Fluid overpressure and stress drop in fault zones

Using the average aspect (length/maximum thickness) ratios of 379 mineral-f illed extension (mode I) veins from an active fault zone, the fluid overpre ssure, during their development, with reference to the minimum compressive principal stress, sigma(3), is estimated at 20 MPa. Emplacement of such vei ns increases sigma(3) and can generate a temporary stress barrier to the pr opagation of subsequent hydrofractures. On meeting a subhorizontal stress b arrier, vertically propagating hydrofractures may change into water sills w here the fluid pressure is at or above lithostatic. In this model, stress b arriers, and thus water sills, can form at any depth in, and in any type of , fault zones. For such a high fluid pressure, the product of the coefficie nt of sliding friction and the normal stress in the Modified Griffith Crite rion becomes essentially zero and the driving stress associated with faulti ng equal to twice the in situ tensile strength of the host rock. For typica l in situ tensile strengths of 2-3 MPa, the driving stresses for slip on ov erpressured fault planes is 4-6 MPa. These results are in good agreement wi th the commonly measured average static stress drops of 3-6 MPa during eart hquakes.