Article

Mapping the Lower Mantle: Determination of Lateral Heterogeneity in P Velocity up to Degree and Order 6

July 1984
Journal of Geophysical Research Atmospheres 89(NB7):5929-5952

July 1984
89(NB7):5929-5952

Authors:

Harvard University

The data from the International Seismological Centre bulletins for the years 1964-79 are used to derive a 3-D model of lateral variations of the P velocity in the lower mantle. Particular attention is given to the problem of weighting the individual observations in order to avoid, as much as possible, the bias due to the uneven distribution of sources and receivers. The resulting model shows a high level of perturbations just below the 670-km discontinuity and just above the core-mantle boundary. It also predicts well the large-scale pattern of observed travel time residuals for various source regions except for the distinct effects of the subduction zones. While the inferences made here with respect to the origin of the gravest orders of the geopotential field are not conclusive, they point the way in which results from seismology can be used to address some of the basic questions of geodynamics.-from Author

An Integrated Geophysical Approach to Investigating Thermal and Chemical Variations in Earth's Mantle

Thesis

Jan 2018

Ross Maguire

The Earth’s mantle is heterogeneous as a result of melting, differentiation, plate subduction, and whole-mantle convection throughout geologic time. Our current picture of the mantle has been informed largely by mapping variations in seismic wavespeed. However, it is challenging to infer the thermochemical nature of the mantle from seismic images because they are often poorly resolved, and velocity variations cannot be uniquely related to either temperature or composition. In this thesis, I take a multi-disciplinary approach that combines constraints from geodynamics, mineral physics, and seismology, in order to investigate how ther- mal and compositional Earth models are compatible with seismic observations. I focus primarily on thermal upwellings (i.e., mantle plumes), and assess how these features can be seismically imaged. In chapter 2, I model plume development in a compressible mantle using physics-based simulations of flow in the mantle, and calculate the travel time delays of P waves and S waves propagating through the narrow plume tails in the lower mantle. In chapter 3, I investigate whether or not mantle plume tails can be seismically imaged using common seismic tomog- raphy approaches. I analyze how imaging artifacts can affect our interpretations of the deep mantle below hotspots and find optimal imaging configurations that will maximize resolution of plume tails. In chapter 4, I analyze images of the mantle beneath the Samoa hotspot in global tomography model S40RTS (Ritsema et al., 2011). Specifically, I explore the range of temperatures and compositions that can explain the observed seismic velocity variations and determine if obser- vations are consistent with a lower mantle plume origin of Samoan volcanism. In chapter 5 I investigate the origin of large low velocity provinces (LLVPs) above the core-mantle-boundary beneath Africa and the Pacific, which are thought to be anomalously hot and compositionally distinct mantle domains. I test the hypoth- esis that the anomalies represent an accumulation of recycled oceanic crust above the core by comparing LLVPs resolved in S40RTS to dynamic mantle mixing simulations of Brandenburg et al. (2008). Chapter 6 focuses on how lateral vari- ations of temperature and composition affect the seismic structure of the mantle transition zone (MTZ). I use P-to-S receiver functions to image the strengths and depths of mineralogical phase changes in the MTZ beneath the United States, and relate these observations to the physical conditions of the transition zone using constraints from experimental and theoretical mineral physics.

Physicogeochemical Evolution of Melts of Superplumes Uplift from the Lower Mantle to the Transition Zone: Experiment at 26 and 20 GPa

Article

Jul 2021

Basal Mantle Flow Over LLSVPs Explains Differences in Pacific and Indo‐Atlantic Hotspot Motions

Article

Full-text available

Jan 2024

Surface hotspot motions are approximately a factor of two faster in the Pacific than the Indo‐Atlantic, and the Indo‐Atlantic large low shear velocity province (LLSVP) appears to be significantly taller than the Pacific LLSVP. Hypothesizing that surface hotspot motions are correlated with the motion of plume sources on the upper surface of chemically distinct, intrinsically dense LLSVPs, we use 3D spherical mantle convection models to compute the velocity of plume sources and compare with observed surface hotspot motions. No contrast in the mean speed of Pacific and Indo‐Atlantic hotspots is predicted if the LLSVPs are treated as purely thermal anomalies and plume sources move laterally across the core‐mantle boundary. However, when LLSVP topography is included in the model, the predicted hotspot speeds are, on average, faster in the Pacific than the Indo‐Atlantic, even when modest topography is assigned to both LLSVPs (e.g., 100–300 km). The difference in mean hotspot speed increases to a factor of two for larger and laterally variable LLSVP topography estimated from seismic tomographic model S40RTS (up to 1,100–1,500 km for the Indo‐Atlantic region vs. 700–1,400 km for the Pacific region) and our results also broadly reproduce the convergence of Pacific hotspots toward the center of the Pacific LLSVP. These largescale features of global hotspot motions are only reproduced when ambient mantle material flows over large, relatively stable topographical features, suggesting that LLSVPs are chemically distinct and intrinsically dense relative to ambient mantle material.

Basal Mantle Flow Over LLSVPs Explains Differences in Pacific and Indo-Atlantic Hotspot Motions

Preprint

Aug 2023

Surface hotspot motions are approximately a factor of two faster in the Pacific than the Indo-Atlantic, and the Pacific large low shear velocity province (LLSVP) appears to be significantly shorter than the Indo-Atlantic LLSVP. Hypothesizing that surface hotspot motions are correlated with the motion of plume sources on the upper surface of chemically distinct, intrinsically dense LLSVPs, we use 3D spherical mantle convection models to compute the velocity of plume sources and compare with observed surface hotspot motions. No contrast in the mean speed of Pacific and Indo-Atlantic hotspots is predicted if the LLSVPs are treated as purely thermal anomalies and plume sources move laterally across the core-mantle boundary. However, when LLSVP topography is included in the model, the predicted hotspot speeds are, on average, faster in the Pacific than the Indo-Atlantic, even when modest topography is assigned to both LLSVPs (e.g., 100-300 km). The difference in mean hotspot speed increases to a factor of two for larger and laterally variable LLSVP topography estimated from seismic tomographic model S40RTS (up to 1100-1500 km for the Indo-Atlantic region versus 700-1400 km for the Pacific region) and our results also broadly reproduce the convergence of Pacific hotspots toward the center of the Pacific LLSVP. These largescale features of global hotspot motions are only reproduced when ambient mantle material flows over large, relatively stable topographical features, suggesting that LLSVPs are chemically distinct and intrinsically dense relative to ambient mantle material.

