Joint Analysis of GOCE Gravity Gradients Data with Seismological and Geodynamic Observations to Infer Mantle Properties
Abstract:Joint inversion of the observed geoid and seismic velocities has been commonly used to constrain the viscosity profile within the mantle as well as the lateral density variations. Recent satellite measurements of the second-order derivatives of the Earth’s gravity potential give new possibilities to understand these mantle properties.
We use lateral density variations in the Earth's mantle based on slab history or deduced from seismic tomography. The main uncertainties are the relationship between seismic velocity and density -the so-called density/velocity scaling factor- and the variation with depth of the density contrast between the cold slabs and the surrounding mantle, introduced here as a scaling factor with respect to a constant value.
The geoid, gravity and gravity gradients at the altitude of the GOCE satellite (about 255 km) are derived using geoid kernels for given viscosity depth profiles.
We assume a layered mantle model with viscosity and conversion factor constant in each layer, and we fix the viscosity of the lithosphere. We perform a Monte Carlo search for the viscosity and the density/velocity scaling factor profiles within the mantle which allow to fit the observed geoid, gravity and gradients of gravity. We test a 2-layer, a 3-layer and 4-layer mantle. For each model, we compute the posterior probability distribution of the unknown parameters, and we discuss the respective contributions of the geoid, gravity and gravity gradients in the inversion.
Finally, for the best fit, we present the viscosity and scaling factor profiles obtained for the lateral density variations derived from seismic velocities and for slabs sinking into the mantle.