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 ((Formula presented.)) influence fault (in-)stability by performing (Formula presented.) -perturbation experiments on simulated carbonate fault gouges under fluid-drained, hydrothermal conditions. Our results indicate that in the velocity-neutral or -weakening regime, perturbing (Formula presented.) can reinforce frictional instability, leading to accelerated slow slips or enhanced stick-slip events. This effect is particularly pronounced when the excitation period ((Formula presented.)) approaches or exceeds the characteristic recurrence period ((Formula presented.)) 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 (Formula presented.) is close to (Formula presented.) — notably between 0.5π and 1π in radian, corresponding to maximum destressing rate and minimum (Formula presented.), respectively. Additionally, short-period ((Formula presented.)) 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.