Crustal faults are subjected to ubiquitous stress oscillations, such as Earth tides, reservoir cyclic loading, and passing tectonic waves. Although these perturbations typically operate at stress levels far below static yielding strength, they drive progressive degradation (fatigue) that can eventually trigger seismogenic rupture. However, effectively linking microscopic damage accumulation to macroscopic fault instability remains a significant challenge. To bridge this scale gap, we conducted uniaxial compression tests on saw-tooth granite faults under both monotonic and cyclic loading conditions, integrating in-situ micro-CT imaging and acoustic emission (AE) monitoring. This multi-physics approach allows for a direct correlation between micro-scale damage mechanisms and macro-scale fault energetics. Our results demonstrate that the loading path fundamentally regulates the spatiotemporal evolution of fractures. Monotonic loading promotes localized fracture propagation characterized by high spatial heterogeneity. In contrast, cyclic loading fosters diffuse damage accumulation, resulting in widespread, spatially uniform crack networks and progressive stiffness degradation. Seismologically, the cyclic regime exhibits a higher b-value and significantly lower mainshock radiated energy. Energy budget analysis reveals that while frictional work dominates dissipation in both regimes (>70%), the complex, tortuous micro-structures generated by cyclic loading significantly enhance frictional dissipation. Consequently, seismic radiation efficiency is markedly suppressed under cyclic loading (peaking at 0.6%) compared to the monotonic regime (2.5%). This study establishes a physical link between micro-scale fatigue evolution and macro-scale seismogenic potential, providing new insights into how sub-critical cyclic loading modulates energy dissipation patterns and rupture dynamics in natural faults.

Fault,In-situ Micro-CT imaging,acoustic emission