The Spatial Shape of Avalanches

In disordered elastic systems, driven by displacing a parabolic confining potential adiabatically slowly, all advance of the system is in bursts, termed avalanches. Avalanches have a finite extension in time, which is much smaller than the waiting-time between them. Avalanches also have a finite extension $\ell$ in space, i.e. only a part of the interface of size $\ell$ moves during an avalanche. Here we study their spatial shape $\left< S(x)\right>_{\ell}$ given $\ell$, as well as its fluctuations encoded in the second cumulant $\left< S^{2}(x)\right>_{\ell}^{{\rm c}}$. We establish scaling relations governing the behavior close to the boundary. We then give analytic results for the Brownian force model, in which the microscopic disorder for each degree of freedom is a random walk. Finally, we confirm these results with numerical simulations. To do this properly we elucidate the influence of discretization effects, which also confirms the assumptions entering into the scaling ansatz. This allows us to reach the scaling limit already for avalanches of moderate size. We find excellent agreement for the universal shape, its fluctuations, including all amplitudes.