Inner core anisotropy inferred by direct inversion of normal mode spectra

The spectra of 25 inner-core-sensitive normal modes observed following eight recent major (M-w > 7.5) earthquakes are inverted for anisotropic structure in the inner core, using a one-step inversion procedure. The mode data are combined with PKP(DF)-PKP(AB) differential traveltime data and the inner core is parametrized in terms of general axisymmetric anisotropy, allowing structure beyond restrictive transversely isotropic models with radially varying strength. The models obtained are in good agreement with previous ones derived through the intermediate step of computing splitting functions. Splitting functions predicted for the inner core model determined using the one-step, direct inversion of all mode spectra agree well with those obtained from non-linear inversion of individual modes. We discuss the importance of handling the perturbation to radial isotropic structure appropriately in order to align the observed and predicted spectra properly. We examine the effect of using existing tomographic mantle models to correct for mantle effects on the inner core modes Versus a mantle model derived by us using a relatively small number of mantle-sensitive modes, and show that the latter leads to a significantly better lit to the inner core data. Our ability to fit the inner core spectral data degrades appreciably if an isotropic layer thicker than 100-200 km is imposed at the top of the inner core.