Shear-wave splitting illuminates mantle flow and subduction zone dynamics but is typically inferred near stations or earthquakes, limiting studies in sparsely instrumented regions away from earthquakes. Where stations or earthquakes are present, fast splitting directions are often parallel to the nearest trench, which has yet to be fully understood and reconciled with geodynamic flow predictions. Here we use a novel technique that leverages surface-reflected PS and PPS waves to overcome limitations in coverage beneath the Philippine Sea plate, which is surrounded on all sides by subducting slabs. Upper mantle deformation in this region, best explained by mantle flow below 250 km depth, is generally different from the absolute plate motion direction and instead shaped by the surrounding subduction zones. We interpret our observations as likely evidence for trench-parallel flow in the mantle wedge beneath the Ryukyu, Nankai, and parts of the Izu-Bonin subduction zones, as well as toroidal flow around the Ryukyu and Mariana slab edges. Beneath the Pacific Plate, near the Mariana subduction zone, flow is trench-perpendicular. Our results are generally consistent with flow patterns predicted by previous geodynamic models, and provide a new constraint on subduction-driven deformation in the region.

The bottom of Earth's mantle hosts strong seismic wave speed heterogeneities. These are commonly detected via forward modeling of seismic waveforms, which can include time‐consuming waveform synthesis and visual inspection. Furthermore, such imaging has been most commonly carried out with waves that have limited global coverage. In this work, we investigate the efficacy of the diffracted S () wavefield, which has global coverage to map core‐mantle boundary heterogeneity. We implement a Kirchhoff migration algorithm to objectively investigate the presence or absence of postcursors to , caused by ultralow velocity zones (ULVZs) and other sharp velocity contrasts. Our approach makes use of the expected moveout of ULVZ‐generated postcursors as a function of distance from great‐circle path at the base of the mantle. We investigate epicentral distances , where includes asymptotic S/ScS up to diffraction. We test the algorithm using synthetic waveforms calculated for models that include lowermost mantle wavespeed heterogeneity via a recently proposed hybrid simulation approach. Our results demonstrate that the migration approach, when applied to a single event, can well resolve the location of heterogeneity structures in the azimuthal direction, but is less accurate at constraining the along‐great circle path location. To locate ULVZ structure accurately, heterogeneity maps from different earthquakes with crisscrossing raypaths are combined. Lastly, we provide real‐data proof‐of‐concept examples which detect ULVZs with different sizes that have been proposed in past work. These include the Hawaiian ULVZ, which is roughly 1,000 km across and a ULVZ beneath the Himalayas with a lateral extent of only 200 km.

Summary Seismic anisotropy can inform us about convective flow in the mantle. Shear waves traveling through azimuthally anisotropic regions split into fast and slow pulses, and measuring the resulting shear-wave splitting provides some of the most direct insights into Earth’s interior dynamics. Shear-wave splitting is a constraint for path-averaged azimuthal anisotropy and is often studied regionally. Global compilations of these measurements also exist. Such compilations include measurements obtained using different data processing methodologies (e.g., filtering), which do not necessarily yield identical results, and reproducing a number of studies can be challenging given that not all provide the required information, e.g., about the source location. Here, we automatically determine SKS, SKKS and PKS shear-wave splitting parameters from a global dataset. This dataset includes all earthquakes with magnitudes ≥5.9 from 2000 to the present, collected from 24 data centers, totaling over 4,700 events and 16 million three-component seismograms. We obtain approximately 90,000 robust measurements for “fast azimuth”, φ, and delay time, δt, and 210,000 robust null measurements. Results generally agree with previous work but our measurements allow us to identify hundreds of “null stations” below which the mantle appears effectively isotropic with respect to azimuthal anisotropy, which are important for some splitting techniques. We make all measurements publicly available as a data product, along with detailed metadata. This serves two purposes: ensuring full reproducibility of results and providing all necessary information for future systematic use of our measurements, in tomography applications or comparisons with geodynamic flow predictions.

