Do Normal Stress Vibrations Stabilize or Destabilize Faults? Experimental Perspectives

The effects of stress perturbations on friction are crucial for understanding earthquake triggering. Previous experimental studies have primarily been conducted at room temperature, where fault gouge materials typically exhibit velocity‐strengthening and frictionally stable behaviour. In this study, we investigate how variations in effective normal stress () influence fault (in‐)stability by performing ‐perturbation experiments on simulated carbonate fault gouges under fluid‐drained, hydrothermal conditions. Our results indicate that in the velocity‐neutral or ‐weakening regime, perturbing can reinforce frictional instability, leading to accelerated slow slips or enhanced stick‐slip events. This effect is particularly pronounced when the excitation period () approaches or exceeds the characteristic recurrence period () associated with pre‐perturbation instabilities. Stress drops of the resulting events can have larger amplitudes than expected from quasi‐steady‐state. When cyclic perturbations are imposed, slip events tend to synchronize at specific phases when is close to — notably between 0.5π and 1π in radian, corresponding to maximum destressing rate and minimum , respectively. Additionally, short‐period () perturbations can induce significant shear stress reduction (or fault weakening), with magnitudes comparable to the stress drops from stick slips, yet they are surprisingly associated with acoustically quiet slow slip, suggesting a stabilizing effect. These findings underscore the critical role of perturbation period in controlling fault response. In the context of induced seismicity, our results imply that cyclic or monotonic fluid injections should be carefully designed, considering both perturbation amplitude and period. Properly turned cyclic injections could potentially mitigate seismic risk by promoting quiet, slow slip over seismic fault slip.