Evidence for a Kilometer‐Scale Seismically Slow Layer Atop the Core‐Mantle Boundary From Normal Modes

Geodynamic modeling and seismic studies have highlighted the possibility that a thin layer of low seismic velocities, potentially molten, may sit atop the core-mantle boundary but has thus far eluded detection. In this study we employ normal modes, an independent data type to body waves, to assess the visibility of a seismically slow layer atop the core-mantle boundary to normal mode center frequencies. Using forward modeling and a data set of 353 normal mode observations we find that some center frequencies are sensitive to one-dimensional kilometer-scale structure at the core-mantle boundary. Furthermore, a global slow and dense layer 1–3 km thick is better-fitting than no layer. The well-fitting parameter space is broad with a wide range of possible seismic parameters, which precludes inferring a possible composition or phase. Our methodology cannot uniquely detect a layer in the Earth but one should be considered possible and accounted for in future studies.