Abstract Temporal changes near the core‐mantle boundary (CMB) would trigger insights into ongoing thermal and chemical interactions between the Earth's core and mantle. Here, we search for multidecadal temporal changes in the CMB topography and heterogeneity using waveform similarity analysis of core‐reflected phases (PcP, ScP, and ScS) from global repeating earthquakes. Most well‐resolved observations reveal no temporal changes; however, we observe clear PcP waveform difference in the last event of a repeating earthquake triplet sampling the region beneath Siberia. Seismic array analysis indicates that this discrepancy results from interference from a local earthquake, which can easily be misinterpreted as a CMB‐related changes. These findings suggest that dynamical evolution at the CMB either occurs over timescales longer than several decades; any faster transient processes remain invisible to current seismic detection limits.

The physical origin of temporal changes in inner-core waveforms remains debated, partly because waveform variations may arise from bulk motion of volumetric heterogeneity, changes in boundary structure, or changes in scattering strength. We use 2D acoustic wavefield simulations to isolate end-member waveform responses to translated volumetric heterogeneity, shifted rough-boundary structure, and changes in scattering strength in both forward- and back-scattering scenarios. The simulations show that coda decorrelation depends strongly on the direction of structural motion relative to the raypath. In forward scattering, along-ray shifts of volumetric heterogeneity preserve high waveform correlation, whereas transverse shifts produce substantially stronger decorrelation. We refer to these mechanism-dependent patterns in cross-correlation, time shift, and amplitude ratio as waveform fingerprints, while emphasizing that they are qualitative diagnostic tendencies rather than necessarily unique waveform matches. A comparison with South Sandwich Islands waveforms recorded at YKA and ILAR shows that the observed cross-correlation trends fall within the range of the modeled end-member behavior, with YKA retaining higher coherence and ILAR showing stronger decorrelation. If the stronger decorrelation reflects transverse motion of volumetric heterogeneity, the associated effective heterogeneity scale is likely of order tens of kilometers, but this estimate remains model dependent.

The inner core has been inferred to change its rotation rate or shape over years to decades since the discovery of temporal variability in seismic waves from repeating earthquakes that travelled through the inner core. Recent work confirmed that the inner core rotated faster and then slower than the rest of Earth in the last few decades; this work analysed inner-core-traversing (PKIKP) seismic waves recorded by the Eielson (ILAR) and Yellowknife (YKA) arrays in northern North America from 121 repeating earthquake pairs between 1991 and 2023 in the South Sandwich Islands. Here we extend this set of repeating earthquakes and compare pairs at times when the inner core re-occupied the same position, revealing non-rotational changes at YKA but not ILAR between 2004 and 2008. We propose that these changes originate in the shallow inner core, and so affect the inner-core-grazing YKA ray paths more than the deeper-bottoming ray paths to ILAR. We thus resolve the long-standing debate on whether temporal variability in PKIKP waves results from rotation or more local action near the inner-core boundary: it is tentatively both. The changes near the inner-core boundary most likely result from viscous deformation driven by coupling between boundary topography and mantle density anomalies or traction on the inner core from outer-core convection.

The largest deep-focus earthquakes typically occur in isolated settings, making their aftershock sequences crucial for understanding the interaction between these deep events, the subducting slabs, and their surroundings. The exceptionally deep Mw 7.9 Bonin Islands earthquake, which occurred at a depth of 680 km in a region previously thought to be aseismic, provides a rare opportunity to investigate the behavior and mechanism of deep earthquakes. Using an array-based beamforming technique, we flag numerous false detections in previous work, detect additional aftershocks following the Bonin Islands earthquake, and relocate using backprojection. The aftershocks are located outside the mainshock rupture zone and are aligned along a plane that extends from the mainshock rupture, suggesting a deformation migration across a pre-existing zone of weakness. These observations are compatible with the hypothesis of a thin metastable olivine wedge within the lower part of the Izu–Bonin slab, which may play a role in the deep earthquake mechanism. We reject a previous claim of deeper aftershock activity within the lower mantle. Our results further the understanding of the structure of the mantle and the physical mechanisms driving deep-focus earthquakes.

