Marine geology and tectonics Marine geology

9.2.1

9.2.1.1 Marine geology and tectonics Marine geology

The geology of the Baltic Sea comprises bedrock covered by sediments, as shown in Atlas Map GE-01-Espoo. The morphology of the bedrock is a result of fluvial and glacial erosion, with troughs and valleys created by erosion of less resistant bedrock layers forming pronounced seabed features.

The bedrock is covered by Quaternary sediment deposits formed during the last ice age and during different post-glacial Baltic Sea development stages /30/. The deposits are dominated by glacial till comprised of a mixture of grain sizes, from clay to boulders, varying in thickness from a few metres to several tens of metres. These till deposits are hard and possess high strength as a result of the pressure of the overlying ice. Late-glacial and post-glacial sediments occur upon the glacial deposits. The late-glacial sediments are mainly clay, silt and sand. These deposits are covered by even younger deposits of mainly clay and silt.

The distribution of sediments in the sea floor is a result of the Quaternary geological history of the Baltic Sea and the subsequent sediment dynamic in the marine environment. Bedrock without a cover of younger sediments is found only in nearshore areas in the northern Baltic Proper and the Gulf of Finland or where steep slopes are present on the seabed. Exposed till is found on top of or at the sides of topographical heights and on steep slopes at the sea floor.

Tectonics

The Baltic Sea is situated on the Eurasian continental plate, providing relatively stable geological conditions. The area is nearly devoid of earthquake activity in global terms /31/. However, seismic activity in the form of small-scale earthquakes occurs occasionally. This activity is mainly the result of stress release in the lithosphere caused by the uplift following the deglaciation at the end of the latest ice age. Along the proposed NSP2 route, the recent relative uplift varies between less than 3 mm/year to about -1 mm/year.

Atlas Map GE-03-Espoo shows incidents of earthquakes measured in the Baltic Sea during the period 2002-2015 in Finland, Sweden and Denmark, as well as the location of the so-called Tornquist Zone (a 30-50 km wide zone of extensive faulting developed in late Cretaceous/early

Tertiary time). All recorded incidents have a magnitude below 5 on the Richter scale, confirming the low seismic activity in the area.

A probabilistic seismic hazard assessment carried out for the NSP route corridor in 2007 concluded that the seismic hazard along the pipeline route is low /33/. This assessment is considered to remain valid for the proposed NSP2 route, due to the proximity to the NSP route.

During a marine geological mapping in 2005, the Geological Survey of Sweden (SGU) discovered scars of two submarine landslides in the south-east Baltic Sea. An additional landslide scar was identified in the Swedish EEZ in 2014. The location of the scars, in glacial sediments in areas of very gently sloping sea floor, strongly suggest that the slides were triggered by palaeoseismic activity, probably at the very end of the Late Weichselian or during the Early Holocene geological time periods /32/. Landslides have not been reported in the Baltic Sea in recent geological time.

9.2.1.2 Bathymetry

The bathymetry of the Baltic Sea is determined by the geological settings and history as outlined above. The bathymetry is the subsea landscape, which is important for both the design of the pipeline route and for the marine life in the Baltic Sea.

The Baltic Sea is a semi-enclosed area connected to the surrounding oceans through the shallow-water and narrow Danish straits, which connect the brackish shallow-water of the Baltic Sea with the oceanic water of the North Sea. The bathymetry is characterised by basins separated by sills /34/, with a maximum depth of 459 m and average depth of 52 m /28/, /29/. Two sills in the transition zone between the North Sea and the Baltic Sea (the Darss Sill, with a water depth of 18 m, and the Drogden Sill, with a water depth of 8 m) effectively limit the inflow of saline, oxygen-rich water to the Baltic Sea to rare occurrences of storms from the west (see Section 9.2.2).

The proposed NSP2 route runs across several of the Baltic Sea sub-basins from the Gulf of Finland in the north-east to the south-western Baltic Sea (see Figure 9-1 and Atlas Map BA-01-Espoo). A depth profile showing the bathymetry along the proposed NSP2 pipeline route from the Russian landfall to the German landfall is shown in Figure 9-2. Figure 9-3 and Figure 9-4 show the detailed bathymetry at the Russian and German landfalls respectively.

Figure 9-1 Bathymetry of the Baltic Sea, showing the preferred NSP2 route option and the various sub-basins. The Darss Sill and the Drogden Sill are shallow-water thresholds controlling the inflow of saline water to the Baltic Sea.

Figure 9-2 Water depth per KP along the NSP2 route, from the Russian landfall to the German landfall.

Figure 9-3 Bathymetry near Russian landfall area.

As shown in Figure 9-3, the bathymetry near the Russian landfall area increases smoothly from 0 m at the landfall to a depth of approximately 40 m at a distance of 30 km from the landfall.

Figure 9-4 Bathymetry near German landfall area

As shown in Figure 9-4, the area near the German landfall is a shallow water region (in general depths less than 20 m) comprising the Pomeranian Bay and the Greifswalder Bodden. These areas are characterised by shallow banks (the Oderbank (not in figure) and the Boddenrandschwelle respectively) and artificially built navigation channels /35/.