TIME VARIATIONS IN GRAVITY AND INFERENCES ON THE EARTH’S STRUCTURE AND DYNAMICS JACQUES HINDERER 1 and DAVID CROSSLEY 2 1 Institut de Physique du Globe de Strasbourg (CNRS- ULP UMR 7516) 5, rue Descartes, 67084 Strasbourg, France, e-mail: jhinderer@eost.u-strasbg.fr; 2 Department of Earth and Atmospheric Sciences, Saint Louis University, 3507 Laclede Ave., St. Louis, MO 63103, USA Abstract. 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. The spectral range which is observable with superconducting gravimeters is broad and goes from the seismic frequency band to periods longer than one year. We first investigate the seismic and sub-seismic bands with a special attention 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 bands, we show how accurate measurements allow us to infer constraints on various phenomena such as mantle (an-)elasticity, as well as ocean and atmospheric loading. The observation of the Free Core Nutation resonance in the diurnal frequency band is re- viewed and indirectly suggests an increase in the ellipticity of the core-mantle boundary with respect to its hydrostatic value. A similar resonance is also theoretically predicted in the diurnal band 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 amplitude and lack of a nearby tidal frequency. Oceanic and atmospheric loading mechanisms induce gravity changes over a wide spectral range and we present some recent progress in this field. Finally, because superconducting gravimeters have high calib- ration stability and small long-term instrumental drift, they can easily measure long period gravity variations due to polar motion and hydrogeology. Keywords: gravity, geodynamics, fluid core, tides, superconducting gravimeter 1. Introduction The elasto-gravitational deformation of the Earth and subsequently the vertical gravity changes measured at the Earth’s surface are caused by many geophysical phenomena: seismic free oscillations, possible gravito-inertial core modes includ- ing the translational mode of the solid inner core (Slichter triplet), lunisolar tides, Earth’s rotation changes, atmospheric and oceanic loading, and hydrogeology. The spectrum of the gravity changes varies from short periods (microseismic noise around 1–10 sec) to periods longer than 1 year (435 day Chandlerian contribu- tion from the Earth’s polar motion). Figure 1 from Crossley and Hinderer (1995) summarizes the spectrum which is observable with superconducting gravimeters (SG) thanks to their high sensitivity (close to 1 nanogal or 10 −11 m.s −2 ) and very low instrumental drift (less than 10 microgal per year). Surveys in Geophysics 21: 1–45, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands.