Tectonic, Climatic and Anthropogenic effect in Western Europe by Repeated Absolute Gravity Measurements

Van Camp, Michel 1; de Viron, Olivier 2; Lecocq, Thomas 1; Hinzen, Klaus Guenter5; Quinif, Yves 3; Williams, Simon 4; Camelbeeck, Thierry 1
1. Royal Observatory of Belgium, Brussels, Belgium. 2. IPGP-U. Paris Diderot, Paris, France. 3. U. Mons, Mons, Belgium. 4. Proudman Oceanographic Laboratory, Liverpool, UK. 5. Earthquake Geology, Cologne University, Germany; mvc@oma.be

In continental plate interiors, tectonic deformations are small and the associated ground surface movements remain close to or below the accuracy of current geodetic techniques, and at the limit of the noise level. An absolute gravimeter is an appropriate tool to quantify slow vertical movements, as this instrument, based on length and time standards, is drift free and does not depend on any terrestrial reference frame. Repeated absolute gravity (AG) measurements have been performed in Oostende (Belgian coastline) and at 8 stations along a southwest-northeast profile across the Belgian Ardennes and the Roer Valley Graben (Germany), in order to estimate the tectonic deformations in the area. After 7-13 years (depending on the station), we find evidence that the gravity rates of change are no larger than a µGal per year and result from a combination of anthropogenic, climatic, tectonic, and Glacial Isostatic Adjustment (GIA) effects. This demonstrates the importance of precisely modeling the GIA effects in order to investigate intraplate tectonic deformations at the sub-millimeter level. This study also shows that AG measurements, repeated once or twice a year, can resolve gravity changes at the 2 nm/s²/yr level (equivalent to 1 mm/s² vertical velocity) after 10 years, even in difficult conditions, provided that the gravimeter is carefully maintained.