CRUSTAL AND MANTLE STRUCTURE BENEATH EASTERN EURASIA FROM FINITE FREQUENCY SEISMIC TOMOGRAPHY (FFST)

Technical Report

Full-text available

Jan 2005

An accurate, high-resolution 3-D earth model is crucial to the seismic calibration for nuclear monitoring. We use the newly developed Finite Frequency Seismic Tomography (FFST) approach to construct the crustal and mantle structure beneath eastern Eurasia. Traditionally, travel times of seismic waves are calculated based on the ray theory, which is valid strictly for infinite-frequency waves. Observed seismic waves, however, are finite frequency signals. As a result of scattering and diffractive effects, travel times of realistic finite frequency waves are sensitive to 3-D structure around the geometrical rays, and the travel time shifts caused by heterogeneities diminish gradually from the heterogeneities to receivers-a phenomena named the wave front healing. The ray theory ignores this ubiquitous effect of the propagation of realistic seismic waves. Tomographic inversions based on it, therefore, tend to underestimate the magnitudes of heterogeneities. The newly developed FFST utilizes the 3-D Born-Fréchet sensitivity kernels of the travel times of finite-frequency seismic waves. The new method accounts for the wave front healing, off-ray scattering and other non-geometrical diffraction phenomena, and significantly improves the resolution of the velocity heterogeneity. In addition to the new methodology, we will use a more comprehensive data set than in previous studies to construct the new earth model beneath eastern Eurasia. We will collect data from the publicly accessible sources e.g., Incorporated Research Institutions for Seismology (IRIS), Global Seismographic Network (GSN), the Program for the Array Seismic Studies of the Continental Lithosphere (PASSCAL), and International Monitoring System (IMS) stations. Efforts will also be made to collect data sets from other networks in the region such as the Japanese Broadband Seismograph Network (F-net), the Japanese International Seismic Network (JISNET in Indonesia), and the Taiwan Broadband Seismic Network. Access to other unique sources including permanent and portable seismic stations throughout the study area further improves the station coverage over eastern Eurasia. This is the beginning of a three-year effort to improve seismic calibration in eastern Eurasia. We will first develop a 3-D FFST velocity model using body waves, along with model uncertainties. Travel time corrections and modeling error surfaces will be derived and applied to relocating ground truth (GT5) events. The final model will be developed using joint body and surface waves. From the 3D velocity models, we will also simulate the full wave propagation in the transition zone. To facilitate data integration, a project database and a web-based tool are being implemented. 479 27th Seismic Research Review: Ground-Based Nuclear Explosion Monitoring Technologies

Can Plate Bending Explain the Observed Faster Landward Motion of Lateral Regions of the Subduction Zone After Major Megathrust Earthquakes?

Article

Full-text available

Mar 2023

Greater landward velocities were recorded after six megathrust earthquakes in subduction zone regions adjacent to the ruptured portion. Previous explanations invoked either increased slip deficit accumulation or plate bending during postseismic relaxation, with different implications for seismic hazard. We investigate whether bending can be expected to reproduce this observed enhanced landward motion (ELM). We use 3D quasi‐dynamic finite element models with periodic earthquakes. We find that afterslip downdip of the brittle megathrust exclusively produces enhanced trenchward surface motion in the overriding plate. Viscous relaxation produces ELM when a depth limit is imposed on afterslip. This landward motion results primarily from in‐plane elastic bending of the overriding plate due to trenchward viscous flow in the mantle wedge near the rupture. Modeled ELM is, however, incompatible with the observations, which are an order of magnitude greater and last longer after the earthquake. This conclusion does not significantly change when varying mantle viscosity, plate elasticity, maximum afterslip depth, earthquake size, megathrust locking outside of the rupture, or nature and location of relevant model boundaries. The observed ELM consequently appears to reflect faster slip deficit accumulation, implying a greater seismic hazard in lateral segments of the subduction zone.

A dynamic 2000–540 Ma Earth history: From cratonic amalgamation to the age of supercontinent cycle

Article

Feb 2023
EARTH-SCI REV

Establishing how tectonic plates have moved since deep time is essential for understanding how Earth’s geodynamic system has evolved and operates, thus answering longstanding questions such as what “drives” plate tectonics. Such knowledge is a key component of Earth System science, and has implications for wide ranging fields from core-mantle-crust interaction and evolution, geotectonic phenomena such as mountain building and magmatic and basin histories, the episodic formation and preservation of Earth resources, to global sea-level changes, climatic evolution, atmospheric oxygenation, and even the evolution of life. In this paper, we take advantage of the rapidly improving database and knowledge about the Precambrian world, and the conceptual breakthroughs, both regarding the presence of a supercontinent cycle and possible dynamic coupling between the supercontinent cycle and mantle dynamics, in order to establish a full-plate global reconstruction from 540 Ma back to 2000 Ma. We utilise a variety of global geotectonic databases to constrain our reconstruction, and use palaeomagnetically recorded true polar wander events and global plume records to help evaluate competing geodynamic models and also provide new constraints on the absolute longitude of continents and supercontinents. After revising the configuration and life span of both supercontinents Nuna (1600—1300 Ma) and Rodinia (900—720 Ma), we present a 2000—540 Ma animation, starting from the rapid assembly of large cratons and supercratons (or megacontinents) between 2000 Ma and 1800 Ma. This occurred after a billion years of dominance by small cratons, and kick-started the ensuing Nuna and Rodinia supercontinent cycles and the emergence of stable, hemisphere-scale (long-wavelength) degree-1/degree-2 mantle structures. We further use the geodynamicly-defined type-1 and type-2 inertia interchange true polar wander (IITPW) events, which likely occurred during Nuna (type-1) and Rodinia (type-2) times as shown by the palaeomagnetic record, to argue that Nuna assembled at about the same longitude as the latest supercontinent Pangaea (320—170 Ma), whereas Rodinia formed through introversion assembly over the legacy Nuna subduction girdle either ca. 90◦ to the west (our slightly preferred model) or to the east before the migrated subduction girdle surrounding it generated its own degree-2 mantle structure by ca. 780 Ma. Our interpretation is broadly consistent with the global LIP record. Using TPW and LIP observations and geodynamic model predictions, we further argue that the Phanerozoic supercontinent Pangaea assembled through extroversion on a legacy Rodinia subduction girdle with a geographic centre at around 0◦E longitude before the formation of its own degree-2 mantle structure by ca. 250 Ma, the legacy of which is still present in present-day mantle. (the paper is of OPEN ACCESS at http://dx.doi.org/10.1016/j.earscirev.2023.104336)

