On modelling of postglacial gravity change

Doctoral thesis, 2013

Glacial isostatic adjustment (GIA) is the Earth's response to glacial-induced load variations on its surface. This phenomenon can today be observed in, for example, North America and Fennoscandia. Different observables, such as vertical and horizontal deformation of the crust, relative sea level change and disturbances in the gravity field, contribute different and complemental information on the phenomenon. Knowledge of the gravitational component is important for understanding the underlying geodynamical processes. Further, accurate predictions of the gravity change are important for e.g. reductions of geodetic observations to a reference epoch. During the last decade, efforts to observe the surface gravity change in Fennoscandia have been intensified and the observational accuracy successively improved. This offers new possibilities at the same time as it puts new demands on modelling. The purpose of this thesis is to study some aspects of the modelling of GIA-induced gravity change. We show that gravity stations close to the sea are affected by non-tidal sea level variations and that the direct attraction from the sea water constitutes a crucial contribution. Accurate modelling of the direct attraction from sea water is an intricate matter. We use different methods to model the direct attraction from GIA-induced sea level change and show that standard methods are not adequate. Further we solve the forward GIA modelling problem and show numerically how predictions of the gravity change are affected using different approximate methods. We also investigate the relation between gravity change and vertical displacement as they are predicted with the earth model depending on different sets of structural and rheological parameters. A linear relation is used as a reference. Deviations from the linear approximation are small, especially in Fennoscandia. The relation differs more between different regions included in the study than between different earth models within each region. The thesis also includes an overview of observational efforts to determine the postglacial land uplift and gravity change in Fennoscandia, as well as a general discussion on some modelling issues that form the background for the motivation of the thesis.