Inner core structure: Constraints from frequency dependent seismic anisotropy

Seismic waves sampling the inner core have revealed that it is anisotropic, propagation being faster and attenuation stronger for waves travelling parallel to the Earthʹs rotation axis, relative to those parallel to the equatorial plane. Here, the anisotropy in attenuation is investigated in the frequency range 0.2–2 Hz, in a region of the uppermost inner core sampled by rays of various orientations. A spectacular amplitude decay with frequency is observed for polar paths whereas it is very mild for equatorial paths, leading to a much stronger anisotropy at high frequency than at low frequency. A velocity dispersion is also observed for polar paths, with a propagation faster at high frequency than at low frequency. These observations may provide important constraints on the texture of the inner core, and help to better understand its growth mechanism.