  In order to better understand the interaction between pore-fluid overpressure
and failure patterns in rocks we consider a porous elasto-plastic medium in
which a laterally localized overpressure line source is imposed at depth below
the free surface. We solve numerically the fluid filtration equation coupled to
the gravitational force balance and poro-elasto-plastic rheology equations.
Systematic numerical simulations, varying initial stress, intrinsic material
properties and geometry, show the existence of five distinct failure patterns
caused by either shear banding or tensile fracturing. The value of the critical
pore-fluid overpressure at the onset of failure is derived from an analytical
solution that is in excellent agreement with numerical simulations. Finally, we
construct a phase-diagram that predicts the domains of the different failure
patterns and at the onset of failure.
