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.

Steamboat Geyser in Yellowstone National Park is the tallest active geyser on Earth and is believed to have hydrologic connection to Cistern Spring, a hydrothermal pool ~100 m southwest from the geyser vent. Despite broad scientific interest, rare episodic Steamboat eruptions have made it difficult to study its eruption dynamics and underground plumbing architecture. In response to the recent reactivation of Steamboat, which produced more than 115 eruptions since March 2018 already, we deployed a dense seismic nodal array surrounding the enigmatic geyser in summer 2019. The array recorded an abundant 1-5 Hz hydrothermal tremor originating from phase-transition events within both Steamboat and Cistern. To constrain the spatiotemporal distribution of the tremor sources, an interferometric-based polarization analysis was developed. The observed tremor locations indicate that the conduit beneath Steamboat is vertical and extends down to ~120 m depth and the plumbing of Cistern includes a shallow vertical conduit connecting with a deep, large, and laterally offset reservoir ~60 m southeast of the surface pool. No direct connection between Steamboat and Cistern plumbing structures is found. The temporal variation of the tremor combined with in situ temperature and water depth measurements of Cistern, do reveal the interaction between Steamboat and Cistern throughout the eruption/recharge cycles. The observed delayed responses of Cistern in reaction to Steamboat eruptions and recharges suggest the two plumbing structures might be connected through a fractured/porous medium instead of a direct open channel, consistent with our inferred plumbing structure.