Active faulting and paleoseismology along the Bree fault, lower Rhine graben, Belgium

Paleoseismic analysis of the 10-km-long Bree fault scarp in the lower Rhine graben yields numerous lines of evidence of earthquake activity in the Holocene and late Pleistocene. This active normal fault, a part of the Feldbiss fault system, dips 70 degrees NE and is expressed at the surface by a prominent NW-SE trending 7 to 20 m high scarp, formed since the deposition of the Maas River main terrace <700 kyr. B.P. Trenches and geophysical prospecting show that the fault, which is known to have similar to 100 m of vertical. offset since the late Pliocene, breaks late Pleistocene and Holocene deposits. Ground-penetrating radar, seismic refraction, and electric tomography suggest that at shallow depth the amount of displacement is larger than the youngest vertical offset visible in the trenches and corresponds to cumulative fault displacements. The analysis of 36 leveling profiles across the scarp indicates that its height can be classified into three groups, likely corresponding to different events. A morphologic dating gives approximate ages of 2 +/- 1.5 kyr B.P., 14 +/- 5 kyr B.P., and 41 +/- 6 kyr B.P. for the past three surface-faulting earthquakes. Analysis of faulted stratigraphy and earthquake-induced deformation structures exposed in trenches suggests the occurrence of three large earthquakes during the past 45x10(3) years and yields 0.07 mm/yr of relative vertical deformation rate. The most recent seismic event occurred between A.D. 610 and 890. The first identification of an active fault with surface ruptures in the lower Rhine graben area emphasizes that large earthquake sources exist within intraplate Europe and that at least some of these events are preserved in the geologic record.