On varying b-values with depth: results from computer-intensive tests for Southern California

Abstract : The Gutenberg-Richter b-value is thought to reflect the stress conditions in the crust; therefore, spatial and/or temporal variations of the b-value can provide important information regarding crustal tectonics. We investigate the variation of b-value with depth in seven selected areas of Southern California. A previous study provided a detailed mapping of the variations of b with depth in California; our study is less systematic than this study. Our approach is more similar to the regional one used by Mori & Abercrombie. In comparison to these previous studies, our investigation indicates that the variability of b is often not statistically significant and that the decrease of b with depth should be interpreted with caution. The seismic catalogues used are subsets of a set of about 100000 seismic events recorded by the Southern California Seismic Network (SCSN) and relocated by Richards-Dinger & Shearer. We study the performance of Utsu's test compared to bootstrap tests for comparison of b-values. The results of our investigation also raise the question of the relevancy of Utsu's test when comparing b-values. Both simulations and real cases show that the Utsu's test is biased towards rejection of the null hypothesis in favour of the hypothesis that b-values are significantly different.