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.

In the framework of EGS, using hydraulic stimulation increases pore pressure into the existing fracture networks and can be responsible for slip reactivations. In order to predict the seismic or aseismic character of these slips, it is important to determine the influence of the gouge characteristics on the slip behaviour. Lab or in-situ testing can be very helpful, but experience has shown that it is complicated to install local sensors inside a mechanical contact and that some data remains impossible to obtain. In this study, we are focusing on the physics of the contact inside a granular gouge to understand the mechanisms of slip triggering by using numerical modelling. We implement a 2D granular fault gouge with Discrete Element Modelling (DEM). The model involves two rough surfaces representing the rock walls separated by a granular gouge with realistic angular particles created by the wear of previous slips. A displacement-driven model with dry contact is primarily studied to observe the peak of static friction (shape, slope or duration). In order to spotlight the aseismic and seismic patterns, a second model is carried out to add the stiffness of the rocks (or loading apparatus). Dedicated post processing tools are also used in order to analyse the spatial distribution of the solid fraction and the force chains patterns bringing a new understanding of physics from a granular point of view. Representing realistic morphology of particles (angular shapes) brings an additional level of control with respect to most of the simulations reported in the literature using circular grains. Angular grains lead to higher friction coefficients with different global behaviours. The initial solid fraction of the gouge appears to be a good indicator of the friction peak we have at a slip triggering (Fig.). The more compacted the gouge is, the more effort is needed to disturb the initial grains assembly and to reach a steady state regime. With fractal size distribution, we have also studied the mechanical contribution of small particles in the media. Friction coefficient results let us conclude that the presence or not of very small particles is not as influent as the initial solid fraction of the gouge. Finally, adding a stiffness on the upper rock wall provides information on the transition from stable sliding to slow-slip/stick-slip behaviour.