Notes:

Shear failure in rock is controlled by rock strength, internal friction, and applied external load (net stress). In the area around the fracture that is affected by leakoff, the pore fluid pressure may be greatly elevated. In the extreme, the pore pressure may be close to the frac fluid pressure in the open fracture. In this case, one of the net stresses will be near zero and possibly the second principal stress may be considerably reduced. This sets up a condition where shear failure and slip can occur even if the rock were originally intact.