion in the frequency range from 0.01 to 10,000 Hz. The major features of the resistivity section are as follows: (1) a 50-m-thick surficial resistive (1000-10,000 Omega m) layer corresponding to the lava layer; (2) a relatively conductive (10-1000 Omega m) layer from the surface to 200 m depth, corresponding to Quaternary sedimentary and volcanic rocks; (3) beneath the Quaternary formations, a thick (2-4 km) conductive (1-500 Omega m) layer of altered Tertiary volcanics distributed widely in the volcanic edifice; (4) a resistive block (500-1,000 Omega m) in the Tertiary layer from 200 to 400 m in depth beneath the summit crater, forming a striking contrast to the rest of the conductive Tertiary layer and probably indicating a cooling intrusion at shallow depth; (5) pre-Tertiary formations consisting of a high resistivity (1000-10,000 Omega m) layer overlying a conductive layer, with the top of the Pre-Tertiary becoming shallow to the northeast; and (6) no deep conductive anomalies corresponding to a magma chamber inferred from petrological studies. (C) 2001 Elsevier Science Ltd All rights reserved.