A review of geophysical research: Perspective into the Indian Ocean Geoid Low

Article

Feb 2023
EARTH-SCI REV

Seismic structure beneath Southeast Asia from adjoint waveform tomography

Thesis

Full-text available

Jun 2022

Deborah Wehner

Seismic tomography has played a crucial role in the illumination of deep Earth structure. Most existing tomographic methods are based on seismic ray theory and hence do not fully account for the true physics of wave propagation. Recent computational advances allow us to embrace the full complexity of seismic wave propagation by accurately solving the 3-D seismic wave equation numerically. This can account for effects such as wavefront healing, interference, scattering and (de)focusing, which are often ignored or not properly captured by other methods such as ray tracing. Thus, such methodologies are particularly suitable for strongly heterogeneous regions such as Southeast Asia, where large variations in elastic parameters are likely to be present. Here, an unprecedented dataset and access to sizeable computational resources allow their application to Southeast Asia for the first time. In the first part of this thesis, a continental-scale seismic model of the lithosphere and underlying mantle beneath Southeast Asia obtained from adjoint waveform tomography (often referred to as full-waveform inversion or FWI) is presented. FWI is a non-linear imaging method, where an initial model is updated in order to minimise the difference between observed and predicted waveforms. Based on > 3,000 h of analysed waveform data gathered from 13,000 unique source-receiver pairs and filtered at periods between 20 – 150 s, isotropic P-wave velocity, radially anisotropic S-wave velocity and density are imaged via an iterative non-linear inversion that begins from a 1-D reference model. At each iteration, the full 3-D wavefield is determined through an anelastic Earth, accommodating effects of topography, bathymetry and ocean load. SASSY21, the final model after 87 iterations, appears to be robust since it is able to explain true-amplitude data from events and receivers not included in the inversion. The new model reveals detailed anomalies down to the mantle transition zone, including multiple subduction zones. The most prominent feature is the (Indo-)Australian plate descending beneath Indonesia, which is imaged as one continuous slab along the 180 curvature of the Banda Arc. The tomography confirms the existence of a hole in the slab beneath Mount Tambora and locates a high S-wave velocity zone beneath northern Borneo that may be associated with subduction termination in the mid-late Miocene. A previously undiscovered feature beneath the east coast of Borneo is also revealed, which may be a signature of postsubduction processes, delamination or underthrusting from the formation of Sulawesi. In the second part of this thesis, SASSY21 is used as a starting model to obtain a more refined image of the eastern Indonesian region, using seismic data filtered at periods from 15 – 150 s. In this study, the fluid ocean is accounted for explicitly by solving a coupled system of the acoustic and elastic wave equation. This is computationally more expensive but allows seismic waves within the water layer to be simulated, which becomes important at shorter periods. The effects arising from surface topography, bathymetry and the fluid ocean on synthetic waveforms become pronounced at periods ≤ 20 s. In particular, surface elevation can result in a considerable phase advance and change in amplitude of the surface wave train, and has an effect on both horizontal and the vertical seismogram components for this simulation setup. The fluid ocean results in a phase delay as well as a change in amplitudes and duration of the surface wave train, and affects both the radial and vertical components. At periods ≤ 20 s, accounting for the fluid ocean explicitly can lead to more realistic lithospheric velocities and a more refined image compared to the commonly used ocean load approximation, even at greater depths. Furthermore, it allows for an improved waveform match for source-receiver paths passing partially or entirely through oceanic regions. The final model, SASSIER22, after 34 iterations reveals a convergent double-subduction along the southern segment of the Philippine Trench, which was not evident in the starting model and transitions to a divergent system in the Molucca Sea further south. A more detailed illumination of the slab beneath the North Sulawesi Trench subduction zone reveals a pronounced positive wavespeed anomaly down to 200 km depth, consistent with the maximum depth of seismicity, and a more diffuse but aseismic positive wavespeed anomaly that continues to the 410 km discontinuity.

Global centroid moment tensor solutions in a heterogeneous earth: The CMT3D catalog

Article

Jul 2022

For over forty years, the Global Centroid-Moment Tensor (GCMT) project has determined location and source parameters for globally recorded earthquakes larger than magnitude 5.0. The GCMT database remains a trusted staple for the geophysical community. Its point-source moment-tensor solutions are the result of inversions that model long-period observed seismic waveforms via normal-mode summation for a one-dimensional (1-D) reference earth model, augmented by path corrections to capture three-dimensional (3-D) variations in surface wave phase speeds, and to account for crustal structure. While this methodology remains essentially unchanged for the ongoing GCMT catalog, source inversions based on waveform modeling in low-resolution three-dimensional (3-D) earth models have revealed small but persistent biases in the standard modeling approach. Keeping pace with the increased capacity and demands of global tomography requires a revised catalog of centroid-moment tensors, automatically and reproducibly computed using Green functions from a state-of-the-art 3-D earth model. In this paper, we modify the current procedure for the full-waveform inversion of seismic traces for the six moment-tensor parameters, centroid latitude, longitude, depth, and centroid time of global earthquakes. We take the GCMT solutions as a point of departure but update them to account for the effects of a heterogeneous earth, using the global three-dimensional wavespeed model GLAD-M25. We generate synthetic seismograms from Green functions computed by the spectral-element method in the 3-D model, select observed seismic data and remove their instrument response, process synthetic and observed data, select segments of observed and synthetic data based on similarity, and invert for new model parameters of the earthquake’s centroid location, time, and moment tensor. The events in our new, preliminary database containing 9,382 global event solutions, called CMT3D for “3-D centroid-moment tensors”, are on average 4 km shallower, about 1 s earlier, about 5 percent larger in scalar moment, and more double-couple in nature than in the GCMT catalog. We discuss in detail the geographical and statistical distributions of the updated solutions, and place them in the context of earlier work. We plan to disseminate our CMT3D solutions via the online ShakeMovie platform.

Effect of Mantle Viscosity Structures on Simulations of Geoid Anomalies in the Ross Sea Area

Article

Full-text available

Jun 2022
PURE APPL GEOPHYS

The effect of mantle viscosity structures on numerical simulations of geoid anomalies in the Ross Sea is investigated. Four relatively new S-wave seismic tomography models, SEMUCB_WM1, TX2019slab, S40RTS, and SEISGLOB2, are applied to convert seismic wave velocity anomalies into density anomalies. These density anomalies constitute the buoyancy-driven terms in mantle convection equations used to numerically simulate geoid anomalies corresponding to structures with different upper-to-lower mantle viscosity ratios. When the simulated geoid anomalies fit the observations best globally, the correlation coefficients between the geoid anomalies in the Ross Sea and the observed values are 0.56, 0.57, 0.57, and 0.46, respectively, for the four tomographic models. The best fits between the simulated geoid anomalies and observations in the Ross Sea area were obtained for upper-to-lower mantle viscosity ratios of 1:35, 1:45, 1:30, and 1:30 using the SEMUCB_WM1, TX2019slab, S40RTS, and SEISGLOB2, respectively. Within the range of the mantle viscosity structures selected for the simulation experiment, as the viscosity of the lower mantle gradually increases from the value at which the numerical results best fit the observations to the maximum value, the correlation coefficient between the simulated geoid anomalies and observations decreases faster for the Ross Sea area than on the global scale. We speculate that the lower mantle below the Ross Sea area may contain materials with lower viscosity than those in other mantle regions at the same depth.

Dupal Anomaly and Identification using Nd‐Hf Isotopes

Article

Apr 2022

The Dupal anomaly has attracted widespread attention since being discovered and is regarded as the most direct manifestation of mantle inhomogeneity at present. From the initially defined anomalies limited to the southern hemisphere to the global scale, the criteria for identifying anomalies defined by Pb isotopes have also been adjusted, providing an important method and reference for the study of the mantle evolution. Pearce and Peate (1995) proposed the method of Nd‐Hf isotope and element ratio to identify the Dupal anomaly. The Nd‐Hf method also offers a possible way to discriminate the mantle region of arc magmatism through the correction of Nd in the subduction process. This paper introduces the concepts and determination methods of the Dupal anomaly, and reports new Hf isotopic data of MORB‐type rocks with Dupal signature in the several Tethys ophiolites. Our results of Nd‐Hf method are in good agreement with those of previous Pb isotope identification. Moreover, origins and their controversy of Dupal anomaly are reviewed, and possible internal connections between Dupal anomalies and the two Large Low Shear Velocity Provinces (LLSVPs) in the lower mantle are discussed in depth. Further studies on origin and evolution of the Dupal anomaly are suggested, especially using integrated approach of Hf‐Nd and Pb isotopes.

101 geodynamic modelling: How to design, interpret, and communicate numerical studies of the solid Earth

Article

Full-text available

Mar 2022
SE

Geodynamic modelling provides a powerful tool to investigate processes in the Earth's crust, mantle, and core that are not directly observable. However, numerical models are inherently subject to the assumptions and simplifications on which they are based. In order to use and review numerical modelling studies appropriately, one needs to be aware of the limitations of geodynamic modelling as well as its advantages. Here, we present a comprehensive yet concise overview of the geodynamic modelling process applied to the solid Earth from the choice of governing equations to numerical methods, model setup, model interpretation, and the eventual communication of the model results. We highlight best practices and discuss their implementations including code verification, model validation, internal consistency checks, and software and data management. Thus, with this perspective, we encourage high-quality modelling studies, fair external interpretation, and sensible use of published work. We provide ample examples, from lithosphere and mantle dynamics specifically, and point out synergies with related fields such as seismology, tectonophysics, geology, mineral physics, planetary science, and geodesy. We clarify and consolidate terminology across geodynamics and numerical modelling to set a standard for clear communication of modelling studies. All in all, this paper presents the basics of geodynamic modelling for first-time and experienced modellers, collaborators, and reviewers from diverse backgrounds to (re)gain a solid understanding of geodynamic modelling as a whole.

