  The complexity of the frictional dynamics at the microscopic scale makes
difficult to identify all of its controlling parameters. Indeed, experiments on
sheared elastic bodies have shown that the static friction coefficient depends
on loading conditions, the real area of contact along the interfaces and the
confining pressure. Here we show, by means of numerical simulations of a 2D
Burridge-Knopoff model with a simple local friction law, that the macroscopic
friction coefficient depends non-monotonically on the bulk elasticity of the
system. This occurs because elastic constants control the geometrical features
of the rupture fronts during the stick-slip dynamics, leading to four different
ordering regimes characterized by different orientations of the rupture fronts
with respect to the external shear direction. We rationalize these results by
means of an energetic balance argument.
