Surface-rupturing history of the Bree fault scarp, Roer Valley graben: Evidence for six events since the late Pleistocene

Since 1996 paleoseismological investigations have been used to develop the surface- rupturing history of the Bree fault scarp, the morphologically bes t-defined segment of the southwestern border fault of the Roer Valley grabe n in northeastern Belgium. The first studies determined that the escarpment is associated with a surface fault, and they exposed evidence for three su rface displacements since about 40 ka BP. The most recent event probably oc curred between 1000 and 1350 yr cal BP. Geophysical and trenching studies a t a new site near the southeastern end of the fault scarp reconfirmed the c oincidence of the frontal escarpment with a shallow normal fault, which dis places the Middle Pleistocene 'Main Terrace' of the Maas River, as well as overlying coversands of Saalian to late Weichselian age. Different amounts of displacement shown by the two youngest coversand units indicate two disc rete faulting events, but primary evidence for the coseismic nature of thes e events is sparse. Radiocarbon and optically stimulated luminescence datin g constrain the age of these events to the Holocene and between 14.0 +/- 2. 3 ka BP and 15.8 +/- 2.9 ka BP, respectively. In addition, four older surfa ce-rupturing events are inferred from the presence of four wedge-shaped uni ts of reworked Main Terrace deposits that are interbedded with coversand un its in the hanging wall of the trench and in shallow boreholes. These wedge s are interpreted as colluvial wedges, produced by accelerated slope proces ses in response to rejuvenation of the fault scarp, most probably in a peri glacial environment. Luminescence dating indicates that five out of a total of six identified faulting events are younger than 136.6 +/- 17.6 ka. The antepenultimate event was the largest faulting event, associated with a tot al fault displacement in excess of 1 m. Thus, the newly investigated trench site represents the longest and most complete record of surface rupturing recovered so far along the Bree fault scarp. This study also demonstrates t he viability of the paleoseismological approach to identify past large eart hquakes in areas of present-day moderate to low seismic activity.