  Recent experiments indicate that frictional sliding occurs by the nucleation
of detachment fronts at the contact interface that may appear well before the
onset of global sliding. This intriguing precursory activity is not accounted
for by traditional friction theories but is extremely important for friction
dominated geophysical phenomena such as earthquakes, landslides or avalanches.
Here we simulate the onset of slip of a three dimensional elastic body resting
on a surface and show that experimentally observed frictional precursors depend
in a complex non-universal way on the sample geometry and the loading
conditions. Our model satisfies Archard's law and Amontons' first and second
laws, reproducing with remarkable precision the real contact area dynamics, the
precursors' envelope dynamics prior to the transition to sliding, and the
normal and shear internal stress distributions close to the slider-substrate
interface. Moreover, it allows to assess which experimental features can be
attributed to the elastic equilibrium, and which are attributed to the
out-of-equilibrium dynamics, suggesting that precursory activity is an
intrinsically quasi-static physical process. A direct calculation of the
evolution of the Coulomb stress before and during precursors nucleation shows
large variations across the sample, which helps to explain why earthquake
forecasting methods based only on accumulated slip and Coulomb stress
monitoring are often ineffective.
