Seismological Landscape of the Core-Mantle Boundary

At nearly 3000 km depth, the core-mantle boundary (CMB) marks the largest physical change within the Earth, separating the liquid iron outer core and the solid silicate mantle. On the mantle side, large-scale convection provides the driving forces for plate tectonics whereas the rapid convection in the outer core sustains Earth's magnetic field. The interplay of these dynamic processes at the CMB produces a range of structural imprints at scales from tens to thousands of kilometres. Seismological studies of these structures offer critical insights into core and mantle dynamics, leading ultimately to better constraints on the history of our planet. In particular, accurate seismological estimates of density variations are required to model mantle flow and to distinguish between a thermal or compositional origin of mantle heterogeneity. Topography on the CMB itself also plays a critical role in the mechanical coupling between the outer core and mantle, affecting outer core flow while also reflecting mantle processes. The relationship between CMB topography and density variations further helps to understand the origin of seismic anomalies in the deep mantle. Whole Earth oscillations or normal modes are key for probing Earth's deep interior since they have the potential to resolve density variations, as well as velocity and topography structure. Particularly, Stoneley modes that are confined to the liquid-solid interface of the CMB provide an invaluable tool for determining CMB topography and lower mantle density structure. Here, we review recent normal mode studies focused on the imaging of large-scale CMB structure. We present the results of a straightforward model space search, in which we determine the probability of different density and CMB topography models while incorporating our data uncertainties. We demonstrate that the Stoneley mode data are optimally fit when the density of the LLSVPs is lower than the surrounding mantle, underlain by an elevated CMB. We discuss the implications of these results in light of different geodynamic scenarios.