Stick-slips are spontaneous, unstable slip events during which a natural or man-made system transitions from a strong, sticking stage to a weaker, slipping stage. Stick-slips were proposed by Brace and Byerlee (1966) as the experimental analogue of natural earthquakes. We analyze here the mechanics of stick-slips along brittle faults by conducting laboratory experiments and by modeling the instability mechanics. We performed tens of shear tests along experimental faults made of granite and gabbro that were subjected to normal stresses up to 14.3 MPa and loading velocities of 0.26-617 micron/s. We observed hundreds of spontaneous stick-slips that displayed shear stress drops up to 0.66 MPa and slip-velocities up to 14.1 mm/s. The pre-shear and post shear fault surface topography were mapped with atomic force microscopy at pixel sizes as low as 0.003 micron^2. We attribute the sticking phase to the locking of touching asperities and the slipping phase to the brittle failure of these asperities, and found that the fault asperities are as strong as the inherent strength of the host rock. Based on the experimental observations and analysis, we derived a mechanical model that predicts the relationships between the measured stick-slip properties (stress-drop, duration, and slip-distance) and asperity strength.

A large earthquake unlocks a fault-zone via dynamic rupture while releasing part of the elastic energy stored during the interseismic stage. As earthquakes occur at depth, the analyses of earthquake physics rely primarily on experimental observations and conceptual models. A common view is that the earthquake instability is necessarily related to the frictional weakening that is commonly observed in shear experiments under seismic slip velocities. However, recent experiments with frictional interfaces in brittle acrylics and rocks have explicitly demonstrated that no characteristic frictional strength exists; a wide range of stresses (‘overstresses’) are sustained prior to rupture nucleation. Moreover, the experimentally observed singular stress-fields and rupture dynamics are precisely those predicted by fracture mechanics. We therefore argue here that earthquake dynamics are best understood in terms of dynamic fracture mechanics: rupture dynamics are driven by overstresses, but not directly related to the fault frictional properties.