Waveform search for the innermost inner core

Waveforms of the PKIKP seismic phase are inconsistent with the effects of a sharp boundary at 300 km radius of an inner-most inner core. Seismograms synthesized for the distance range 150° to 180° in models having a discontinuity at 300 km radius in the inner core exhibit focused diffractions around the innermost sphere at antipodal range that are inconsistent with observed PKIKP waveforms. A few examples, however, can be found of PKIKP complexity near 165°, consistent with a triplication created by a 475 km radius discontinuity. A rapid change in the magnitude and variance of seismic attenuation of PKIKP whose rays turn between 400 and 600 km radius is also observed, consistent with an innermost sphere of low, regionally uniform, seismic attenuation. Models predicting waveforms in agreement with observed waveforms have either a transition in elastic and anelastic properties spread over a depth interval greater than 100 km or an innermost sphere of differing elastic properties that exceeds 450 km radius. The weak but suggestive evidence for an innermost inner core may simply represent a transition in fabric, spread over 100 km or more, centered near 500 km radius. This change may signify the end stage of solidification, flow and re-crystallization, resulting in the highest ordering and largest grain sizes of intrinsically anisotropic crystals.