Normal Mode Insights into the Long Wavelength Velocity and Density Structure of the Lowermost Mantle

Shear wave velocity (Vs) models of the lowermost mantle are dominated by two large-low-shear velocity provinces (LLSVPs). The LLSVPs cover about half of the core surface and extend hundreds of kilometers up into the mantle. These large features inevitably have a significant effect on the dynamics of Earth's mantle and have been interpreted as both thermal superplumes and thermochemical piles. To resolve the debate regarding the possible presence of chemical heterogeneity on these large length scales, accurate seismic characterization of the long wavelength compressional wave velocity (Vp) and density structure is required. Earth's normal modes provide an invaluable tool for probing the Earth's deep interior since they are global in character and affected by density variations in addition to velocity. In particular, Stoneley modes, confined to solid-liquid interfaces such as the core-mantle boundary (CMB), are primarily sensitive to structures in the D'' region. Observations of these modes increase the depth resolution and provide unique constraints on long wavelength structures in the deep mantle. We combine recent normal mode splitting function measurements of 143 modes including 33 modes dominantly sensitive to Vp and 9 CMB Stoneley modes with independent constraints from body wave travel times and surface wave phase velocity maps. We invert jointly for lateral Vs and Vp variations in the Earth mantle and analyze the resulting SP12RTS model in terms of the ratio R=dlnVs/dlnVp and the correlations between seismic velocities. In addition, we study the density structure of the LLSVPs by forward modeling of possible density scenarios for the lowermost mantle. We discuss our results in the light of thermal versus thermochemical variations in the Earth's mantle.