Mean magnitude variations of earthquakes as a function of depth: Different crustal stress distribution depending on tectonic setting

Abstract

The mean magnitude of earthquakes in the Gulf of Corinth is found to increase strongly with depth (b‐value decreases), whereas the dip of fault planes decreases. The b‐value difference of 0.25, between shallow and deep earthquake distributions, is based on about 7,000 events and therefore is statistically highly significant. The same is true in California, but opposite patterns are observed in southern Iceland and in western Nagano, Japan. Because large mean magnitudes (low b‐values) are indicative of relatively high stress levels, we propose that in the detachment layer at about 9 ± 2 km depth, earthquakes are generated at higher stresses than in the shallower parts of the crust. The correlation of low b‐values with low faulting dips can be taken as line of evidence that low b‐values map high stress regimes.