P-wave propagation in transversely isotropic media: II. Application to inner core anisotropy: Effects of data averaging, parametrization and a priori information

The main goal of this study is to check the existence of an innermost inner core with a radius of about 300 km and a different anisotropy [Ishii, M., Dziewonski, A.M., 2003. Distinct seismic anisotropy at the centre of the Earth. Phys. Earth Planet. Int. 140, 203–217]. We first compute the sensitivity kernels of PKP(DF), the seismic P-wave which samples the inner core, in a transversely isotropic medium. Owing to their long paths, PKP(DF) phases have a Fresnel zone inside the inner core which is as large as the innermost inner core radius for a 2 s dominant period. To enhance the inner core cylindrical structure and improve the signal-to-noise ratio in the data, PKP(DF) traveltime residuals from bulletins are computed for summary rays using two different stacking procedures. These data are then inverted to obtain radial anisotropic models of the inner core with a symmetry axis parallel to Earthʹs rotation axis, using both finite-frequency and ray theories. We do not observe significant differences between the two theories because of the dominant effect of PKP(DF) residuals averaging, and of the model parametrization. However, the inversions show that the method to average traveltime residuals as well as the a priori assumptions both influence the final model. The same data can be explained by three families of models which all exhibit anisotropy with fast axis parallel to Earthʹs rotation axis in the external part of the inner core (from the inner core boundary to a radius around 450 km) but which reveal very different innermost inner core structures: (I) a weak anisotropy with a slow symmetry axis parallel to Earthʹs rotation axis; (II) a nearly isotropic innermost inner core; (III) a strong anisotropy with a fast symmetry axis parallel to Earthʹs rotation axis. These models have very different implications for the origin of the anisotropy and the history of Earthʹs core.