The topic of VILMA, Viscoelastic Lithosphere and Mantle model, is the reduction of global GRACE, GPS and altimetry data with respect to the glacial-isostatic adjustment applying a 3D viscoelastic earth model.
The project is set up for the development of a numerical code, which enables the consideration of lateral variations of viscosity in the lithosphere and mantle for modeling of global deformations and gravity changes.
The delayed response of the earth to the last glaciation (> 10,000 yr before present) as well as to decadal (< 100 yr) and recent glacial changes (< 10 yr) is evident in fossil sea-level indicators, the secular trends in gravity, GPS, tide-gauge and altimetry observations, and must be modeled by a viscoelastic mantle rheology. Predicting the glacially induced response allows us to separate it out from the observations that include other contributions, such as hydrologically and oceanographically induced changes. The latter occur over time scales of up to several years, for which an elastic earth model is adequate. Lateral variations in the viscoelastic structure of the Earth's lithosphere and mantle, which are related to plate-tectonic units on spatial scales of thousands of kilometers, have to be considered on the global scale intended in this program. Two approaches are possible to model the integral response of the earth to glacial changes on the individual plate-tectonic units. The traditional approach modeling the earth's response to glacial changes based on the loading of one-dimensional (1-D) earth models applied only to particular plate-tectonic units, followed by the superposition of the individual responses. This approach has the drawback that the integration of the individual responses to determine the global signal is not straightforward for the gravity variations observed by CHAMP/GRACE because coupling effects between the loading processes cannot be considered. In the alternative approach, a 3-D earth model is used to compute the global-scale response of the earth forced by all loads simultaneously. Considering, in addition to lateral variations in viscosity, a non-hydrostatic stress field at the initial state allows the linkage to present-time mantle convection models. Another reason for using a 3-D earth model is that such a model is also applicable to regions near tectonic boundaries, such as Patagonia, Antarctica, Svalbard, Alaska and Iceland. Along all these boundaries, recent glacial changes are observed and their modeling with a 1-D earth model is inadequate.