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.

Fault zone usually presents a granular gouge, coming from the wear material of previous slips. Considering a mature fault gouge with mineral cementation between particles, we aim to understand the influence of these cohesive links on slip mechanisms. As cohesion is difficult to follow and to quantify with Lab or in-situ experiments, we choose to use Discrete Element Method that has already shown its ability to represent granular gouges with relevant kinematics and rheology. In this work, we consider a dry cohesive contact model in 2D (2x20mm²) involving two rough surfaces representing the rock walls separated by the granular gouge (5000 particles, grain size 27- 260 μm). A step forward compared to literature is to add cohesion on real angular and faceted grains that modifies contact between particles. Focusing on physics of contacts inside the granular gouge, we explore contact interactions and friction coefficient between the different bodies. To represent the cementation we set up a Bonded Mohr-Coulomb law, considering that inter-particular bridges and particles are made with the same material. This numerical model is displacement-driven and is implemented to study the peak of static friction (shape, slope, duration) under a confined pressure of 40 MPa. Depending on the compacity and on the cohesion level of each model, the peak strength may be sharp, short, and intense for dense and highly cohesive cases or smooth, delayed with moderate amplitude for mid-dense and moderately cohesive cases. Three main behaviors are observed: a non-cohesive regime where the added cohesion is too small to truly disturb the global slip mechanism (Couette flow), an intermediate cohesive regime with clusters of cohesive grains, changing the granular flow and acting on slip weakening mechanisms (Riedel shear band R1) and an ultra-cohesive regime where gouge behaves as a brittle material with several Riedel shear bands emergence. We also investigate the role of cohesive bonds in energy budget, focusing on fracture energy term. Three mechanisms are playing a role in fracture energy evolution: the rupture of cohesive bonds, the dilatancy of the gouge and Coulomb dissipations due to friction. These factors are linked to the initial percentage of cohesion inside the sample and help characterize the mechanisms at stake in the initiation of sliding.

Fault zones are usually composed of a granular gouge, coming from the wear material of previous slips, which contributes to friction stability. Once considering a mature enough fault zone that has already been sheared, different types of infill materials can be observed, from mineral cementation to matrix particles that can fill remaining pore spaces between clasts and change the rheological and frictional behaviors of the gouge. We aim to understand and reproduce the influence of grain-scale characteristics on slip mechanisms and gouge rheology (Riedel bands) by employing the Discrete Element Method. A 2D-direct shear model is considered with a dense assembly of small polygonal cells of matrix particles. A variation of gouge characteristics such as interparticle friction, gouge shear modulus or the number of particles within the gouge thickness leads to different Riedel shear bands formation and orientation that has been identified as an indicator of a change in slip stability (Byerlee et al., 1978). Interpreting results with slip weakening theory, our simulated gouge materials with high interparticle friction or a high bulk shear modulus, increase the possible occurrence of dynamic slip instabilities (small nucleation length and high breakdown energy). They may give rise to faster earthquake ruptures.