Horizontal deformations of the Earth's crust in the Baltic region from GPS data

The horizontal deformations of the Earth's crust were identified from Global Positioning System (GPS) data measured during the years 1992 and 2003. A tensor analysis method was applied to the results using a finite element modelling approach. The network consists of 19 triangular segments of 15 GPS sites located in Estonia, Latvia, Lithuania and northeast Poland. The relative lengthening varies from -0.93x 10(-6) to + 0.58x 10(-6). The dilatational strain rate changes from -3.2x10(-8) yr(-1) to + 1.5x10(-8) yr(-1). Three strain provinces, i.e. West Lithuanian-West Latvian, East Lithuanian-East Latvian, and NW Estonian, separated by the Middle Lithuanian and Paldiski-Pskov transitional zones, are identified. These provinces closely correlate with the major lithotectonic domains of the crystalline basement thus pointing to the importance of lithospheric compositional heterogeneities for the strain pattern. In the West Lithuanian-West Latvian province the horizontal strain is dominated by NNW-SSE extension at -0.5x10(-8) to -1.5x10(-8) yr(-1), while the second principle strain rate axis is compressional. The East Lithuanian-East Latvian province is dominated by the contractional deformations; the strain rate reaches -3 x 10(-8) yr(-1) with a N(N)W-S(S)E orientation of maximum compression. The north-eastern part of Estonia is subject to a bi-axial extension, which is also revealed in the Middle Lithuanian transitional zone. The NW-SE oriented Paldiski-Pskov transitional zone is characterised by stress field rotation. The identified strain rates are compatible to those obtained from other cratonic areas (e.g. Fennsocandian Shield, North America, and India). Furthermore, the domination of extensional deformations in the western and northern parts of the Baltic region correlate with the GPS data from the Fennsocandian shield. The identified pattern of the horizontal deformation is important for understanding seismic processes in the Baltic region. For instance, increased seismicity in easternmost Lithuania and adjacent Belarus is related to high compressional strain rates. Furthermore, the inversion of recent vertical movements is characteristic for the compression-dominated eastern province, as defined by precise geodetic levelling data, while no inversion is reported from extension dominated areas. A rotation of the stress field is recognized in the Kaliningrad District. Furthermore, the highest dilatational strain is measured the Kaliningrad District attaining +1.5x10(-8) yr(-1) and may be causally related to the strong Kaliningrad earthquakes of 2004.