Attenuation and scattering in the innermost inner core

Abstract

Attenuation of PKIKP waves transmitted through the inner core of Earth is modeled to constrain the depth of a transition between an outer and inner inner core having a change in elastic anisotropy. Using reference waveforms inverted from earthquake source-time functions and an average mantle attenuation operator, fits to observed PKIKP waveforms are determined for a two layered model of P wave attenuation in the inner core in which 1/Q_P at 1 Hz has a sharp decrease in a 100 km thick transition zone centered at 650 km radius from Earth's center, coincident with the radius estimated for a change in elastic anisotropy. The attenuation of broadband PKIKP waveforms is found to be dominated by viscoelastic rather than by scattering attenuation. Bounds for parameters describing the spatial spectrum of heterogeneity of the inner inner core, consistent with the scattered coda of both forward scattered PKIKP and back-scattered PKiKP, are estimated to between 4 and 6 km at 2 % and 1 to 2 km at 1 %, for the corner scale length and P velocity fluctuation assuming an exponential autocorrelation. Strong antipodal focusing of scattered waves is observed and predicted from this heterogeneity, particularly within 0.5 great circle degrees of the antipode. Hypotheses for the transition in anelasticity and anisotropy of the - inner inner core include changes in the inner core's composition, superionic state, and/or a change in its heterogeneity texture or crystalline lattice structure.