Seismic anisotropy can inform us about convective flow in the mantle. Shear waves traveling through azimuthally anisotropic regions split into fast and slow pulses, and measuring the resulting shear-wave splitting provides some of the most direct insights into Earth's interior dynamics. Shear-wave splitting is a constraint for path-averaged azimuthal anisotropy and is often studied regionally. Global compilations of these measurements also exist. Such compilations include measurements obtained using different data processing methodologies (e.g., filtering), which do not necessarily yield identical results, and reproducing a number of studies can be challenging given that not all provide the required information, for example, about the source location. Here, we automatically determine shear-wave splitting parameters from core-refracted SKS, SKKS and PKS waves from a global data set. This data set includes all earthquakes with magnitudes >= 5.9 from 2000 to the present, collected from 24 data centers, totaling over 4700 events and 16 million three-component seismograms. We obtain approximately 90 000 robust measurements for 'fast azimuth', phi, and delay time, delta t, and 210 000 robust null measurements. Results generally agree with previous work but our measurements allow us to identify hundreds of 'null stations' below which the mantle appears effectively isotropic with respect to azimuthal anisotropy, which are important for some splitting techniques. We make all measurements publicly available as a data product, along with detailed metadata. This serves two purposes: ensuring full reproducibility of results and providing all necessary information for future systematic use of our measurements, in tomography applications or comparisons with geodynamic flow predictions.

Seismic waves traversing the interior of the planet reveal continent-sized regions with distinct physical properties. Researchers are investigating what they are, how they got there, and what their role is in Earth’s dynamic convecting interior.

SUMMARY Seismic traveltime anomalies of waves that traverse the uppermost 100–200 km of the outer core have been interpreted as evidence of reduced seismic velocities (relative to radial reference models) just below the core–mantle boundary (CMB). These studies typically investigate differential traveltimes of SmKS waves, which propagate as P waves through the shallowest outer core and reflect from the underside of the CMB m times. The use of SmKS and S(m-1)KS differential traveltimes for core imaging are often assumed to suppress contributions from earthquake location errors and unknown and unmodelled seismic velocity heterogeneity in the mantle. The goal of this study is to understand the extent to which differential SmKS traveltimes are, in fact, affected by anomalous mantle structure, potentially including both velocity heterogeneity and anisotropy. Velocity variations affect not only a wave's traveltime, but also the path of a wave, which can be observed in deviations of the wave's incoming direction. Since radial velocity variations in the outer core will only minimally affect the wave path, in contrast to other potential effects, measuring the incoming direction of SmKS waves provides an additional diagnostic as to the origin of traveltime anomalies. Here we use arrays of seismometers to measure traveltime and direction anomalies of SmKS waves that sample the uppermost outer core. We form subarrays of EarthScope's regional Transportable Array stations, thus measuring local variations in traveltime and direction. We observe systematic lateral variations in both traveltime and incoming wave direction, which cannot be explained by changes to the radial seismic velocity profile of the outer core. Moreover, we find a correlation between incoming wave direction and traveltime anomaly, suggesting that observed traveltime anomalies may be caused, at least in part, by changes to the wave path and not solely by perturbations in outer core velocity. Modelling of 1-D ray and 3-D wave propagation in global 3-D tomographic models of mantle velocity anomalies match the trend of the observed traveltime anomalies. Overall, we demonstrate that observed SmKS traveltime anomalies may have a significant contribution from 3-D mantle structure, and not solely from outer core structure.

