Earthquake nucleation on rough faults

Earthquake nucleation is currently explained by rate-and-state stability analysis, which successfully models the behavior of laboratory simulated faults with constant thickness gouge layers.However roughness is widely observed on natural faults and its influence on earthquake nucleation is little explored.Here we conduct frictional sliding experiments with different roughnesses on granite samples at upper crustal conditions .We observe a wide range of behaviors, from stable sliding to stick-slip, depending on the combination of roughness parameters and normal stress.Stick-slip is repeatedly observed in velocity-strengthening regimes, and increases in normal stress stabilize slip; these features are not fully predicted by current stability analysis.We derive a new instability criterion which matches our observations, based on fracture energy considerations and the size of weak patches created by fault roughness.