Global postseismic gravity changes of a viscoelastic Earth

A spherically symmetric Earth model with a viscoelastic mantle and an invis cid core is used to study the gravitational perturbation due to a lithosphe ric shear dislocation in the coseismic and postseismic regime. Our approach is based on a quasi-static solution of the equilibrium equations taking ad vantage of the normal mode technique for a layered incompressible Earth wit h linear viscoelastic rheology. We perform a systematic study of space and time evolution of gravity changes associated with a wide selection of seism ic sources investigating the effects of several different mantle viscosity profiles. Our results show that the gravitational signal is sensitive not o nly to the viscosity of the asthenosphere but also to that of the mantle, i ncluding the portion below the 670 km discontinuity. As a case study, we co mpute the coseismic and postseismic gravity perturbation due to the 1964 Al aska earthquake. The last section is devoted to the analysis of the postsei smic relaxation rates of the gravitational perturbation following that eart hquake for several different viscosity profiles of the mantle. Our results show that experimental attempts to obtain reliable detection of static and quasi-static gravitational perturbation associated with seismic events woul d be of great importance in understanding the dynamic and viscosity profile of both upper and lower mantle.