The dynamics of Earth’s D” layer at the base of the mantle plays an essential role in Earth’s thermal and chemical evolution. Mantle convection in D” is thought to result in seismic anisotropy; therefore, observations of anisotropy may be used to infer lowermost mantle flow. However, the connections between mantle flow and seismic anisotropy in D” remain ambiguous. Here we calculate the present-day mantle flow field in D” using 3D global geodynamic models. We then compute strain, a measure of deformation, outside the two large-low velocity provinces (LLVPs) and compare the distribution of strain with previous observations of anisotropy. We find that, on a global scale, D” material is advected towards the LLVPs. Strain is highest at the core-mantle boundary (CMB) and decreases with height above the CMB. Material outside the LLVPs mostly undergoes lateral stretching, with the stretching direction often, but not always, aligning with mantle flow direction. Strain generally increases towards the LLVPs and reaches a maximum at their edges, although models that consider recrystallization suggest that anisotropy may actually be weaker near LLVP edges. The depth-averaged strain in D” is >1.5 in almost all regions, consistent with widespread observations of seismic anisotropy. The mantle flow field and strain in D” outside of LLVPs are not very sensitive to LLVP density but are strongly controlled by local density and viscosity variations outside the LLVPs. Flow directions inferred from anisotropy observations often (but not always) align with predictions from geodynamic modeling calculations.

Within the last decade, thin ultra-low velocity zone (ULVZ) layering, sitting directly on top of the core-mantle boundary (CMB), has begun to be investigated using the flip-reverse-stack (FRS) method. In this method, pre- and post-cursor arrivals that are symmetrical in time about the ScS arrival, but with opposite polarities, are stacked. This same methodology has also been applied to high velocity layering, with indications that ultra-high velocity zones (UHVZs) may also exist. Thus far, studies using the FRS technique have relied on 1-D synthetic predictions to infer material properties of ULVZs. 1-D ULVZ models predominantly show a SdS precursor that reflects off the top of the ULVZ and an ScscS reverberation within the ULVZ that arrives as a post-cursor. 1-D UHVZ models are more complex and have a different number of arrivals depending on a variety of factors including UHVZ thickness, velocity contrast, and lateral extent. 1-D modelling approaches assume that lower mantle heterogeneity is constant and continuous everywhere across the lower mantle. However, lower mantle features display lateral heterogeneity and are either finite in extent or display local thickness variations. We examine the interaction of the ScS wavefield with ULVZs and UHVZs in 2.5-D geometries of finite extent. We show that multiple additional arrivals exist that are not present in 1-D predictions. In particular, multipath ScS arrivals as well as additional post-cursor arrivals are generated. Subsequent processing by the FRS method generates complicated FRS traces with multiple peaks. Furthermore, post-cursor arrivals can be generated even when the ScS ray path does not directly strike the heterogeneity from above. Analysing these predictions for 2.5-D models using 1-D modelling techniques demonstrates that a cautious approach must be adopted in utilization and interpretation of FRS traces to determine if the ScS wavefield is interacting with a ULVZ or UHVZ through a direct strike on the top of the feature. In particular, traveltime delays or advances of the ScS arrival should be documented and symmetrical opposite polarity arrivals should be demonstrated to exist around ScS. The latter can be quantified by calculation of a time domain multiplication trace. Because multiple post-cursor arrivals are generated by finite length heterogeneities, interpretation should be confined to single layer models rather than to interpret the additional peaks as internal layering. Furthermore, strong trade-offs exist between S-wave velocity perturbation and thickness making estimations of ULVZ or UHVZ elastic parameters highly uncertain. We test our analysis methods using data from an event occurring in the Fiji-Tonga region recorded in North America. The ScS bounce points for this event sample the CMB region to the southeast of Hawaii, in a region where ULVZs have been identified in several recent studies. We see additional evidence for a ULVZ in this region centred at 14 degrees N and 153 degrees W with a lateral scale of at least 250 km x 360 km. Assuming a constant S-wave velocity decrease of -10 or -20 per cent with respect to the PREM model implies a ULVZ thickness of up to 16 or 9 km, respectively.

