Cohesion and Initial Porosity of Granular Fault Gouges control the
Breakdown Energy and the Friction Law at the Onset of Sliding
Abstract
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.