Using the average aspect (length/maximum thickness) ratios of 379 mineral-filled extension (mode I) veins from an active fault zone, the fluid overpressure, during their development, with reference to the minimum compressive principal stress, &sgr;3, is estimated at 20 MPa. Emplacement of such veins increases &sgr;3 and can generate a temporary stress barrier to the propagation of subsequent hydrofractures. On meeting a subhorizontal stress barrier, vertically propagating hydrofractures may change into water sills where the fluid pressure is at or above lithostatic. In this model, stress barriers, 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 coefficient of sliding friction and the normal stress in the Modified Griffith Criterion becomes essentially zero and the driving stress associated with faulting equal to twice the in situ tensile strength of the host rock. For typical in situ tensile strengths of 2--3 MPa, the driving stresses for slip on overpressured fault planes is 4--6 MPa. These results are in good agreement with the commonly measured average static stress drops of 3--6 MPa during earthquakes. ¿ 1999 American Geophysical Union