Ultra-low velocity zones (ULVZs) are anomalous structures, generally associated with decreased seismic velocity and sometimes an increase in density, that have been detected in some locations atop the Earth's core-mantle boundary (CMB). A wide range of ULVZ characteristics have been reported by previous studies, leading to many questions regarding their origins. The lowermost mantle beneath Antarctica and surrounding areas is not located near currently active regions of mantle upwelling or downwelling, making it a unique environment in which to study the sources of ULVZs; however, seismic sampling of this portion of the CMB has been sparse. Here, we examine core-reflected PcP waveforms recorded by seismic stations across Antarctica using a double-array stacking technique to further elucidate ULVZ structure beneath the southern hemisphere. Our results show widespread, variable ULVZs, some of which can be robustly modeled with 1-D synthetics; however, others are more complex, which may reflect 2-D or 3-D ULVZ structure and/or ULVZs with internal velocity variability. Our findings are consistent with the concept that ULVZs can be largely explained by variable accumulations of subducted oceanic crust along the CMB. Partial melting of subducted crust and other, hydrous subducted materials may also contribute to ULVZ variability. Earth's core-mantle boundary (CMB), the interface between the solid silicate mantle and the molten iron-rich outer core, is associated with a range of anomalous structures, including ultra-low velocity zones (ULVZs). While generally associated with reduced seismic wave velocities and sometimes increased density, prior studies have reported a wide range of ULVZ characteristics, leading to many questions regarding their origins. The lowermost mantle beneath the southern hemisphere provides a unique environment to study ULVZs because it is located away from regions of large-scale mantle upwelling and downwelling. Our study uses core reflected P-waves (PcP) recorded by seismic stations in Antarctica to investigate this portion of the CMB for ULVZ presence. We find widespread evidence for variable ULVZ structure. Some of the imaged ULVZs can be modeled with a single layer, but others are more complex. We suggest that the ULVZs beneath the southern hemisphere are predominantly associated with subducted oceanic crust that has variable accumulations along the CMB. In some regions, hydrated subducted materials may also experience partial melting, which may contribute to the complicated ULVZ structures imaged in some locations. Core-reflected P-waves are used to investigate ultra-low velocity zones in the lowermost mantle beneath the southern hemisphere Results show widespread evidence for variable ultra-low velocity zones, some of which may indicate layered or gradational structure Variable accumulations of subducted oceanic crust with localized partial melting can explain these anomalous structures

Convective flow in the deep mantle controls Earth's dynamic evolution, influences plate tectonics, and has shaped Earth's current surface features. Present and past convection-induced deformation manifests itself in seismic anisotropy, which is particularly strong in the mantle's uppermost and lowermost portions. While the general patterns of seismic anisotropy have been mapped for the upper mantle, anisotropy in the lowermost mantle (called D '') is at an earlier stage of exploration. Here we review recent progress in methods to measure and interpret D '' anisotropy. Our understanding of the limitations of existing methods and the development of new measurement strategies have been aided enormously by the availability of high-performance computing resources. We give an overview of how measurements of seismic anisotropy can help constrain the mineralogy and fabric of the deep mantle. Specifically, new and creative strategies that combine multiple types of observations provide much tighter constraints on the geometry of anisotropy than have previously been possible. We also discuss how deep mantle seismic anisotropy provides insights into lowermost mantle dynamics. We summarize what we have learned so far from measurements of D '' anisotropy, how inferences of lowermost mantle flow from measurements of seismic anisotropy relate to geodynamic models of mantle flow, and what challenges we face going forward. Finally, we discuss some of the important unsolved problems related to the dynamics of the lowermost mantle that can be elucidated in the future by combining observations of seismic anisotropy with geodynamic predictions of lowermost mantle flow. Earthquakes cause waves that travel through Earth's interior and are recorded by distant seismometers. These seismic waves behave differently depending on the material that they pass through, revealing Earth's material properties. At the very bottom of the mantle, seismic waves sometimes travel at different speeds depending on their direction. This material property is called seismic anisotropy and is caused by material deformation and flow. Global patterns of mantle flow, which is directly connected to surface processes such as the movement of tectonic plates or hotspot volcanism, can therefore be inferred from seismic anisotropy. Recent years have seen advances in anisotropy imaging in the lowermost mantle as well as in numerical calculations of flow in Earth's lowermost mantle. We review the methods that are used to infer lowermost mantle anisotropy. We further give an overview of previous results and interpretations, which include seismic anisotropy caused by upwelling plumes and ancient slab remnants. Future improvements in the fields of seismology, geodynamics, and mineral physics are needed to improve our understanding of global deep mantle flow. Measurement techniques for deep mantle anisotropy have been improved substantially in the last decade These improvements enable inferences of deep mantle flow with increasing confidence Knowledge of mantle dynamics elucidates the drivers of flow, relationships among structures, and Earth's dynamic evolution

