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The core‐mantle boundary (CMB) and the outermost core are dynamic and heterogeneous regions with time‐dependent flows. We examine two seismic raypaths, diffracted P and PKP precursors—both replete with scattering, with precisely repeating earthquakes. These earthquakes, occurring in the South Sandwich Islands, were recorded on the Yellowknife array in Canada, the Alice Springs array in Australia, and the Eilson Array in United States for the past 30 years. In all the most resolved cases, five for diffracted P and 19 for PKP precursors, we observe 1–2 Hz scattered waves that exactly repeat within the resolution of our study for more than 10 s. Although the absence of observable changes is unsurprising, it imposes constraints on potential temporal variations near the CMB. This suggests that any dynamic processes in this region might either be too subtle to detect, even with high‐frequency waves, or occur on different timescales. Plain Language Summary: Earthquakes generate waves that travel through the Earth's interior, revealing details about its structure. In this study, we examined how certain high‐frequency waves behave as they pass near the boundary between the Earth's core and mantle. By analyzing waves from repeating earthquakes, which have almost identical focal mechanism and locations, in the South Sandwich Islands, recorded by stations in Canada and Australia, we found no noticeable changes over time. This suggests that the core‐mantle boundary remains stable, at least within the time frame and sensitivity of our study. These findings help us better understand the dynamics of the Earth's deep interior and guide future research in this area. Key Points: We analyzed two high‐frequency seismic phases from repeating earthquakes to seek temporal changes near the core‐mantle boundary (CMB)No significant temporal changes were observed in the scattered waves recorded by seismic arrays in Canada and AustraliaThe absence of detectable changes constrains the dynamic and mineralogical processes at the CMB, and informs future, more sensitive studies [ABSTRACT FROM AUTHOR]

We observe high‐frequency scatterers consistent with the interpretation of a tabular high‐velocity structure under the Indian Ocean as an ancient subducted slab. We use a previously rarely used raypath, P waves scattered in the slab into PKP waves (P∗ ${\mathrm{P}}^{\ast }$PKP), from 12 earthquakes and explosions in five locations recorded on the antique LASA (Large Aperture Seismic Array) located in Montana, United States. The scatterers concentrate in the mantle transition zone and ∼ ${\sim} $1,500 km depths, in the locations where the fast anomalies in the tomography broaden and strengthen. Our inference that the slab lingers in the upper‐ and mid‐mantle despite subducting and detaching more than 130 million years ago suggests that models of slabs sinking into the mantle may have to accommodate such long‐term stagnation. Plain Language Summary: Through study of a novel raypath of seismic waves identified on a large, high‐quality seismographic array, we find evidence of an ancient subducted slab residing beneath the Indian Ocean. Our investigation reveals the presence of high‐frequency scatterers nestled approximately 500 and 1,500 km deep. Notably, these scatterers locate within fast anomalies observed in tomographic imaging. Despite having undergone subduction and detachment over 130 million years ago, this slab remains suspended mid‐mantle. Such longevity challenges prevailing models of subduction dynamics by indicating protracted stagnation of subducted lithospheric material within the Earth's mantle. Key Points: A unique raypath P*PKP is utilized to detect heterogeneities in the mantleStrong scattering coincides with a hypothesized ancient subducted slab beneath the Indian OceanThis Southeast Indian Slab may have stagnated more than 100 Ma, which is unusually long [ABSTRACT FROM AUTHOR]

The rate that Earth's inner core rotates relative to the mantle and crust has been debated for decades. Nonrotational processes, including internal deformation and flow in the outer core, have also been proposed to explain observed seismic changes. The observed changes thus far have been so inconsistent and weak as to hamper convincing interpretation. Here, we examine waves backscattered from within the inner core, which can more robustly evaluate rotation, from two nuclear tests 3 years apart in Novaya Zemlya, Russia. We have extended our previous analysis of these explosions using precise station corrections and the full Large Aperture Seismic Array, thus revealing how the time shifts depend on slowness and lag time and halving our rotation rate estimate. Our derived 0.07°/year inner core superrotation rate from 1971 to 1974 is more robust and slower than most previous estimates and may require interesting reinterpretations of localized signals previously interpreted as inner core rotation. Plain Language Summary: The Earth's solid inner core, which is surrounded by the liquid outer core, is not attached to the overlying mantle. It is only inhibited from differentially spinning by gravitational, magnetic, and viscous coupling through the outer core. Rotation has been inferred, but studies do not agree on the rate nor the time variation in the rate nor even on whether differential spinning takes place. By comparing backscattering from the inner core due to two large nuclear tests in the same place 3 years apart, we measure the motion of the inner core with unprecedented resolution and find very slow motion, 0.07° superrotation per year from 1971 to 1974. This slow rate, if persistent, has difficulty explaining observations in other studies of localized core changes, and so may require additional dynamic physics be active near the inner core boundary. Key Points: We precisely measure the rotation rate for at least a sizeable portion of the inner core from repeated megaton nuclear blastsThe rotation rate is about the rotation pole and just 0.07° ± 0.02°/year, slower than most previous estimatesOnly simple rotation is inconsistent with observations, so inner core deformation may be taking place as well as a rigid rotation [ABSTRACT FROM AUTHOR]