The solid inner core, suspended within the liquid outer core and anchored by gravity, has been inferred to rotate relative to the surface of Earth or change over years to decades based on changes in seismograms from repeating earthquakes and explosions 1,2 . It has a rich inner structure 3-6 and influences the pattern of outer core convection and therefore Earth's magnetic field. Here we compile 143 distinct pairs of repeating earthquakes, many within 16 multiplets, built from 121 earthquakes between 1991 and 2023 in the South Sandwich Islands. We analyse their inner-core-penetrating PKIKP waves recorded on the medium-aperture arrays in northern North America. We document that many multiplets exhibit waveforms that change and then revert at later times to match earlier events. The matching waveforms reveal times at which the inner core re-occupies the same position, relative to the mantle, as it did at some time in the past. The pattern of matches, together with previous studies, demonstrates that the inner core gradually super-rotated from 2003 to 2008, and then from 2008 to 2023 sub-rotated two to three times more slowly back through the same path. These matches enable precise and unambiguous tracking of inner core progression and regression. The resolved different rates of forward and backward motion suggest that new models will be necessary for the dynamics between the inner core, outer core and mantle. Matching seismic waveforms show that the inner core of Earth gradually super-rotated from 2003 to 2008, and then more slowly sub-rotated from 2008 to 2023 back through the same path.

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 & lowast; ${\mathrm{P}}<^>{\ast }$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 similar to ${\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. 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. A unique raypath P*PKP is utilized to detect heterogeneities in the mantle Strong scattering coincides with a hypothesized ancient subducted slab beneath the Indian Ocean This Southeast Indian Slab may have stagnated more than 100 Ma, which is unusually long

AbstractThe 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.

Arriving at the Seismo Lab in 1981,I first worked with Hiroo Kanamori on the seismotectonics of the New Hebrides(Vidale and Kanamori,1983)(I don't remember why).Then with Rob Clayton I worked on finite-difference methods of modeling seismic waves,specifi-cally how to bump second-order methods to fourth-order and encode the inner loop in assembly language to make it run twice as fast(Vidale,1990).

ABSTRACTSedimentary basins in the Puget Sound region, Washington State, increase ground-motion intensity and duration of shaking during local earthquakes. We analyze Pacific Northwest Seismic Network and U.S. Geological Survey strong-motion recordings of five local earthquakes (M 3.9–6.8), including the 2001 Nisqually earthquake, to characterize sedimentary basin effects within the Seattle and Tacoma basins. We observe basin-edge generated surface waves at sites within the Seattle basin for most ray paths that cross the Seattle fault zone. We also note previously undocumented basin-edge surface waves in the Tacoma basin during one of the local earthquakes. To place quantitative constraints on basin amplification, we determine amplification factors by computing the spectral ratios of inside-basin sites to outside-basin sites at 1, 2, 3, and 5 s periods. Ground shaking is amplified in the Seattle basin for all the earthquakes analyzed and for a subset of events in the Tacoma basin. We find that the largest amplification factors in the Seattle basin are produced by a shallow earthquake located to the southwest of the basin. Our observation suggests that future shallow crustal and megathrust earthquakes rupturing west of the Puget Lowland will produce greater amplification within the Seattle basin than has been seen for intraslab events. We also perform ground-motion simulations using a finite-difference method to validate a 3D Cascadia velocity model (CVM) by comparing properties of observed and synthetic waveforms up to a frequency of 1 Hz. Basin-edge effects are well reproduced in the Seattle basin, but are less well resolved in the Tacoma basin. Continued study of basin effects in the Tacoma basin would improve the CVM.

We investigate the differential rotation of Earth’s inner core relative to the mantle using pairs of precisely located nuclear explosions. We find that the inner core subrotated at least 0.1° from 1969 to 1971, in contrast to superrotation of ~0.29° from 1971 to 1974. These observations contradict models of steady inner core rotation and models that posit much faster rotation rates. The reversal of polarity, timing, and rotation rates is consistent with a model of oscillations about an equilibrium with gravitational locking of the mantle and inner core due to lateral density variations. The model, which has a 6-year period, can explain the variation in the length of day, which has oscillated fairly steadily for the past decades. Inner core oscillation would also allow interpretations of causal connections between inner core and mantle lateral variations, which are problematic if the inner core consistently superrotates.