The dynamics of Earth's D '' layer at the base of the mantle plays an essential role in Earth's thermal and chemical evolution. Mantle convection in D '' is thought to result in seismic anisotropy; therefore, observations of anisotropy may be used to infer lowermost mantle flow. However, the connections between mantle flow and seismic anisotropy in D '' remain ambiguous. Here, we calculate the present-day mantle flow field in D '' using 3D global geodynamic models. We then compute strain, a measure of deformation, outside the two large-low velocity provinces (LLVPs) and compare the distribution of strain with previous observations of anisotropy. We find that, on a global scale, D '' materials are advected toward the LLVPs. The strains of D '' materials generally increase with time along their paths toward the LLVPs and toward deeper depths, but regions far from LLVPs may develop relative high strain as well. Materials in D '' outside the LLVPs mostly undergo lateral stretching, with the stretching direction often aligning with mantle flow direction, especially in fast flow regions. In most models, the depth-averaged strain in D '' is >0.5 outside the LLVPs, consistent with widespread observations of seismic anisotropy. Flow directions inferred from anisotropy observations often (but not always) align with predictions from geodynamic modeling calculations.

SUMMARY Structure of the inner core is often measured through traveltime differences between waves that enter the inner core (PKPdf) and waves that travel through the outer core only (PKPab and PKPbc). Here we extend the method to converted waves PKSdf and SKPdf and compare results to PKP wave measurements. PKSdf and SKPdf have a very similar path to PKPdf and if velocity variations are present in the inner core, all three wave types should experience them equally. Since traveltime deviations can be due to velocity changes (either isotropic or anisotropy) as well as wave path deviations born from heterogeneity, we simultaneously investigate wave path directions and traveltimes of PKP, SKP and PKS waves for several source-array combinations. One of the path geometries is the anomalous polar corridor from South Sandwich to Alaska, which has strong traveltimes anomalies for PKPdf relative to more normal equatorial path geometries. Here we use array methods and determine slowness, traveltime and backazimuth deviations and compare them to synthetic data. We find that path deviations from theoretical values are present in all wave types and paths, but also that large scatter exists. Although some of the path deviations can be explained by mislocation vectors and crustal variations, our measurements argue that mantle structure has to be considered when investigating inner core anisotropy. Our polar path data show similar traveltime residuals as previously published, but we also find slowness residuals for this path. Interestingly, SKPdf and PKSdf for the South Sandwich to Alaska path show traveltime residuals that are partly opposite to those for PKPdf, possibly due to an interaction with a localized ultra-low velocity zone where waves enter the core.

Abstract We compile and make publicly available a global digital database of body wave observations of seismic anisotropy in the D′′ layer, grouped using the method used to analyze deep mantle anisotropy. Using this database, we examine the global distribution of seismic anisotropy in the D′′ layer, evaluating the question of whether seismic anisotropy is more likely to be located at the edges of the two large‐low velocity provinces (LLVPs) in Earth's mantle than elsewhere. We show that this hypothesis lacks statistical justification if we consider previously observed lowermost mantle anisotropy, although there are multiple factors that are difficult to account for quantitatively. One such factor is the global lowermost mantle ray coverage for different phases that are commonly used to detect deep mantle anisotropy in shear wave splitting studies. We find that the global ray coverage of the relevant seismic phases is highly uneven, with LLVP edges and their interiors less well‐sampled than the global average.