Plume-Induced Subduction Initiation: Revisiting Models and Observations

Article

Full-text available

Nov 2021

Subduction initiation induced by a hot and buoyant mantle plume head is unique among proposed subduction initiation mechanisms because it does not require pre-existing weak zones or other forces for lithospheric collapse. Since recognition of the first evidence of subduction nucleation induced by a mantle plume in the Late Cretaceous Caribbean realm, the number of studies focusing on other natural examples has grown. Here, we review numerical and physical modeling and geological-geochemical studies which have been carried out thus far to investigate onset of a new subduction zone caused by impingement of a mantle plume head. As geological-geochemical data suggests that plume-lithosphere interactions have long been important - spanning from the Archean to the present - modeling studies provide valuable information on the spatial and temporal variations in lithospheric deformation induced by these interactions. Numerical and physical modeling studies, ranging from regional to global scales, illustrate the key role of plume buoyancy, lithospheric strength and magmatic weakening above the plume head on plume-lithosphere interactions. Lithospheric/crustal heterogeneities, pre-existing lithospheric weak zones and external compressional/extensional forces may also change the deformation regime caused by plume-lithosphere interaction.

Structure de la lithosphère de l'océan Indien occidental révélée par la méthode des fonctions récepteur

Thesis

May 2021

Anthony Dofal

Le sud-ouest de l'océan Indien est un lieu unique pour comprendre comment le magmatisme influence la structure de la lithosphère. En effet, plusieurs événements magmatiques ont successivement accompagné sa formation (e.g. les points chauds Marion et Réunion). Ce travail a pour objectif d'étudier la structure de la lithosphère de cette région à partir des données sismologiques obtenues à des stations terrestre et de fond de mer issues du projet RHUM-RUM. Nous avons calculé des fonctions récepteur (RFs) à partir de ces données et avons appliqué 1) des méthodes d'inversion linéaires et non linéaires afin d'obtenir les profils de vitesse des ondes S aux stations sismiques (respectivement, l'algorithme de voisinage et l'inversion conjointe des RFs et de courbe de dispersion d’ondes de surface), et 2) l'approche de l’H-k stacking pour caractériser la nature de la lithosphère sous le réseau. Dans le Canal du Mozambique, la structure et la nature de la lithosphère ont été étudiées sous les îles Glorieuses, Mayotte, Juan de Nova et Europa. Nous montrons que le magmatisme a largement influencé la structure lithosphérique en fournissant une quantité importante de matériel magmatique à la base de la croûte sous Mayotte (10 km), Europa (6 km) et probablement Juan de Nova. Ce sous-placage est très probablement le résultat d'événements magmatiques successifs survenus depuis la dislocation du Gondwana. Une nature océanique de la lithosphère a été déterminée sous Glorieuses et Europa. En revanche, une lithosphère de nature continentale est trouvée à Juan de Nova et Mayotte, impliquant qu'une partie de l'archipel des Comores se serait développée sur un bloc crustal continental abandonné lors du démantèlement du Gondwana. Un modèle de stockage et de transfert magmatique au sein de cette lithosphère est aussi proposé. Autour de la partie sud de la trace du point-chaud Réunion, nos résultats révèlent que le sous-placage magmatique est contraint par la faille transformante de Mahanoro-Wilshaw à l'ouest et la Dorsale Centrale Indienne (CIR). Cette zone présente une bathymétrie surélevée de 500 à 1000 m à celle des bassins voisins. L'épaisseur du sous-placage montre des variations importantes au sein de ce bloc, de 10 km sous le plateau des Mascareignes à moins de 4 km sous La Réunion et au sud-est de l'île. Ces variations, combiné aux âges et aux reconstructions cinématiques, suggère que le plateau des Mascareignes et son sous-placage résultent d'une production magmatique massive due à l'interaction entre le point chaud Réunion et les dorsales environnantes. Au contraire, le sous-placage associé à la construction de l'île de la Réunion et des monts sous-marins voisins semble être issus uniquement de la production magmatique du point-chaud. Ces deux dynamiques dessinent une évolution de la production magmatique du point-chaud Réunion, liée à la distance entre le point-chaud et les dorsales environnantes. Enfin, nous avons entrepris une caractérisation de la lithosphère le long de la CIR et de la dorsale sud-ouest Indienne (SWIR). Nous montrons que malgré une interaction connue du point chaud Réunion avec la CIR, aucun épaississement crustal n'est observé. Le long de la SWIR, nos résultats suggèrent que la discontinuité de vitesse majeure est située dans le manteau lithosphérique, et pourrait correspondre à un front de serpentinisation à 7 km de profondeur. Enfin, nous nous sommes intéressés aux processus contrôlant l'épaisseur du sous placage sous les îles volcaniques. L’analyse d’un ensemble de paramètres décrivant la production volcanique et la dynamique lithosphérique pour 14 édifices montre que l'épaisseur du sous-placage est d'abord contrôlée par un volcanisme statique et long, puis par une production volcanique et magmatique élevée. Enfin, nous montrons que les anomalies bathymétriques de la lithosphère sont positivement corrélées à l'épaisseur du sous-placage, révélant que cette couche joue un rôle majeur dans l'équilibre lithosphérique.

Tomography across scales: Knowledge transfer from seismology to imaging breast tissue with ultrasound

Thesis

Full-text available

Mar 2020

Naiara Korta Martiartu

Wave propagation is extensively used to understand the internal structure of media that are not accessible to direct observations. Seismology and medical ultrasound imaging are good examples of this. The former uses observations of seismic waves at the Earth’s surface to increase our knowledge about its interior. This is crucial, for instance, to improve our understanding about the Earth’s dynamics and evolution. Medical ultrasound, on the other hand, uses observations of acoustic waves, emitted and recorded at the surface of human bodies, to visualize internal body structures. This has become an essential screening tool, useful for diagnostic examination. This thesis presents an interdisciplinary work between seismology and medical ultrasound. In particular, we focus on transferring knowledge from seismic tomography to Ultrasound Computer Tomography (USCT), an emerging technology that holds great potential for early-stage breast cancer diagnosis. Here, the human breast is surrounded by transducers that collect transmitted and reflected ultrasound signals. This information is then used to obtain 3D quantitative images of acoustic tissue properties, which enable non-invasive tissue characterization and improve the specificity of standard imaging modalities. Current challenges in USCT mostly consist in providing a diagnostic tool with high accuracy (comparable to magnetic resonance imaging) and affordable computational and acquisition cost for clinical practice, the target being a maximum time of 15 minutes per patient. Despite the vastly different scale, seismic and medical ultrasound tomography share fundamental similarities that allow us to address these challenges from the stand point of the seismologist. We first introduce finite-frequency traveltime tomography to medical ultrasound. In addition to being computationally tractable for 3D imaging at high frequencies, the method has two main advantages: (1) It correctly accounts for the frequency dependence and volumetric sensitivity of traveltime measurements, which are related to off-ray-path scattering and diffraction. (2) It naturally enables out-of-plane imaging and the construction of 3D images from 2D slice-by-slice acquisition systems. Our method rests on the availability of calibration data measured in water, used to linearize the forward problem and to provide analytical expressions of cross-correlation traveltime sensitivity. We present a memory-efficient implementation suitable for arbitrarily large-scale domains, and we discuss its extension to amplitude tomography. To adapt existing acquisition systems to new imaging techniques, we then introduce optimal experimental design methods. These provide a systematic and quantitative framework ito (1) evaluate the quality of different designs in terms of uncertainties in the estimated tissue parameters and (2) optimize the configuration with respect to predefined design parameters, for example the position of transducers on the scanning device. Our first application presents a cost-effective 3D configuration of transducers optimized for transmission tomography. This is useful to analyze appropriate quality measures for USCT experiments and explore computationally tractable optimization approaches. The multi-modality capability of USCT, however, requires careful designs that simultaneously provide accurate images for both transmission (e.g., velocity) and reflection (reflectivity) information. We therefore extend the formulation to jointly optimize the experiment for transmission and reflection data. Here we focus on image reconstruction methods with linear(ized) observable-parameter relationship, for which quality measures are analytically given and independent of breast properties. This is crucial for optimizing USCT devices prior to any data acquisition. Methods investigated within this thesis are validated using experimental data. These contributions represent innovative solutions for USCT and ultimately serve to foster the knowledge and technology transfer between seismology and medical imaging, which may benefit imaging methods on all scales.

