Complex seismic anisotropy in the top of the Earth's inner core beneath Africa

Abstract

We report complex seismic anisotropy in the top 80 km of the Earth's inner core beneath Africa. The anisotropy in the top 80 km of the inner core is constrained using differential travel times, amplitude ratios, and waveforms of the PKiKP‐PKIKP phases sampling Africa along various directions. The differential PKiKP‐PKIKP time residuals (relative to the Preliminary Reference Earth Model [PREM]) along the polar paths are larger than those along the equatorial paths by 0–1.4 s, indicating the presence of seismic anisotropy in the top 80 km of the inner core. Furthermore, the observations along the polar paths show complex regional variations beneath Africa: the differential PKiKP‐PKIKP travel time residuals vary from 1.2 s beneath eastern Africa, to −0.1 s beneath central Africa, and to −0.2 to 0.8 s beneath western Africa. A correlation between small PKIKP/PKiKP amplitude ratios and large differential PKiKP‐PKIKP travel time residuals is observed. The waveform data are spatially binned into six groups to constrain the regional dependence of velocity and attenuation anisotropy in the top 80 km of the inner core. Overall, the seismic data can be explained by an isotropic upper inner core (UIC) overlying an anisotropic lower inner core (LIC) in the top 80 km of the inner core across Africa. The thickness of the isotropic UIC varies from 0 to 50 km, and the P velocity transition from the isotropic UIC to the anisotropic LIC is sharp, with velocity increases laterally varying from 1.6% to 2.2%. The attenuation structure along the polar paths has a Q value of 600 for the isotropic UIC and Q values varying from 150 to 400 for the anisotropic LIC. The complex seismic anisotropy in the top of the inner core is found in a region where a rapid change of the inner core boundary (ICB) between 1993 and 2003 was discovered (Wen, 2006) and may be explained by complex alignments of iron crystals, resulting from a localized anomalous solidification of the inner core.