Stresses in planar normal faulting: shallow compression caused by fault-plane mismatch

Shallow horizontal compressive stress often occurs near the top of the hangingwall plate in numerical modelling of planar normal faulting, extending horizontally for up to 14 km from the fault plane. This compression is attributed to a potential downward mismatch between the dips of the opposing fault planes which would rotate differentially if unconstrained to remain in contact. The mismatch is suppressed by downward increasing lithostatic pressure which applies equal but opposite couples to the fault planes forcing them to remain in contact. This gives rise to shallow compression and deep tension on both sides of the fault. The potential mismatch originates partly from differential loading on opposite sides of the fault, but also from the anti-symmetrical shapes of the footwall and hangingwall plates. These two contributions oppose each other in normal faulting but reinforce in reverse faulting. The modelling also reveals large fault-parallel compressive stress adjacent to the footwall. This compression acts as a seal inhibiting fluid flow across the fault and preventing upflow adjacent to the footwall. In contrast, smaller fault-parallel tension adjacent to the hangingwall provides a low pressure channel for upward fluid flow adjacent to the fault, giving rise to a zone of weakness. Strain relief in this weak zone, in response to shallow mismatch compression, may explain the so-called normal fault drag near the top of the hangingwall.