Уточнение гипоцентров землетрясений методом двойной разности

Article

Dec 2020

S.E. Kazimova

In the last decade, significant advances have been made in the theory and application of seismic tomography. These include refinements in model parameterization, 3D ray tracing, an inversion algorithm, sharing local, regional, and teleseismic data, and adding transformed and reflected waves to tomographic inversion. Explorations have shown that with the help of seismotomography it is possible to obtain reliable data on the deep structure of the Earth, its thickness, the mutual arrangement of layers, as well as tectonic structures identified in the earth’s crust. Due to a significant increase in the number of seismic stations in the Republican Seismic Survey Center and equipping them with modern instruments of the MacOs system (made by “Kinemetrics”), it was possible to obtain a large volume of observed seismic material and solve rather complex methodological issues, which is relevant today. The aim of this article is to redefine the data of the hypocenters of earthquakes that occurred on the territory of Azerbaijan for the period 2010‑2019 (ml>2.0) and calculate the velocity model of the earth’s crust using algorithms that are not included in the mandatory processing when compiling a catalog of seismic events. The catalog data were taken from the “Bureau of Earthquake Research” of the Republican Seismic Survey Centerof Azerbaijan National Academy of Sciences. Research methods. Within the framework of present work, using the double difference method, we redefined the location of seismic events, showing that the obtained positions of the epicenters are lined up in systems of linear chainsalong the main and feathering faults, which is consistent with the relief and geological concepts. Results. Comparing the values of the velocities with the values of the one-dimensional velocity model, it was found that at depths of 5‑10 km, there is good convergence in the regions of the Greater Caucasus. The middle Kura depression is mainly characterized by low velocities compared to the one-dimensional velocity model. At a depth of 15 km, the eastern part of the Middle Kura depression is characterized by good convergence, but in the western part high velocities are noted. The maximum convergence of velocities was noted at a depth of 35 km В последнее десятилетие были достигнуты значительные успехи в теории и применении сейсмотомографии. К ним относятся уточнения в параметризации модели, трас- сировка трехмерных лучей, алгоритм инверсии, совместное использование локальных, региональных и телесейсмических данных, а также добавление преобразованных и отраженных волн в томографическую инверсию. Исследования показали, что с помощью сейсмотомографии можно получить достоверные дан- ные о глубинном строении Земли, ее толщине, взаимном расположении слоев, а также о тектонических структурах, выявленных в земной коре. Благодаря значительному увеличению числа сейсмических стан- ций в РЦСС, оснащению их современными приборами системы MacOs (фирсы «Кинеметрикс»), удалось получить большой объем наблюденного сейсмического материала и решить довольно сложные методи- ческие вопросы, что является актуальным на сегодняшний день. Целью данной статьи является пере- определение данных гипоцентров землетрясений произошедших на территории Азербайджана за период 2010‑2019 гг. (ml>2,0) и вычислению скоростной модели земной коры с использованием алгоритмов, не входящих в обязательную обработку при составлении каталога сейсмических событий. Данные каталога были взяты в «Бюро исследований землетрясений» РЦСС при НАНА. Методы исследования. В рамках дан- ной работы методом двойных разностей мы переопределили положения сейсмических событий, показав, что полученные положения эпицентров выстраиваются в системы линейных цепочек, положение которых согласуется с рельефом и геологическими представлениями, располагаясь вдоль главного и оперяющих разломов. Результаты работы. Сопоставляя значения скоростей со значениями одномерной скоростной модели, установлено что на глубинах 5‑10 км наблюдается хорошая сходимость в областях Большого Кавказа. Среднекуринская депрессия в основном характеризуется малыми скоростями по сравнению с одномерной скоростной моделью. На глубине 15 км восточная часть Среднекуринской депрессии характе- ризуется хорошей сходимостью, однако в западной части отмечены завышенные скорости. Максимальная сходимость скоростей отмечена на глубине 35 км

Pannotia: Evidence for its existence and thermal legacy

Conference Paper

Jan 2020

Deflating the LLSVPs: Bundles of Mantle Thermochemical Plumes Rather Than Thick Stagnant “Piles”

Article

Full-text available

Sep 2020
TECTONICS

Based on SEMUCB‐WM1 tomographic model, validated by other recent models, and fluid mechanics constraints, we show that the large low shear velocity provinces (LLSVPs) present at the base of the Earth's mantle beneath the Pacific and Africa do not extend as compact, uniform structures very high above the core‐mantle boundary. In contrast, they contain a number of well‐separated, low‐velocity conduits that extend vertically throughout most of the lower mantle. The conceptual model of compact piles, continuously covering the areal extent of the LLSVPs, is therefore not correct. Instead, each LLSVP is composed of a bundle of thermochemical upwellings probably enriched in denser than average material. It is only when the tomographic model is filtered to long wavelengths that the two bundles of plumes appear as uniform provinces. Furthermore, the overall shape of the LLSVPs is probably controlled by the distribution of subducted slabs, and due to their thermochemical nature, the position of both LLSVPs and individual upwelling dynamics should be time dependent. There is also evidence for smaller plumes originating near the CMB in the faster than average regions of the voting map of Lekic et al. (2012, https://doi.org/10.1016/j.epsl.2012.09.014) as well as other, barely resolved, weaker plumes within the LLSVPs. These finer‐scale features are starting to be resolved tomographically owing to improvements in full waveform modeling of body waves, including diffracted S waves (Sdiff) and waves multiply reflected on the core‐mantle boundary (ScS) and their codas.

