  The Mohr-Coulomb criterion is widely used in geosciences to relate the state
of stress at failure to the observed orientation of the resulting faults. This
relation is based on the assumption that the fault occurs along a plane that
maximizes the Coulomb stress. Here, we test this hypothesis using an elastic,
progressive damage model that implements the Mohr-Coulomb criterion at the
local scale. We find that the orientation of the fault is not given by the
Mohr-Coulomb criterion. Instead, for minimal disorder, it corresponds to the
most unstable mode of damage in the model, which we determine through a linear
stability analysis of the homogeneously damaged state. Our simulations show
that microstructural disorder significantly affects the orientation of the
fault, which, however, remains always far from the Mohr-Coulomb prediction.
