Abstract

Volcanic eruptions threaten more than one in ten people worldwide, with the greatest risk at volcanoes reawakening after long quiescence. In these settings, eruptions are often explosive and nearby communities may be unprepared. Unrest commonly reflects magma accumulating in shallow storage, deforming the surrounding crust until it fails; a magma-filled fracture can then propagate to the surface and trigger eruption. Forecasting such events therefore depends on tracking deformation and estimating when the crust will rupture. Here we introduce a framework to track deformation by inferring the temporal evolution of elastic strain energy and mean stress during unrest, combining geodetic deformation with seismicity. Applied to Campi Flegrei (2011–2025) and the 2018 Sierra Negra eruption, the inferred mean stress follows a characteristic sequence: an initial linear increase consistent with predominantly elastic deformation, followed by a transition to inelastic behaviour as energy loss to fracturing rises and ultimately exceeds the elastic energy input. Thereafter, the mean stress declines, indicating progressive weakening driven by seismicity-induced damage. This pattern mirrors laboratory observations of bulk failure in extension and is consistent with the development of new pathways for magma and fluids. Our approach helps distinguish deformation regimes at reawakening volcanoes and supports hazard assessment during prolonged unrest.