Meso-mechanical analysis of deformation characteristics for dynamically triggered slip in a granular medium

The deformation characteristics of a sheared granular layer during stick-slip are studied from a meso-mechanical viewpoint, both in the absence and in the presence of externally applied vibration. The ultimate goal is to characterize the physics of dynamic earthquake triggering, where one earthquake, i.e., slip on one fault, is triggered via the seismic waves radiated by another spatially and temporally distant seismic event. Toward this goal, we performed Discrete Element Method simulations of a two-dimensional packing of disks, mimicking a mature geologic fault. These simulations were used to investigate the affine and non-affine deformations inside the granular layer and their spatial-temporal evolution across the stick-slip cycle. The simulation results show that slip in general is accompanied by the appearance of localized regions with high values of both affine and non-affine deformations. These regions are temporally correlated and are mainly concentrated in a shear zone at the interface between the granular layer and the driving block. Dynamic triggering is found to initiate slip when vibration is applied late in the stick-slip cycle, when the system is close to a critical state. It is also found that vibration itself introduces a large amount of affine and non-affine strains, which leads to the initiation of slip at lower shear stress than an equivalent slip event without vibration.