Frictional motion is mediated by rapidly propagating ruptures that detach the ensemble of contacts forming the frictional interface between contacting bodies1–7. These ruptures are similar to shear cracks. When this process takes place in natural faults, these rapid ruptures are essentially earthquakes8,9. Although fracture mechanics describe the rapid motion of these singular objects, the nucleation process that creates them is not understood10–19. Here we fully describe the nucleation process by extending fracture mechanics to explicitly incorporate finite interface widths (which are generally ignored20,21). We show, experimentally and theoretically, that slow steady creep ensues at a well-defined stress threshold. Moreover, as slowly creeping patches approach the interface width, a topological transition takes place in which these creeping patches smoothly transition to the rapid fracture that is described by classical fracture mechanics22–26. Apart from its relevance to fracture and material strength, this new picture of rupture nucleation dynamics is directly relevant to earthquake nucleation dynamics; slow, aseismic rupture must always precede rapid seismic rupture (so long as the initial defect in the interface is localized in both spatial dimensions). The theory may provide a new framework for understanding how and when earthquakes nucleate.
