Boundary-element studies of an active strike-slip fault zone subject to fault-parallel loading of 6 MPa show tensile stress concentration in large areas around the fault-zone tips. In these areas, tensile stress exceeds typical in situ tensile strengths of rocks, resulting in the formation or reactivation of tensile fractures. These fractures curve toward the tips of the fault zone, and if interconnected they increase the rock permeability. Fault slip also increases the temporary permeability of the fault zone, by as much as many orders of a magnitude. Its effects on the surrounding groundwater flow, however, is normally small if the fault trends at a high angle to the groundwater flow but gradually increases as the angle between the flow and the fault decreases. When the trend of the fault zone and the groundwater flow coincide, the upstream part collects groundwater whereas the downstream part expels it. It follows that the yield of springs decreases in the upstream part, but increases in the downstream part. ¿ 2000 American Geophysical Union