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. Normal modes are long-period oscillations of the whole Earth as it vibrates after large earthquakes. The frequency that a mode oscillates at depends on the interior structure of the Earth. Research suggests a global and thin layer of anomalous composition and low seismic wave speeds may have formed at the base of Earth's mantle, but would be difficult to observe seismically. We test and quantify the effect of this layer on the frequencies at which normal modes vibrate. We then compare these predictions to a large data set of normal mode frequency measurements to examine whether such a layer is consistent with observed data. We find that not only is a layer of 1-3 km thickness permitted by the modes, but that a layer being present improves the fit to the data. There are a wide range of parameters that adequately fit the data set so we cannot be specific about its properties. Furthermore a layer is likely not a unique way to improve the model. A seismically slow layer at the core-mantle boundary has implications for processes in the mantle and outer core and the interaction between them. Normal mode center frequencies are a sensitive data type to detect seismically anomalous thin layers at the core-mantle boundaryA slow and dense layer on the order of 1 km thick atop the core-mantle boundary can improve the fit to normal mode dataThe inclusion of a layer is likely not a unique way to improve the 1D model and layer properties remain uncertain