The dynamical laws which guarantee conservation of momentum and continuity of traction across a tangential slip surface are the same in an infinite, homogeneous, elastic medium which is subject to a large, uniform, compressive isotropic stress as in one which is not. It follows that many of the results from the theory of seismic faulting are applicable in this case. Any consideration which explicitly involves the concept of energy is different in the two instances, because in the former there are additional second order terms which arise in calculating the work done against the initial stress. The relative contribution of these terms to the net elastic strain energy released by a typical seismic fault appears to be no more than a few percent.