The role of loading path on fault reactivation: a laboratory perspective

Slip along pre-existing faults in the Earth’s crust occurs whenever the shear stress resolved on the fault plane overcomes fault frictional strength, potentially generating catastrophic earthquakes. The coupling between shear stress and normal stress during fault loading depends on 1) the orientation of the fault within the stress field and 2) the tectonic setting. In compressional settings, a load-strengthening path occurs because along thrust faults the increase in shear stress is coupled with an increase in effective normal stress. On the contrary, in extensional settings, the increase in shear stress is coupled with a decrease in effective normal stress, resulting in load-weakening paths for normal faults.Analytical approaches to evaluate the potential for fault reactivation are generally based on the assumption that faults are ideal planes, characterized by zero thickness and constant friction, embedded in homogeneous isotropic elastic media. However, natural faults typically host thick fault cores and highly fractured damage zones, which can compact or dilate under different loading paths (i.e., different coupling between normal and shear stress). In addition, in most laboratory friction experiments, the fault is loaded under constant or increasing normal stress and at optimal orientation for reactivation. Here, we present laboratory experiments simulating reactivation of thick gouge-bearing faults that experienced different loading paths.Our results show that the differential stress required for reactivation strongly differs from theoretical predictions, and unfavourably oriented faults appear systematically weaker, especially when a thick gouge layer is present. Before reactivation fault zone compacts in load-strengthening paths whereas dilation is observed in load-weakening path. Upon fault reactivation at comparable normal stress, load-strengthening promotes stable creep  whereas load-weakening results in accelerated slip. Our study highlights the importance of fault thickness and loading path in fault hydromechanical coupling and stability with significant implications for fluid circulation within fault zones and earthquake mechanics.