We investigate the very long‐wavelength, global pattern of surface heat flux anomalies within the context of whole‐mantle and layered‐mantle anelastically compressible internal loading theories. Since the internal loading framework does not yield a direct estimate of the geotherm, we argue that accurate predictions for the surface heat flux may nevertheless be obtained by assuming that it is linearly related to the radial component of flow velocity at shallow depth in the mantle. The mantle convective circulation is assumed to be driven by density heterogeneity inferred from global seismic tomography models. Best results for the pattern of surface heat flux anomalies are obtained for models that significantly impede the circulation at a depth of 670 km. Total variance reductions of 60–65% (degree 1–5) are obtained when the viscosity profile includes a low‐viscosity asthenosphere. Within the context of our modeling assumptions, however, whole‐mantle circulation models provide best descriptions of the long‐wavelength nonhydrostatic gravity data. In order to resolve the gravity‐heat flux impasse that is revealed herein, we consider the possibility of modifying the a priori global seismic models employed in the calculations. We show that the rigidly layered‐mantle internal loading theory is equivalent to a theory in which no explicit flow‐blocking boundary condition is imposed at 670 km but in which the buoyancy field inferred from the a priori tomographic model is supplemented by flow‐blocking heterogeneity in the form of an appropriately constrained sheet mass load. We develop a general mathematical formalism describing how the introduction of appropriately constrained sheet mass loads allows the exact reconciliation of a number of a priori constraints or hypotheses concerning the structure of the circulation. Using this formalism, we explore the extreme nonuniqueness that not only characterizes internal loading theory inferences of the depth profile of mantle viscosity but also inferences of the radial style of the circulation. On this basis, we suggest that great caution is warranted with respect to tomography‐based inferences of mantle properties. Based on a viscosity profile whose depth dependence is close to that independently inferred within the context of postglacial rebound studies, we present plausible resolutions of the gravity‐heat flux impasse effected either within the framework of whole‐mantle or layered‐mantle circulation models.

We present SubMachine, a collection of web-based tools for the interactive visualization, analysis, and quantitative comparison of global-scale data sets of the Earth's interior. SubMachine focuses on making regional and global-scale seismic tomography models easily accessible to the wider solid Earth community, in order to facilitate collaborative exploration. We have written software tools to visualize and explore over 30 tomography models-individually, side-by-side, or through statistical and averaging tools. SubMachine also serves various nontomographic data sets that are pertinent to the interpretation of mantle structure and complement the tomographies. These include plate reconstruction models, normal mode observations, global crustal structure, shear wave splitting, as well as geoid, marine gravity, vertical gravity gradients, and global topography in adjustable degrees of spherical harmonic resolution. By providing repository infrastructure, SubMachine encourages and supports community contributions via submission of data sets or feedback on the implemented toolkits. , Shephard , G. E. , Domeier , M. , & Tsekhmistrenko , M. ( 2018 ). SubMachine: Web‐based tools for exploring seismic tomography and other models of Earth's deep interior . Geochemistry, Geophysics, Geosystems , 19 , 1464 – 1483 . 10.1029/2018GC007431 PMC6109961 30174559 1 Introduction Seismic tomography is a powerful geophysical imaging method that has been yielding increasingly detailed structural information about the Earth's deep interior. Applied on a planetary scale, it uses seismic waves, generated by tens to thousands of moderate to large earthquakes, to sample and estimate the 3‐D spatial distribution of heterogeneities in the crust and mantle. Such heterogeneities cause seismic waves to propagate at slightly faster or slower velocities than average ambient mantle or crust, the structure of which is reasonably well known (Dziewonski & Anderson, 1981 ; Kennett & Engdahl, 1991 ; Kennett et al., 1995 ). Although seismic velocity anomalies (dv/v) are of secondary interest per se, they correlate with density, temperature, and compositional anomalies, which are the drivers of heat and material flows in the solid Earth. Due to the vast amounts of data and the heavy computational demands, generating a whole‐mantle tomography model is a major, nonroutine effort. The appearance of SubMachine's home page is shown in Figure 1 . Figure 1 Screenshot of the home page of the SubMachine web portal ( http://submachine.earth.ox.ac.uk ). The first release includes functionalities for the visualization, analysis and quantitative comparison of over 30 global and regional seismic tomography models, as well as complementary data sets that are pertinent to interpreting mantle structure and evolution. Section 2 explains SubMachine's architecture and its functionalities: visualization of global tomography models, i.e., volumetric data sets, and related statistics (section 2.1 ), and the comparison of models through tomographic “vote maps” which are generated by applying a one‐bit (binary digit) thresholding operation to two or more tomography models and then stacking them (Shephard et al., 2017 ; section 2.2 ). Section 3 explains the visualization of static and time‐dependent observations and models that are pertinent to the interpretation of mantle structure and are complementary to seismic tomography. 3 Complementary Data Sets 3.1 Tectonic Plate Reconstructions To facilitate the linking of mantle structure with plate motion histories, SubMachine provides the functionality to overlay reconstructed plate boundaries (subduction zones, ridges, and transform boundaries) and/or coastlines on seismic tomography models and vote maps. These reconstructions present different, relative and absolute plate motion histories, and their corresponding publications are listed in Table 2 . Comparisons between mantle structure and plate reconstructions have broad applications. Global‐scale geoid observations, mainly from satellites, are used in many geodetic, oceanographic and geophysical applications, and can serve to constrain subsurface structure in mantle convection models or joint seismic‐geoid tomographies (Simmons et al., 2009 ). The smooth but irregular shape of the geoid is due to the uneven distribution of mass within and on the surface of the solid Earth. A positive gravity anomaly is caused by a mass excess and results in a geoid high relative to the reference ellipsoid. In each 1° cell, boundary depth, compressional velocity (Vp), shear velocity (Vs) and density are given for eight layers: water, ice, sediment layers (upper, middle, and lower), and crystalline crust (upper, middle, and lower). Bathymetry, topography, and ice thickness are derived by 3.4 Normal Mode Observations Normal modes, the solid Earth's free oscillations, are frequency‐split due to 3‐D heterogeneities that break the spherical symmetry. Some of this frequency splitting is caused by the same mantle heterogeneities that are imaged by body and surface‐wave tomography models. Hence, it is pertinent to compare to normal mode splitting observations, which are visualized as splitting function maps. These maps represent the local radial average (depending on the sensitivity kernel) of the underlying heterogeneity sampled by a particular normal mode below each point. Mantle structure can be linked with plate motion histories by overlaying surface reconstructions of paleo‐plate boundaries on depth slices from seismic tomography. Various ways for comparing tomography models were discussed, e.g., side‐by‐side comparison of any slice through any number of tomography models, in a uniform, customizable format; quantitative model comparison via the histograms and velocity‐depth profiles; model comparison by creating vote maps. We welcome community input on features and model contributions. Source code is in a GitHub repository and can be made available upon request.

