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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]

During earthquakes, slip is often localized on preexisting faults, but it is not well understood how the structure of crustal faults may contribute to slip localization and energetics. Growing evidence suggests that the crust along active faults undergoes anomalous strain and damage during large earthquakes. Seismic and geodetic data from the Calico fault in the eastern California shear zone reveal a wide zone of reduced seismic velocities and effective elastic moduli. Using seismic traveltimes, trapped waves, and interferometric synthetic aperture radar observations, we document seismic velocities reduced by 40%-50% and shear moduli reduced by 65% compared to wall rock in a 1.5-km-wide zone along the Calico fault. Observed velocity reductions likely represent the cumulative mechanical damage from past earthquake ruptures. No large earthquake has broken the Calico fault in historic time, implying that fault damage persists for hundreds or perhaps thousands of years. These findings indicate that faults can affect rock properties at substantial distances from primary fault slip surfaces, and throughout much of the seismogenic zone, a result with implications for the amount of energy expended during rupture to drive cracking and yielding of rock and development of fault systems.

Author(s): Justin L. Rubinstein (corresponding author) [1]; John E. Vidale [1]; Joan Gomberg [2]; Paul Bodin [1]; Kenneth C. Creager [1]; Stephen D. Malone [1] Non-impulsive seismic radiation or 'tremor' [...]

Author(s): Miaki Ishii (corresponding author) [1, 3]; Peter M. Shearer [1, 3]; Heidi Houston [2]; John E. Vidale [2] The disastrous Sumatra-Andaman earthquake of 26 December 2004 was one of [...]

Author(s): John E. Vidale (corresponding author) [1, 2]; Doug A. Dodge [3]; Paul S. Earle [1] The finding that the Earth's inner core might be rotating faster than the mantle [...]

The core-mantle boundary (CMB) is the lower bound on mantle convention, and the lowermost mantle must underlie at leas a thermal boundary layer. There may be a chemical heterogeneity just above the CMB. The lowermost mantle has two types of seismic structure, layering in P and S wave velocities and weak fine-scale heterogeneity from scattered waves, preceding the seismic phase PKP. Observations of scattered PKP waves needing large velocity perturbations close to the CMB are presented. Their position is noted beneath low-velocity lower mantle and close to a patch showing slow basal P velocities.

Geophysical research using data recorded at US and Canadian regional seismographic networks following the May 1992 Chinese nuclear test at Lop Nor reveals a seismically fast layer 130 kilometers (km) thick and 300 km wide at the boundary of the core and mantle. The resolution needed to observe such a mass was obtained by organizing data from 14 regional networks of short-period vertical-component seismometers. The data suggest that the boundary region between mantle and core cannot be viewed as a single homogeneous layer and may be as structurally complex as the crust.

The geometry of an earthquake fault is a function of its depth as observed by the equi-dimensional geometry of the fault which deviates from an elongated structure on increase in depth from the surface of the earth. At shallower depths, the episodic or non-steady faulting geometry is attributed to the increase in the heterogeneity of temperature and composition found near the surface of the earth. A change in the state of faulting from frictional to transformational at intermediate depths affects rupture characteristics such as velocity and geometry. Features of rupture are also affected by transitions, alpha-olivine to beta-phase at a depth of 390 km, beta phase to gamma phase at a depth of 500 km or other phase transitions occurring within the subducting slabs.

Seismic arrays are used to study the interior of the earth, its crust, mantle and core. A study of the seismic discontinuities in earth's interior, which cause changes in the upper mantle and core mantle boundaries, reveal the way earth and its heat transport mechanisms were formed. Seismic arrays are used to detect nuclear explosions and improve the seismic signal capacity. Seismic array studies are useful for analyzing data and in processing softwar computers.

The structure of a 350-kilometer-long portion of the core-mantle boundary (CMB) beneath the northeast Pacific was determined by measuring seismic waves that change from shear waves to compression waves after reflecting off the CMB. This section of the CMB is characterized by an amplitude of less than 500 meters, an absence of sharp radial gradients in the 400 kilometers above the boundary and a mantle-to-core transition of less than 1 kilometer in thickness. Hence the mantle in this region is probably not chemically heterogeneous.

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]