The numerical modelling of the behaviour of concrete gravity dams under earthquake ground motions requires an appropriate representation of the foundation rock. Many models used in earthquake analysis of dams assume linearity of behaviour, which can be conveniently treated by frequency domain methods. Earthquake analysis of surface structures normally involves the consideration of an upward propagating stress wave, which is applied at the base of the model. Fluid flow calculations were switched off during the dynamic loading stage. Since the earthquake has a short duration compared with the characteristic time scale of the water flow through the foundation, this simplification is acceptable. The effect of the joint normal displacements on the joint water pressures during the earthquake were not taken into account. This simplification was made for reasons of economy of computation. The present two-dimensional distinct element model allows rock discontinuities to be represented explicitly in the seismic analysis of concrete gravity dams.