Influence of Grain Fracture and Shape Evolution on Fault Gouge Strength in Particle Dynamics Simulations

The San Andreas Fault and other large-displacement fault zones contain significant amounts of fault gouge that might dramatically reduce frictional strength during dynamic slip. Based on laboratory experiments, gouge strength is thought to be influenced by several factors, including interparticle sliding friction, shear zone dilation, and grain fracture. Numerical simulations using the distinct element method (DEM) have easily demonstrated the role of interparticle friction and dilation, but have yielded coefficients of sliding friction far below those measured in the lab. Enhanced particle rolling, and differences in grain characteristics have been thought to be major factors causing the apparent disparity between laboratory observations and numerical studies of granular friction. To better reproduce the conditions of laboratory experiments and to naturally reduce particle rolling, new numerical experiments have been designed in which round particles are bonded into triangular grains of different size. The aggregate grains are allowed to break if the stress exceeds maximum grain strength. Initial results show that grain fracture is a dominant deformation mechanism when angular grains interlock, and fault friction increases to the level observed in laboratory experiments. The progressive break up of large grains produces many smaller angular grains, and ultimately, round particles, at which point interparticle rolling becomes the dominant deformation mechanism, and fault friction is reduced. This progression may not be representative of real materials, as fracturing of real grains generates smaller angular particles that continue to interlock, and inhibit particle rolling. However, field observations have shown significant grain rounding in fine-grained gouge, suggesting that angular grains become more rounded after longer slip due to wear; this effect is not reproduced in short-displacement laboratory experiments. Our initial results confirm that particle shape and configuration determine the degree of particle interlocking, and therefore the dominant deformation mechanisms. The active deformation mechanisms are strongly displacement dependent and play an important role in controlling fault strengthening or weakening.