Koen Verbeeck

Hydrogeological investigations at the Membach station, Belgium, and application to correct long periodic gravity variations

[1] A comprehensive hydrogeological investigation regarding the influence of variations in local and regional water mass on superconducting gravity measurements is presented for observations taken near the geodynamic station of Membach, Belgium. Applying a regional water storage model, the gravity contribution due to the elastic deformation of the Earth was derived. In addition, the Newtonian gravity effect induced by the local water mass variations was calculated, using soil moisture observations taken at the ground surface (about 48 m above the gravimeters). The computation of the gravimetric effect is based on a digital elevation model with spatially discretized rectangular prisms. The obtained results are compared with the observations of a superconducting gravimeter (SG). We find that the seasonal variations can be reasonably well predicted with the regional water storage model and the local Newtonian effects. Shorter-period effects depend on the local changes in hydrology. This result shows the sensitivity of SG observations to very local water storage changes.

Pseudo-3D imaging of a low-slip-rate, active normal fault using shallow geophysical methods: The Geleen fault in the Belgian Maas River valley

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 ∼15 m and ∼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 rever...

A Model of Composite Seismic Sources for the Lower Rhine Graben, Northwest Europe

Abstract The Lower Rhine Graben (LRG) straddling the border zone of Belgium, the Netherlands, and Germany, is an active tectonic structure in continental northwest Europe. It is characterized by northwest–southeast oriented normal faults, and moderate but rather continuous seismic activity. Many faults have been mapped in the LRG, but so far a model of fault hierarchy or fault segmentation has been lacking. In the frame of a European database of seismogenic sources, we have devised a seismic‐source model for the LRG consisting of so‐called composite seismic sources. Each composite seismic source may encompass one or more segments, but it is unlikely that a segment would extend across more than one source. We distinguish 15 seismic sources based on major stepovers, bifurcations, gaps, and important changes in strike, dip direction, or slip rate. The sources are partitioned into one or more informal fault sections, each with an associated surface trace. For each source, we describe the limits and the composing fault sections, and present the geological arguments for their existence. We have compiled all relevant data concerning the seismic‐source parameters required for the database, putting lower and upper bounds on strike, dip, rake, slip rate, and depth, and an upper bound on earthquake magnitude. This source model should provide a new basis for modeling seismic hazard, as well as for guiding further paleoseismic studies in the LRG. Online Material: Detailed maps of the composite sources in the Lower Rhine Graben, 3D views of the fault model, and a table with parameters of earthquake focal mechanisms and detailed information sheets for each composite seismic source.

Morphotectonics and Past Large Earthquakes in Eastern Belgium

Tectonic landforms are generally modest in stable plate interiors characterized by low strain rates and rare earthquakes. Nevertheless, specific investigations identified such landforms in Belgium, which is located in the most seismically active region of stable Europe northwest of the Alps. Here, we present two active fault zones among the best documented in continental Europe, and whose geomorphology is related to earthquake activity in eastern Belgium. The 12-km-long Bree fault scarp is readily identified in the flat alluvial landscape of the Campine. It results from the activity since the Middle Pleistocene of the Geleen fault bounding the Roer Valley Graben (RVG) to the west. At its base, a ~1-m-high scarplet corresponds to the surface rupture associated with a Holocene large normal-faulting earthquake. Combined geomorphological and paleoseismic investigations allowed evaluating the average slip rate and the return period of large earthquakes on the Geleen fault during the Late Pleistocene. Extending across a more animated landscape in NE Ardenne, owing also to lower slip rates, the Hockai Fault Zone (HFZ) is morphologically less conspicuous. Microseismicity, geology and geomorphology provide consistent evidence of recent activity of this fault zone. The location of the M = 6 ¼ 18 September 1692 earthquake, the largest historical earthquake recorded in this part of Europe, on the northern HFZ suggests that the fault morphology could have resulted from sporadic bursts of large earthquakes.