The solid inner core is at the center of the Earth, gravitationally held within the liquid outer core. It is one of the most dynamic parts of Earth's interior. Since the initial claim of inner core differential rotation relative to the mantle, its existence and rate have been challenged for over two decades. Here, we re-examine the seismic records of two megaton nuclear tests in Novaya Zemlya, Russia, three years apart, from the Large Aperture Seismic Array in Montana, USA. Using an improved static time correction from an antipodal earthquake, we refine the resolution of the beamforming of PKiKP and its inner-core scattered coda. Then, we measure the slight time shifts (tenths of seconds) between the inner-corescattered waves from the two events with moving-time-window cross-correlation. Applying a novel back-projection method, we locate the inner-core regions that scatter the energy within the PKiKP coda based on its slowness and the lapse time. We then measure the inner core rotation, first assuming alignment with Earth's rotation axis, then finding the best-fitting differential rotation axis with a grid search. The rotation rates here are robust and consistent across the many scattered arrivals throughout the inner core scattering wavetrain. Our results indicate 0.10 degrees/year inner core super-rotation rate from 1971 to 1974 aligned with Earth's rotation axis, or 0.125 degrees/year with the rotation axis tilting about 8 degrees from the Earth's rotation axis, which yields a marginally better fit to the observed time shifts. (C) 2021 Elsevier B.V. All rights reserved.

The seismological properties of Earth’s inner core are key to understanding its composition, dynamics and growth history. Within the inner core, fine-scale heterogeneity has previously been identified from backscattering of high-frequency compressional waves. Here we use historical earthquake and explosion data from the Large Aperture Seismic Array, USA, between 1969 and 1975 to build a 3D map of heterogeneity from the inner-core boundary to 500 km depth and determine the geographical distribution of the scatterers across the 40% of the inner core that is visible to the array. Our model has two regions of strong scattering, one beneath eastern Asia and the other beneath South America, both located where past local surveys have identified scattering. We suggest that these loci of strong, fine-scale heterogeneities may be related to random alignments of small, inner-core crystals due to fast freezing. These areas, which have been identified as having high attenuation and lie beneath colder areas of the core–mantle boundary, potentially provide constraints on the dynamics of the inner core and the motions in the outer core, with downwelling in the mantle and outer core possibly associated with strong scattering and inner-core heterogeneity.

Studying the seismicity triggering response of fault systems to periodic stress fluctuations can improve our understanding of earthquake nucleation, rupture failure processes, and local stress states. Geothermal fields are well known to be susceptible to triggering, as the injection and extraction activities change the local stress and fluid flow conditions. Here, we examine the modulation of earthquakes by Earth tides within California's Coso geothermal field (CGF) and its vicinity. To maximize our resolution to detect modulation of small earthquakes, we take advantage of the new Quake Template Matching catalog in southern California, which has nearly twice as many events in the Coso region as the standard catalog and is complete down to about magnitude 0.3. We observe strong tidal triggering of earthquakes within the CGF, even though the fluctuations of tidal stresses are small (similar to 2 kPa). The tidally-triggered earthquakes tend to occur near the time of maximum tensile tidal stress. The signal is strongest near the edges of the zone of new production wells, suggesting fluid pressure gradients encourages triggering at tidal periods. (c) 2021 Elsevier B.V. All rights reserved.

This benefit-cost analysis (BCA) assesses the economic value of the implementation and operation of an earthquake early warning system (EEWS) over a 50-year period in the state of Washington. Based on a cost avoidance approach to assess benefits, this analysis compares the expected economic losses under earthquake scenarios with and without an operating EEWS. The FEMA's earthquake economic loss software program, Hazards US Multi-Hazard (HAZUS-MH), is used to make these comparisons. This analysis demonstrates the net benefits of an EEWS for Washington residents. A Monte Carlo simulation predicts a positive net present value for Washington residents at a 98% probability. A mean savings of $289 million is predicted from the implementation and operation of the proposed EEWS, an amount that considers only some of the potential benefits.

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.