Assessing Intrinsic versus Scattering Attenuation in Earth’s Inner Core

Abstract:

The inner core is known to contain lateral heterogeneities at a variety of scales. While hemispherical and 1000 km scale variations are established, the distribution, scales and sharpness of finer scale (~ 1 – 10 km) heterogeneities are still not well constrained. We study the influence on P waves transmitted through the inner core (PKIKP) of several proposed models of heterogeneity in the upper inner core previously determined from the coda of pre-critical P wave reflections from the inner core boundary (PKiKP). Waveform envelopes are synthesized by AxiSEM, which generates synthetics in 2.5D material structure while running within a 2D computational domain. We examine tradeoffs in the parameters describing the inner core heterogeneity spectrum and intrinsic attenuation to fit PKIKP coda envelopes in the distance range 155^o to 180^o. We find that the inclusion of 2 km sized heterogeneities described by an exponential auto correlation function having 0.5% or 1.2% rms perturbation in P velocity in a 100 km layer at the top of the inner core is insufficient to strongly affect PKIKP waveforms and coda. If this type of model is applied to the entire inner core, a model having a 1.2% perturbation begins to affect the PKIKP waveforms, reducing pulse heights by an average value of 0.2 % and increasing pulse widths by 0.2 %, contributing to the total apparent attenuation in addition to intrinsic attenuation. Another model, which also fits precritical PKiKP, having an rms perturbation of 2% and a larger autocorrelation length (12 km) tends to attenuate the PKIKP waveform significantly and decreases the amplitude ratio of the initial PKIKP pulse and its coda, inconsistent with observations. These results suggest that the effects of scattering on PKIKP waveforms are small, and that intrinsic viscoelasticity dominates the apparent attenuation inferred from PKIKP waveforms at ranges beyond 155^o