Slow fault unlocking: where is the threshold between fault weakening and fault healing?

Slow slip along tectonic faults during creep events, slow earthquakes and earthquake nucleation phases may last from days to months/years. Over these time scales, the stress levels that a fault can withstand before slipping may depend from the competition between time-dependent mechanisms that degrade the rock strength (fault weakening) and re-strengthening processes (fault healing). In fault rocks, these time-dependent processes and their competing kinetics are still poorly constrained by laboratory experiments. We used stress relaxation experiments to investigate the stress threshold at which fault weakening processes may outpace fault healing processes. We studied experimental fault gouges of a large variety of crustal rocks, including some natural fault gouges, with both a biaxial and triaxial apparatus. After a phase of deformation under constant slip velocity, experimental fault rocks are allowed to creep and relax the accumulated stress for a standardized time, before being reloaded at constant rate. We found that shear stress measured at the end of the relaxation time, is linearly dependent with the normal stress imposed on the fault, thereby defining a Coulomb-type envelope. We interpret this envelope to define the fault strength under imposed stress conditions (creep envelope), in contrast with classical envelopes derived from experiments carried out under imposed strain rate conditions. For each rock and under identical experimental conditions, the creep envelope is systematically lower (9-20%) than the envelope determined under imposed strain rate conditions. We suggest that the creep envelope represent the stress threshold above which fault weakening processes, such as sub-critical crack growth, are effectively faster than healing processes, ultimately leading to failure and fault slip. Therefore, subcritical processes leading to fault slip may be effectively constrained by relaxation experiments and slip on portions of tectonic faults leading to processes of earthquake nucleation or stable creep may occur at significantly lower stresses than previously predicted.