Time variations in gravity and inferences on the Earth's structure and dynamics

This paper reviews how the study of the surface gravity changes is able to provide useful information on the Earth's structure and global dynamics. Th e spectral range which is observable with superconducting gravimeters is br oad and goes from the seismic frequency band to periods longer than one yea r. We first investigate the seismic and sub-seismic bands with a special at tention paid to the gravity detection of core modes in the liquid core and to the Slichter mode of translation of the solid inner core. In the tidal b ands, we show how accurate measurements allow us to infer constraints on va rious phenomena such as mantle (an-)elasticity, as well as ocean and atmosp heric loading. The observation of the Free Core Nutation resonance in the d iurnal frequency band is reviewed and indirectly suggests an increase in th e ellipticity of the core-mantle boundary with respect to its hydrostatic v alue. A similar resonance is also theoretically predicted in the diurnal ba nd for the rotation of the solid inner core (Free Inner Core Nutation) but we show that its detection is much more difficult because of the small ampl itude and lack of a nearby tidal frequency. Oceanic and atmospheric loading mechanisms induce gravity changes over a wide spectral range and we presen t some recent progress in this field. Finally, because superconducting grav imeters have high calibration stability and small long-term instrumental dr ift, they can easily measure long period gravity variations due to polar mo tion and hydrogeology.