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IS THE RURRAND FAULT (LOWER RHINE GRABEN, GERMANY) RESPONSIBLE FOR THE 1756 DÜREN EARTHQUAKE SERIES?

Winandy, Jonas (1), Christoph Grützner (1), Klaus Reicherter (1), Thomas Wiatr (1), Peter Fischer (2), Thomas Ibeling (3)

(1) RWTH Aachen, Neotectonics and Natural Hazards Group, Aachen, Germany (jonas.winandy@rwth-aachen.de, +49 241 8096358),

(2) Köln University, Geographisches Institut, Köln, Germany (3) Archäologische Grabungen und Sondagen GBR, Köln, Germany Abstract (Is the Rurrand Fault (Lower Rhine Graben, Germany) responsible for the 1756 Düren earthquake series?): In 1756, several strong earthquakes (M5-6.1) occurred close to Düren (Lower Rhine Graben, LRG) in Germany. The Rurrand Fault in the LRG located in the middle between Aachen and Cologne possibly indicates the Düren earthquake sequence. This fault is one of the most prominent NW-SE trending normal faults with a morphological expression in the area within the Lower Rhine Graben. Holocene sediments with significant offsets covered by thin colluvial sediments were found and a complex fault geometry was observed during archaeological excavations. DC geoelectrics and georadar were applied in order to image the deeper parts of the fault zone. Radiocarbon and luminescence dating of sediment samples are in progress, but the morphological expression of the fault, the shallow depths of the offset sediments, and geophysical data allow concluding on recent seismicity along this active fault with at least four surface-rupturing events. Key words: earthquake, geophysics, Rhine Graben, Rurrand Fault INTRODUCTION: THE DÜREN EARTHQUAKES 1755/1756 The area between Aachen and Cologne in western Germany was hit by a series of earthquakes in 1755/1756. On 18th February, 1756, the strongest event took place most likely west of the city of Düren, leaving two people dead (some reports claim three fatalities) and causing significant damages also in Aachen, Cologne, and nearby villages. Chimneys were destroyed in up to 70 km distance (Liège, Belgium), light damages were recorded in Brussels, Gießen and Osnabrück (200 km distance). A landslide was triggered 15 km SW from Düren. The shaking was felt as far as 400 km from the epicentre in London, Magdeburg, Halle, Paris, and Strasbourg (Meidow, 1995). Epicentral intensities of VIII are reported by Skupin et al. (2008) for the Eschweiler area (15 km W of Düren). A magnitude of 6.3 is assumed for the main event by Skupin et al. (2008), Meidow (1995) assumes ML=6.1. Our study shows that the Rurrand Fault was possibly activated during the Düren earthquake sequence. GEOLOGY AND TECTONIC SETTING The Lower Rhine Embayment (LRE) underwent subsidence from Miocene to recent, accompanied by uplift of the Rhenish Massif to the southwest and east of the study area. Tertiary and Quaternary sediments of more than 1 km thickness were deposited and also include lignites which are extracted in open pit mines. Fluvial and aeolian Pleistocene-Holocene sediments as well as loess cover wide areas.

The Lower Rhine Embayment and especially its western part is one of the tectonically most active areas in Germany and dominated by the Lower Rhine Graben. NW-SE trending normal faults form a horst and graben structure with a number of single blocks (Krefeld-, Köln-, Venlo-, Erft-, and Rur blocks, from NE to SW). The faults show offsets of more than 50 m in the Quaternary. Düren is situated in a NW-SE striking graben, which is flanked by the Rurrand Fault in the NE and the Stockheimer Sprung in the

Fig. 1: Neotectonics and historical earthquakes in the study area, the Lower Rhine Embayment. RRF: Rurrand Fault; SSF: Stockheimer Sprung

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SW (Fig. 1). The Rurrand Fault is a NW-SE striking normal fault dipping to the SW and expressed by an escarpment. A large number of damaging earthquakes since Karolingian times has been reported for this area. The most recent one was the 1992 Roermond earthquake which reached ML=5.9. Recent studies reported that active faults in the study area are characterized by recurrence periods in the order of tens of ka (Skupin et al., 2008; Camelbeeck et al., 2007), and that present day aseismic slip is assumed for the Rurrand Fault, resulting form the lowering of the groundwater level due to the nearby mining activities (Vanneste & Verbeeck, 2001). Active faults in Germany are often not visible in the field due to relatively high erosion rates. Therefore, the seismic hazard might be under-estimated. The Rurrand Fault was trenched already only approx. 2-3 km away, and only Pleistocene faulting evidence was found (Skupin et al., 2008). METHODS

Due to construction works for a new highway, extensive archeological excavations have proven findings from Roman times until recent. At this occasion, the Rurrand Fault was trenched in several places, where we mapped layer offsets, sediment deformation, and structural data (Fig. 2). The trench walls were sketched and photographed. Ground penetrating radar (GPR) and electric resistivity measurements have been applied in order to image deeper sediment structures and to map the fault trace. We used the GSSI 100, 270, and 400 MHz antennas with the SIR 3000 controller, a survey wheel and a GPS data logger for georadar

measurements. Data processing was done with ReflexW by Sandmeier Scientific Software. The geoelectrics data were gathered with the 4-point-light system (Lippmann), and 80 electrodes with 1.5 m spacing. Schlumberger, Dipole-dipole and Wenner configurations were applied. Soil samples were taken for radiocarbon and luminescence dating, which is currently in progress. RESULTS

The GPR data revealed sediment layers inclined towards the fault (Fig. 3). The fault cropped out 2 m away from the GPR profile, allowing a direct comparison. Down to a depth of 4 m several reflectors dip towards the NW.

