  Slip at a frictional interface occurs via intermittent events. Understanding
how these events are nucleated, can propagate, or stop spontaneously remains a
challenge, central to earthquake science and tribology. In the absence of
disorder, rate-and-state approaches predict a diverging nucleation length at
some stress $\sigma^*$, beyond which cracks can propagate. Here we argue for a
flat interface that disorder is a relevant perturbation to this description. We
justify why the distribution of slip contains two parts: a powerlaw
corresponding to `avalanches', and a `narrow' distribution of system-spanning
`fracture' events. We derive novel scaling relations for avalanches, including
a relation between the stress drop and the spatial extension of a slip event.
We compute the cut-off length beyond which avalanches cannot be stopped by
disorder, leading to a system-spanning fracture, and successfully test these
predictions in a minimal model of frictional interfaces.
