Global postseismic deformation in a stratified viscoelastic earth: effects on Chandler wobble excitation

The principal purpose of this paper is to examine whether movement of matter in a stratified viscoelastic earth can enhance the excitation of the Chandler wobble. We have constructed analytically a two-layer model consisting of an elastic lithosphere overlying a Maxwell viscoelastic mantle in order to calculate explicitly the temporal evolution of the inertia tensor from earthquake faulting. It is found that the ability of the viscoelastic flows to produce an additional increment of the inertia tensor depends critically on R, the ratio of the inner radius to the outer radius. Only for R = or > 0.7 is there a substantial increase. For the earth (R = 0.98) an amplification of between four and six is obtained from the models considered. However, the time scales over which this enhancement of the moment of inertia takes place is about 10 Maxwell times, thus making this model unlikely to excite the Chandler wobble for values of the mantle viscosity derived from postglacial uplifts. This phenomenon of strain amplification occurs by virtue of 'transient membrane mechanics', as the ratio between the horizontal wavelength of the deformation field is large compared with the elastic shell. By means of a similarity argument we have found that for angular orders <40-45 'transient membrane mechanics' can be important in global postseismic deformation in the earth. This mode of transient deformation arises from the three- dimensional nature of the problem and hence cannot be modeled with conventional flat earth models.