Postseismic displacements and strains observed at the earth's surface can be explained through the relaxation of a viscoelastic asthenosphere underlying a purely elastic crust. A theory is presented for modeling the effects of postseismic relaxation on a spherically symmetric earth. For an earthquake point source located in the upper elastic layer, the displacement field is decomposed into its toroidal and spheroidal components. A linear (Maxwell) rheology in the viscoelastic layers is assumed, enabling the use of the correspondence principle for the solutions of the equations of static equilibrium. Displacement and strain fields are then calculated using normal mode summation. Computational results are presented for two simple earthquake sources: a strike slip fault and a uniaxial thrust fault. The patterns of postseismic displacements and strains are found to depend strongly on both the earth model and earthquake source geometry. In particular, the elastic plate thickness, asthenosphere thickness, fault type, and fault length each play a major role in determining the spatial pattern of postseismic relaxation effects. Asthenospheric viscosity controls the temporal pattern of relaxation.