Abstract Gradual localization of deformation preceding catastrophic failure can produce precursory signals associated with a phase transition that may be present prior to earthquakes. However, due to the unclear origin of these precursors and the complexity of the environmental conditions, detecting such preparatory signals remains challenging. Here we present the spatio‐temporal evolution of surface strain measured using fiber‐optic sensing during triaxial experiments in wet and dry conditions. We identify a power‐law distribution of strain increments where the largest magnitude diverges toward failure. This suggests a critical phase transition with the emergence of failure precursors. However, criticality is only observed in dry conditions and disappears with pressurized pore fluids, where the largest strain increment accelerates exponentially, consistent with a first‐order transition. Our results highlight that progressive damage features criticality for failure prediction, but elevated fluid pressures may shift this behavior to abrupt rupture.

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