(CCP) of Sp receiver revealed variations of lithospheric thickness across short horizontal receiver reflection, towards quantifying the interpretability of Sp receiver functions, especially in settings where large lateral structure variations are present. Using the spectral element method, we model wave propagation and S -to- P conversion through simple synthetic models with varying velocity interface topography. We systematically explore the effects of wave frequency content, seismometer spacing and illumination geometry on CCP stacked Sp receiver functions in settings where velocity interface depth varies laterally. We observe that the resolving power of Sp receiver functions decreases with decreasing frequency content, and that upward deflections of velocity interfaces are more difficult to observe than are downward deflections, an asymmetry that primarily arises due to corner diffractions. Furthermore, we document how the relationship between the angle of illumination and the orientation of the topography of the velocity interfaces largely determines the apparent interface slope and strongly affects the amplitude of Sp phases in the CCP stacks. Indeed, under certain illumination geometries, strong velocity contrasts across a dipping lithosphere–asthenosphere boundary may not produce detectable Sp phases at the surface. Furthermore, diffractions arising from corners of interface topography can produce artefacts in CCP stacks that masquerade as mid-lithospheric impedance jumps or drops, as well as gently sloped sublithospheric impedance drops. We find that estimates based on Fresnel zone arguments might, in some cases, underestimate the true resolution, and that they are likely to be only appropriate for situations in which abrupt lateral variations in structure do not produce waveform complexities. These results imply that the interpretation of Sp receiver functions and CCP stacks is not straightforward and that care must be exercised when inferring the presence or absence of lithospheric velocity interfaces.

We present a new approach for evaluating existing crustal models using ambient noise data sets and its associated uncertainties. We use a transdimensional hierarchical Bayesian inversion approach to invert ambient noise surface wave phase dispersion maps for Love and Rayleigh waves using measurements obtained from Ekström (2014). Spatiospectral analysis shows that our results are comparable to a linear least squares inverse approach (except at higher harmonic degrees), but the procedure has additional advantages: (1) it yields an autoadaptive parameterization that follows Earth structure without making restricting assumptions on model resolution (regularization or damping) and data errors; (2) it can recover non‐Gaussian phase velocity probability distributions while quantifying the sources of uncertainties in the data measurements and modeling procedure; and (3) it enables statistical assessments of different crustal models (e.g., CRUST1.0, LITHO1.0, and NACr14) using variable resolution residual and standard deviation maps estimated from the ensemble. These assessments show that in the stable old crust of the Archean, the misfits are statistically negligible, requiring no significant update to crustal models from the ambient noise data set. In other regions of the U.S., significant updates to regionalization and crustal structure are expected especially in the shallow sedimentary basins and the tectonically active regions, where the differences between model predictions and data are statistically significant.

S U M M A R Y High-resolution models of seismic velocity variations constructed using body-wave tomography inform the study of the origin, fate and thermochemical state of mantle domains. In order to reliably relate these variations to material properties including temperature, composition and volatile content, we must accurately retrieve both the patterns and amplitudes of variations and quantify the uncertainty associated with the estimates of each. For these reasons, we image the mantle beneath North America with P-wave traveltimes from USArray using a novel method for 3-D probabilistic body-wave tomography. The method uses a Transdimensional Hierarchical Bayesian framework with a reversible-jump Markov Chain Monte Carlo algorithm in order to generate an ensemble of possible velocity models. We analyse this ensemble solution to obtain the posterior probability distribution of velocities, thereby yielding error bars and enabling rigorous hypothesis testing. Overall, we determine that the average uncertainty (1σ ) of compressional wave velocity estimates beneath North America is ∼0.25 per cent dVP/VP, increasing with proximity to complex structure and decreasing with depth. The addition of USArray data reduces the uncertainty beneath the Eastern US by over 50 per cent in the upper mantle and 25–40 per cent below the transition zone and ∼30 per cent throughout the mantle beneath the Western US. In the absence of damping and smoothing, we recover amplitudes of variations 10–80 per cent higher than a standard inversion approach. Accounting for differences in data coverage, we infer that the length scale of heterogeneity is ∼50 per cent longer at shallow depths beneath the continental platform than beneath tectonically active regions. We illustrate the model trade-off analysis for the Cascadia slab and the New Madrid Seismic Zone, where we find that smearing due to the limitations of the illumination is relatively minor.

