New Appraisals of GIA Modelling and Space Gravity (GRACE) Data Treatment

Abstract

The new generation of space gravity data, from GRACE and the incoming GOCE, rejuvenated our interest on some basic issues in GIA modelling, and made us to conceive new approaches in space gravity data treatment. Once cleared for hydrological effects, for those of the atmosphere and oceans, and present day mass imbalance, the signal from the deep interior is that from the GIA response to Pleistocene deglaciation. We have thus reconsidered the basic physics underlying the differential equations controlling the GIA readjustment of the mantle, following the philosophy of pushing the treatment of these differential equations as far as possible from the analytical standpoint. We obtained, among other findings, that the contribution of the denumerably infinite compressional D-modes can be dealt with accurately by normal mode approach, disclosing the very intimate nature of the compressional D-mode cluster point, which we demonstrated not contributing at all to deformation: our results definitely prove the correctness of normal mode approach, even for compressible mantle models. In parallel with these new theoretical achievements, we implemented a new procedure aimed at deriving a weighted surface mass distribution in water equivalent, starting from an initial guess, to exctract the secular gravity effects of present-day phenomena. Once these gravity effects from the various Earth's compartments except its interior are carefully removed from GRACE data, we remain with a gravity pattern where the effects of PGR are clearer, ready to be compared with viscoelastic model predictions. This cleared gravity pattern allows us to pursue a global preliminary viscosity inversion, greatly improved with respect to those based on un-cleared gravity data. Theoretical achievements in viscoelastic modelling are prerequisites to better understand many Solid Earth phenomena, GIA in the first place, and we show that our data treatments can improve interpretation of space gravity data