On upper mantle heterogeneity and anisotropy as mapped by inversion of global surface wave data

Category Geophysic
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Khan, Amir
Holding Date 23 September 2008

We jointly invert fundamental mode Love and Rayleigh dispersion curves, for the Earth’s mantle composition, thermal state, P and S wave anisotropy at different locales on the Earth, based on self-consistent thermodynamic calculations. The method consists of four parts:
1. The composition of the silicate Earth is modeled by the chemical system CaO-FeO-MgO-Al2O3-SiO2. Given these parameters, in addition to a geotherm, we calculate stable mineral modes, elastic properties, bulk density at the prevailing physical conditions using Gibbs free energy minimisation. Voigt-Reuss-Hill averaging is subsequently employed to compute bulk radial isotropic seismic P and S wave velocity profiles.
2. Assuming shear attenuation to be a thermally activated process, we used our thermodynamic calculations to estimate radial bulk and shear attenuation profiles, and employed these to incorporate anelastic contributions to P and S wave velocities.
3. Anisotropic P and S wave velocities are determined from the isotropic ones by employing the relations ξ = (Vsh/Vsv)2, φ = (Vpv/Vph)2 , η = F/(A-2L), Vs = (2Vsv2 +Vsh2)/3 and Vp = (Vpv2 +4Vph2 )/5, where ξ, φ and η are anisotropy parameters that we also invert for.
4. From these radial profiles, i.e. of Vsh, Vsv, Vph, Vpv, ρ and η, synthetic Love and Rayleigh wave dispersion curves are calculated. The dispersion curves have been extracted from global surface wave velocity maps, covering the period range from 40-300 s.
Given this scheme, the data for each locale are jointly inverted using a Markov chain Monte Carlo algorithm, from which a range of compositions, temperatures and radial profiles of anisotropy parameters, fitting data within uncertainties, are obtained. The method has several advantages over standard approaches, in particular that no scaling relationships between Vs and Vp and rho and Vs are introduced as is commonly done, implying that the full sensitivity of Rayleigh and Love waves to the parameters Vs, Vp and rho is accounted for.
In this particular study we investigate the structure of the mantle at a number of locales beneath the Pacific Ocean.