Anisotropy of Earth's inner core intrinsic attenuation from seismic normal mode models

The Earth's inner core, the slowly growing sphere of solid iron alloy at the centre of our planet, is known to exhibit seismic anisotropy. Both normal mode and body wave studies have established that, when the global average is taken, compressional waves propagate faster in the North-South direction than in the equatorial plane. Recent body wave studies also indicate that this fast direction may be more attenuating, and interpret this anisotropic attenuation in terms of anisotropic scattering due to inner core texturing. Here we use the Earth's normal modes to study the attenuation anisotropy of both compressional and shear waves in the inner core. As normal modes have wavelengths several orders of magnitude longer than estimates of inner core grain size, any attenuation anisotropy quantified using normal modes must reflect the anisotropy of intrinsic (viscoelastic) attenuation of the crystalline inner core alloy. By inverting zonal anelastic and elastic normal mode splitting function coefficients of twenty inner core sensitive modes, we construct models of inner core intrinsic attenuation and velocity anisotropy. We find that, for compressional waves, the North-South direction is both fast and more strongly attenuating. The existence of intrinsic inner core attenuation anisotropy can be interpreted in terms of anisotropic Zener relaxation in the metallic alloy comprising the inner core. Such anisotropic Zener relaxation has only been observed in the presence of solute atoms, and is thus entirely consistent with the presence of a few atomic per cent of light elements in the Earth's inner core.