Supercritical fluid flow through permeable window and phase transitions at volcanic brittle–ductile transition zone

Abstract Hydrothermal fluid migration and supercritical phase transitions around the brittle–ductile transition are key to understanding volcanic activity and geothermal energy, but it remains challenging due to limited subsurface imaging. Here we conducted active-source seismic surveys at Kuju volcano, Japan, applying extended common-reflection-surface analysis to image magma-related and hydrothermal structures. We further estimated earthquake hypocenters, focal mechanisms, and seismic velocity. A continuous horizontal reflector could indicate a low-permeability seal confining supercritical fluids, while a low-amplitude reflection zone reveals fractures disrupting the seal. This permeable window, located at the magma margin, coincides with earthquake clusters of various focal mechanisms, suggesting upward fluid migration. A plume-shaped zone with low P-wave and S-wave velocity ratio (Vp/Vs) above the permeable window indicates thermally-altered formations and gas release from phase transitions. These results provide 3D visualization of the interplay among trapped supercritical fluid, its migration through permeable window, and related seismicity near the brittle-ductile transition.