Irregular Transition Layer Beneath the Earth's Inner Core Boundary From Observations of Antipodal PKIKP and PKIIKP Waves

Abstract

Standard Earth models assume a simple uniform inner core boundary (ICB) separating the liquid iron outer core from the solid iron inner core. Metallurgical and geodynamic experiments, however, predict lateral variations along this boundary originating from thermochemical and geodynamic instabilities during solidification. We search for evidence of this lateral heterogeneity by exploiting the sensitivity of antipodal PKIIKP waveforms to the shear wave velocity structure of the uppermost inner core beneath their reflection points on the underside of the ICB. Measuring PKIIKP/PKIKP energy ratios from 33 rare antipodal seismograms in the 178o to 180o distance range, we find this ratio varying between 0.1 and 1.1. Synthetic seismograms demonstrate that a laterally homogeneous liquid‐solid ICB cannot account for this variability. Observations instead support a spatially variable ICB transition consisting of either (1) gradients in seismic velocities and density in which they smoothly increase from those at the outer core to those in the bulk of the inner core over a maximum depth of 10 km or (2) a layered transition with localized double discontinuities in velocities and densities separated by 4–10 km. A layered transition can generate a coda following PKIKP if shear velocity is small (<2 km/s) in the transition. Our results imply that the ICB is not uniform and might appear patchy with lateral rigidity variations. Nonuniform small‐scale structural features that we infer to be present at the ICB are consistent with nonlinear solidification mechanisms driven by small‐scale outer core convection in the lowermost outer core.

Anisotropy at the Inner Core Boundary

“…Our model has the virtue of not requiring any new properties of the inner or outer core, simply the evolution of a known solid property, the inner core's anisotropy, as the core crystallizes. Similarly, lateral changes in the ICB's properties (Attanayake et al, 2018;Krasnoshchekov et al, 2005) or topography (Cao et al, 2007;de Silva et al, 2017) may also modulate PKiKP amplitudes, but anisotropy's first-order effect on amplitudes suggests that a parsimonous approach would first attribute amplitude changes to changes in anisotropy to before invoking other effects.…”

A Search for Large‐Scale Variations in the Fine‐Scale Structure of Earth's Inner Core

“…Many studies have reported evidence for topography or other types of lateral heterogeneity at the ICB (e.g., Attanayake et al., 2018; Blom et al., 2015; Cao et al., 2007; Dai et al., 2012; Koper & Dombrovskaya, 2005; Krasnoshchekov et al., 2005; Song & Dai, 2008; Stroujkova & Cormier, 2004; Tanaka & Tkalčić, 2015; Tian & Wen, 2017; Waszek & Deuss, 2015; Wen, 2006; Zou et al., 2008), raising the possibility that pre‐critical PKiKP coda waves may be generated at the ICB as opposed to within the volume of the inner core. For instance, Poupinet and Kennett (2004) proposed that PKiKP coda waves recorded at distances less than 45° at the Warramunga array in Australia were created by complex reverberation or resonance effects at the ICB.…”

Analyses of anomalous amplitudes of antipodal PKIIKP waves

“…(5) We determined the PKIIKP time window, using a 10-s time window centered on the theoretical arrival time of PKIIKP (Figure 3a). The 10-s threshold was chosen to reduce the interference of the neighboring phase PKP-C diff when picking the PKIIKP phase (Attanayake et al, 2018). The peak amplitude near the beginning portion of the time window was selected as the determinant of PKIIKP arrival time.…”