Planetary Seismology using Single-Station and Small-Aperture Arrays: Implications for Mars and Ocean Worlds

Thesis

Jan 2020

Angela Marusiak

Studying geophysical station deployment on Earth is essential preparation for future geophysical experiments elsewhere in the solar system. Here, I investigated how single-station seismometers and small-aperture seismic arrays in analog settings can quantify instrument capabilities, develop methodologies to detect and locate seismicity, and constrain internal structure. First, I used a single-station seismometer in Germany to study how the NASA InSight mission could constrain core depth. I showed that InSight could recover the Martian core within ±30 km if ≥ 3 events are located within an epicentral distance uncertainty of < ±1 degree. Increasing the number of detected events reduces core depth uncertainty, and higher signal-to-noise events will not affect core depth uncertainty or recovery rate. Next, I used environmental analogs in Earth's cryosphere to quantify how seismometer placement on a mock-lander would affect instrument performance and seismic science results for a future surface mission to an icy ocean world. If mock-lander instruments were unprotected from the wind, noise levels were 50 dB higher than those on the ground. However, once seismometers were shielded via burial, noise performances were similar to the ground-coupled seismometers, although spacecraft resonances were found at frequencies ~100 Hz. For icy ocean worlds lacking atmospheres, I showed that deck-mounted flight-candidate seismometers recorded ground motion comparably to surface-deployed instrumentation, with responses similar to terrestrial seismometers at frequencies > 0.1 Hz. Finally, I investigated seismicity detection capabilities of single-station and small-aperture seismic arrays. Small-aperture arrays were more effective at distinguishing low-frequency seismic events from noise and had fewer false positive events than a single-station. The Greenland site detected a higher percentage of teleseismic and regional tectonic events while the Gulkana Glacier, Alaska site observed more high frequency events. The high frequency seismicity was interpreted as originating from moulins, drainage events, icequakes, and rockfalls. Both sites had very high frequency events (> 100 Hz) that came from poles left in the field. These studies inform landing site selection criteria, such that there were trades between detecting local seismicity at the expense of seeing more distant events, and detecting larger teleseismic events that inform on deeper internal structure.

Seismic Tomography 2024

Article

May 2024

Seismic tomography is the most abundant source of information about the internal structure of the Earth at scales ranging from a few meters to thousands of kilometers. It constrains the properties of active volcanoes, earthquake fault zones, deep reservoirs and storage sites, glaciers and ice sheets, or the entire globe. It contributes to outstanding societal problems related to natural hazards, resource exploration, underground storage, and many more. The recent advances in seismic tomography are being translated to nondestructive testing, medical ultrasound, and helioseismology. Nearly 50 yr after its first successful applications, this article offers a snapshot of modern seismic tomography. Focused on major challenges and particularly promising research directions, it is intended to guide both Earth science professionals and early-career scientists. The individual contributions by the coauthors provide diverse perspectives on topics that may at first seem disconnected but are closely tied together by a few coherent threads: multiparameter inversion for properties related to dynamic processes, data quality, and geographic coverage, uncertainty quantification that is useful for geologic interpretation, new formulations of tomographic inverse problems that address concrete geologic questions more directly, and the presentation and quantitative comparison of tomographic models. It remains to be seen which of these problems will be considered solved, solved to some extent, or practically unsolvable over the next decade.

Thermobaric Conditions of Organic Matter Catagenesis in the Russian Segment of Barents Region Archipelagos (Franz Josef Land, Novaya Zemlya)

Article

Sep 2023

The natural bitumen (NB) localization finds in the archipelagos of Russian Arctic Western segment is considered. The nature of their appearance in connection with the geothermal regime of the subsurface is discussed. Based on numerical simulation, temperatures and heat flow density are calculated in 2D geometry along seismogeological profiles and in 3D geometry for the Franz Josef Land isometric structure. It is concluded that all the noted NB manifestations are genetically related to hydrothermal activity, the signs of which are adequately recorded in the geotemperature field.

Hamiltonian Monte Carlo based elastic full-waveform inversion of wide-angle seismic data

Article

Mar 2024

Full-waveform inversion (FWI) of seismic data provides quantitative constraints on subsurface structures. Despite its widespread success, FWI of data around the critical angle is challenging because of the abrupt change in amplitude and phase at the critical angle and the complex waveforms, especially in the presence of a sharp velocity contrast, such as at the Moho transition zone (MTZ). Furthermore, the interference of refracted lower crustal (Pg) and upper mantle (Pn) arrivals with the critically reflected Moho (PmP) arrivals in crustal and mantle studies makes the application of conventional FWI based on linearized model updates difficult. To address such a complex relationship between the model and data, one should use an inversion method based on a Bayesian formulation. Here, we propose to use a Hamiltonian Monte Carlo (HMC) method for FWI of wide-angle seismic data. HMC is a non-linear inversion technique where model updates follow the Hamiltonian mechanics while using the gradient information present in the probability distribution, making it similar to iterative gradient techniques like FWI. It also involves procedures for generating distant models for sampling the posterior distribution, making it a Bayesian method. We test the performance and applicability of HMC based elastic FWI by inverting the non-linear part of the synthetic seismic data from a three-layer and a complex velocity model, followed by the inversion of wide-angle seismic data recorded by two ocean bottom seismometers (OBS’s) over a 70 Ma old oceanic crustal segment in the equatorial Atlantic Ocean. The inversion results from both synthetic and real data suggest that HMC based FWI is an appropriate method for inverting the non-linear part of seismic data for crustal studies.

从地震到星震

Article

Feb 2024

GEODYNAMICS

Article

Dec 2023

The conventional approach to constructing a three-dimensional distribution of the Earth's masses involves using Stokes constants incrementally up to a certain order. However, this study proposes an algorithm that simultaneously considers all of these constants, which could potentially provide a more efficient method. The basis for this is a system of equations obtained by differentiating the Lagrange function, which takes into account the minimum deviation of the three-dimensional mass distribution of the planet's subsoil from one-dimensional referential one. An additional condition, apart from taking into account the Stokes constants, for an unambiguous solution to the problem is to specify the value of the function on the surface of the ellipsoidal planet. It is possible to simplify the calculation process by connecting the indices of summation values in a series of expansions to their one-dimensional analogues in the system of linear equations. The study presents a control example illustrating the application of the given algorithm. In its implementation, a simplified variant of setting the density on the surface of the ocean is taken. The preliminary results of calculations confirm the expediency of this approach and the need to expand such a technique with other conditions for unambiguously solving the inverse problem of potential theory. Objectives. To create and implement the algorithm that takes into account the density of the planet’s subsoil on its surface. Method. The mass distribution function of the planet's subsoil is represented by a decomposition into biorthogonal series, the coefficients of decomposition which are determined from a system of linear equations. The system of equations is obtained from the condition of minimizing the deviation function of the desired mass distribution from the initially determined two-dimensional density distribution (PREM reference model). Results. On the basis of the described algorithm, a three-dimensional model of the density distribution of subsoil masses in the middle of the Earth is obtained, which takes into account Stokes constants up to the eighth order inclusively and corresponds to the surface distribution of masses of the oceanic model of the Earth. Its concise interpretation is also presented.

Representation of three-dimensional mass distribution of the Earth's interior by biorthogonal series and its use for studying internal structure of the planet

Article

Dec 2023

Speculations on the Paleozoic legacy of Gondwana amalgamation

Article

Dec 2023
GONDWANA RES

Tectonic evolution of a sequence of related late Permian transtensive coal-bearing sub-basins, Mongolia: A global wrench tectonics portrait

Article

Full-text available

Dec 2023

During the late Permian in Mongolia, inertia-driven transtensive reactivation of primordial fracture zones gave rise to the development of a sequence of related, but isolated, fault-bounded sub-basins; some of these became the locus of substantial peat accumulation that evolved into economically important coal deposits. The present study focuses on late Permian coal measures in two widely separated areas: Area 1: located in central Mongolia, developed along the southern margin of the Mongol-Transbaikalian Seaway. The late Permian coal sequence forms a c. 420 m thick middle part of a Permo-Triassic succession which spans c. 2,600 m. The V-shaped, fault-bounded NE oriented sub-basin evolved under transtensive conditions. The thick infill records a transition from shallow marine and humid coal forming depositional environments during the late Permian to relatively arid desolate terrestrial conditions during early Triassic times, considered here to mark the dramatic drainage of the Mongol-Transbaikalian Seaway across the Permo-Triassic boundary. Area 2: situated in southern Mongolia, is a NE oriented elongate sub-basin, bounded by two wrench faults, which formed under transtensive conditions. Thickness of the late Permian coal-bearing strata is c. 650 m. The sedimentary strata record a transition from a humid coal-bearing environment to predominantly marine conditions. Both study areas are located proximal to two controversial suture zones. However, the zones do not show the presumed shortening, major thrusting, regional metamorphism and given the complete absence of tuffs within the studied Permo-Triassic successions it could be argued that the sutures are not only cryptic but non-existent.

