Geophysical evidence of neotectonic activity on the Aller Valley Fault system in northern Germany

Northern Germany is commonly regarded as a low seismicity area, but a number of historic earthquakes with intensities of up to VII have occurred in this region during the last 1200 years. The Aller Valley fault system, with a length of about 250 km, is one of the major fault systems in northern Germany. It strikes NW-SE and extends from the Magdeburg area over Wolfsburg across Lower Saxony to the area of Bremen and Oldenburg, close to the border to the Netherlands. This fault was highly active in the Mesozoic. Reflection seismic profiles of the petroleum industry show that during the Triassic it was a normal growth fault, which was inverted during Late Cretaceous compression. In addition, a large number of earthquakes have occurred close to the Aller Valley Fault system between AD997 and 1576.

We carried out seven, high-resolution, shear(S)-wave reflection seismic profiles accompanied by georadar in an area of the Aller Valley Fault system near Lehringen in Lower Saxony. Shear waves propagate up to twelve times slower than P-waves in unconsolidated sediments, making it the ideal tool to investigate the near-surface. The geological map displays a rhomboidal outcrop of Eemian sediments in this area, which we hypothese is a pull-apart basin.

The S-wave seismic profiles image a number of Eemian and Weichselian depocentres at depths of 10-30 m that are progressively displaced north-eastwards by a series of steep to vertical faults that propagate from depth. The georadar data provide a high-resolution imaging of the upper 5 m of the Weichselian sediments and support the findings of the seismics. In some georadar profiles, fault structures in the Weichselian sediments are imaged, indicating that the faults must still have been active after sedimentation. OSL-dating of a hand drill core has substantiated the geological interpretation. We postulate that the recent fault activity is due to glacial isostatic adjustment.