Effects of fluid-flow on the strength of gouge-filled fault zones: liquefaction and dilatant hardening

Slip within natural groundwater-saturated fault zones often occurs within a layer of unconsolidated granular fault gouge. The strength and sliding stability of such a fault will be determined by friction on grain surfaces, the geometry of the network of grain contacts, and the effective confining stress (confining stress minus fluid pressure) that creates this force network. Since shear in granular materials is always accompanied by dilation and compaction, local pore fluid pressure perturbations are created in shearing regions that both affect the granular interactions and drive fluid flow. Discontinuous numerical models of granular material have been used to explore both grain-scale and fault-scale properties and deformation. We use such a model to explore deformation patterns within the shearing gouge, and how the localization of shear bands varies with the flow of pore fluids under different boundary conditions and properties (permeability, porosity) of both the gouge and the surrounding country rock. We conducted a range of numerical experiments using the Goren et al. (JGR, 2011) coupled granular/fluid model. Dynamic stresses and grain rearrangements are modeled using the discrete element method. Local variations in porosity and permeability (taken to by a Kozeny-Karman function of porosity) are restricted onto a finite-difference grid for solving pore fluid pressure. Pressure gradients on the grid are interpolated to form drag forces on individual grains, fully coupling the two phases together. A set of non-cohesive grains is confined between rough-walled undeformable but permeable fault blocks. The layers we modeled were all approximately 70 grains thick, but mean gouge permeability was varied to simulate different grain sizes. A mean effective confining stress was applied to the layer, and the confining fault blocks were sheared either at a constant rate, or with a variable shear force that allows stick-slip behavior. Dilation and compaction in shearing regions of the gouge layer generate fluid pressure perturbations. The magnitude of these perturbations varies with the intensity and rate of pore volume change and the permeability of the gouge (k). With decreasing mean effective stress (N), positive pressure perturbations are more likely to reach the level of liquefaction. During liquefaction, system-spanning force chains in the granular skeleton are lost and the layer begins to compact at rate proportional to Nk. After a small amount of compaction, force chains reform and drive a rapid dilation of the layer that eventually generates large negative pressure perturbations. While shear strength of the layer drops to essentially zero during the liquefied periods, during the rapid dilation phase shear strength briefly spikes to several times the typical background value. This rapid variation in strength points to the de-stabilizing nature that pore fluid flow can have on fault zones. We will catalogue the conditions under which liquefaction and dilatant hardening become important effects in natural fault zones and present a detailed view of the liquefaction process.