Predicted effects of laterally varying viscosity on GIA-induced horizontal motions, including its possible impact on GPS plate motion estimates

GPS measurements have been used to study the Earth's tectonic plate motion, for which the surface motion caused by GIA is a source of noise that should be modeled and removed from the plate motion estimates. In addition, GPS measurements have also been used to constrain GIA models. For both applications, laterally varying (3-D) viscosity structure might have a significant impact on the modeling results of the GIA-induced present-day surface motions, especially for the horizontal component. In this study, we use a 3-D finite-element model to study the viscoelastic response of a compressible Earth to surface loads. By computing GIA results for a 3-D viscosity profile derived from a realistic seismic tomography model, and comparing with results computed for 1-D averages of that 3-D profile, we investigate the effects of 3-D viscosity structure on the GIA-induced present-day surface motion at areas where GIA signals are important, and we discuss how these effects can complicate efforts to use GPS observations to estimate plate motion, and to constrain 1-D GIA models.