Seismic Structure of Earth’s Crust in Finland

Abstract

During the last three decades, the Earth’ crust in Finland has been extensively studied using seismic refraction/wide angle surveys, near vertical reflection profiling and tomographic measurements, with profiles and networks covering most of the territory of Finland. A major change in the seismic structure of the crust occurs along the Lake Ladoga – Bothnian Bay zone. In the velocity models NE of the zone the crustal thickness varies from 40 to 48 km, whereas in SW underneath the Svecofennian orogen, the thickness can reach values over 60 km. In general, all the crust is reflective, but there is a clear change across the zone. In SW lower crustal reflectivity is diffuse whereas in the north it is strong, with a clear base of reflectivity at the MOHO. Based on reflectivity, the crust is composed of blocks with sizes varying from tens of kilometers to a couple of hundred kilometers. This block structure is also visible in the tomographic 3D velocity models.

The seismic studies have significantly improved understanding of the crustal structure and evolution of the Fennoscandian Shield, showing that the same plate tectonic processes which are active today in the Earth formed the Fennoscandian shield 2 billion years ago.