The mathematical description of fluid-saturated rocks developed by Biot and others has been applied to earthquake dynamics, providing a quantitative relation between rock properties and some seismic precursors which are being widely evaluated as a basis for earthquake prediction. This relation is a quantitative expression of the dilatancy-diffusion model of earthquake dynamics, with one significant refinement in concept: the dilatancy here is a consequence of the nonlinear stress-strain curve for rocks, rather than a result of crack formation as the rock approaches failure. Solution gas plays a key role. The computed imbalance in fluid pressure following the stress-release by an earthquake provides a starting point for describing the time sequence and spatial migration of aftershocks.