The southern Baltic Sea between the German and Polish coast in the south and the Danish and Swedish coast in the north comprises the well-investigated structural-geological situation along the Tornquist Fan area. Different tectonic features tell the story of this polyphase activated weakness zone, like the Paleozoic Tornquist Zone in the NW, Mesozoic NE trending fault zones or salt structures with crestal grabens above, such as various Late Cretaceous to Paleogene inversion structures.

Time-migrated reflection seismic sections indicate neotectonic activity and fault reactivation, a fact supported by recent earthquakes and ongoing isostatic movements of up to 1 cm/a. Due to the reduced resolution of this thin Quaternary layer within the seismic sections, the fault activation could not be dated more precisely so far. Stress variations of the existing background stress are assumed to result from glacial isostatic adjustment (GIA) processes within the lithosphere. This study analyses fault reactivation due to GIA, covering the past 200 ka. For the first time, this glacially triggered faulting could be studied and compared on a variety of faults that differ in their strike and dip direction, age, depth, and character. Using finite element simulations, we tested if and when faults were reactivated during ice advances and retreats within the European Saalian and Weichselian glacial phases.

So-called ‘blind’ faults, which are not visible at the Earth’s surface may be the source of unexpected and potentially disastrous earthquakes and thus represent a major hazard, especially in urban areas. Detecting such hidden faults is highly important for the seismic hazard assessment of a region, but often remains a challenge, because the faults are covered by young sediments. To overcome this limitation, we visualize blind faults in the area of the Sorgenfrei-Tornquist Zone in northern Denmark with shear-wave seismic reflection surveys. The faults on the seismic surveys are interpreted based on systematic reflector offsets, the presence of continuous transparent zones that separate individual fault blocks, abrupt lateral changes in the reflector pattern, and the presence of fault shadows. The seismic surveys give evidence for a near-surface strike-slip fault system, based on the presence of flower structures and the dominance of steep faults. The distribution of the faults on the seismic surveys and the occurrence of fault-related shear-deformations bands that occur in outcrops along the nearby seas-cliff, together with fault-related basins that are developed in the study area, indicate that the northern boundary fault of the Sorgenfrei-Tornquist Zone is not an isolated fault, but a wider fault array. The study demonstrates that the shear-wave reflection seismic method is a powerful tool to image near-surface faults and is very suitable for palaeoseismological studies. The significantly improved resolution of shear-wave seismic surveys compared to those of the common P-wave reflection method is an advantage, especially in the case of small fault displacements.