Combined effects of fault orientation and surface roughness on fluid-induced reactivation: Experimental insights and implications for induced seismicity

Fluid injection can induce fault slip and seismicity, but the joint influence of fault orientation and surface roughness on the hydromechanical reactivation of mechanically unfavourable faults has not been systematically quantified. To address this problem, triaxial shear–fluid injection experiments were conducted on saw-cut granite faults with different inclinations to the maximum principal stress ( β = 30°, 45°, 60°) and two surface roughness levels (100-grit and 10,000-grit). A displacement-driven stage followed by constant-rate injection was used to distinguish mechanical from hydromechanical controls. Under displacement loading, larger β and higher roughness increase friction and system stiffness, allowing greater elastic strain accumulation prior to instability. During injection-driven reactivation, all faults ultimately exhibit abrupt stick–slip; however, the critical pore pressure required for reactivation rises systematically with β and is highest for smooth faults. This indicates that unfavourably oriented or hydraulically tight faults demand substantial overpressure due to elevated normal stress and limited fluid diffusion. Mechanistically, fault orientation governs the resolved stress state, while surface roughness influences both mechanical contact stiffness and hydraulic transmissivity; together they regulate elastic strain-energy accumulation, pore-pressure build-up, and the transition to unstable slip. These findings highlight the necessity of incorporating fault geometry and surface structure into injection design and induced seismic hazard assessment, particularly to mitigate delayed, high-energy reactivation events. • The interplay between orientation and surface roughness governs slip behaviour and reactivation stability. • Slip on unfavourably oriented or smooth faults requires elevated overpressure from impeded fluid diffusion. • Unfavourably oriented or rough faults store more strain energy, leading to high-energy ruptures.