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

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