Hydrothermal and Magmatic System of a Volcanic Island Inferred From Magnetotellurics, Seismicity, Self-potential, and Thermal Image: An Example of Miyakejima (Japan)

Marceau Gresse , Makoto Uyeshima , Takao Koyama , Hideaki Hase , Koki Aizawa , Yusuke Yamaya , Yuichi Morita , Derek Weller , Tawat Rung-Arunwan , Takayuki Kaneko , Yoichi Sasai , Jacques Zlotnicki (1) , Tsuneo Ishido , Hideki Ueda , Maki Hata

1 OPGC - Observatoire de Physique du Globe de Clermont-Ferrand

Phreatic and phreatomagmatic eruptions represent some of the greatest hazards occurring on volcanoes. They result from complex interactions at a depth between rock, water, and magmatic fluids. Understanding and assessing such processes remain a challenging task, notably because a large-scale characterization of volcanic edifices is often lacking. Here we focused on Miyakejima Island, an inhabited 8-km-wide stratovolcano with regular phreatomagmatic activity. We imaged its plumbing system through a combination of four geophysical techniques: magnetotellurics, seismicity, self-potential, and thermal image. We thus propose the first comprehensive interpretation of the volcanic island in terms of rock properties, temperature, fluid content, and fluid flow. We identify a shallow aquifer lying above a clay cap (<1 km depth) and reveal its relation with magmatic-tectonic features and past eruptive activity. At greater depths (2-4.5 km), we infer a seismogenic resistive region interpreted as a magmatic gas-rich reservoir (≥370°C). From this reservoir, gases rise through a fractured conduit before being released in the fumarolic area at ∼180°C. During their ascent, these hot fluids cross a ∼1.2-km-long liquid-dominated zone causing local steam explosions. Such magmatic-hydrothermal interaction elucidates (i) the origin of the long-period seismic events and (ii) the mixing mechanism between magmatic and hydrothermal fluids, which was previously observed in the geochemical signature of fumaroles. Our results demonstrate that combining multidisciplinary large-scale methods is a relevant approach to better understand volcanic systems, with implications for monitoring strategies.

Mots clés: magnetotellurics; seismicity; self-potential; electrical resistivity; hydrothermal system; Miyakejima volcano

Domaines: Planète et Univers [physics]; Volcanologie

Marceau Gresse, Makoto Uyeshima, Takao Koyama, Hideaki Hase, Koki Aizawa, et al.. Hydrothermal and Magmatic System of a Volcanic Island Inferred From Magnetotellurics, Seismicity, Self-potential, and Thermal Image: An Example of Miyakejima (Japan). Journal of Geophysical Research : Solid Earth, 2021, 126, ⟨10.1029/2021JB022034⟩. ⟨insu-03712634⟩