Seismic Evidence for a Complicated Inner Core Boundary

We present new seismic evidence for strong lateral heterogeneity near the inner core boundary (ICB). As a probe of the ICB we examined the amplitudes of waves (PKiKP) reflected from the ICB at precritical distances. Specifically, we selected a distance range of 50^o-90^o, in which the PKiKP amplitude is especially sensitive to the shear velocity at the top of the inner core. At these distances the theoretical reflection coefficient at the ICB is nearly zero, and so observations of significant PKiKP energy are inconsistent with the standard model (PREM) for the density and velocity jumps at the ICB. We searched for PKiKP waves using data recorded by the short-period, small-aperture arrays of the International Monitoring System (IMS), for which coherence based stacking procedures can be used to high frequencies. We selected about 180 source-array combinations that had the highest potential for generating PKiKP waves and used a sliding window, time-domain beamforming process for phase identification. In 23 cases PKiKP was positively identified, in 32 cases the data were inconclusive, and in the remainder PKiKP was definitely not present. For data in the positive category we measured PKiKP/P amplitude ratios from optimally tuned array beams, and for data in the inconclusive and negative categories we measured upper bounds on PKiKP/P amplitude ratios from appropriately formed beams. In all cases we corrected the ratios for the effect of the source mechanism using Harvard CMT solutions. The positive PKiKP data are clustered geographically and most have PKiKP/P amplitude ratios 1-2 orders of magnitude larger than predicted by PREM; however, many of the negative data have upper bound PKiKP/P ratios that are significantly lower. We carried out a series of robustness tests to affirm the anomalous PKiKP/P amplitude ratios, and also found evidence that the relevant P waves are not unusually small. Although mantle effects cannot be completely ruled out, the most straightforward explanation for our observations is that the ICB has significant lateral variations in structure. This implies that the sedimentation and compaction processes responsible for growing the inner core are not uniformly simple, and that some of the heterogeneity present deeper within the inner core may reflect this complexity.