Earthquake depth distributions in continental crust can be related to factors which determine the depth of the transition from brittle to ductile failure. These factors include temperature, strain rate and lithology. The San Andreas Fault Zone of Northern and Central California is an ideal place to test the relative importance of these factors since there is abundant earthquake data and our understanding of the tectonic interaction of the Pacific and North American plates provides constraints for several parameters. In the San Andreas Fault Zone we have found an approximately 3 km increase from Northern to Central California of the median depth of events and the depth above which 90% of events occur. In addition, depth distributions of earthquakes vary systematically from Northern to Central California. Both the maximum depth of earthquakes and the depth of the peak of seismic activity are shallower in Northern California. Utilizing Critical Strength Envelopes we have found that, unlike the oceanic case, this variation is not thermally controlled. Rather, a combination of low strain rate and low stress level cause the seismicity in Northern California to be shallow relative to Central California. ¿ American Geophysical Union 1988