Normal Mode Constraints on Anelastic Structures in the Earth's Lower Mantle

Free oscillations, or normal modes, of the Earth provide important constraints on large-scale structures in the mantle, both elastic and anelastic. In addition to shear-wave (Vs), compressional-wave velocity (Vp) and density (ρ), normal modes are sensitive to perturbations in attenuation, or loss of energy. Attenuation is key in imaging partial melt, water, grain size differences and temperature. Surface waves have imaged the upper mantle attenuation, but lower mantle attenuation is still unknown. Normal mode observations of attenuation in the lower mantle provide constraints on the origin and nature of the two lower mantle Large Low Shear wave Velocity Provinces (LLSVPs). Scattering and focussing, which are hard to separate from intrinsic attenuation in the case of body waves, are included by cross-coupling between normal modes.

Here, we will use normal mode spectra to make tomographic models of 3D variations in shear-wave velocity and shear attenuation qμ = 1/Qμ. Normal mode spectra can be inverted in two ways, using either 1) a direct spectrum one-step inversion or 2) a two-step inversion with splitting function measurements as intermediate step. We will image attenuation using the first method. In synthetic tests, we are able to recover Vs and Qμ structure very well. In our real data inversions, we find anti-correlation in the upper mante, i.e. strong attenuation in low-velocity zones, agreeing with the idea of ridges being hotter and containing melt. In the lower mantle, we find weak attenuation in the center of LLSVPs, and stronger attenuation in the ring around them, which is in agreement with results for the two-step splitting function inversion.