Anisotropic attenuation in the Earth’s inner core: normal mode observations and models

The Earth's inner core, the elusive and intriguing centremost part of our planet, mainly comprises iron and grows by solidification of the liquid outer core surrounding it. By arguments of density and lower-than-expected S-wave velocity, some alloying light elements must also be present in the inner core. Seismological studies show that compressional wave velocity in the inner core is anisotropic, with a fast North-South (polar) direction. Body wave studies also hint at attenuation anisotropy: for compressional waves, the faster polar direction appears more attenuating than the equatorial plane. Such results are usually interpreted in terms of anisotropic scattering by texturing of inner core material, though it is not possible to distinguish between intrinsic and scattering attenuation using body waves alone. In this talk, I shall use the Earth's long-wavelength normal modes, which are not attenuated by scattering, to elucidate and quantify the attenuation anisotropy. I shall discuss an extended normal mode technique that allows for mapping of three-dimensional variations in intrinsic attenuation, and show how this can be used to model attenuation anisotropy in the inner core. I shall show that the Earth's inner core exhibits anisotropic intrinsic attenuation, which I interpret in terms of relaxation processes in metal alloys.