Gravity and seismic measurements collected on granitic plutons cut by the San Andreas, Garlock, and San Jacinto faults of central and southern California show that these faults are bounded by belts of anomalous rock extending 2--5 km on both sides of the surface traces of the faults. Borehole and petrographic studies show that anomalously low densities and seismic velocities in this zone are due to both fracturing and chemical alteration. Fracturing, water circulation, and alteration are inexorably interrelated. High fracture densities are required to provide the permeability that allows water to reach and react with rocks hundreds to thousands of meters below the surface. Alteration products stable to seismogenic depths include montmorillonite (MO). Since the products of this alteration include expansive clays, this process is self-limiting. Formation of MO is facilitated by sparse water, whereas kaolinite and other clays can form if the amount of water exceeds the quantity needed for MO. Earthquakes periodically reopen the blocked fractures, a mechanism necessary in generating the broad, deep zone of pervasive alteration inferred from geophysical measurements. These geophysical and petrological patterns are clues not only to the evolution of these faults but also to the dynamics of mechanisms currently operating within the seismogenic zone. This deep circulation model predicts that the strength of a fault segment varies in time. Timing of major earthquakes may be due as much to temporal strength variations as to stress events in the crust. Significant preseismic changes in the physical properties of the crust are probably confined to the zone of fractured rock immediately adjacent to the fault.