Soil Dynamics and Earthquake Engineering 20 (2000) 405±414 www.elsevier.com/locate/soildyn Long-term seismicity in regions of present day low seismic activity: the example of western Europe Thierry Camelbeecka,*, Pierre Alexandrea, Kris Vannestea, Mustapha Meghraouib aRoyal Observatory of Belgium, avenue circulaire 3, B-1180, Brussels, Belgium bEcole et Observatoire des Sciences de la Terre, rue Rene Descartes 5, F-67084 Strasbourg, France Abstract In western Europe, the knowledge of long-term seismicity is based on reliable historical seismicity and covers a time period of less than 700 years. Despite the fact that the seismic activity is considered as low in the region extending from the Lower Rhine Embayment to England, historical information collected recently suggests the occurrence of three earthquakes with magnitude around 6.0 or greater. These events are a source of information for the engineer or the scientist involved in mitigation against large earthquakes. We provide information relevant to this aspect for the Belgian earthquake of September 18, 1692. The severity of the damage described in original sources indicates that its epicentral intensity could be IX (EMS-98 scale) and that the area with intensity VII and greater than VII has at least a mean radius of 45 km. Following relationships between average macroseismic radii and magnitude for earthquakes in stable continental regions, its magnitude Ms is estimated as between 6.0 and 6.5. To extend in time our knowledge of the seismic activity, we conducted paleoseismic investigations in the Roer Graben to address the question of the possible occurrence of large earthquakes with coseismic surface ruptures. Our study along the Feldbiss fault (the western border of the graben) demonstrates its recent activity and provides numerous lines of evidence of Holocene and Late Pleistocene large earthquakes. It suggests that along the 10 km long Bree fault scarp, the return period for earthquakes with magnitude from 6.2 to 6.7 ranges from 10,000 to 20,000 years during the last 50,000 years. Considering as possible the occurrence of similar earthquakes along all the Quaternary faults in the Lower Rhine Embayment, a large earthquake could occur there each 500± 1000 years. These results are important in two ways. (i) The evidence that large earthquakes occur in western Europe in the very recent past which is not only attested by historical sources, but also suggested by paleoseismic investigations in the Roer Graben. (ii) The existence of a scienti®c basis to better evaluate the long-term seismicity in this part of Europe (maximal magnitude and return period) in the framework of seismic hazard assessment. q 2001 Elsevier Science Ltd. All rights reserved. 1. Introduction measurable. The fault length along which the rupture propa- gates has a minimum value of the order of 10 km, which A large part of tectonic deformation is relaxed during corresponds to a Mw 6.0 magnitude. earthquakes, which result from the unstable slippage of The densely populated areas of northwest Europe, where two crustal blocks along a fault zone. The affected fault seismic activity is apparently low and very destructive area can vary from a few m2 for the weakest detected earth- earthquakes virtually unknown, would be at high risk quakes to some thousands of km2 for the larger ones. In from the occurrence of such a large earthquake. Ordinary continental zones, the seismogenic layer, where earthquakes dwellings and industries can already be at risk from small originate, extends from a shallow depth to depth ranging earthquakes when they occur close to vulnerable cities. Two from 10 to 25 km depending on the considered region. recent examples are the Ms 4.6 1983 LieÁge (Belgium) The base of the seismogenic layer corresponds to the earthquake [14] and the Ms 5.3 1992 Roermond (The mechanical limit under which tectonic deformations occur Netherlands) earthquake [3] which caused in the epicentral as a plastic ¯ow. Earthquakes are quali®ed as large when areas losses estimated in excess of 100 million Euros. These they affect the whole seismogenic layer. In this case, surface two examples focus the importance to evaluate the potential faulting can occur and the surface coseismic deformation is (in term of magnitude and return period) for large earth- quakes in this part of Europe. * Corresponding author. Fax: 132-2-373-0339. In the region between the Lower Rhine Embayment and E-mail address: [email protected] (T. Camelbeeck). England, there is no surface rupturing earthquake known in 0267-7261/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S0267-7261(00)00080-4 406 T. Camelbeeck et al. / Soil Dynamics and Earthquake Engineering 20 (2000) 405±414 Fig. 1. Map of the region considered in this study. Historical earthquakes with estimated epicentral intensity of VII or greater are indicated. The data for the instrumental period (1910±1995) are taken from the catalogue of the