Stick‐slip motion in simulated granular layers

Abstract

[1] Two-dimensional numerical simulations of shear in a gravity-free layer of circular grains were conducted to illuminate the basic mechanics of shear of granular layers (such as layers of fault gouge). Our simulated granular layers exhibit either stable (steady state) or unstable (stick slip) motion. The transition from steady to stick-slip sliding depends on loading velocity and applied confining stress in a way similar to a simple model of a block on a frictional surface. We investigate the conditions which lead to naturally occurring stick-slip behavior and study in detail the systems behavior prior to and during slip events. Matching our numerical results to a spring block model, the system of grains was found to have bulk static and dynamic coefficients of friction that differ by about 0.1. This differing static and dynamic friction emerged spontaneously, from the collective behavior of grains, and was not prescribed a priori via a frictional rule between grain contacts. Results show that the micromechanics of contact forces is responsible for stick-slip behavior: During the ''stuck'' phase, and in preparation for slip, more and more grain contacts which carry low forces slide, resulting in accelerating internal stress release. When enough of the low-force contacts frictionally slide, the granular layer weakens and losses rigidity, leading to motion of contacts that carry larger forces and large-scale slip. Our results may have implications to the understanding of the stability of gouge layers and are thus related to the underlying physics of earthquakes.

Micromechanics and statistics of slipping events in a granular seismic fault model

“…3. In agreement with experimental results of a similar system [11], as well as with numerical results found in two dimensions [18], we find either stick-slip motion at high pressure and small loading velocity, or continuous flow at low pressure and high loading velocity.…”

“…In fact, the non linear behaviour could be explained considering that small perturbations applied to a granular system modify the contact force network, setting up a memory mechanisms which could be responsible of the spatio-temporal correlations between earthquakes. Numerical simulations of granular models for seismic faults allow to investigate the mechanical properties of the system at a level of spatial and temporal resolution not accessible experimentally [13,19,14,15,16,17,18,20]. Some studies investigate the effect of fragmentation of the gouge [19,14,15], others focus on the mechanisms at the basis of the unjamming transition [16,17,18,20].…”

“…Numerical simulations of granular models for seismic faults allow to investigate the mechanical properties of the system at a level of spatial and temporal resolution not accessible experimentally [13,19,14,15,16,17,18,20]. Some studies investigate the effect of fragmentation of the gouge [19,14,15], others focus on the mechanisms at the basis of the unjamming transition [16,17,18,20]. An important open question concerns the ability of granular fault models to reproduce fundamental phenomenological laws for seismic occurrence, as for instance the Gutenberg-Richter (GR) law [21].…”

Intermittent Granular Dynamics at a Seismogenic Plate Boundary

“…4, ξ, is associated with the average granule or bubble size [17], suggesting that the length scale associated with the seismogenic plate boundary is roughly 90 km. This value is 4-5 orders of magnitude larger than granular fault gauge [13], indicating that plate boundaries may be treated as granular systems at a macroscopic scale with the length scale ξ associated with the average mesh size of the fault network. Furthermore, we show that the heavy tails of the fluctuation distributions do not depend on the form of major fault accommodating most of the deformation between the two plates -the San Andreas fault (details in the SI).…”

“…Dynamic models of activity at seismogenic plate boundaries have generally focused on the physics within narrow (< 1m) individual fault shear zones [10][11][12][13][14]. However while large faults (e.g.…”