Deep sedimentary basins amplify long‐period shaking from seismic waves, increasing the seismic hazard for cities sited on such basins. We perform 3‐D simulations of point source earthquakes distributed around the Seattle and Tacoma basins in Washington State to examine the dependence of basin amplification on source azimuth, depth, and earthquake type. For periods between 1 and 10 s, the pattern of amplification is spatially heterogeneous and differs considerably with the source‐to‐site azimuth. For close‐in earthquakes, the greatest basin amplification occurs toward the farside of the basin and ground motions from crustal earthquakes experience greater amplification than those from more vertically incident, deeper intraplate earthquakes. Love and Rayleigh waves form similar spatial patterns for a given source location, although the magnitude of amplification varies. The source dependence of basin amplification is an important factor for seismic hazard assessment, in both the Seattle and Tacoma basins, and by extension for deep sedimentary basins worldwide. Plain Language Summary: Sedimentary basins amplify the ground shaking due to an earthquake. This increases the seismic hazard for cities located above such basins. Therefore, determining the amount and distribution of shaking amplification is critical to estimating the hazard associated with future earthquakes. We use computer simulations to determine how an earthquake's location, depth, and focal mechanism affect ground shaking for communities above the Seattle and Tacoma sedimentary basins, Washington State. We find that the amount of shaking amplification strongly depends on where the earthquake is located, as well as the earthquake type. This has important implications for seismic safety (e.g., for buildings and infrastructure) located above deep sedimentary basins worldwide. Key Points: Basin amplification within the Seattle and Tacoma basins is strongly dependent on earthquake‐to‐site azimuth, source mechanism, and depthFor close‐in sources, peak long‐period basin amplification is generally focused on the farside of the basin relative to the sourceImpact of source‐dependent basin amplification needs to be considered for estimates of seismic hazard in sedimentary basins worldwide [ABSTRACT FROM AUTHOR]

Abstract: Volcanic deep long‐period earthquakes (DLPs) have been observed for a long time but remain poorly understood. In this study, 22 catalog DLPs were taken as templates to search for repeating events at Mount St. Helens. During the period from 2007 to 2016, we have detected 277 DLPs. Three templates are isolated DLPs, while all other templates produced matches, identifying loci of repeated activity. Overall, the DLPs show no significant correlation with either the timing of the subduction zone tremor and slow slip west of Mount St. Helens, or the shallow seismicity, or distant earthquakes. Temporal analysis shows an elevated rate of DLPs at times of compressional volumetric tidal stress, suggesting a possible association with magmatic and/or fluid activity. We also observed variable S wave polarization of the DLPs in the most productive DLP source region, indicating source mechanisms are not identical despite the waveform similarity that allowed their detection. [ABSTRACT FROM AUTHOR]

Abstract: We present a catalog of offshore seismicity generated from Cascadia Initiative (CI) ocean‐bottom seismometer data. Earthquakes were detected within the CI data using a short‐time‐average/long‐time‐average trigger and located using 1‐D velocity models developed from seismic reflection/refraction surveys. The catalog, which contains 271 earthquakes with magnitude 0.4–4.0 along the coasts of Vancouver Island, Washington, Oregon, and Northern California, spans all 4 years of the ocean bottom seismometer deployment and shows distinct along‐strike variations in seismicity consistent with structural observations from recent active source seismic reflection/refraction studies. Seismicity is sparse off Vancouver Island and Washington (49°N–46°N) but increases off northern and central Oregon, corresponding to a roughened, more deformed subducting slab. Widespread earthquakes are observed at near‐interface depths between 46°N and 45°N, though an increase in underthrust sediment thickness between 45°N and 43°N likely restricts seismicity to scattered asperities on the plate interface. South of 43°N, where both the overriding and subducting plates are severely deformed approaching the Mendocino triple junction, seismicity is abundant. We locate an additional 440 events in the Juan de Fuca plate seaward of the deformation front. The higher seismicity south of 46°N is consistent with more extensive intraplate deformation. Along with the complex stress field induced by the Mendocino triple junction, our observations imply that the smoothness and degree of hydration of the incoming plate, which are linked to the amount of underthrust sediment and extent of intraplate deformation, are major contributing factors to the distribution of microseismicity in the Cascadia subduction zone. [ABSTRACT FROM AUTHOR]