Visco-Elastic Response of the Earth to Seasonal Loading

Abstract:

Elastic deformation of the Earth caused by seasonal hydrological loading is now well established. Vertical displacements recorded at continuous stations of the Global Positionning System (cGPS) are fairly well predicted as the response of an elastic spherical and layered model to large variations of continental water storage. Here we investigate deviations from this model in a more realistic elastic-viscous Earth. We use a spectral method to compute displacements at the surface of a spherical elastically-layered lithosphere resting on a viscous asthenosphere, imposing a surface load derived from the Gravity and Recovery Climate experiment (GRACE) equivalent water height data. The model predicts a phase lag of several tens of days between a purely eastic response and a visco-elastic one for viscosities less than about 10x10^18 Pa.s. This time lag is most prominent in the horizontal components and for loads of spherical harmonic degrees 10-20. Vertical displacements are only weakly sensitive to the presence of a viscous asthenosphere. We validate this model by observational data sets in regions of significant seasonal loading variations and low asthenospheric viscosity.