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.

How cratons, the ancient cores of continents, evolved since their formation over 2.5 Ga ago is debated. Seismic tomography can map the thick lithosphere of cratons, but its resolution is low in sparsely sampled continents. Here we show, using waveform tomography with a large, newly available dataset, that cratonic lithosphere beneath Africa is more complex and fragmented than seen previously. Most known diamondiferous kimberlites, indicative of thick lithosphere at the time of eruption, are where the lithosphere is thin today, implying surprisingly widespread lithospheric erosion over the last 200 Ma. Large igneous provinces, attributed to deep-mantle plumes, were emplaced near all lithosphere-loss locations, concurrently with or preceding the loss. This suggests that the cratonic roots foundered once modified by mantle plumes. Our results imply that the total volume of cratonic lithosphere has decreased since its Archean formation, with the fate of each craton depending on its movements relative to plumes.

We launched an array of nine freely floating submarine seismometers near the Galápagos islands, which remained operational for about two years. P and PKP waves from regional and teleseismic earthquakes were observed for a range of magnitudes. The signal-to-noise ratio is strongly influenced by the weather conditions and this determines the lowest magnitudes that can be observed. Waves from deep earthquakes are easier to pick, but the S/N ratio can be enhanced through filtering and the data cover earthquakes from all depths. We measured 580 arrival times for different raypaths. We show that even such a limited number of data gives a significant increase in resolution for the oceanic upper mantle. This is the first time an array of floating seismometers is used in seismic tomography to improve the resolution significantly where otherwise no seismic information is available. We show that the Galápagos Archipelago is underlain by a deep (about 1900 km) 200-300 km wide plume of high temperature, with a heat flux very much larger than predicted from its swell bathymetry. The decrease of the plume temperature anomaly towards the surface indicates that the Earth's mantle has a subadiabatic temperature gradient.

