SUMMARY OF PALEOSEISMIC RESEARCH ON A BORDER FAULT OF THE ROER VALLEY RIFT SYSTEM, BELGIUM

The Roer Valley Rift System (RVRS), a system of NW-SE oriented normal faults in the border area of Belgium, the Netherlands, and Germany, was one of the first tectonic structures in intraplate Europe north of the Alps where paleoseismic investigations were conducted. Early investigations focused on the Bree fault scarp, a portion of the SW border fault that is well expressed in the morphology in Belgium, and provided evidence that large surface-rupturing earthquakes with magnitudes of M_W=6.3 or larger have occurred during the Holocene and late Pleistocene. At one site, six paleoearthquakes were identified, five of which occurred in the past ~125 kyr. This indicates that deformation in the RVRS is a near-continuous process. Over the past several years, we extended the investigation to the adjacent section of the fault in the Belgian Maas River valley. The surficial sediments in this area are much younger (mainly late Weichselian to Late Glacial), and thus record less cumulative offset. Consequently, the geomorphic expression of the fault is subdued. Using electric-resistivity tomography and ground-penetrating radar, we were able to identify the fault in the shallow subsurface, and we found evidence for a left stepover a few hundreds of meters wide. Two trenches were excavated south of this stepover. We found evidence for a late Holocene paleoearthquake in both trenches. Radiocarbon and OSL dating constrain the event between 2.5 ± 0.3 and 3.1 ± 0.3 kyr, and between 2790 ± 20 and 3770 ± 50 calibrated years before AD 2005, respectively. Thin-section analysis confirmed our identification of the prefaulting soil and the overlying scarp-derived colluvium, which are primary coseismic evidence. In both trenches this event is associated with liquefaction, including a series of sand blows and a gravel dike. These features are compelling evidence for strong co-seismic shaking. In one trench, we identified a second paleoearthquake which was OSL-dated between 15.9 ± 1.1 and 18.2 ± 1.3 ka kyr. The ages obtained for the two paleoearthquakes in the Maas River valley are in relatively good agreement with those obtained on the Bree fault scarp. This raises the possibility that the investigated border fault defines a single, 30-km-long rupture segment, capable of producing M_W=6.7 earthquakes.