Fault gouge evolution and its dependence on normal stress and rock strength—Results of discrete element simulations: Gouge zone micromechanics

Abstract

We simulate the breakdown of fault blocks using the Distinct Element Method (DEM) in two dimensions in order to study the frictional strength, mechanical behavior, and stress and strain state of evolving fault gouge and their dependence on normal stress σn and uniaxial compressive strength σucs. The fault blocks were generated by adding breakable elastic bonds between adjacent circular particles to obtain a given σucs in a range from 100 to 260 MPa and deformed in a direct shear configuration over a range of σn from 10 to 100 MPa. The simulated shear zones experienced two growth stages: fast and slow. During the fast growth stage, the shear zones attain a peak friction value within the first 2% shear strain, which is enhanced in low‐σn and high‐σucs experiments. The residual sliding friction decreases nonlinearly with increasing ratio of σn/σucs. The variation in friction is related to volume strain and localized deformation, both of which are dependent on strain, σn, σucs, and associated variations in gouge properties. The complex correlations reflect the complexity and diversity of micromechanical processes in response to changing physical and structural properties of fault gouge as it evolves, with implications for fault behavior through time and space.