Volcano-tectonic earthquake focal mechanisms reveal fluid-induced stress changes driving hydrothermal system development at Mount Ontake

Abstract Volcano-tectonic earthquakes are shear faulting that releases elastic strain energy within the shallow crust beneath volcanoes. While their focal mechanisms are widely used to infer stress states, they have rarely been applied to investigate hydrothermal system evolution. Here, we show that focal mechanisms of volcano-tectonic events associated with the 2024–2025 unrest at Mt. Ontake reveal the development of a hydrothermal circulation system. Under the regional strike-slip stress, vertical fractures acted as preferred fluid pathways. Fluid pressurization along them reoriented the local stress, making the minimum principal stress vertical and enabling fluid intrusion into horizontal fractures. Subsequent fluid pressurization further modified the local stress. This feedback between evolving fluid pathways and stress changes explains the diverse focal mechanisms observed during the unrest. Events inconsistent with the regional stress indicate increased magmatic-hydrothermal activity. Our findings highlight that focal mechanisms provide a physically grounded tool for characterizing hydrothermal processes and improving eruption forecasting.