Overview

• Researchers mapped deep fluid systems in 3D beneath the Kuju region of Kyushu, resolving the brittle–ductile transition where fluids accumulate.
• The study shows that high-pressure supercritical fluids can be trapped, migrate through permeable windows and fractures, and undergo phase changes that alter fault stress.
• Dense seismometer data combined with machine learning detailed earthquake distributions and mechanisms beyond the resolution of earlier electromagnetic surveys.
• Analyses identified reservoirs sealed beneath cap layers, pathways that vent fluids upward, and a clear statistical correlation between heavy rain and increased seismicity.
• The findings, published in Communications Earth & Environment, suggest improvements to hazard models and geothermal site targeting, with commercial use constrained by extreme deep-drilling challenges; the work was supported by NEDO and JSPS.