Gravity Variations and Ground Deformations Resulting from Core Dynamics

Abstract

Fluid motion within the Earth’s liquid outer core leads to internal mass redistribution. This occurs through the advection of density anomalies within the volume of the liquid core and by deformation of the solid boundaries of the mantle and inner core which feature density contrasts. It also occurs through torques acting on the inner core reorienting its non-spherical shape. These in situ mass changes lead to global gravity variations, and global deformations (inducing additional gravity variations) occur in order to maintain the mechanical equilibrium of the whole Earth. Changes in Earth’s rotation vector (and thus of the global centrifugal potential) induced by core flows are an additional source of global deformations and associated gravity changes originating from core dynamics. Here, we review how each of these different core processes operates, how gravity changes and ground deformations from each could be reconstructed, as well as ways to estimate their amplitudes. Based on our current understanding of core dynamics, we show that, at spherical harmonic degree 2, core processes contribute to gravity variations and ground deformations that are approximately a factor 10 smaller than those observed and caused by dynamical processes within the fluid layers at the Earth’s surface. The larger the harmonic degree, the smaller is the contribution from the core. Extracting a signal of core origin requires the accurate removal of all contributions from surface processes, which remains a challenge. Article Highlights Dynamical processes in Earth's fluid core lead to global gravity variations and surface ground deformations We review how these processes operate, how signals of core origin can be reconstructed and estimate their amplitudes Core signals are a factor 10 smaller than the observed signals; extracting a signal of core origin remains a challenge

Uncertainty of low-degree space gravimetry observations: surface processes versus internal signal

“…Let us underline the importance of our results. Considering that the predicted core signals would have an amplitude of 10 −11 for C 2,0 and 5 × 10 −12 for other degree two coefficients (Dumberry, 2010b;Dumberry & Mandea, 2021b), they account respectively for 2 mm and 1 mm signals in EWH when projected onto the Earth's surface in a grid format. Dumberry and Mandea (2021b) also predicted a gravity signal in the S 2,2 coefficient with an amplitude below 2×10 −10 and at a time period around 10 to 30 years.…”

“…Another interesting approach is to look at SH coefficients. Dumberry and Mandea (2021a) predicted that a potential core signal might be present from degree 2 onward to higher degrees with decreasing amplitudes.…”

“…Other authors have proposed to improve the knowledge of the dynamical processes of the Earth's core. Dumberry (2010a); Dumberry and Mandea (2021a) predicted a gravity perturbation generated by various core processes that might be observable on the low degrees of the gravity field. No signature of these perturbations has yet been observed in the gravity variations.…”

“…Dynamical core processes disturb the gravity field through the direct Newtonian effect of mass anomalies in the liquid core, and through indirect effects, such as changes in the rotation vector of the solid Earth. Dumberry and Mandea (2021a) provided a review of the surface deformation and gravity variations induced by core dynamics, as well as a quantification of the expected amplitudes. Different mechanisms are covered including mass anomalies in the core, inner-core reorientation and pressure fluid acting at the CMB (Gillet et al, 2020).…”

Uncertainty of low-degree space gravimetry observations: surface processes versus internal signal

“…Other authors have proposed to improve the knowledge of the dynamical processes of the Earth's core. Dumberry (2010a); Dumberry and Mandea (2021) predicted a gravity perturbation generated by various core processes that might be observable on the low degrees of the gravity field. No signature of these perturbations has yet been observed in the gravity variations.…”