Mapping of Quaternary faults by geophysical methods in the Lower Rhine Graben

Northwestern Europe is usually considered as a relatively stable continental with a low seismic hazard. After the Ms=5.3 1992 Roermond earthquake, a seismotectonic program was launched in Belgium to identify possible active faults along the Belgian side of the Roer valley (northwestern branch of the Rhine graben). After a preliminary geological and geomorphological study, it was decided to investigate a 10 km long and 15 to 20 m high scarp in the area of Bree (Fig. 1). This latter has generally been attributed to the NW-SE Quaternary fault system (and particularly to the Feldbiss fault) separating the Campine plateau to the west from the Roer valley to the east (Paulissen et al., 1985). This fault was investigated by seismie reflection profiles (Demyttenaere and Laga, 1988) which pointed out more than 600 m of offset in Neogene deposits. However, no evidence of Holocene or latest Pleistocene activity along this fault was shown before. Beside a detailed geomorphological investigation, this one has included the excavation of two 80 m long trenches across the Bree fault scarp, which have revealed the faulting and the flexuration of Holocene formations, and the evidence of at least three paleoearthquakes (Camelbeeck and Meghraoui, 1996). Even when a geomorphological expression of an active fault is present in the topography, the fault trace is often difficult to locate with accuracy , as a consequence of the complexity of faulting and of the erosion processes. Non-destructive methods - like geophysical prospecting techniques - may be then used for mapping faults beneath superficial sediments. Ground-penetrating radar and high-resolution reflection seismic techniques have recently been applied by Cai et al. (1996) and Williams et al. (1995), respectively, with the aim of imaging Quaternary faulting. This paper presents some results of a geophysical campaign performed in Bree before trenching and which included refraction seismics, electromagnetic and electrical profiling, electrical tomography, ground penetrating radar and high resolution seismic reflection.