1. Background to >10 years of BIFROST activities Jan M. Johansson 1, Hans-Georg Scherneck 1, Rüdiger Haas 1, Sten Bergstrand 1 Martin Lidberg 1,2, Lotti Jivall 2, and Bo Jonsson 2 1. Onsala Space Observatory, SE-439 92 Onsala, Sweden, Email: jmj@oso.chalmers.se 2. National Land Survey, Gävle, Sweden The BIFROST project monitors movements of the earth crust in three dimensions In the BIFROST (Baseline Inferences for Fennoscandian Rebound Observations Sea-level and Tectonics) project continuous GPS observations are used to determine three-dimensional motion at about 100 stations (see figure to the right). The project has been active since 1993 and is based on data obtained from the GPS networks in the northern Europe and the IGS global network. In addition occasional GPS field campaigns have been carried out in order to include measurements at tide gauge sites. For the GPS data analysis we use the GIPSY, Bernese, and GAMIT software packages. Ice and Ocean Load The maximum glaciation stage of the Pleistocene occurred 18 kyr ago. Shown to the right are three later stages of deglaciation at 10,9 and 8 kyr before present. The right column shows the change of the accompanying ocean load. The receding ice edge and the increasing sea level cause the ocean load to increase while the land uplift displaces sea water outward from the rebound area. The load model is fully consistent with the visco-elastic self- graviting planet. GPS Time Series, daily processing since August 1993 We produce time series of daily determinations of geodetic positions. In the figures below the bottom diagrams show the vertical, the middle the east and the top the north position, respectively. The GPS results are shown in black with green bars representing the standard error. The motion has been reduced with respect to the rigid plate that moves together with the most important European stations. Predictions based on a model are shown in red. The model comprises one constant rate of motion in each component and occasional offsets at times when antenna configuration was changed. In the predictions we also include a regional common mode of motion that is derived from the measurements at neighboring stations using an Empirical Orthogonal Functions method. The fitted rate is shown in each diagram. The left figure shows the case of a southern station (Onsala) far away from the postglacial rebound center, and the right one located more centrally (Sundsvall). Deformation Rates The map below shows observed vertical motion in colors and horizontal motion by red arrows. The blue arrows denote predictions from a model for visco-elastic isostatic adjustment. GPS and Tide Gauges Vertical motion of the crust relative to the gravity center of the Earth is inferred from GPS. This motion is compared to tide gauge observations in the figure below. Tide gauges observe the change of land level relative to the ocean. The change of the gravity potential has been modeled out. Plotting one data set against the other reveals: 1) residual unmodeled gravity field effects as a deviation from a uniform slope and 2) a regional change of the sea level as a negative intercept on the abscissa. From the plot the sea level appears to rise with about 1 mm/year.