2. Paleoseismological studies

2.1. The Feldbiss Fault in Belgium

2.1.5. Trench analysis

Successful identification of the surface fault with geophysical methods was in each of the sites followed by the excavation of a perpendicular trench, allowing for the direct observation of fault displacements and related sediment deformation.

2.1.5.1. Site 1

The trench on site 1 was about 60 m long and only 2 m deep, limited by a shallow depth of the water table. Figure 7 gives an overview of the sediment patterns exposed on the south-east wall of trench 1. The profile is dominated by a homogeneous, structureless sand layer, the top of which is in most places reduced. Within the sand layer, two thin gravel horizons can be discerned, both showing a vertical displacement of about 0.6 m (Fig. 8). This discontinuity is exactly aligned with the frontal escarpment at the surface, and with the geophysical anomalies at depth. A second argument for the fault's existence is found in the larger thickness of the reduced top layer in the downthrown block, which also indicates that the displacement occurred more recently than the main phase of soil development; erosion removed part of the reduced soil horizon from the elevated block. In addition, the many plant roots that have been preserved in this place within the sand layer, point to the existence of several fissures, through which the roots could penetrate the sediment more easily.

Fig. 7 - Sediment patterns drawn from the south-east wall of trench 1.

Fig. 8 - Photograph (top) and corresponding log (bottom) of the fault zone in trench 1. Note the two displaced gravel horizons (marked in green and blue on the photograph).

Together, these different observations point to the continuation of the Feldbiss Fault up to the Earth's surface. According to preliminary ¹⁴C-datings (Fig. 8), this co-seismic displacement would have occurred approximately between 1000 and 1350 years ago.