The importance of the third dimension in granular shear

[1] We present new numerical results showing the importance of the third dimension in simulations of granular shear. Our 3-D numerical models of gouge layers under shear exhibit friction levels notably higher than 2-D models and values that approach those of recent laboratory experiments on spherical beads. Fluctuations in friction are directly related to grain reorganization normal to shear. In 3-D, these fluctuations are reduced due to significant particle motion in the third dimension, leading to a steady frictional sliding curve. Since these fluctuations are small compared with 2-D models, our 3-D simulation offers potential to investigate small amplitude but extremely important second order rate and state friction effects that fundamentally control the strength and stability of fault zones.