A damage mechanics model for shear failure under compressive loading is used to calculate the shear strength of a fault. Based on field studies of the structure of natural fault zones, the distribution of starter flaws (initial damage) is assumed to be fractal with dimension Df=2.6. For this fractal dimension, the largest flaws dominate the fracture process at low confining stress and the shear strength scales as the inverse square root of the largest flaw size (as in tensile loading). At higher levels of confining stress appropriate to the base of the seismogenic zone, the strength becomes independent of the distribution of flaw sizes and depends only on the density of starter flaws. When the initial damage is sufficiently high, the damage initiation surface coincides with the failure surface, and the fault zone appears to obey the same friction law which controls slip on the individual microfractures. The initial damage corresponding to the fractal distribution of flaws measured in a natural fault zone is large enough for this to occur. ¿ American Geophysical Union 1992