Dynamic mantle density heterogeneities and global geodetic observables

Abstract

S U M M A R YWe investigate the influence of mantle dynamics on low degree deformations of the Earth at geological timescale. We first compute surface deformations, and discuss the analytical form of the tangential surface displacement induced by internal loads, in a reference frame related to the centre of mass of the planet. We use the theoretical Love numbers formalism since the Earth has a viscoelastic behaviour at geological timescale.Then we quantify degree-one and degree-two deformations induced by upwelling domes and subducted plates sinking into the mantle. We use a simple model in which the slabs are modelled as blobs diving vertically through the mantle, and in which the domes are assumed to be stable over the last 120 Ma. Their location is modelled from seismic tomography within the lower mantle. The temporal evolutions of the J 2 gravitational potential coefficient and of the geocentre motion are plotted since 120 Ma.We find that:(1) The mantle density heterogeneities within the mantle can explain the present-day nonhydrostatic flattening of the Earth. However they vary at a too slow timescale to significantly perturb theJ 2 coefficient.(2) Although there is a significant discrepancy of about a few hundred metres between the centre of figure and the centre of mass of the Earth, the secular variation of the geocentre motion is one order of magnitude smaller than the one induced by surface loads.

ITRF2008 plate motion model

“…They separated the origin drift error and geocenter velocity and found respectively −0.5 mm/a and −0.8 mm/a (for both GIA and PDMT) [ Wu et al , 2011; X. Wu, personal communication, 2012] equivalent to 1.3 mm/a in total for . However, we note that the first value depends on OBP and GRACE data and the second is in the upper limit of other published values [ Greff‐Lefftz et al , 2010; Métivier et al , 2010]. As a consequence, we prefer estimating in the following our own ORB as an independent study.…”

Geocenter motion and its geodetic and geophysical implications

“…3 and 4. Historical geocenter motion due to n = 1 internal load-induced deformation has also been studied over even longer time scales using the Maxwell viscoelastic theory (Greff-Lefftz and Legros, 1997;Greff-Lefftz et al, 2010).…”

“…This unique, very long-wavelength component is important (critically so in many cases) for understanding global mass redistribution processes such as sea level rise, atmospheric and ocean circulation, present-day ice mass imbalance, continental hydrology, ocean tides, glacial isostatic adjustment (GIA), and geodynamic processes of the Earth's core and mantle (e.g., Stolz, 1976;Trupin et al, 1992;Dong et al, 1997;Greff-Lefftz andLegros, 1997, 2007;Watkins and Eanes, 1997;Chen et al, 1999;Blewitt et al, 2001;Wu et al, 2002;Blewitt and Clarke, 2003;Chambers et al, 2004;Klemann and Martinec, 2009;Greff-Lefftz et al, 2010).…”

ITRF2008: an improved solution of the international terrestrial reference frame

“…We recall that there is a significant Z-translation rate of 1.8 mm/yr between ITRF2000 and ITRF2005 . A few studies (Greff-Lefftz 2000;Greff-Lefftz et al 2010;Métivier et al 2010) have shown that geophysical phenomena, such as present day ice melting or postglacial rebound, cannot lead to an increase of the geocenter velocity along the Z-axis larger than 0.3 mm/yr in a decade. Given the fact that the ITRF2008 results show negligible translation rates with respect to ITRF2005, the Z-translation rate between ITRF2005 and ITRF2000 is most likely an indication of an imprecise origin determination of the ITRF2000 solution.…”