Royal Observatory of Belgium for epicenters in Belgium and from the International Seismological Center for epicenters outside Belgium. the historical account. Despite that fact, from their estimated 2. Seismic activity in the region between the Lower magnitude around 6.0 [11], the earthquakes having occurred Rhine Embayment and the North Sea in 1382 in the southern North Sea and in 1580 in the Chan- nel can be considered as large earthquakes. In this paper, we Studies of long-term seismicity in any region of the world present arguments for the occurrence of a third large histor- require the longest historical perspective possible. In terms ical earthquake since the 14th century, the Verviers of the available documents, the seismic history of the (Belgium) event of September 18, 1692. In contrast to the studied region (Fig. 1) before the instrumental period, can two other events, having occurred at sea, detailed informa- be divided into three periods: tion of damage in the epicentral area has been found, which can be valuable in terms of mitigation for future large earth- 1. Before 700: the silence of Roman sources and the nearly quakes in this part of Europe. total absence of local sources have as a consequence that In the considered studied area, the more active region with even strong earthquakes stay unknown. the occurrence since 1350 of at least six earthquakes with 2. From 700 to 1350: sources allow a catalogue of the felt estimated Ms magnitude greater than 5.0 is the Lower Rhine earthquakes to be established, but the available informa- Embayment, mainly the Roer Graben. The strong subsidence tion gives very few data to estimate local intensities and of the Roer Graben during the last 150,000 years [7], the epicenters. Quaternary faults and associated morphology and the seismic 3. From 1350 to 1900: sources from different origins activity are evidence of recent and present day crustal defor- (chronicles, annotations, parish registers, account regis- mation. ters, etc.) give us more details on local effects and allow Since 1995, we have undertaken detailed paleoseismic reliable estimation of damages and the perceptibility area investigations along the Feldbiss fault [5,6,17] on the of the earthquakes. It is then possible to determine the western border of the Roer Graben to evaluate its more probable epicentral area of the earthquakes and also to recent activity in terms of large paleoearthquakes. We estimate their magnitude by comparison with recent present the main results of the study and we discuss their earthquakes for which the magnitude has been instru- impact in terms of future seismic hazard assessment. mentally determined [2,9]. T. Camelbeeck et al. / Soil Dynamics and Earthquake Engineering 20 (2000) 405±414 407 Fig. 2. Cumulative annual rate of earthquakes as a function of magnitude for the region plotted in Fig. 1. Even though instrumental recordings of earthquakes began (Belgium) earthquake of June 11, 1938 (Ms 5.0) and at the end of the last century, the reliability of the calculated two events having occurred in the Lower Rhine Embay- earthquake parameters were very unsatisfactory at least up ment, the Euskirchen (Germany) earthquake of March 14, to 1965 for the epicenter location and to 1980 for the focal 1951 (Ms 5.3) and the Roermond (The Netherlands) earth- depth determination [4]. Before 1960, with the exception of quake of April 13, 1992 (Ms 5.3). larger shocks, macroseismic reports often provide a better During the historical period, several strong earthquakes epicenter location than those calculated from arrival data occurred, mainly in the Lower Rhine Embayment, but the read on the seismograms. It is important, therefore, to discri- real importance of some of them was neglected until very minate the reliable from the less reliable data in the catalo- recently. Melville et al. [11] studied the historical earth- gue when we want to use it for seismic hazard assessment. quake activity in the Channel and the southern North Sea The catalogue of earthquakes established at the Royal and concluded that the events on May 21, 1382 and April 6, Observatory of Belgium for the region de®ned in Fig. 1 is 1580 probably both had a magnitude around 6.0. By their supposed to be complete for Ms magnitude greater than 4.7 estimated magnitude, these earthquakes should be consid- since 1350 (all historical events with epicentral intensity VII ered as large earthquakes rupturing the whole seismogenic or more), Ms 3.3 since 1911 and Ms 1.8 since 1985. layer. The annual cumulative frequency of earthquakes as a Recently found original historical sources suggest that the function of magnitude (Ms) for the whole region (Fig. 2) earthquake having occurred on September 18, 1692 in the was calculated taking into account earthquakes with magni- Belgian Ardenne is probably the strongest known earth- tude greater than 4.7 since 1350, with magnitude ranging quake in the studied region [1].