The response of planetary bodies to perturbations with periods of less than several days is conventionally studied assuming elastic constitutive behaviour. The defining feature of elastic solids is the proportionality between stress and strain. The phenomena governed by the associated field theory of elastodynamics can be classified into the propagation of seismic waves, free oscillations, and short-period body and load tides. For the latter two classes, it is necessary to account also for gravity and the presence of initial stress in planetary interiors. This is accomplished by the field theory of gravitational elastodynamics (GED).
