Palaeoseismic structures in Quaternary sediments of Hamburg (NW Germany), earthquake evidence during the younger Weichselian and Holocene

Abstract

Investigations at a construction site in Hamburg (NW Germany) exposed palaeoseismic structures from moderate to strong earthquakes. Based on the large size of large blowout clastic dykes being up to 2.0 m wide and 2.5 m high, as well as the occurrence of infill structures, erratics/rafts of up to 9 kg in weight, seismites s.s. and decimetre-scale folds or seismoslumps, the magnitude of the earthquakes could be in the order of up to M ≥ 6. This is significantly higher than previously assumed for prehistorical seismic events in NW Germany. The structures are assigned to earthquakes possibly related to NW–SE-oriented faults that are evident in Lidar and SAR surface interpretations. Organic material from blowout-related infill bowls, assumed to be synchronously formed collapse depressions, are analysed for 14C ages. The dates reveal ages between 31,500 and 1200 14C cal a BP, indicating five prehistoric earthquakes. Accordingly, three of these earthquakes occurred before and after the main Weichselian glaciation phase (ca. 31,350, 26,850 and 18,980 14C cal a BP), and two occurred during the Subatlantic (ca. 4900 and 1200 14C cal a BP). The Weichselian events indicate seismic activity in the ice marginal zone during or following interstadials/Dansgaard–Oeschger events, as well as suggesting that advancing ice sheets foster earthquakes.