DEM Analyses of Cemented Granular Fault Gouges at the Onset of Seismic Sliding: Peak Strength, Development of Shear Zones and Kinematics

Abstract

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 varying amount of mineral cementation between particles, we aim to understand the influence of the strength 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 angular and faceted grain shapes. Focusing on the physics of shear accommodation inside the granular gouge, we explore the effect of an increase of cementation on effective friction (i.e. stress ratio) within the fault. We find that brittleness and the overall shear strength are enhanced with cementation, especially for dense materials. For the investigated data range, three types of cemented material are highlighted: a poorly cemented material (Couette flow profile, no cohesion), a cemented material with aggregates of cemented particles changing the granular flow and acting on slip weakening mechanisms (Riedel shear bands R), and a highly-cemented material behaving as a brittle material (with several Riedel bands followed by fault-parallel shear-localization Y). Effective friction curves present double weakening shapes for dense samples with enough cementation. We find that the effective friction of a cemented fault cannot be directly predicted from Mohr–Coulomb criteria because of the heterogeneity of the stress state and kinematic constraints of the fault zone.