Magma prospecting in Usu volcano, Hokkaido, Japan, using magnetotelluric soundings

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. W 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 Ω m) layer corresponding to the lava layer; (2) a relatively conductive (10–1000 Ω 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 Ω m) layer of altered Tertiary volcanics distributed widely in the volcanic edifice; (4) a resistive block (500–1,000 Ω 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 Ω 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.