The thickness of faults: From laboratory experiments to field scale observations

To assess the role of the fault thickness on its mechanical behavior, we first present the results of an experimental modeling of a thick fault core. Our laboratory setup consists in an annular simple shear apparatus in which we can apply very large shear displacements (50 m) to 100 particle thick granular samples. Thanks to a window in the apparatus, pictures of the microstructures can be continuously taken during shear. We observe from a Correlation Image Velocimetry technique that a significant strain field exists outside of the observable shear band. This strain field, though of small magnitude compared to that existing inside the shear band, is very structured and extends in a region much wider than expected from individual static observations (i.e. wider than the directly observable shear band). Moreover, this strain field controls most of the evolution of the shear strength of the fault. We then propose plausible comparisons of our experimental results to geological observations of fault cores in the region of Aigion (Corinth Gulf, Greece). The studied faults indeed display spectacular indurated fault planes lying on weakly cohesive material. Signatures of cementation, clay mineral distribution and porosity profile of one of the studied fault cores are included and discussed in the light of the experimental results. Our observations suggest that the maximum shear strain during earthquakes might occur not in the center, but on the border of the fault cores. It is presumably localized in a transition zone which exhibits a significant cementation owing to a process of mechanical smearing by fine particles. This zone may also act as a very low permeability layer responsible for a channeling of the fluid flow. Such a scheme of progressive multi sub-localizations, is different from classical descriptions of faults and consistent with a layering of the core consisting of separated zones of high strains or large cataclastic flows.