Eos, Vol. 77, No. 42, October 15, 1996 VOLUME 77 NUMBER 42 OCTOBER 15,1996 PAGES 405-416 Large Earthquakes in Northern Europe More Likely Than Once Thought PAGES 405, 409 T. Camelbeeck and M. Meghraoui Northern Europe would typically be classi- fied as a relatively stable continental region where seismic hazard is low. Indeed, in com- parison to active zones in the Mediterranean, seismic activity is low in northern Europe and the Lower Rhine graben region. Until re- cently, no one would have suspected that the normal faults related to graben structures in the Lower Rhine embayment could produce large earthquakes. Yet, on April 13,1992, an M s = 5.3 earthquake struck the city of Roer- mond in the Netherlands. This moderate earthquake is considered to be one of the ma- jor historical seismic events in this region, and it is stimulating concern among seismolo- gists and geologists. Fundamental questions are being asked about fault behavior in the re- gion and the earthquake distribution in time and space. The Royal Observatory of Belgium re- cently launched a seismotectonic program to estimate the true seismic hazard. A major as- pect of this program was to identify active faults and conduct associated paleoseismic studies along the Belgian side of the Roer Val- ley, which is the northwestern branch of the Rhine graben system (Figure 1). After a thor- ough field investigation using geologic and geomorphological methodologies coupled with geophysical prospection (radar and electric tomography), we were able to iden- tify active faults and correlate Holocene sur- face faulting with historical and prehistorical earthquakes in trenches. The Roer Valley, which crosses three countries (Belgium, The Netherlands, and Germany), is bounded by two north-north- west, south-southeast trending Quaternary Royal Observatory of Belgium, Department of Geodynamics, 3, Avenue Circulaire, B-1180 Brussels, Belgium normal fault systems. The eastern boundary is defined by the Peel boundary fault, along which the 1992 Roermond earthquake oc- curred [Camelbeeck and van Eck, 1994], and the western boundary is defined by the Feld- biss fault zone, which is partly located in Bel- gium. Evidence of recent tectonic activity along the Feldbiss fault zone is visible on seis- mic profiles that show more than 600 m of off- set in Neogene deposits [Demyttenaere and Laga, 1988].AlthoughAhorner[ 1975] demon- strated the existence of the " ...Rhenish seis- moactive zones..." and recommended a comprehensive analysis of Quaternary struc- tures and background seismicity, coseismic movements were considered to be improb- able, and active faults remain largely uniden- tified. Active Faulting and Evidence of Coseismic Displacements Are the Quaternary normal faults along the Roer graben capable of producing an earthquake larger than the Roermond event? Paleoseismology is a fundamental tool for ad- dressing this critical question and relating tec- tonic activity and seismicity in this area. On the basis of previous seismotectonic studies [Camelbeeck, 1993], we conducted detailed geological and geomorphological investiga- tions of fault scarps in the Roer graben, searching for the slightest evidence of late Pleistocene and Holocene movement along the main Quaternary faults. Near the town of Bree (Belgian Limburg, Figure 1), and along the Feldbiss fault zone, a prominent north-northwest, south-southeast trending fault scarp separates the Campine Plateau to the west from the Roer Valley to the east [Paulissen et al, 1985]. The geomor- phic expression of the scarp consists of ~10-km-long escarpment that has 15-20 m of vertical topographic relief (Figure 2). In de- tail, the scarp is multiple, and the frontal fault trace consists of an en echelon geometry that suggests a component of left-lateral slip. The fault dips 75°NE and offsets young deposits and alluvial terraces in the flat valley. Level- ing profiles along the frontal fault scarp yield an average vertical displacement of 0.6 m, which may correspond to the last coseismic event along the Bree fault (Figures 2 and 3a). Two 80-m trenches were excavated across the Bree fault scarp to characterize the re- cent faulting and related paleoearthquakes (Figures 3b and 3c). The two trenches re- vealed outstanding coseismic features includ- ing faulted and flexured Holocene alluvium, evidence of at least three historical and pre- historical seismic events, liquefaction struc- tures, and minor normal faulting. Despite the 2.5-m-deep water table in trench I, two well- sorted gravel horizons intercalated with sandy-clay deposits were excavated that have 0.5 m of vertical net slip (Figures 3a and 3b); the alluvial deposits are flexured, and the maximum displacement is 0.80 m. This offset agrees with the geomorphic measure- ments of the fault scarps, indicating the pres- ence of at least two tilted and displaced surfaces with a maximum vertical offset of 1.70 m at this site that are likely due to pre- vious coseismic displacements (Figure 3a). Paleosols and sedimentary deposits contain- ing organic matter, charcoal and peat depos- its suggest the last coseismic event occurred between 432 and 941 A.D. In the second trench (Figure 3c), the fault cuts a succession of coarse gravels with an in- tercalated clay horizon, a sandy unit showing a prominent flexure, and colluvial deposits. Near the surface, sandy-gravel lenses (dated at 402-206 B.C.) are cut by the fault and juxta- posed against a dark brown, fine, sandy clay that likely represents the last colluvium de- posited. In the downthrown block, fine- grained sand and clay, upon which admixed coarse gravels and sandy clay form channel structures, contain minor normal faults and prominent liquefaction features. Some nor- mal faults cut the fluvial deposits and show evidence of at least three paleoearthquakes. The paleoseismic information joining both the trench and geomorphic observations pro- vides clear evidence of surface faulting with a historical coseismic displacement, and This page may be freely copied. Eos, TRANSACTIONS, AMERICAN GEOPHYSICAL UNION