The seismic velocities for the San Andreas fault zone in central California are interpreted in the light of new experimental data for the velocities of compressional and shear waves in fault gouges collected from central California. Under confining pressures the velocities of seismic waves in fault gouge are in the same range as those in a velocity model for the San Andreas fault. Numerical modeling of density in interpreting the gravity anomalies across the fault suggests a zone of relatively low density at depth. These findings indicate that fault gouge may exist down to a depth of 10 km. If this is the case, the mechanical behavior of the San Andreas fault would be greatly influenced by the gouge.