The upper mantle degree 2: Constraints and inferences from global mantle wave attenuation measurements

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Abstract
We present the results of an analysis of global lateral variations in anelasticity of the upper mantle, as measured from very long period Rayleigh waves observed on the GEOSCOPE network. Four consecutive wave trains are used on each record to eliminate uncertainty on the amplitude at the source and to take into account, in a linear manner, focussing effects due to lateral variations in elastic parameters. Local estimates of attenuation are obtained using an inversion method based on the introduction of a correlation length, as an alternative to spherical harmonics expansion. Comparison of phase velocity and attenuation maps obtained at various periods shows a strong correlation of high‐Q and high‐velocity regions (respectively low Q and low velocity) both for great circle and minor arc data, at periods around 200 s. The detailed analysis of the degree 2 pattern, well resolved in the attenuation maps around this period, shows that it originates in the depth range 250–500 km and indicates that of the upper mantle degree 2 pattern observed in shear velocity is likely to be of thermal origin, possibly related to the topmost part of the large‐scale convective system. Corrections for dispersion due to anelastic attenuation in degree 2 lead to significant decrease of corresponding lateral variations in shear velocity and a better agreement in phase and amplitude of the upper mantle degree 2 pattern with that observed in the geoid. This study shows that it is important to consider correcting presently available upper mantle tomographic models for the effects of intrinsic attenuation at long periods, before performing any comparison with short‐period data or other geophysical parameters.