Intraplate Deformations Measured using an Absolute Gravimeter Across the Ardenne and the Roer Graben (North-western Europe)

Given the importance of measuring present-day crustal deformation for the interpretation of the seismotectonics in the Lower Rhine Embayment, the Royal Observatory created a profile of absolute gravity (AG) sites across the Belgian Ardenne and the Roer Graben to infer vertical crustal movements. This eight- station profile is 140 km long and measurements have been made biannually since 1999. This should allow one to detect the spatial extension of the uplift expected from the tectonic activity but also to discriminate it from long-wavelength phenomena like glacial isostatic adjustment (GIA). GIA models predict that Belgium is on the subsiding peripheral bulge of the GIA. We present the results of AG measurements performed in Ostend and across the Belgian Ardenne and the Roer Graben. At all stations except Jülich, there are no detectable gravity variations higher than 11.4 nm s-2 at the 95% confidence level. This does contradict predictions from GIA models or the GPS observations. Intraplate deformation in active tectonic structures such as the Roer Graben and GIA around the peripheral bulge is close to or below the accuracy of current geodetic techniques. It will be necessary to observe for longer periods before interpreting the GPS and AG data in terms of tectonic rates of deformation and produce convincing arguments in favour of one (or more) of the hypotheses concerning the origin of the crustal movement. Repeated AG campaigns should constrain the gravity rate of change with an uncertainty of 1 nm/s² (or 0.5 mm of vertical movement) after 15-25 years. AG measurements can constrain vertical deformation only but due to the methodology, an absolute gravimeter potentially return to any undisturbed site even after, say, a century and make a relevant measurement. Two important stations are Jülich, in the graben and Membach, which is collocated with a superconducting gravimeter, on the western border. Jülich, located in an industrialised area, suffers from high noise, partially due to the vibrations imparted to the unconsolidated soil by opencast brown coal mines. To prevent the mines from flooding, continuous water pumping has been carried out for the past decades, causing a subsidence of more than 1 cm/yr. Despite the significant high frequency noise, we are able to observe a trend of 31.7±5.8 nms-2/yr. This rate of change is similar to that predicted for Fennoscandia due to GIA or at plate boundaries: therefore this station acts as a case study. In addition dozens of surrounding wells allow us to investigate the influence of hydrological variations on gravity. In Membach, more than 110 AG measurements have been completed since 1996. There is no detectable movement corresponding to gravity changes higher than 1.6±0.7 nms-2/yr (0.8 mm/yr) at the 2σ level. In addition, hydrogeological investigations are being performed (see Dassargues et al. session H07). The removal of seasonal and higher frequency effects from the gravity signal will improve the ability to monitor long- term gravity changes in this area.