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A Search for Large-scale Variations in the Fine-scale Structure of Earth's Inner Core
  • Sin-Mei Wu,
  • Guanning Pang,
  • Keith D. Koper
Sin-Mei Wu
University of Utah
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Guanning Pang
Cornell University

Corresponding Author:[email protected]

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Keith D. Koper
University of Utah
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A hemispherical pattern in inner core attenuation (Q) has been observed from the apparent Q of inner core transmitted waves (PKPDF). How the elastic scattering (QS) originating from kilometer-scale inner core heterogeneities relates to large-scale apparent Q variations remains elusive. Such inner core scattered energy (ICS) is characterized by emergent, long-lasting, high-frequency (1-4 Hz) coda immediately following pre-critical reflections (PKiKP) from the inner core boundary (ICB). In this study, we develop a framework to systematically investigate ICS and examine its hemispherical pattern using data from two arrays that sample opposing sides of the Pacific quasi-hemispherical boundary. We use all the viable data from earthquakes (Mw{greater than or equal to}5.8) within the past 2-3 decades recorded at distances of 50-75 degrees by YKA and ILAR-small-aperture (~10-20 km) seismic arrays situated in northwestern North America. Two independent waveform stripping methods are applied to extract ICS from the background energy, and various stacks are performed to identify ICS and its spatial pattern. We found that individual beams lacking clear evidence of PKiKP coda waves reveal ICS characteristics when stacked together, implying that ICS is ubiquitous. We used a modified phonon-based simulation to reinforce the idea that ICS is primarily created by volumetric heterogeneity within the inner core as opposed to ICB topography. With simplified two-layer ICS models, our results suggest that ICS within the eastern quasi-hemisphere is slightly stronger than in the western quasi-hemisphere, although intra-hemispherical variations are as significant and our sampling is limited to patches of the northern hemisphere.