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.

Seismic wave amplitudes have tremendous sensitivity to subduction structure; however, they are affected by attenuation, scattering and focusing, and have therefore been sparsely used compared with traveltimes. We measure and model teleseismic body wave amplitudes recorded at a dense broadband array in the Washington Cascades (iMUSH). These data show anomalous amplitude variations with complex azimuthal dependence at the low frequency of 0.05 Hz, accompanied by significant multipathing. We demonstrate using spectral-element numerical simulations that focusing of the teleseismic wavefield by the Juan de Fuca slab is responsible for some of the amplitude anomalies. The focusing effects can contaminate the apparent differential attenuation measurements and produce at least 20% of the inferred attenuation signal. The focusing results in complex azimuthal patterns that produce different phase and amplitude variations than does intrinsic attenuation, which should allow separation of elastic (focusing) and anelastic effects. Our results indicate that the amplitudes are sensitive to the subducting slab geometry and subduction structure, and can be used to refine seismic images. Ubiquitous focusing effects are observed along the arc, suggesting a continuous Juan de Fuca slab from Canada to northern California.

The 18 March 2020 M 5.7 Magna earthquake near Salt Lake City, Utah, offers a rare glimpse into the subsurface geometry of the Wasatch fault system—one of the world’s longest active normal faults and a major source of seismic hazard in northern Utah. We analyze the Magna earthquake sequence and resolve oblique-normal slip on a shallow (30–35º) west-dipping fault at ~9–12 km depth. Combined with near-surface geological observations of steep dip (~70º), our results support a curved, or listric, fault shape. High-precision aftershock locations show the activation of multiple, low-angle (<30º) structures, indicating the existence of a complicated fault system. Our observations provide the first direct evidence for the deep structure of the Wasatch fault system, and suggest that ground shaking in the Salt Lake City region in future Wasatch fault earthquakes may be higher than previously estimated.