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SUMMARY A numerical model is developed to simulate faults and shear zones in rocks. It consists of a 2‐D set of soft spheres in plane‐strain compression tests. The soft spheres obey Newton's equations of motion and initially interact with viscoelastic forces. The fracturing process is simulated by the transition from ‘attractive‐repulsive’ forces to solely ‘repulsive’ forces. The behaviour of the solid is studied by varying two independent parameters: the density of pre‐existing fractures and the confining pressure. The density of pre‐existing fractures controls the intrinsic cohesion of the rock. A transition from brittle to ductile behaviour is generated by letting this parameter vary. The deformation is localized along narrow shear zones when the solid is intact. As the cohesion decreases, the deformation becomes more homogeneous. The effect of the confining pressure is then studied for different cohesions. In loose media a variation of the stress drop in stress–strain curves is observed.

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