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, com-ing 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 fric-tion (i.e. stress ratio) within the fault. We find that brittleness and the overall shear strength are enhanced with cementation, espe-cially 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.

Laboratory Earthquakes Simulations—Typical Events, Fault Damage, and Gouge Production

“…In the present case, stick‐slip patterns arise naturally from the interactions between rock elasticity and (a) its calibrated fracture properties (i.e., asperity breakage in the very first stages of the loading), (b) its progressive damage, (c) the production of a gouge in the interface, and (d) the granular rheology of this third body layer (Aharonov & Sparks, 2004; Houdoux et al., 2021; Lherminier et al., 2019), all of which being consequences of the bulk properties of the host rock. This observation confirms that the typical friction laws used in numerical simulations of earthquake nucleation and propagation (e.g., slip‐weakening or rate‐and‐state friction models) are mere consequences of a large number of local phenomena taking place within the interface (Casas et al., 2022; Mollon et al., 2021), and are by no means to be considered as first‐principles.…”

“…The geological third body is the fault gouge, which originates from the degradation and wear of the host rock during sliding. Several authors have used DEM or FDEM (combined Finite‐Discrete Element Method) to represent fault gouge, simulate its seismic shearing, and investigate its frictional response as a third body (Aharonov & Sparks, 2004; Casas et al., 2022; Cho et al., 2008; Da Cruz et al., 2005; Dorostkar et al., 2017; Gao et al., 2018; Guo & Morgan, 2004; Mair & Marone, 1999; Mair et al., 2002; Mollon et al., 2021; Morgan, 1999; Morgan & Boettcher, 1999; Zhao et al., 2012), following the same logic as in the tribological literature. Another area relevant to earthquakes mechanics that has been investigated with DEM is the fracturing of intact rock.…”

Laboratory Earthquakes Simulations - Typical Events, Fault Damage, and Gouge Production

“…This observation confirms that the typical friction laws used in numerical simulations of earthquake nucleation and propagation (e.g. slip-weakening or rate-and-state friction models) are mere consequences of a large number of local phenomena taking place within the interface [Mollon et al 2021, Casas et al 2022, and are by no means to be considered as first-principles.…”

“…The geological third body is the fault gouge, which originates from the degradation and wear of the host rock during sliding. Several authors have used DEM to represent fault gouge, simulate its seismic shearing, and investigate its frictional response as a third body [Morgan 1999, Mair and Marone 1999, Morgan & Boettcher 1999, Mair et al 2002, Guo & Morgan 2004, Aharonov and Sparks 2004, Da Cruz et al 2005, Cho et al 2008, Zhao et al 2012, Dorostkar et al 2017, Gao et al 2018, Mollon et al 2021, Casas et al 2022, following the same logic as in the tribological literature. Another area relevant to earthquakes mechanics that has been investigated with DEM is the fracturing of intact rock.…”

How vorticity and agglomeration control shear strength in soft cohesive granular flows

“…This software generalizes the conventional Discrete Element Method (DEM) and allows simulating complex granular materials, with either rigid or highly deformable grains with arbitrary shapes, within the framework of large deformation hyperelasticity in 2D. The versatility of this software allows it to be used in several fields, for example in mechanical engineering [46,47] or geomechanics [48].…”