Postglacial rebound and Earth's viscosity structure from GRACE

The Gravity Recovery and Climate Experiment (GRACE) satellite mission was launched in March 2002 and has an expected 5-year lifetime. One potential application of GRACE measurements of time-variable gravity will be to isolate the postglacial rebound signal, which can then be used to estimate the Earth's viscosity structure. In this paper we present a sensitivity analysis of simulated GRACE data, designed to assess the accuracy with which those data can be used to recover a simple model of Earth viscosity. We find that without combining with any other data type, but ignoring complications caused by uncertainties in the global ice loading history, GRACE data alone would allow us to determine the viscosity of a uniform lower mantle layer and an upper mantle/transition zone layer to within +/-30-40% and to estimate lithospheric thickness to within +/-15-20%. GRACE will have a harder time differentiating between the separate viscosities of the transition zone and upper mantle, but accuracies of within a factor of 2 might still be achievable for those parameters. Errors in the ice loading history could significantly degrade these viscosity estimates, particularly for the transition zone and upper mantle. The accuracy of recovery of the true Earth viscosity will depend in part on how well the model parameterization used for the grid search can represent the true Earth structure. However, combining GRACE data with data from other more traditional measurements of postglacial rebound has the potential of dramatically improving viscosity estimates throughout the Earth, particularly in the lower mantle.