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

Abstract

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.

The emerging picture of a complex core-mantle boundary

“…The frequencies that normal modes oscillate at provide constraints on Earth’s internal structure. Recent work suggests that normal mode centre frequency observations are consistent with a dense and slow kilometre-scale layer atop the CMB 18 .…”