The structure of the Earth's deep mantle is a result of complex processes that are influenced by surface tectonics through the subduction of oceanic lithosphere and by core dynamics through the heat flow across the core-mantle-boundary. The other way around the structures in the deep mantle affect the Earth's surface by feeding mantle plumes that sustain volcanism. By modulating the heat flow at the CMB the mantle also affects the dynamics of the core and the magnetic field.These processes focus in the D'' layer that marks the mysterious few hundred kilometers directly above the core-mantle-boundary which contain dominant features like the Large Low Shear Velocity Provinces and features with rather extreme properties like the Ultra Low Velocity Zones. Knowledge of the structural features in the D'' layer is of importance for the understanding of long- and short-term processes in our direct environment at the surface of the Earth.The remoteness of D'' layer more than 2,500 kilometers below the surface poses challenges for geophysical investigations and limits the resolution of seismological imaging. Seismic tomography with surface waves and normal modes therefor locate the large scale features, only. Detailed wavefield analysis and modeling of particular seismic phases, often based on array observations provide more detailed information about locally dominating structures and their contrasts. For the characterization of distributed small scale structures that can be referred t Investigating small-scale deep-mantle structure, the stories told by high frequency scattered waves Christoph Sens-Schönfelder Christoph Sens-Schönfelder Christoph Sens-Schönfelder GFZ Helmholtz Centre for Geosciences, Potsdam, Germany (sens-schoenfelder@gfz.de) GFZ Helmholtz Centre for Geosciences, Potsdam, Germany (sens-schoenfelder@gfz.de) Hide The structure of the Earth's deep mantle is a result of complex processes that are influenced by surface tectonics through the subduction of oceanic lithosphere and by core dynamics through the heat flow across the core-mantle-boundary. The other way around the structures in the deep mantle affect the Earth's surface by feeding mantle plumes that sustain volcanism. By modulating the heat flow at the CMB the mantle also affects the dynamics of the core and the magnetic field. These processes focus in the D'' layer that marks the mysterious few hundred kilometers directly above the core-mantle-boundary which contain dominant features like the Large Low Shear Velocity Provinces and features with rather extreme properties like the Ultra Low Velocity Zones. Knowledge of the structural features in the D'' layer is of importance for the understanding of long- and short-term processes in our direct environment at the surface of the Earth. The remoteness of D'' layer more than 2,500 kilometers below the surface poses challenges for geophysical investigations and limits the resolution of seismological imaging. Seismic tomography with surface waves and normal modes therefor locate the large scale features, only. Detailed wavefield analysis and modeling of particular seismic phases, often based on array observations provide more detailed information about locally dominating structures and their contrasts. For the characterization of distributed small scale structures that can be referred to as heterogeneity even wavefield analysis fails due to the superposition of waves scattered at different locations of the heterogeneous material. Such heterogeneity can for instance represent remnants of oceanic crust that has been subducted down to the CMB. Despite the complexity of signals generated by distributed heterogeneity the analysis of high frequency scattered waves provides constraints on the presence structures at short length scales of a few kilometers in the deep mantle. I review the theoretical basics of scattering theory and the observational evidence for deep Earth distributed heterogeneity. I discuss new observations of high frequency seismic waves scattered in the deep mantle together with limitations in the interpretation imposed by the nature of the scattered wavefield. How to cite: Sens-Schönfelder, C.: Investigating small-scale deep-mantle structure, the stories told by high frequency scattered waves, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14881, https://doi.org/10.5194/egusphere-egu26-14881, 2026.

Large low velocity provinces (LLVPs) dominate Earth's lowermost mantle, but their detailed thermochemical nature remains a topic of discussion. In particular, it is unclear to what extent the bridgmanite to post-perovskite phase transition is able to explain their seismic velocity characteristics. Robust constraints on the origin of these seismic structures would shed light on large-scale mantle dynamics and Earth's thermal and chemical evolution. Here, we examine the combined effects of temperature, chemical heterogeneity and phase transitions on lowermost mantle tomographic signatures. To investigate this, we calculate synthetic seismic velocities expected from a range of scenarios for the stability of post-perovskite combined with models of different lowermost mantle temperatures and compositions using recent thermodynamic data. These are filtered to account for limited tomographic resolution, allowing for quantitative comparisons between our synthetic seismic velocities and a recent Backus-Gilbert based tomography model. Crucially, this model provides robust ratios and correlations of velocity anomalies derived from nearly identical Vp and Vs resolution, and includes uncertainty quantification that accounts for both data and theoretical errors. Given the tomographic uncertainties and limited resolution, our comparisons focus on globally and depth averaged seismic characteristics, which capture the effects of lateral compositional and mineralogical variability. By rejectin

