(Why) Are there powerlaw earthquakes?

Laws of friction have been studied since 500 years, yet their microscopic underpinning still eludes us. We do not understand how slip events are nucleated, nor what controls the distribution of their magnitude; questions that are central in earthquake science. I provide a novel framework to capture these phenomena by considering how continuum descriptions (rate-and-state laws) are perturbed by disorder. It predicts the existence of power-law distributed slip events whose size diverges at a critical stress, and that nucleate global slip (massive earthquakes, preceded by fracture-like front) if they extend past a critical length. I confirm these predictions in a minimal model of a frictional interface. In addition, the coupling between the (rate-and-state) flow properties of the interface and criticality yields a prediction for the relationship between the size and the duration of the avalanche (earthquake).