Fault zones usually present a granular gouge, coming from the wear material of previous slips. This layer contributes to friction stability and plays a key role in the way elastic energy is released during sliding. Considering a mature fault gouge with a change in the percentage of mineral cementation between particles, we aim to understand the influence of interparticle bonds on slip mechanisms by employing the Discrete Element Method. We consider a direct shear model without fluid in 2D, based on a granular sample with realistic grain sizes and shapes. Focusing on the physics of contacts inside the granular gouge, we explore contact interactions and effective friction coefficient within the fault. Brittleness is enhanced with cementation and even more with dense materials. For the investigated data range, three types of cemented material are highlighted: a mildly cemented material (Couette flow, no cohesion), a cemented material with agglomerates of cemented particles changing the granular flow and acting on slip weakening mechanisms (Riedel shear bands R1), and an ultra-cemented material behaving as a brittle material (with several Riedel bands followed by shear-localization). Effective friction curves present double weakening shapes for dense samples with enough cementation. We find that effective friction of a cemented fault cannot be predicted from Mohr-Coulomb criteria because of the specific stress state and kinematic constraints of the fault zone.

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.