Active fault zones and groundwater flow

Boundary-element studies of an active strike-slip fault zone subject to fau lt-parallel loading of 6 MPa show tensile stress concentration in large are as around the fault-zone tips. In these areas, tensile stress exceeds typic al in situ tensile strengths of rocks, resulting in the formation or reacti vation of tensile fractures. These fractures curve toward the rips of the f ault zone, and if interconnected they increase the rock permeability. Fault slip also increases the temporary permeability of the fault zone, by as mu ch as many orders of a magnitude. Its effects on the surrounding groundwate r flow, however, is normally small if the fault trends at a high angle to t he groundwater flow but gradually increases as the angle between the flow a nd the fault decreases. When the trend of the fault zone and the groundwate r flow coincide, the upstream part collects groundwater whereas the downstr eam part expels it. It follows that the yield of springs decreases in the u pstream part, but increases in the downstream part.