Elasto-Gravitational Deformation, Relative Gravity Changes and Earth dynamics

Abstract

SUMMARY We consider the elasto-gravitational deformation of an earth model with fluid parts (core, ocean, atmosphere). A Love number formalism is used in order to express the global deformation induced by volume potentials (tides, variations in the Earth’s rotation) or boundary conditions (in pressure, transverse stress, load) at the interfaces between the fluid layers and the solid mantle. The induced disturbances in gravity are expressed with the help of generalized gravimetric factors and special attention is paid to resonance processes. Various applications related to global Earth dynamics are developed and numerically quantified. The surface gravity changes caused either by a variation in the direction of the Earth’s rotational axis (polar motion) or in the rotation rate (length of the day) are compared with the lunisolar gravimetric tides. The loading effect due to the oceans is found to increase the oceanless gravimetric factor by about 2.5 per cent. Two simplified approaches are proposed to model the gravity perturbations originating in the fluid core (e.g. due to buoyancy core modes); it is shown that a (degree 2 planetary-scale) pressure field acting at the core-mantle boundary of about 80 Pa is able to induce a 10 ngal surface signal. Similarly, gravity contributions from the atmosphere (or oceans) are derived using outer pressure, load or (spheroidal) shear stress gravimetric factors; a (degree 2) surface pressure and transverse traction of 100 Pa causes a gravity change of 0.27 and 0.14 pgal, respectively. Finally, we point out that, in addition to the solar gravitational tide, there are several other contributions (rotational, meteorological) appearing in the annual gravity signal by different gravimetric factors. We therefore conclude that high-precision relative gravimetry can provide useful information not only about the Earth’s physical properties but also about its dynamical behaviour.