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Cohesion and Initial Porosity of Granular Fault Gouges control the Breakdown Energy and the Friction Law at the Onset of Sliding
  • Nathalie Casas,
  • Guilhem Mollon,
Nathalie Casas
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Guilhem Mollon

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Earthquakes happen with frictional sliding, by releasing excess stresses accumulated in the pre-stressed surrounding medium. The geological third body (i.e. fault gouge), originating from the wear of previous slips, contribute to friction stability and plays a key role in the energy released. An important part of slip mechanisms are influenced by gouge characteristics and environment. The study of several types of gouge, as mixtures of different initial porosity and cohesive contact law, allows to link fault gouge properties to its rheological behavior. In this paper, a cohesive fault gouge segment is modeled in 2D with DEM. The analyses of friction coefficient evolution, gouge kinematics and force chains within the gouge highlight the main mechanisms acting on fracture processes. A link is made between the initial state of the gouge and the ductile or brittle character of the whole granular flow. For the investigated data range, three regimes are highlighted: a mildly cohesive regime (ductile behavior), a cohesive regime with agglomerates formation and Riedel shear bands and an ultra-cohesive regime with several Riedel bands followed by ultra-localization (brittle behavior). As a result of this study, the total macroscopic friction generated during the shearing is proposed to be a combination of three contributions: Coulomb friction, dilation and decohesion process. A simplified model is built up to represent these contributions and to be implemented in dynamic rupture modelling at higher scale. The Breakdown energy appears to be controlled by the intensity of these three mechanisms and their associated slip distance.