Changes in gravity and ground deformations caused by core dynamics
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.
For all of these processes, The typical gravity changes and ground deformations are approximately a factor 10 smaller than those observed at the Earth's surface, which are domaninatly the result from surface processes. It is difficult to make accurate predictions of signals originating from the core. The signal caused by pressure variations at the core-mantle boundary (CMB) from core flows is perhaps that which can be most readily predicted.
Flows near the surface of the Earth's fluid core can be reconstructed
on the basis of the observed secular variation of the magnetic field.
This reconstruction is non-unique and depends on assumptions on the
dynamical regime inside the core but parts of the core flows can be
nevertheless recovered somewhat robustly. In Figure 1 below, an
example of the horizontal flow in year 2000 is shown. The dominant force
balance near the surface of the core is one between the Coriolis
acceleration and pressure gradients: this is referred to as a
geostrophic balance. Lateral variation of pressure at the surface of the core pushes
and pulls on the mantle. The latter can deform, and because the core
and mantle have different densities, a radial
displacement of the core-mantle boundary leads to an internal density
change. Also shown in Figure 1 are the pressure at the CMB, gravity change at the Earth's surface and radial deformations of the CMB and surface. As core flows change with time, all of these also vary with time.
Figure 1:Example of a flow map at the CMB for the year 2000, and its associated geostrophic pressure, vertical displacement at the CMB and surface, and gravity change at the surface. The figure is from Dumberry and Mandea, Surveys in Geophysics, 2021. |
Some of my papers on this topic
Dumberry, M. and Mandea, M., 2021, Gravity variations and ground deformations resulting from core dynamics, Surveys in Geophysics, https://doi.org/10.1007/s10712-021-09656-2.
Rosat, S., Gillet, N., Boy, J.-P., Couhert, A. and Dumberry, M., 2021, Interannual variations of degree 2 from geodetic observations and surface processes, Geophysical Journal International, 225, 200-221.
Gillet, N., Dumberry, M. and Rosat, S., 2021, The limited contribution from outer core dynamics to global deformations at the Earth's surface, Geophysical Journal International, 224, 216-229.
Dumberry, M., 2010, Gravity variations induced by core flows, Geophys. J. Int., 180, 635-650.
Dumberry, M., 2008, Decadal Variations in gravity caused by a tilt of the inner core, Geophysical Journal International, 172, 921-933.
Dumberry, M. and Bloxham, J., 2004, Variations in the Earth's gravity field caused by torsional oscillations in the core. Geophys. J. Int., 159, 417-434.