A systematic test of the hypothesis that the b value varies with depth in California

We spatially map the shallow to deep b value ratio in the crust in California. Previous studies of the frequency magnitude distribution, as a function of depth, for selected crustal regions indicated that b decreases from b > 1.1 in the 0-5 km depth range to b < 0.8 in the depth range 7-15 km. Our detailed mapping confirms that this pattern can be established at the 99% significance level for about 32% of the entire seismically active crust. About 2% of the crust displays the opposite b-gradient. One such area is the San Francisco Bay Area. We speculate that differences in stress levels are the main factor controlling the depth dependency of b. These results confirm that the b value should not always be considered a constant in studies such as seismic hazard estimations.