In a detailed site survey for paleoseismic trenching, we applied shallow geophysical prospecting techniques, including ground-penetrating radar (GPR), electric resistivity tomography (ERT), and resistivity mapping to identify, locate, and visualize in 3D the Geleen fault, an active normal fault bordering the Roer Valley graben in northeast Belgium. Because of a low slip rate, the geomorphic expression of this fault is very faint in the relatively young deposits of the Maas River valley. ERT profiles show the fault as a broad, near-vertical anomaly characterized by sharp lateral resistivity contrasts, with an associated vertical offset of sediment layers. We observed offsets of similar to 15 m and similar to 5 m for the base and top, respectively, of a middle-late Pleistocene fluvial gravel layer. Shallow ERT and GPR profiles indicate that younger sediments are also affected by faulting, but the amount and sense of offset appear to be conflicting: ERT profiles show a near-surface, high-resistivity layer with an apparent reverse offset, and GPR profiles portray the fault as a sharp and laterally consistent disruption of reflectors, often without a clearly identifiable offset. Resistivity maps at different depths map the fault as a narrow, linear, lateral resistivity gradient matching the anomalies observed with other techniques. This method proved to be efficient in determining the precise position and orientation of dip-slip faults, and could potentially be very useful for the identification of lateral changes in fault geometry, such as splays and step-overs. Subsequent trenching confirmed the presence of a normal fault at the location predicted by the geophysical survey. Correlation with the sediments exposed on the trench walls demonstrated that, close to the surface, resistivity and dielectric permittivity contrasts mostly occur in a postdepositional soil, which developed differently on either side of the fault. This explains why shallow geophysical variations do not reflect the true fault offset.
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