Three-dimensional inversion of audio-magnetotelluric data acquired from the crater area of Mt. Tokachidake, Japan

Earth, Planets and Space

Ryo Tanaka¹  Takeshi Hashimoto¹  Yusuke Yamaya²  Ryo Takahashi³  Noritoshi Okazaki³  Makoto Tamura³  Toru Mogi⁴ 

[1] Institute of Seismology and Volcanology, Faculty of Science, Hokkaido University, N 10 W8, 060-0810, Sapporo, Japan;Renewable Energy Research Center, Fukushima Renewable Energy Research Institute, AIST, Machiikedai 2-2-9, 963-0298, Koriyama, Japan;Research Institute of Energy, Environment and Geology, Hokkaido Research Organization, N19 W12, Kita-Ku, 060-0819, Sapporo, Japan;Volcanic Fluid Research Center, Tokyo Institute of Technology, Ookayama 2‑12‑2, Meguro‑Ku, 152‑8551, Tokyo, Japan;

关键词: Mt. Tokachidake;    Resistivity structure;    Three-dimensional inversion;    Volcanic hydrothermal system;   

DOI  :  10.1186/s40623-021-01502-4

来源: Springer

【 摘 要 】

Subvolcanic hydrothermal systems can lead to hydrothermal eruptions as well as unrest phenomena without an eruptive event. Historical eruptions and recent unrest events, including ground inflation, demagnetization, and a gradual decrease in the plume height, at Mt. Tokachidake, central Hokkaido, Japan, are related to such a subvolcanic hydrothermal system. This study investigates the three-dimensional (3-D) resistivity structure of Mt. Tokachidake to image its subvolcanic hydrothermal system. A 3-D inversion of the magnetotelluric data, acquired at 22 sites around the crater area, was performed while accounting for the topography. Our resistivity model was characterized by a high-resistivity layer at a shallow depth (50–100 m) and two conductors near the active crater and dormant crater. The high-resistivity layer was interpreted to be composed of dense lava, which acts as a caprock surrounding the conductor. The high conductivity beneath the active crater can be explained by the presence of hydrothermal fluid in fractured or leached zones within the low-permeability lava layer, as the sources of ground inflation and demagnetization were identified within the conductive zone immediately beneath the resistive layer. The resistivity structure was used to estimate the volume of hydrothermal fluid within the pore space. The minimum volume of hydrothermal fluid beneath the active crater that can explain the resistivity structure was estimated to be 3 × 106 m3. This estimate is comparable to the water volume that was associated with the long runout and highly fluidized lahar in 1926. The resistivity structure and volume of hydrothermal fluid presented in this study can be used as a reference for further numerical simulations, which aim to reveal the mechanisms of recent unrest events and assess the risk of hazards, such as lahar.

【 授权许可】

CC BY