Slip tendency is the ratio of the shear stress to the normal stress on a fault or fracture surface:
Slip tendency = (τ)/(σn)
Slip tendency analysis yields useful insights into the nature of the stress tensor and its effects on which fault surfaces may become reactivated and in which direction a fault will slip.
The relative magnitude of the intermediate principal stress can strongly affect the flow of fluid through a fracture system. This image illustrates a fourfold increase of fracture network participation in fluid flow from a stress state with σ2 halfway between σ1 and σ3 (lower) and one where σ2 ≈ σ1 (upper).