Late Quaternary earthquake-related soft-sediment deformation along the Belgian portion of the Feldbiss Fault, Lower Rhine Graben system

Trench investigations along the Bree fault scarp in Belgium, in the framewo rk of a first paleoseismological experience in the area of the Lower Rhine Graben system, have not only demonstrated the existence of seismogenic faul ts almost extending to the present ground surface, but also exposed several types of soft-sediment deformation affecting sandy sediments and soils of inferred late Weichselian to Holocene age. The observed features include as ymmetric folding, small-scale normal faulting, possible sand intrusions, an d small water escape or load structures. Establishing a seismic origin for the individual features is not always possible based on their sedimentary c haracteristics only, particularly due to the potential confusion with perig lacial phenomena, but their abundance, the close association of different d eformational styles, and their vicinity to a known active fault, all seem t o be compatible with an earthquake-induced scenario. The features are to so me extent comparable to those generated during the recent M-S 5.3 Roermond earthquake at the opposite side of the Roer Valley Graben. Stratigraphical and mutual geometrical relationships suggest the occurrence of at least thr ee distinct deformational events since probably 30,000 years, and one event before that time. Since nothing is yet known about their regional distribu tion, however, estimating the associated paleomagnitudes from these soft-se diment deformations is premature. (C) 1999 Elsevier Science B.V. All rights reserved.