Normal-stress unclamping promotes cascading instability in deep mining-induced seismicity

Deep mining-induced seismicity poses a severe dynamic hazard and offers a near-field setting for examining fault interaction under rapidly evolving excavation-induced stress fields. Conventional stress-concentration frameworks may overlook failure paths governed by normal-confinement reduction, frictional weakening, and bedding-controlled weak-plane activation. Here, we investigate a high-energy doublet in a deep coal mine using an integrated workflow that combines shear-tensile-compressive source inversion, stress-tensor reconstruction, kinematic forward modelling, catalogue-level event-pair analysis, and uncertainty-aware Coulomb stress-transfer modelling. The source mechanisms and stress projections reveal selective activation within a structurally anisotropic lacustrine rock mass. Shear-dominated events occur under high resolved shear traction, whereas tensile-sensitive events preferentially develop along bedding-related weak planes where normal confinement is reduced. For the doublet sequence, Coulomb stress decomposition shows that the initiating event imposed a negative coseismic shear-stress change on the preferred receiver plane of the subsequent event, while lowering its compressive normal traction. Because the receiver plane already carried high background shear traction and a high instability coefficient, this unclamping effect likely promoted secondary failure by reducing frictional resistance on a critically stressed weak plane. Uncertainty-aware calculations further show that the favourable Coulomb response is concentrated near the secondary source region, supporting a localized source-receiver stress-transfer pathway. The later event was slightly larger in reported magnitude, calibrated energy, and field impact, a feature consistent with weakened rupture arrest under reduced normal confinement. These results highlight normal-stress unclamping as a key mechanical pathway for cascading instability in deep mining environments and suggest that seismic-hazard assessment should incorporate full-tensor stress evolution, normal-confinement reduction, unloading-front migration, and bedding-controlled weak-plane activation.