Inner core transition zone and anomalous PKP(DF) waveforms from polar paths

Short-period PKP(DF) waves (which travel through the inner core) from polar paths (nearly NS) were observed to have small amplitudes and complex waveforms but the cause has been a mystery. Here we examine both short-period and broadband PKP record sections from a single earthquake in South Sandwich Islands (SSI). The data from polar paths show clear evidence for an inner core triplication. Our waveform modeling suggests a 250-km thick isotropic upper inner core (UIC) overlying an anisotropic lower inner core (LIC) with about 8% anisotropy. The synthetics match the observed waveforms and amplitudes well with a normal value of inner core attenuation. We propose that such a UIC/LIC transition zone structure may be the cause of anomalous DF waveforms along polar paths. The inferred level of anisotropy in the LIC, 2-3 times previous estimates, has important implications for understanding physical properties of the inner core and sources of inner core anisotropy.