Resistivity Structure of the Hiyoriyama Cryptodome at Kuttara Volcano, Hokkaido, Japan

Direct current (DC) electrical and controlled source audio-frequency magnetotelluric (CSAMT) surveys were performed over the Hiyoriyama Cryptodome in Kuttara Volcano, Hokkaido, Japan. Both surveys were performed on the same survey line across the cryptodome in a NE‒SW orientation. Two-dimensional joint inversion of the DC electrical and CSAMT data revealed the underground resistivity structure at depths less than 400m beneath the cryptodome. The resistivity structure suggests that the cryptodome comprises a dacite intrusion of 150m wide and 80m thick (20-50Ω・ m), and overlying pyroclastic deposits that are 10-30m thick (>100Ω・m). The dacite intrusion is underlain by a convex-shaped, low-resistivity layer (<5Ω・m) that is interpreted to be smectite-rich, altered pyroclastic deposits that have been subjected to low-temperature (<200°C) hydrothermal alteration. The low-resistivity layer is underlain by a slightly higher-resistivity layer (10-30Ω・m) that is interpreted to be altered pyroclastic deposits that were subjected to higher-temperature (> 200°C) hydrothermal alteration in a relatively deep, hot region near the conduit of the cryptodome.