Postglacial deformation of bedrock in Finland

Glacial isostatic adjustment controls the three-dimensional deformation in Fennoscandia. Maximum vertical uplift rates based on the GPS measurements are about 11 mm/yr and horizontal motions are up to 2 mm/yr. Tectonic component is about 10% of the land uplift (or 1 mm/yr). Horizontal motions are directed outward from area of the fastest uplift. Horizontal tectonic motions are also less than 1 mm/yr. Seismic activity in Finland is low and heterogeneously distributed and the earthquake density maximums and the areas of postglacial faults have a spatial correlation. Detailed geodetic surveys indicate that crustal deformation occurs unevenly. However, the bedrock in Finland is so fractured that the deformation is distributed over a number of structures and that deformations and displacements along individual structures are very small and difficult to resolve. Fault intersections can form a locked area where stresses large enough to trigger intraplate earthquakes can build up. In the absence of intersections, the pre-existing faults can creep at a lower stress threshold. In Fennoscandia, plate-boundary tectonic stresses drive the regional compressive stress field, but to account for the current level of seismicity the glacial isostatic adjustment has a very important role. Brittle crust is near the point of failure, and, consequently, small changes, like glacial rebound related, (0.1 Mpa) in the state of stress can nucleate earthquakes are sufficient to reactive optimally oriented pre-excising weaknesses. Stress orientations inferred from the strain measurements of the first order triangulation network and seismological stress data shows (a) the dominating ridge-push/mantle drag related compression and, (b) evidence on significant local variations of the surface stress field influenced by the orientation of major fracture zones. Postglacial faults are re-activated old faults and the areas of postglacial faulting are still the most seismically active areas in Fennoscandia. The association of seismicity with glacial rebound suggests that in areas experiencing diminishing rebound, the level of seismicity decreases over time. Therefore, palaeoseismicity and geological modelling have to be combined to predict the likely incidence, magnitude and frequency of future earthquake activity. (orig.)