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 (QSC) originating from kilometer‐scale inner core heterogeneities relates to large‐scale apparent Q variations remains elusive. Such inner core scattered (ICS) energy 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 ≥ 5.8) within the past 2–3 decades recorded at distances of 50°–75° 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.

Focusing Effects of Teleseismic Wavefields by the Subducting Plate Beneath Cascadia

“…To evaluate multipathing effects, we apply frequency‐wave number analysis ( f‐k ) (Wu et al., 2022) to the teleseismic S wavefields recorded on the T component at frequencies of 0.05–0.3 Hz and calculated the power spectrum of the slowness vector in a range of −20–20 s/° at intervals of 0.1 s/°. Arrays with the aperture of iMUSH may have slowness anomalies from near‐array heterogeneity (Bondár et al., 1999).…”