The Accuracy Assessment of Lithospheric Density Models

Article

Full-text available

Sep 2023

The Earth’s synthetic gravitational and density models can be used to validate numerical procedures applied for global (or large-scale regional) gravimetric forward and inverse modeling. Since the Earth’s lithospheric structure is better constrained by tomographic surveys than a deep mantle, most existing 3D density models describe only a lithospheric density structure, while 1D density models are typically used to describe a deep mantle density structure below the lithosphere-asthenosphere boundary. The accuracy of currently available lithospheric density models is examined in this study. The error analysis is established to assess the accuracy of modeling the sub-lithospheric mantle geoid while focusing on the largest errors (according to our estimates) that are attributed to lithospheric thickness and lithospheric mantle density uncertainties. Since a forward modeling of the sub-lithospheric mantle geoid also comprises numerical procedures of adding and subtracting gravitational contributions of similar density structures, the error propagation is derived for actual rather than random errors (that are described by the Gauss’ error propagation law). Possible systematic errors then either lessen or sum up after applying particular corrections to a geoidal geometry that are attributed to individual lithospheric density structures (such as sediments) or density interfaces (such as a Moho density contrast). The analysis indicates that errors in modeling of the sub-lithospheric mantle geoid attributed to lithospheric thickness and lithospheric mantle density uncertainties could reach several hundreds of meters, particularly at locations with the largest lithospheric thickness under cratonic formations. This numerical finding is important for the calibration and further development of synthetic density models of which mass equals the Earth’s total mass (excluding the atmosphere). Consequently, the (long-to-medium wavelength) gravitational field generated by a synthetic density model should closely agree with the Earth’s gravitational field.

Thermobaric Conditions of Organic Matter Catagenesis in the Russian Segment of the Barents Region Archipelagos (Franz Josef Land, Novaya Zemlya)

Article

Sep 2023

Seismological Studies of Deep Earth Structure Using Seismic Arrays in East, South, and Southeast Asia, and Oceania

Chapter

Jun 2023

Improved Characterization of Ultralow‐Velocity Zones Through Advances in Bayesian Inversion of ScP Waveforms

Article

Full-text available

Jun 2023

Ultralow‐velocity zones (ULVZs) have been studied using a variety of seismic phases; however, their physical origin is still poorly understood. Short period ScP waveforms are extensively used to infer ULVZ properties because they may be sensitive to all ULVZ elastic moduli and thickness. However, ScP waveforms are additionally complicated by the effects of path attenuation, coherent noise, and source complexity. To address these complications, we developed a hierarchical Bayesian inversion method that allows us to invert ScP waveforms from multiple events simultaneously and accounts for path attenuation and correlated noise. The inversion method is tested with synthetic predictions which show that the inclusion of attenuation is imperative to recover ULVZ parameters accurately and that the ULVZ thickness and S‐wave velocity decrease are most reliably recovered. Utilizing multiple events simultaneously reduces the effects of coherent noise and source time function complexity, which in turn allows for the inclusion of more data to be used in the analyses. We next applied the method to ScP data recorded in Australia for 291 events that sample the core‐mantle boundary beneath the Coral Sea. Our results indicate, on average, ∼12‐km thick ULVZ with ∼14% reduction in S‐wave velocity across the region, but there is a greater variability in ULVZ properties in the south than that in the north of the sampled region. P‐wave velocity reductions and density perturbations are mostly below 10%. These ScP data show more than one ScP post‐cursor in some areas which may indicate complex 3‐D ULVZ structures.

A review of the influencing factors on teleseismic traveltime tomography

Article

Jun 2023

Teleseismic traveltime tomography is an important tool for investigating the crust and mantle structure of the Earth. The imaging quality of teleseismic traveltime tomography is affected by many factors, such as mantle heterogeneities, source uncertainties and random noise. Many previous studies have investigated these factors separately. An integral study of these factors is absent. To provide some guidelines for teleseismic traveltime tomography, we discussed four main influencing factors: the method for measuring relative traveltime differences, the presence of mantle heterogeneities outside the imaging domain, station spacing and uncertainties in teleseismic event hypocenters. Four conclusions can be drawn based on our analysis. (1) Comparing two methods, i.e., measuring the traveltime difference between two adjacent stations (M1) and subtracting the average traveltime of all stations from the traveltime of one station (M2), reveals that both M1 and M2 can well image the main structures; while M1 is able to achieve a slightly higher resolution than M2; M2 has the advantage of imaging long wavelength structures. In practical teleseismic traveltime tomography, better tomography results can be achieved by a two-step inversion method. (2) Global mantle heterogeneities can cause large traveltime residuals (up to about 0.55 s), which leads to evident imaging artifacts. (3) The tomographic accuracy and resolution of M1 decrease with increasing station spacing when measuring the relative traveltime difference between two adjacent stations. (4) The traveltime anomalies caused by the source uncertainties are generally less than 0.2 s, and the impact of source uncertainties is negligible.

How Mantle Convection Drives the Supercontinent Cycle: Mechanism, Driving Force, and Substantivity

Chapter

Jan 2023

Masaki Yoshida

Formation of Thermochemical Heterogeneities by Core‐Mantle Interaction

Article

Full-text available

Feb 2023

Earth's core‐mantle boundary (CMB) shows a complex structure with various seismic anomalies such as the large low shear‐wave velocity provinces (LLSVPs) and ultra‐low velocity zones (ULVZs). As these structures are possibly induced by chemically distinct material forming a layer above the CMB, models of mantle convection made ad hoc assumptions to simulate the dynamics of this layer. In particular, density and mass were prescribed. Both conditions are critical for the dynamics but hardly constrained. Core‐mantle interaction is considered as one possible origin for this dense layer. For example, diffusion‐controlled enrichment of iron has been proposed. We here apply a chemical gradient between the mantle and the denser core to analyze the penetration of dense material into the mantle. As such, we employ 2D Cartesian models where a thermochemical layer at the base of the mantle develops self‐consistently by a diffusive chemical influx. Our simulations indicate that chemical diffusion is strongly affected by the convective mantle flow. This convection‐assisted diffusion yields a compositional influx mainly in the areas where slabs spread over the bottom boundary and sweep dense material aside to form accumulations with rising plumes atop. Like for a prescribed dense layer this process leads to chemically distinct piles, which are typically smaller (therefore more suited to explain ULVZs) but more persistent due to the constant chemical influx. Combining the influx scenario with the primordial layer can possibly explain the simultaneous existence of LLSVPs and ULVZs along with the observation of a core‐like isotopic composition in the mantle.

