Geological and geophysical evidence for large palaeo-earthquakes with surface faulting in the Roer Graben (northwest Europe)

Abstract

From the analysis of geological, geodetic and geophysical data we provide clear evidence of seismogenic faults capable of producing large earthquakes in intraplate Europe. Previous studies (Paulissen, Vandenberghe & Gullentops 1985; Van den Berg et al. 1994; Geluk et al. 1994) have yielded some constraints on the rate of crustal deformation along the Roer Valley, a graben structure crossing the Netherlands, Belgium and Germany, and have allowed us to address the fundamental questions: can intraplate earthquakes rupture the surface in this part of ‘stable’ continental Europe, and if so, what is their return period? Detailed palaeoseismic investigations have been carried out in Belgium along a 10 km long fault scarp which is the morphological expression of the Feldbiss Fault, the southwestern border fault of the Roer Graben Camelbeeck & Meghraoui 1996). The scarp is multiple and the frontal fault scarp offsets young deposits and alluvial terraces. Field investigations using geological and geomorphological methodologies combined with geophysical prospecting provide evidence of Holocene seismic surface faulting. From 14C dating it is suggested that the last earthquake along the fault scarp occurred between 610 AD and 890 AD. Levelling profiles across the scarp suggest that it produced a vertical coseismic displacement of 0.5-1 m along the scarp. If we suppose that the last surface-faulting earthquake ruptured the whole seismogenic layer (17 km thickness) over a minimum length (10 km) corresponding to the length of the Bree scarp with an average slip of 0.6 m, its seismic moment was at least 3.1×1018 N m (MW=6.3) for an average rigidity μ=3×1010 Pa. By estimating the offset of the main terrace of the Maas River by slippage along the Feldbiss Fault, we calculate the average Late Pleistocene vertical deformation rate as 0.08±0.04 mm yr−1. Palaeoseismic information combining the trench and geomorphic observations suggests the occurrence of two surface-faulting earthquakes during the last 20 kyr. A third dates before 28–35 kyr BP. Then, if the time distribution of earthquakes is uniform, a return period of 12±5 ka and a vertical deformation rate of 0.06±0.04 mm yr−1 are inferred.