Fig. 2: Location of trenches, outcrops and geophysical profiles at the study area.

Fig. 3: 270 MHz GPR profile (64) crossing the fault with an angle of 45°.

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A sharp contrast in reflection amplitudes marks the fault itself. On the footwall, only few layers appear to dip towards the Rurrand Fault. Similar observations were made at other GPR profiles that crossed the fault. The high-frequency antennas allowed identifying tilted sediments and the fault itself at various locations. The resolution of the 100 MHz

antenna was found too low for imaging fault features in this case. Geoelectrics data revealed low-resistivity anomalies (higher conductivities) at the fault zone, which are most likely related to an increase in water content at the fault zone.

Fig. 4: Photograph of the fault, outcropped in an archeological trench (upper image), and reconstruction of the faulting history (lower image). S = sedimentation and erosional stages; EW - EZ = last earthquake events.

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Holocene surface-near sediments with significant offsets covered by thin colluvial sediments were found and a complex fault geometry was observed during the archaeological excavations (Figs. 4 - 6). Some deformation structures seem to be related to liquefaction. The offset of surface-near sediments is in the order of 5 cm (Fig. 5), deeper layers show greater offsets (Fig. 6). Growing displacement of the major fault downsection suggest more than one major, surface-rupturing earthquakes along the Rurrand Fault in the Holocene/Late Pleistocene. We developed a deformation model for the fault, assuming at least four surface-faulting events (EW - EZ) that led to the present day geometry (Fig. 4) and seven stages of seismic quiescence (S1-S7). Roots penetrated the soil at stage four, and liquefaction of fine grained material is likely to have occurred during the last earthquake event. Despite the results from dating are yet to come, we can assume four events since Late Pleistocene, which would result in slightly shorter recurrence periods than estimated by previous studies. The fault may also be responsible for the Düren 1756 events, as the evidence for surface-faulting earthquakes proves that it is capable for destructive events with magnitudes > 5.5. However, only the dating will allow associating the Düren earthquakes with the Rurrand Fault.

CONCLUSION

We found evidence for four surface-faulting events at the active Rurrand Fault close to Düren, which might be responsible for the 1755/1756 earthquake series. Offset sediments clearly point to seismic activity since Late Pleistocene and at least four earthquake events. Geophysical data allowed mapping the fault trace where there were no outcrops (Geoelectrics and GPR) and revealed the fault geometry in depths of up to 6 m (GPR).

References Camelbeeck, T., Vanneste, K., Alexandre, P., Verbeeck, K.,

Petermans, T., Rosset, P., Everaerts, M., Warnant, R. & Van Camp, M. (2007). Relevance of Active Faulting And Seismicity Studies To Assessments Of Long-Term Earthquake Activity and Maximum Magnitude In Intraplate Northwest Europe, between the Lower Rhine Embayment and the North Sea. In: Geological Society of America, Special Paper, vol. 425, pp. 193-224.

Meidow, H. (1995). Rekonstruktion und Reinterpretation von historischen Erdbeben in den nördlichen Rheinlanden unter Berücksichtigung der Erfahrungen bei dem Erdbeben von Roermond am 13. April 1992. Dissertation, Universität Köln, Leverkusen, 305 p.

Skupin, K., Buschhüter, K., Hopp, H., Lehmann, K., Pelzing, R., Prüfert, J., Salamon, M., Schollmayer, G., Techmer, A. & Wrede, V. (2008). Paläoseismische Untersuchungen im Bereich der Niederrheinischen Bucht. Scriptum 17, Krefeld (Geol. Dienst Nordrh.-Westf.), 72 p.

Vanneste, K. & Verbeeck, K. (2001). Paleoseismological analysis of the Rurrand fault near Jülich, Roer Valley graben, Germany: Coseismic or aseismic faulting history? Netherlands Journal of Geosciences / Geologie en Mijnbouw, 80 (3-4): 155-169.

Fig. 5: A) Step-like offset clearly points to seismic deformation instead of soil creep. Displacement is about 5 cm. The offset reddish layer is made up of clayey-silty, loess material, 50 cm below the surface. B) The fault zone is clearly visible in the trenches, not only at the walls, but also on the floor. This enabled a very good correlation with the geophysical data and a precise analysis of subsurface features.

Fig. 6: Offset surface-near layers have been found during the archeological excavations.