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

In the Roer Graben and the Belgian Ardenne, there is a difference of an order of magnitude in the inferred vertical crustal movements deduced from the difference of levelling (~1 mm/yr) and geological information (0.1 mm/yr). Moreover, within such intraplate areas, the tectonic deformation signal is usually smaller than the error of geodetic data.

To better constrain the present-day deformation, absolute gravity measurements using the FG5-202 gravimeter have been conducted along a profile twice a year since September 1999. This 140 km long profile includes 8 stations across the Belgian Ardenne and the Roer Graben. A description of the network is given, and specific difficulties met in some stations are presented. We also discuss the possible causes of gravity variations in this region. The precision, less than 2.0 μGal integrated over more than 24 hours of observations, is quite satisfactory in all stations but Jülich (Germany). The mining activities around Jülich cause an increase in the noise up to 7 æGal integrated over more than 72 hours. However, we observe there a gravity change of 6.4\pm2.6 μGal/yr related to a subsidence of more than 1 cm/yr caused by water pumping.

At this present time, it is still not possible yet to detect any tectonic deformation in the available data. However, after 5 years of observations, we should be able to detect or to constrain any possible long-term trend with an accuracy of 2.5 mm/yr, assuming a height change of 1 mm causes a change in gravity of 0.2 μGal. To ensure that the absolute gravimeter is giving accurate results it has been regularly compared with the superconducting gravimeter installed at the Membach reference station, which belongs to the profile.