A Search for Large-scale Variations in the Fine-scale Structure of
Earth's Inner Core
Abstract
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.