Direct spectrum tomography of the Earth's mantle using normal modes Skip to main content Direct spectrum tomography of the Earth's mantle using normal modes Files E A A M Jagt - thesis.pdf (47.42 MB) Publication date 2022-06-14 Authors Jagt, Elisabeth Anna Alexia Maria Editors Advisors Supervisors Deuss, Arwen DOI https://doi.org/10.33540/1254 Document Type Dissertation Metadata Show full item record Collections Utrecht University Repository License Abstract The thermochemical nature of the lower mantle, and the two Large Low Shear-wave Velocity Provinces (LLSVPs) in particular, remains a topic of active debate. Exclusively imaging seismic velocity anomalies only provides limited ability to distinguish between thermal and compositional origins. Here we use whole Earth oscillations, or normal modes, to study 3D variations in mantle shear wave- (vs), compressional wave- (vp) and bulk sound velocity (vc), density (rho) and shear attenuation (qmu). These observations provide new constraints on the presence of lower mantle chemical heterogeneity. Here, we compare the two-step splitting function inversion method to the less frequently used, computationally more expensive one-step direct spectrum inversion. In theory, the one-step inversion suffers less from non-uniqueness and only requires regularization once. In practice, we find that the average spectral misfits for the one-step inversion are always lower. The ratio between vs and vp anomalies obtained from their joint inversion, proposed to be an indicator of chemical heterogeneity when exceeding a threshold predicted by mineral physics, varies significantly between the two inversion methods. The method of computing the ratio is just as important. We obtain ratios exceeding the threshold in the lower mantle only when dividing the root mean square amplitudes of our vs and vp models, although lower than some previous studies suggest. However, by taking the median ratio from a grid at each depth, we barely exceed the threshold. Instead of relying on these 1D representatives of the ratio, we infer chemical heterogeneity in certain depth ranges based on a wide spread in distributions of vs, vp anomalies and their ratio. Another constraint on the presence of chemical heterogeneity comes from the anti-correlation of vs and vc structure, found in many previous studies. We find (de-)correlation in the lower mantle in joint vs and vc inversions, with only slightly negative correlation for the two-step inversion. We shift towards more negative correlation values when extracting vc from our previously obtained vs and vp models. Lower mantle density structure has remained elusive and controversial in recent decades. In joint inversions for vs, vp, rho and discontinuity topography, we show that a basal layer of excess density is located underneath part of the otherwise lighter-than-average LLSVPs, reconciling previous observations. This dense layer is more robust in the one-step inversion and does not result from high-amplitude ghost patterns that may have plagued earlier normal mode studies. Compositional variations such as iron enrichment may explain this dense but seismically slow layer. Finally, we obtain preliminary results for imaging lower mantle shear attenuation in joint vs and qmu one-step inversions. We first select the best way of inverting for 3D qmu in synthetic tests, before applying this method to real data. Mode selection appears to be crucial, and we also need to properly account for detailed elastic structure to minimize leakage into anelastic structure. The preliminary 3D qmu model shows low attenuation in LLSVPs and high attenuation in the surrounding lower mantle, possibly indicating a dominant role for grain size. Keywords Seismologie , normal modes , inversie , tomografie , aardmantel , dichtheid , demping , LLSVPs , Seismology , normal modes , inversion , tomography , Earth's mantle , density , attenuation , LLSVPs Citation Jagt, E A A M 2022, 'Direct spectrum tomography of the Earth's mantle using normal modes', Doctor of Philosophy, Universiteit Utrecht, Utrecht. https://doi.org/10.33540/1254 URI https://dspace.library.uu.nl/handle/1874/420682

It is generally believed that subduction of lithospheric slabs is a major contribution to thermal heterogeneity in Earth's entire mantle and provides a main driving force for mantle flow. Mantle structure can, on the one hand, be inferred from plate tectonic models of subduction history and geodynamic models of mantle flow. On the other hand, seismic tomography models provide important information on mantle heterogeneity. Yet, the two kinds of models are only similar on the largest (1000 s of km) scales and are quite different in their detailed structure. Here, we provide a quantitative assessment how good a fit can be currently achieved with a simple viscous flow geodynamic model. The discrepancy between geodynamic and tomography models can indicate where further model refinement could possibly yield an improved fit. Our geodynamical model is based on 300 Myr of subduction history inferred from a global plate reconstruction. Density anomalies are inserted into the upper mantle beneath subduction zones, and flow and advection of these anomalies is calculated with a spherical harmonic code for a radial viscosity structure constrained by mineral physics and surface observations. Model viscosities in the upper mantle beneath the lithosphere are ~10 20 Pas, and viscosity increases to ~10 23 Pas in the lower mantle above D " . Comparison with tomography models is assessed in terms of correlation, both overall and as a function of depth and spherical harmonic degree. We find that,

