Formation and evolution of shear structures in sheared granular gouge

SUMMARY Fault cores have layered shear structures that evolve with time. Probing details of the accompanied gouge grain breakage and granular flow can unveil the complex evolving mechanism of fault. Here, irregular breakable grains are incorporated in a numerically simulated sheared granular gouge to show the evolution of shear structures. Granular quantities (e.g. non-affine displacement and local granular temperature) are adopted to depict the microstates of gouge in different macro shear stages and during stick-slips. Granular plasticity (defined as plastic granular flow here) is scattered in the linear stage, while it starts to cluster near the plate in the yield stage. Then, the shear localization becomes the strongest in the unstable shearing stage and weakens in the later stick-slip stage. In the stick phase during the stick-slip stage, some particles are more prone to unlocking due to the geometry of the fault and the arrangement of particles. Small local slips occur between these particles. The accumulation of small local slips between these particles can cause large dynamic failures. Finer grains in the principal slip zone decrease the transmitted shear force and render it easier for the granular gouges to rearrange themselves, causing the shear to localize in these zones. The long-term stick-slip cycles of natural faults result in the layered shear structures in the fault cores. Although the granular gouge between fault planes is composed of discrete particles or grains, the granular mass ruptures like a solid when slip events happen.