A mantle story told with metal and gas Differences in isotopic compositions of trace elements can help identify how regions of Earth's mantle have evolved over time. Mundl et al. identified several ancient domains that have been isolated from mantle homogenization and thus contain signatures of primordial material. Tungsten and helium isotope values indicate fractionation and isolation of these mantle domains just after Earth's formation. The findings help constrain ancient processes such as core formation, but also provide insight into unexplained structures in the lower mantle today. Science, this issue p. 66 Tungsten and helium isotopes from the deep mantle require primordial sequestration of metal. New tungsten isotope data for modern ocean island basalts (OIB) from Hawaii, Samoa, and Iceland reveal variable 182W/184W, ranging from that of the ambient upper mantle to ratios as much as 18 parts per million lower. The tungsten isotopic data negatively correlate with 3He/4He. These data indicate that each OIB system accesses domains within Earth that formed within the first 60 million years of solar system history. Combined isotopic and chemical characteristics projected for these ancient domains indicate that they contain metal and are repositories of noble gases. We suggest that the most likely source candidates are mega–ultralow-velocity zones, which lie beneath Hawaii, Samoa, and Iceland but not beneath hot spots whose OIB yield normal 182W and homogeneously low 3He/4He.

The outer core is arguably Earth’s most dynamic region, and consists of an iron-nickel liquid with an unknown combination of lighter alloying elements. Frequencies of Earth’s normal modes provide the strongest constraints on the radial profiles of compressional wavespeed, VΦ, and density, , in the outer core. Recent great earthquakes have yielded new normal mode measurements; however, mineral physics experiments and calculations are often compared to the Preliminary Reference Earth model (PREM), which is 35 years old and does not provide uncertainties. Here we investigate the thermo-elastic properties of the outer core using Earth’s free oscillations and a Bayesian framework.

A. Mundl1, R. J. Walker1, M. Touboul2, M. G. Jackson3, M. D. Kurz4, J. M. D. Day5, V. Lekic1, and R. T. Helz6 1Department of Geology, University of Maryland, College Park, MD 20742, USA (amundl@umd.edu), 2Ecole Normale Supérieure de Lyon, 69364 Lyon Cedex 7, France, 3Department of Earth Science, University of California Santa Barbara, Santa Barbara, CA, USA, 4Woods Hole Oceanographic Institution, Woods Hole, MA, 02543, USA. 5Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA, 92093, USA, 6U.S. Geological Survey, Reston, VA, 20192, USA.

Scattering of seismic waves can reveal subsurface structures but usually in a piecemeal way focused on specific target areas. We used a manifold learning algorithm called “the Sequencer” to simultaneously analyze thousands of seismograms of waves diffracting along the core-mantle boundary and obtain a panoptic view of scattering across the Pacific region. In nearly half of the diffracting waveforms, we detected seismic waves scattered by three-dimensional structures near the core-mantle boundary. The prevalence of these scattered arrivals shows that the region hosts pervasive lateral heterogeneity. Our analysis revealed loud signals due to a plume root beneath Hawaii and a previously unrecognized ultralow-velocity zone beneath the Marquesas Islands. These observations illustrate how approaches flexible enough to detect robust patterns with little to no user supervision can reveal distinctive insights into the deep Earth.

The large low shear‐wave velocity provinces (LLSVP) are thermochemical anomalies in the deep Earth's mantle, thousands of km wide and ∼1800 km high. This study explores the hypothesis that the LLSVPs are compositionally subdivided into two domains: a primordial bottom domain near the core‐mantle boundary and a basaltic shallow domain that extends from 1100 to 2300 km depth. This hypothesis reconciles published observations in that it predicts that the two domains have different physical properties (bulk‐sound versus shear‐wave speed versus density anomalies), the transition in seismic velocities separating them is abrupt, and both domains remain seismically distinct from the ambient mantle. We here report underside reflections from the top of the LLSVP shallow domain, supporting a compositional origin. By exploring a suite of two‐dimensional geodynamic models, we constrain the conditions under which well‐separated “double‐layered” piles with realistic geometry can persist for billions of years. Results show that long‐term separation requires density differences of ∼100 kg/m3 between LLSVP materials, providing a constraint for origin and composition. The models further predict short‐lived “secondary” plumelets to rise from LLSVP roofs and to entrain basaltic material that has evolved in the lower mantle. Long‐lived, vigorous “primary” plumes instead rise from LLSVP margins and entrain a mix of materials, including small fractions of primordial material. These predictions are consistent with the locations of hot spots relative to LLSVPs, and address the geochemical and geochronological record of (oceanic) hot spot volcanism. The study of large‐scale heterogeneity within LLSVPs has important implications for our understanding of the evolution and composition of the mantle.