Earth's mantle is a layer of silicate rock between the crust and the outer core. It has a mass of 4.01×1024 kg (8.84×1024 lb) and makes up 86% of the mass of Earth. It has a thickness of 2,900 kilometers (1,800 mi) making up about 46% of Earth's radius and 84% of Earth's volume. It is predominantly solid but, on geologic time scales, it behaves as a viscous fluid, sometimes described as having the Because of the temperature difference between the Earth's surface and outer core and the ability of the crystalline rocks at high pressure and temperature to undergo slow, creeping, viscous-like deformation over millions of years, there is a convective material circulation in the mantle. Hot material rises (in a mantle plume) while cooler (and heavier) material sinks downward. Downward motion of material occurs at convergent plate boundaries called subduction zones. Locations on the surface that lie over plumes are predicted to have high elevation (because of the buoyancy of the hotter, less-dense plume beneath) and to exhibit hot spot volcanism. The volcanism often attributed to deep mantle plumes is alternatively explained by passive extension of the crust, permitting magma to leak to the surface: the plate hypothesis. The convection of the Earth's mantle is a chaotic process (in the sense of fluid dynamics), which is thought to be an integral part of the motion of plates. Plate motion should not be confused with continental drift which applies purely to the movement of the crustal components of the continents. The movements of the lithosphere and the underlying mantle are coupled since descending lithosphere is an essential component of convection in the mantle. The observed continental drift is a complicated relationship between the forces causing oceanic lithosphere to sink and the movements within Earth's mantle. Although there is a tendency to larger viscosity at greater depth, this relation is far from linear and shows layers with dramatically decreased viscosity, in particular in the upper mantle and at the boundary with the core. The mantle within about 200 km (120 mi) above the core–mantle boundary appears to have distinctly different seismic properties than the mantle at slightly shallower depths; this unusual mantle region just above the core is called D″ ("D double-prime"), a nomenclature introduced over 50 years ago by the geophysicist Keith Bullen. D″ may consist of material from subducted slabs that descended and came to rest at the core–mantle boundary or from a new mineral polymorph discovered in perovskite called post-perovskite. Earthquakes…

Lateral density variations in the mantle result in convection currents, the slow creeping motion of Earth's solid mantle. At a seafloor spreading ridge Plate tectonics (from Latin tectonicus, from Ancient Greek τεκτονικός (tektonikós) 'pertaining to building') is the scientific theory that Earth's lithosphere comprises a number of large tectonic plates, which have been slowly moving since 3–4 billion years ago. The model builds on the concept of continental drift, an idea developed during the first decades of the 20th century. Plate tectonics c Pl… For much of the first quarter of the 20th century, the leading theory of the driving force behind tectonic plate motions envisaged large scale convection currents in the upper mantle, which can be transmitted through the asthenosphere. This theory was launched by Arthur Holmes and some forerunners in the 1930s and was immediately recognized as the solution for the acceptance of the theory as originally discussed in the papers of Alfred Wegener in the early years of the 20th century. However, despite its acceptance, it was long debated in the scientific community because the leading theory still envisaged a static Earth without moving continents up until the major breakthroughs of the early sixties. Two- and three-dimensional imaging of Earth's interior (seismic tomography) shows a varying lateral density distribution throughout the mantle. Such density variations can be material (from rock chemistry), mineral (from variations in mineral structures), or thermal (through thermal expansion and contraction from heat energy). The manifestation of this varying lateral density is mantle convection from buoyancy forces. How mantle convection directly and indirectly relates to plate motion is a matter of ongoing study and discussion in geodynamics. Somehow, this energy must be transferred to the lithosphere for tectonic plates to move. There are essentially two main types of mechanisms that are thought to exist related to the dynamics of the mantle that influence plate motion which are primary (through the large scale convection cells) or secondary. The secondary mechanisms view plate motion driven by friction between the convection currents in the asthenosphere and the more rigid overlying lithosphere. This is due to the inflow of mantle material related to the downward pull on plates in subduction zones at ocean trenches. Slab pull may occur in a geodynamic setting where basal tractions continue to act on the plate as it dives into the mantle (although perhaps to a greater extent acting on both the under and upper side of the slab). Furthermore, slabs that are broken off and sink into the mantle can cause viscous mantle forces driving plates through slab suction. The…