  The onset of frictional motion is characterized by the transition from
quasi-static stable slip growth to dynamic unstable crack propagation. Here, we
investigate how microscopic heterogeneities influence the macroscopic stability
of disordered interfaces. Building on the physics of depinning, we propose an
analytical framework to estimate the critical length $L_\mathrm{c}$ of the
instability nucleus, and benchmark it against 2D numerical dynamic
calculations. The interplay between material properties and disorder gives
birth to three instability regimes (local, extremal, and homogenized), each
related to different nucleation scenarios.
