2. Paleoseismological studies

2.1. The Feldbiss Fault in Belgium

2.1.2. Geomorphology

South-east of the town of Bree the Feldbiss Fault is expressed in the landscape as a striking NW-SE oriented escarpment that is easily identified on topographical maps (Fig. 3) and aerial photographs. This escarpment attains a maximal denivellation of 15 to 20 m, and constitutes the morphological border between the elevated Campine Plateau, defined by an extensive middle Pleistocene gravel terrace, and the subsiding Bocholt Plain, mainly built up by smaller late Pleistocene gravel terraces and eolian sandy silts (Paulissen, 1973). The slope is clearly visible and quasi-rectilinear over a distance of about 10 km between Bree and Opitter (Fig. 4) and further towards Neeroeteren.

Fig. 3 - Topographical contour map outlining the escarpment associated with the Feldbiss Fault southeast of the town of Bree. Based on maps of the National Geographical Institute.

The position of this morphological escarpment coincides with the upward projection of the Feldbiss Fault, and the features is therefore denoted as the Bree fault escarpment. It becomes less distinct in the Holocene alluvium of the Meuse river SE of Neeroeteren, and also NW of Bree, where it is replaced by several smaller escarpments, corresponding to the division of the tectonic faults at depth.

Fig. 4 - Picture of the rectilinear slope dominating the landscape between Bree and Opitter.

Detailed field mapping and levelling of dip profiles across the escarpment reveal the existence over most of its length of a frontal escarpment with about 0.5 to 3 m of topographical offset (Fig. 5). This frontal escarpment is considered the expression of the recentmost surface rupturing earthquake(s), while the main escarpment represents the cumulative result of earlier fault displacements.

Fig. 5 - Picture of the frontal escarpment superposed onto the main slope.
The city of Bree is in the background.