The mechanisms underlying ETS-driven prostate cancer initiation and progression remain poorly understood due to a lack of model systems that recapitulate this phenotype. We generated a genetically engineered mouse with prostate-specific expression of the ETS factor, ETV4, at lower and higher protein dosage through mutation of its degron. Lower-level expression of ETV4 caused mild luminal cell expansion without histologic abnormalities, and higher-level expression of stabilized ETV4 caused prostatic intraepithelial neoplasia (mPIN) with 100% penetrance within 1 week. Tumor progression was limited by p53-mediated senescence and Trp53 deletion cooperated with stabilized ETV4. The neoplastic cells expressed differentiation markers such as Nkx3.1 recapitulating luminal gene expression features of untreated human prostate cancer. Single-cell and bulk RNA sequencing showed that stabilized ETV4 induced a previously unidentified luminal-derived expression cluster with signatures of cell cycle, senescence, and epithelial-to-mesenchymal transition. These data suggest that ETS overexpression alone, at sufficient dosage, can initiate prostate neoplasia.

Seismic anisotropy has been detected at many depths of the Earth, including its upper layers, the lowermost mantle and the inner core. While upper mantle seismic anisotropy is relatively straightforward to resolve, lowermost mantle anisotropy has proven to be more complicated to measure. Due to their long, horizontal ray paths along the core-mantle boundary (CMB), S waves diffracted along the CMB(S-diff) are potentially strongly influenced by lowermostmantle anisotropy. S-diff waves can be recorded over a large epicentral distance range and thus sample the lowermost mantle everywhere around the globe. S-diff therefore represents a promising phase for studying lowermost mantle anisotropy; however, previous studies have pointed out some difficulties with the interpretation of differential SHdiff-SVdiff traveltimes in terms of seismic anisotropy. Here, we provide a new, comprehensive assessment of the usability of S-diff waves to infer lowermost mantle anisotropy. Using both axisymmetric and fully 3-D global wavefield simulations, we showthat there are cases inwhich S-diff can reliably detect and characterize deep mantle anisotropy when measuring traditional splitting parameters (as opposed to differential traveltimes). First, we analyze isotropic effects on S-diff polarizations, including the influence of realistic velocity structure (such as 3-D velocity heterogeneity and ultra-low velocity zones), the character of the lowermost mantle velocity gradient, mantle attenuation structure, and Earth's Coriolis force. Secondly, we evaluate effects of seismic anisotropy in both the upper and the lowermost mantle on SHdiff waves. In particular, we investigate how SHdiff waves are split by seismic anisotropy in the upper mantle near the source and how this anisotropic signature propagates to the receiver for a variety of lowermost mantle models. We demonstrate that, in particular and predictable cases, anisotropy leads to S-diff splitting that can be clearly distinguished from other waveform effects. These results enable us to lay out a strategy for the analysis of S-diff splitting due to anisotropy at the base of the mantle, which includes steps to help avoid potential pitfalls, with attention paid to the initial polarization of S-diff and the influence of source-side anisotropy. We demonstrate our S-diff splitting method using three earthquakes that occurred beneath the Celebes Sea, measured at many transportable array stations at a suitable epicentral distance. We resolve consistent and well-constrained S-diff splitting parameters due to lowermost mantle anisotropy beneath the northeastern Pacific Ocean.

