The nucleation of frictional ruptures, theory and experiment

Frictional motion is mediated by rapidly propagating ruptures, akin to shear cracks, that detach the ensemble of contacts that form the interface between
contacting bodies. While fracture mechanics describe the rapid motion of these singular objects, the nucleation process that creates them is not currently understood. By extending fracture mechanics to explicitly incorporate finite interface widths, we fully describe the nucleation process. We show, experimentally and theoretically, that slow steady creep ensues at a stress threshold. Moreover, as creeping patches approach the interface width, a topological transition occurs where they smoothly transition to rapid fracture. This new picture of the nucleation dynamics of fracture (and friction) is directly relevant to earthquake nucleation dynamics and the transition from aseismic to seismic rupture in natural faults.