Spectral-finite element approach to viscoelastic relaxation in a spherical compressible Earth: application to GIA modelling

Abstract

SUMMARY The choice of the physical model of postglacial rebound plays a decisive role to derive information about the mantle rheology and viscosity from observed data. In models for the mantle rheology, an incompressible Maxwell material is often assumed in spite of seismic observations showing that the Earth's mantle is composed of compressible material. In this study, in order to assess the influence of compressibility on glacial isostatic adjustment (GIA), the spectral-finite element approach proposed by Martinec is extended to incorporate the effect of compressibility. Using this approach, the present-day velocity field is computed for Peltier's ICE5G/VM2 earth-model/glaciation-history combination considering the sea level equation in the formulation of Hagedoorn et al. The results show that the effect of compressibility on the vertical displacement rate is small whereas the horizontal rates are markedly enhanced. For example, the rate around Fennoscandia and Laurentide becomes twice as large when compressibility is considered. This large difference between the compressible and incompressible models can be reduced by adjusting the elastic rigidity of the incompressible model so that the flexural rigidity becomes approximately the same as that in the compressible model. However, differences of ∼1 mm yr−1 still remain for wavelengths longer than 8000 km. These findings show that when modelling horizontal motion induced by GIA, the influence of compressibility cannot be neglected.