Constraints provided by ground gravity observations on geocentre motions
Rogister, Y and Memin, A and Rosat, S and Hinderer, J and Calvo, M, Constraints provided by ground gravity observations on geocentre motions, Geophysical Journal International, 206, (2) pp. 1431-1439. ISSN 0956-540X (2016) [Refereed Article]
The geocentre motion is the motion of the centre of mass of the entire Earth, considered an isolated system, in a terrestrial system of reference. We first derive a formula relating the harmonic degree-1 Lagrangian variation of the gravity at a station to both the harmonic degree-1 vertical displacement of the station and the displacement of the whole Earth's centre of mass. The relationship is independent of the nature of the Earth deformation and is valid for any source of deformation.We impose no constraint on the system of reference, except that its origin must initially coincide with the centre of mass of the spherically symmetric Earthmodel. Next, we consider the geocentre motion caused by surface loading. In a system of reference whose origin is the centre of mass of the solid Earth, we obtain a specific relationship between the gravity variation at the surface, the geocentre displacement and the load Love number h'1 1, which demands the Earth's structure and rheological behaviour be known. For various networks of real or fictitious stations, we invert synthetic signals of surface gravity variations caused by atmospheric loading to retrieve the degree-1 variation of gravity. We then select six well-distributed stations of the Global Geodynamics Project, which is a world network of superconducting gravimeters, to invert actual gravity data for the degree-1 variations and determine the geocentre displacement between the end of 2004 and the beginning of 2012, assuming it to be due to surface loading. We find annual and semi-annual displacements with amplitude 0.5-2.3 mm.