Seismic images of Earth's interior have revealed two continent-sized anomalies with low seismic velocities, known as the large low-velocity provinces (LLVPs), in the lowermost mantle(1). The LLVPs are often interpreted as intrinsically dense heterogeneities that are compositionally distinct from the surrounding mantle(2). Here we show that LLVPs may represent buried relics of Theia mantle material (TMM) that was preserved in proto-Earth's mantle after the Moon-forming giant impact(3). Our canonical giant-impact simulations show that a fraction of Theia's mantle could have been delivered to proto-Earth's solid lower mantle. We find that TMM is intrinsically 2.0-3.5% denser than proto-Earth's mantle based on models of Theia's mantle and the observed higher FeO content of the Moon. Our mantle convection models show that dense TMM blobs with a size of tens of kilometres after the impact can later sink and accumulate into LLVP-like thermochemical piles atop Earth's core and survive to the present day. The LLVPs may, thus, be a natural consequence of the Moon-forming giant impact. Because giant impacts are common at the end stages of planet accretion, similar mantle heterogeneities caused by impacts may also exist in the interiors of other planetary bodies.

Ultralow velocity zones (ULVZs) are the most anomalous structures within the Earth’s interior; however, given the wide range of associated characteristics (thickness and composition) reported by previous studies, the origins of ULVZs have been debated for decades. Using a recently developed seismic analysis approach, we find widespread, variable ULVZs along the core-mantle boundary (CMB) beneath a largely unsampled portion of the Southern Hemisphere. Our study region is not beneath current or recent subduction zones, but our mantle convection simulations demonstrate how heterogeneous accumulations of previously subducted materials could form on the CMB and explain our seismic observations. We further show that subducted materials can be globally distributed throughout the lowermost mantle with variable concentrations. These subducted materials, advected along the CMB, can provide an explanation for the distribution and range of reported ULVZ properties.

Shear-wave splitting measurements are commonly used to resolve seismic anisotropy in both the upper and lowermost mantle. Typically, such techniques are applied to SmKS phases that have reflected (m-1) times off the underside of the core-mantle boundary before being recorded. Practical constraints for shear-wave splitting studies include the limited number of suitable phases as well as the large fraction of available data discarded because of poor signal-to-noise ratios (SNRs) or large measurement uncertainties. Array techniques such as beamforming are commonly used in observational seismology to enhance SNRs, but have not been applied before to improve SmKS signal strength and coherency for shear wave splitting studies. Here, we investigate how a beamforming methodology, based on slowness and backazimuth vespagrams to determine the most coherent incoming wave direction, can improve shear-wave splitting measurement confidence intervals. Through the analysis of real and synthetic seismograms, we show that (a) the splitting measurements obtained from the beamformed seismograms (beams) reflect an average of the single-station splitting parameters that contribute to the beam; (b) the beams have (on average) more than twice as large SNRs than the single-station seismograms that contribute to the beam; (c) the increased SNRs allow the reliable measurement of shear wave splitting parameters from beams down to average single-station SNRs of 1.3. Beamforming may thus be helpful to more reliably measure splitting due to upper mantle anisotropy. Moreover, we show that beamforming holds potential to greatly improve detection of lowermost mantle anisotropy by demonstrating differential SKS-SKKS splitting analysis using beamformed USArray data.

The carbon and water cycles in the Earth's interior are linked to key planetary processes, such as mantle melting, degassing, chemical differentiation, and advection. However, the role of water in the carbon exchange between the mantle and core is not well known. Here, we show experimental results of a reaction between Fe 3 C and H 2 O at pressures and temperatures of the deep mantle and core‐mantle boundary (CMB). The reaction produces diamond, FeO, and FeH x , suggesting that water can liberate carbon from the core in the form of diamond (“core carbon extraction”) while the core gains hydrogen, if subducted water reaches to the CMB. Therefore, Earth's deep water and carbon cycles can be linked. The extracted core carbon can explain a significant amount of the present‐day mantle carbon. Also, if diamond can be collected by mantle flow in the region, it can result in unusually high seismic‐velocity structures.

I arrived at Caltech in June 1986, with interests in both mineral physics and seismology. I was excited to either study mineral physics with Professor Tom Ahrens, or investigate great earthquakes with Hiroo Kanamori. Upon arriving, I decided seismology would be my path. I was particularly interested in Hiroo’s work on earthquake size,