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In the upper crust, where brittle deformation mechanisms dominate, the development of crack networks subject to anisotropic stress fields generates stress-induced elastic anisotropy. Here a rock specimen of Westerly granite was submitted to differential stress cycles (i.e., loading and unloading) of increasing amplitudes, up to failure and under upper crustal conditions. Combined records of strains, acoustic emissions, and P and S elastic wave anisotropies demonstrate that increasing differential stress promotes crack opening, sliding, and propagation subparallel to the main compressive stress orientation. However, the significant elastic anisotropies observed during loading (>= 20%) almost vanish upon stress removal, demonstrating that in the absence of stress, crack-related elastic anisotropy remains limited (<= 10%). As a consequence, (i) crack-related elastic anisotropies measured in the crust will likely be a strong function of the level of differential stress, and consequently (ii) continuous monitoring of elastic wave velocity anisotropy along faults could shed light on the mechanism of stress accumulation during interseismic loading.

Plain Language Summary In the upper crust, large strains are accommodated by brittle deformation mechanisms, leading to macroscopic faults embedded within a substantially damaged rock medium. The development of crack damage affects both the strength and the elastic and transport properties of rocks. Nowadays, the evolution of rock elastic properties is commonly used to estimate the direction of the maximum stress along faults and evaluate seismic hazard of seismogenic area. Up to now, stress-induced anisotropy was expected to be irreversible and observable by geophysics method even after unloading or exhumation of the rocks. In this study, we demonstrate for the first time that unloading induces an almost complete recovery of both stress-induced anisotropy and stress-induced damage. Our results suggest that elastic properties estimated from wave velocity measurement could then underestimate both damage and anisotropy of the crust under shallow depth conditions.