Research Project Objectives. The overall objective of this project is to refine the understanding of the palaeoenvironmental consequences in the Baltic Sea Basin countries through reconstructions of glacio-isostatic adjustment (GIA) resulting from the advance and retreat of the Fennoscandian Ice Sheet (FIS). This GIA led to earthquakes which, under suitable conditions, left traces in the form of laterally extensive seismites characterised by a high concentration of soft-sediment deformation structures (SSDS). The identification of seismites and their palaeoenvironmental interpretation are based on sedimentological investigations combined with dating of sediments, modelling of tectonic activity during ice advance and retreat in areas around the Baltic Sea Basin, and the development of SSDS triggered by earthquakes.
Study area. The investigations are carried out in Baltic countries, as all these territories form part of Europe where the Pleistocene Fennoscandian glaciations reached their maximum extent. It is obvious that a thick ice cap causes high pressure on the substratum, resulting in crustal subsidence during ice accretion and uplift during the melting of the ice mass – and thus sometimes in earthquakes.
Seismites. Earthquakes can create traces near the epicentre, like fissures and faults, and farther away as deformed layers called seismites (Seilacher, 1969). Seismites are found within 30-40 km of the epicentre, but stronger earthquakes can cause seismites hundreds of kilometres away (Galli, 2000). Seismites form when S-type seismic waves reduce shear resistance in water-saturated clastic sediments (cf Owen & Moretti, 2011). Palaeoseismic studies depend on the recognition of seismites on the basis of their characteristics, including SSDS formed due to liquefaction and fluidization. Seismites have been reported particularly from lacustrine, marine, fluvial, and glacial successions where access to water is available.
Investigations focus not only on the identification of seismites and the analysis of their SSDS, but also on tracing faults in the bedrock which may have been reactivated during glaciation and deglaciation phases. The areas south of the Baltic Sea Basin are currently not affected by significant tectonic activity, so the faults in the vicinity of the Quaternary seismites in this area were most probably reactivated by the changing pressures exerted by the advancing and retreating ice fronts. It is likely that differences in the thickness of the Quaternary sediments near these faults also contributed to their reactivation.
The Fennoscandian Ice Sheet advanced and retreated several times, and its behaviour can be regarded as a proxy of climate change. Approximately 10% of the Earth is currently covered by ice. Fluctuations in the extent of the ice sheet had and still have consequences for the occurrence of earthquakes, and thus also for natural hazards.
References: Galli, P., 2000. New empirical relationships between magnitude and distance for liquefaction. Tectonophysics 324, 169–187. Owen, G., Moretti, M., 2011. Identifying triggers for liquefaction-induced soft-sediment deformation in sands. Sedimentary Geology, 235, 141-147. Seilacher, A., 1969. Fault-graded beds interpreted as seismites. Sedimentology, 13, 15-19.