Response of a spherical viscoelastic Earth to seasonal loading derived from GRACE

Abstract:

Elastic deformation of the Earth caused by seasonal hydrological loading is now well established. We compute the vertical and horizontal deformation induced by large variations of continental water storage at a set of 195 globally distributed continuous Global Positioning System (cGPS) stations. Seasonal loading is derived from the Gravity and Recovery Climate experiment (GRACE) equivalent water height data, where we first accounted for non observable degree-1 components using results from Swenson et al. (2010). We find that, while the vertical displacements are well predicted by the model, the horizontal components are systematically underpredicted and out-of-phase with the observations. We discuss possible contributions to this misfit (thermal expansion, draconitic effects, etc.) and show a dramatic improvement when we do not apply a priori degree-1 coefficients but estimate and apply a posteriori a 6-parameters Helmert transform to the horizontal components. The transformation, mostly a Z translation and rotation, considerably improves the fit in phase and amplitude of the seasonal deformation model to the horizontal GPS measurements and does not affect the fit to the vertical component. We conclude that horizontal misfits result mostly from degree-one deformation plus reference frame differences between model and observations. However, the amplitude of global seasonal horizontal displacement remains slightly underpredicted. We explore several hypothesis including the validity of a purely elastic model derived from seismic estimates at an annual time scale. We show that mantle volume variations due to mineral phase transitions may play a role in the seasonal deformation and, as a by-product, use this seasonal deformation to provide a lower bound of the transient astenospheric viscosity.