The Depth Dependence of Seismic Scattering in the Inner Core.

The high frequency coda following the P wave reflected from the inner core boundary (PKiKP) can be explained by scattering from 1-10 km scale heterogeneities in the uppermost 200-400 km of inner core due to the processes of inner core solidifcation. Observations of depth-dependent elastic anisotropy and viscoelastic attenuation from PKIKP waves transmitted through the whole of the inner core suggest that there may also be an undetected depth dependence of elastic scattering. We investigate how models of the depth dependence of inner core heterogeneity, which include models of mantle scattering, affect the coda of PKIKP up to antipodal range. PKIKP waveforms are simulated using the AxiSEM numerical code, which generates synthetic seismograms in 2.5D material structure while running within a 2D computational domain. Comparisons of data waveforms with synthesized waveforms eliminate models that generate unacceptable levels of PKIKP coda. Throughout the inner core we suggest an upper bound of 2% rms perturbation to P wave velocity and a 12 km corner scale length for an exponential autocorrelation of heterogeneity. Without depth dependence the application of this upper bound model throughout the inner core leads to unacceptable amounts of antipodal focusing of coda for great circle distances greater than 178o.