  We describe a 2D spring-block model for the transition from static to kinetic
friction at an elastic slider/rigid substrate interface obeying a minimalistic
friction law (Amontons-Coulomb). By using realistic boundary conditions, a
number of previously unexplained experimental results on precursory micro-slip
fronts are successfully reproduced. From the analysis of the interfacial
stresses, we derive a prediction for the evolution of the precursor length as a
function of the applied loads, as well as an approximate relationship between
microscopic and macroscopic friction coefficients. We show that the stress
build-up due to both elastic loading and micro-slip-related relaxations depend
only weakly on the underlying shear crack propagation dynamics. Conversely,
crack speed depends strongly on both the instantaneous stresses and the
friction coefficients, through a non-trivial scaling parameter.
