An attenuating, isotropic and heterogeneous uppermost inner core: evidence from global PKiKP-PKIKP amplitude ratio tomography

Abstract

SUMMARY The Earth's inner core is solidifying from the liquid outer core, where convection currents power the geodynamo. Constraining properties of the inner core is fundamentally important, yet seismic models do not reconcile body wave and normal mode attenuation properties. Here, we analyse high signal-to-noise ratio core-refracted (PKIKP) and core-reflected (PKiKP) waves generated by earthquakes recorded globally between 1987 and 2021. These phases are excellent indicators of uppermost inner core (UIC) properties due to their low angular separation at the core–mantle boundary and similar ray paths. We analyse their amplitude ratios (ARs) and time delays (DTs) relative to synthetic waveforms and estimate the required attenuation perturbations in a linearized attenuation tomography. The UIC is cylindrically isotropic and heterogeneous in both seismic velocity and attenuation. Contrary to the paradigm that the inner core is seismically hemispherical, we find more complex patterns of attenuation that correlate with seismic velocities, according to both Akaike criterion and Student's t-test. The UIC beneath NE Asia is weakly attenuating and seismically slower, in contrast to the expected properties associated with the quasi-Eastern Hemisphere. Beneath South America, where the core is thought to grow fastest, AR values are more diverse, possibly reflecting interdendritic melt inclusions, and DTs show an E–W gradient. The UIC is seismically slow and weakly attenuating beneath the Atlantic, but strong anomalies emerge near West Africa. Attenuation slightly changes with depth conceivably implying an increase in melt degree or a change in inner core growth rate with time. These 3-D heterogeneities are inconsistent with simple models of core translation or lopsided growth, indicating that more intricate processes are needed to explain inner core structure and evolution.