A degree 8 mantle shear velocity model from normal mode observations below 3 mHz

Abstract

We present inversions for a new three‐dimensional mantle υs model, MM2_L12D8, using a recently compiled catalogue of ∼2300 normal mode structure coefficients for 90 multiplets below 3 mHz. These inversions demonstrate the capabilities and limitations of existing normal mode data and reveal new images of structures in the midmantle (900–1800 km depth), which is poorly resolved by surface wave and body wave data. Our inversions are distinguished both by efforts to maintain consistency with a variety of seismic models, and hence data sets, and by attempts to characterize the sensitivity of our model to the choice of damping, to unspecified structures, and to data errors. We find that sensitivity to damping is the dominant source of model uncertainty, but MM2_L12D8 proves to be a robust model of υs with amplitude uncertainties less than 35% for most depths and degrees. Other characteristics of MM2_L12D8 include χ2 misfit to normal mode structure coefficients which is 58% smaller than that of the best existing models, greater similarity to existing models than they have to each other, perturbations relative to existing υs models that are largest in the midmantle, and amplitudes that are most consistent with existing models that employ global, rather than local, basis functions. MM2_L12D8 also displays definite images of “slabs” and “plumes” in the midmantle and a spectrum of heterogeneity that is more continuous with depth than in most other models. These characteristics suggest that the midmantle participates in a very long wavelength pattern of circulation that involves at least the whole lower mantle. Inversions for υp and ρ heterogeneities decorrelated from υs structure demonstrate that there is a significant signal from such structures in the normal mode data, but υp and ρ models are much more sensitive to damping than are υs models. The normal mode catalogue must be expanded before normal mode models of υp and ρ approach the reliability of the υs structures in MM2_L12D8. (This model, together with our catalogue of structure coefficients, is available at web site phys‐geophys.colorado.edu/geophysics/nm.dir.).