Modelling of the GIA-induced surface gravity change over Fennoscandia

This paper deals with the modelling of surface gravity change in Fennoscandia, induced by postglacial rebound or Glacial Isostatic Adjustment (GIA). The theoretical foundation is based on the theory introduced by Peltier, 1974 and Peltier, 1976 for a spherical, non-rotating, laterally homogenous, viscoelastic, Maxwell Earth and the solution of the Sea Level Equation, originally introduced by Farrel and Clark (1976), with time-dependent coastline geometry. The ice history is defined by the ice model ICE-5G. Rotational feedback is not included.The sensitivity of predictions of present day gravity rates g˙, with respect to a selection of assumptions and approximations, is investigated numerically. Six model variants are defined: (i) linear relation between g˙ and the vertical deformation rate u˙, (ii) direct attraction expressed in terms of internal and (iii) external harmonic series expansions as well as by (iv) analytical integration over rectangular prisms. For the most rigorous treatment of the direct attraction, the effect of simplified modelling of the sea level is also investigated. These modelling approximations of the sea level change include (v) fixed shorelines and (vi) eustatic sea level change. Predictions of g˙ for the model variants are plotted, compared and discussed.The most rigorous model (iv) and the linear model (i) differ less than 0.03 μGal yr−1 over land and close to, or over, the ocean the difference reaches maximally ∼0.5 μGal yr−1. Due to truncation at 180°, the high frequent nature of the direct attraction is not properly described by models (ii) and (iii). The two simplified sea level modelling approximations (v) and (vi) induce differences, compared to the most rigorous model exceeding 0.2 μGal yr−1 over land, i.e. about one order of magnitude worse than the linear model.

► Comparison of GIA-models with different degrees of complexity.
► A linear relation between u˙ and g˙ is a good approximation inland.
► The treatment of the ocean is important especially close to the coast.
► Green's function for gravity using internal or external series differs significantly.