Stratified anisotropic structure at the top of Earth's inner core: A normal mode study

► We probe the isotropic layer at the top of Earth's inner core using normal modes. ► Normal mode data are compatible with an isotropic layer of up to 275 km thick. ► Normal modes can therefore be reconciled with previous body wave studies. ► We further show seismic data support the presence of an S-wave isotropic layer. Body wave studies support the presence of an isotropic layer at the top of the inner core. Recent normal mode models of inner core anisotropy do not contain such a layer, but instead have anisotropic structure extending to the inner core boundary. Here, we use full-coupling of normal mode oscillations sensitive to inner core structure in order to investigate the discrepancy between models of the inner core developed using these two different types of data. We impose an isotropic layer onto existing normal mode models of inner core anisotropy, and calculate frequencies, quality factors and synthetic seismograms for radial modes, PKIKP modes and PKJKP modes. Using full-coupling allows us to make the first simulations of the effect of an isotropic layer on radial modes. The presence of an uppermost isotropic layer has an effect on the frequencies and attenuation of normal modes. By calculating the misfit between data from four large earthquakes and synthetic seismograms created for models of the inner core with isotropic layers of varying thickness, we find that normal mode data are compatible with the presence of an isotropic layer of up to 275 km thickness at the top of the inner core. Thus, normal modes can be reconciled with previous body wave studies. The influence of such a layer on PKJKP modes is the only way that S-wave isotropy at the top of the inner core can currently be studied; we show, for the first time, that seismological data support the presence of an S-wave isotropic layer.