Traditionally, 2D Discrete Element Models (DEMs) have been preferred over 3D models, for fault gouge simulations, because of the computational cost of solving 3D problems. In order to realistically simulate fault gouge processes it is important to characterise differences between 2D and 3D models and be able to assess whether 2D models are adequate for approximating 3D gouge dynamics. In this paper, 2D and 3D fault gouges are simulated as two rectangular elastic blocks of bonded particles, separated by a region of randomly sized non-bonded spherical gouge particles, sheared in opposite directions by normally-loaded driving plates. The dynamic behavior of multiple model parameterisations is analysed by examining instantaneous macroscopic fault friction (muI) statistics. The response of the mean macroscopic friction is characterised for varying values of inter-particle (microscopic) friction muP in 2D and 3D and for non-rotational and rotational particle dynamics. In the nonrotational models, realistic angular gouge mean macroscopic friction values (E[muI] = 0.6) are obtained in the simulations for a 2D inter-particle friction value of muP = 0.3 and 3D value of muP = 0.2. The rotational models exhibit mean macroscopic friction values of E[muI] = 0.3 (in 2D) and E[muI] = 0.38 (in 3D) for inter-particle friction values muP = 0.3. The 2D rotational macroscopic friction values are in close agreement with comparable 2D glass-rod (E[muI] = 0.3) laboratory experiments. In the 3D case, the simulated mean macroscopic friction values are lower than those of 3D spherical bead laboratory experiments where 0.4
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