A benchmark study for glacial isostatic adjustment codes

The study of Glacial Isostatic Adjustment (GIA) is gaining an increasingly im-portant role within the geophysical community. Understanding the response of the Earth to loading is crucial in various contexts, ranging from the the interpretation of modern satellite geodetic measurements (e. g. GRACE and GOCE) to the projections of future sea level trends in response to climate change. Modern modeling approaches to GIA are based on various techniques that range from purely analytical formulations to fully numerical methods. Despite various teams independently investigating GIA, we do not have a suitably large set of explicitly validated numerical results through which they may be validated; a community benchmark data set would clearly be valuable. Following the example of the mantle convection community, here we present, for the first time, the results of a benchmark study of codes designed to model GIA. The approaches benchmarked are based on significantly different codes and different techniques. This effort is performed within the Working Group 4 (WG4) of the ESF COST Action ES0701 “Improved constraints on models of Glacial Isostatic Adjustment”. Our aims are: i) testing the codes currently in use by the various teams, ii) establish a sufficiently large set of agreed results, iii) correct possible systematic errors embedded in the various physical formulations or computer implementations, and iv) facilitate the dissemination of numerical tools for surface loading studies to the community and to young scientists. The test computations are mainly based on models with spherical symmetry and Maxwell rheology and include inputs from various different methods and solution techniques: viscoelastic normal modes, spectral finite–elements and finite–elements. The tests involve the loading and tidal Love numbers and their relaxation spectra, the deformation and gravity variations driven by surface loads characterized by simple geometry and time–history, and the rotational fluctuations in response to glacial unloading. In spite of the significant differences in the numerical methods employed, the test computations show a satisfactory agreement between the results provided by the participants. Most of the existing misfits have been addressed during the preparation of the manuscript, some others are currently the subject of analysis within the WG4 community.

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