Analyse des données GNSS de l’Afrique de 1994 à 2017.6 : caractérisation de mouvement et déformation active

Thesis

Sep 2020

Raphaël NGALULA Mukandila

Le but de cette thèse est la détermination d’un champ de vitesse GNSS actuel de l’Afrique afin de caractériser sa cinématique et ses déformations actives. Différents modèles cinématiques, sismotectoniques ont montré que la plaque Afrique était subdivisée en deux grandes plaques Nubie et Somalie. La densification des données GNSS sur le Rift Est Africain et les blocs tectoniques orientaux connexes a en outre permis de séparer la plaque somalienne de 3 autres sous-plaques. Aujourd’hui, la densification des données géodésiques sur la plaque Nubie a permis de la subdiviser en 4 blocs tectoniques majeurs. Les cœurs de ces blocs sont respectivement formés des trois cratons principaux archéens et un méta-craton qui sont séparés les uns des autres par de ceintures orogéniques néo-protérozoïques et des monts-sous-marins. Le long de ces ceintures se sont formées des zones de cisaillement, des failles actives et du volcanisme actif constituant des lignes de déformation continues (LDC). Le long de LDC, les régimes déformation prédits par la géodésie sont cohérent avec les caractéristiques sismotectoniques de ces régions.

Thermal evolution of the southern part of the Barents Sea (the Fedynsky Arch) (In Russian. English abstract)

Article

Full-text available

Oct 2022

The distribution of thermal conductivity, radiogenic heat generation and heat flow in the Barents Sea southern part, including the Fedynsky Arch, is analyzed. Models of deep temperatures controlling the catagenesis of organic matter thermal conditions are calculated. A 3D temperature model was built up to a 30 km depth, which allowed us to demonstrate cross-sectional temperature maps at various depths in the Earth’s crust. A comparison of the Barents Sea thermal field and seismotomographic model was carried out, which showed that the seismotomographic anomalies are caused by thermal inhomogeneities.

Mitigating the effect of errors in source parameters on seismic (waveform) tomography

Article

Full-text available

Aug 2022

We investigate the effect of errors in earthquake source parameters on the tomographic inverse problem and propose mitigation strategies for avoiding artefacts caused by such errors. In global catalogues, mislocation errors can amount to tens of kilometres both horizontally and vertically, while fault plane uncertainties can be of the order of tens of degrees. We conduct a perturbation study investigating both errors in the source location and in the moment tensor. Spatial perturbations of 5 km and fault plane perturbations of 5○ result in measured timeshifts of the order of 0.5 to several seconds, which in five iterations lead to artefacts with amplitudes of the order of 0.5–1% spanning up to several hundreds of kilometres. Larger perturbations (e.g. 20 km) lead to artefacts similar in amplitude (∼5%) to the features judged to be interpretable in tomographic models. This can be further exacerbated by the cumulative effect of systematic errors. Mitigation strategies consist of removing a region around the source from the gradient and discarding traces where amplitudes are comparatively small. The most problematic type of error is a horizontal mislocation, because its imprint is not removed by such measures – discarding a ‘suspicious’ event may be the only option if no trustworthy ground truth is available. Although this study focuses on (adjoint) waveform tomography, a large part of the results are equally valid for any other type of imaging method that is based on time- and/or phaseshift measurements. The concerns and mitigation strategies presented here therefore have broader applicability.

WUS256: An Adjoint Waveform Tomography Model of the Crust and Upper Mantle of the Western United States for Improved Waveform Simulations

Article

Full-text available

Jul 2022

We report a new model (WUS256) of radially anisotropic seismic wavespeeds of the crust and upper mantle of the western United States (WUS) obtained from adjoint waveform tomography for the purpose of improving synthetic waveform fits to observed data. WUS256 is based on inversion of over 94,000 waveforms from 72 earthquakes recorded by nearly 3,400 stations. We started with the SPiRaL global model (Simmons et al., 2021, https://doi.org/10.1093/gji/ggab277) and waveforms in the period band of 50–120 s. We followed a conservative multiscale inversion approach with eight stages and 256 total inversion iterations which enabled monotonic misfit reduction to 20‐s minimum‐period waves. WUS256 relied on time‐frequency (TF) phase misfits and a trust region limited memory Broyden–Fletcher–Goldfarb–Shanno (L‐BFGS) optimization. Hessian‐vector products were used to qualitatively assess model resolution. Results indicate that WUS256 has good coverage of the continental regions to depths of about 150 km and is able to resolve features on lateral scales of about 200 km. We quantify waveform fits by the reduction in TF and normalized amplitude difference misfits between WUS256 and the SPiRaL starting model. WUS256 significantly improves waveform fits with misfit reduction ≥ $\ge $64% for both inversion and validation data sets compared to the SPiRaL starting model and shows even better fits compared to other models. Waveform fits illustrate that WUS256 reproduces body‐waves, fundamental mode surface waves as well as late arriving dispersed and/or scattered short period surface waves. The improvement in waveform fit indicates that WUS256 can be used to reproduce path effects on regional complete waveforms and moment tensor inversions.

Retracer l’évolution de la Terre silicatée par le couplage des systèmes lanthane-cérium et samarium-néodyme

Thesis

Feb 2021

Claudine Israel

Les terres rares (REE) sont des éléments chimiques partageant des comportements très similaires lors des processus magmatiques. Lithophiles et réfractaires, ils se concentrent dans la fraction silicatée de la Terre. Parmi les REE, quatre éléments constituent deux systématiques isotopiques de longue vie : 138La-138Ce (T1/2 = 295.5 Ga) et 147Sm-143Nd (T1/2 = 106 Ga). Ces systèmes permettent d’apporter une contrainte temporelle à l’évolution de la composition des réservoirs silicatés (croûte – manteau) et de comprendre leur évolution. Par ailleurs, le cérium est sensible aux conditions d’oxydation du milieu et donc présente un comportement différent des autres REE. Dans cette thèse, j’ai couplé les mesures isotopiques Ce-Nd pour mieux contraindre la formation des réservoirs et leur évolution au cours du temps, comme par exemple l’impact du recyclage lors de la subduction sur la composition du manteau.Les très faibles variations du rapport 138Ce/142Ce expliquent pourquoi la systématique 138La-138Ce a été très peu étudiée. Ainsi, la composition isotopique en cérium des réservoirs silicatés, notamment celle de la croûte continentale, n’est pas bien définie. Les mesures développées par spectrométrie de masse à thermo-ionisation (utilisation des amplificateurs 1013 Ω) ont permis d’augmenter considérablement la précision. Dans le cadre de cette thèse, j’ai ainsi mesuré 135 échantillons de contextes divers représentant les principaux réservoirs silicatés étudiés. L’analyse de basaltes de rides médio-océaniques et de sédiments éoliens a permis d’obtenir une estimation des compositions isotopiques respectives du manteau supérieur appauvri et de la croûte continentale supérieure. La variabilité isotopique du manteau en Ce-Nd est précisée par les mesures sur des basaltes d’îles et de rides océaniques. Ils forment la tendance mantellique. La comparaison de cette tendance avec les modèles de mélange issus du bilan de masse croûte-manteau pour les REE montre que la complémentarité manteau-croûte est insuffisante pour expliquer la variabilité de composition isotopique du manteau. La tendance mantellique est mieux reproduite lorsque les modèles intègrent la contribution du recyclage ancien de croûte océanique et de sédiments océaniques, formés avant l’oxygénation des océans (>2 Ga). Le bilan de masse pour les REE implique également que la croûte continentale moyenne à inférieure présente des compositions isotopiques en Ce-Nd distinctes de la tendance mantellique et de la croûte continentale supérieure. Pour vérifier ce modèle, j’ai analysé des roches continentales d’origine superficielle à profonde, d’âges et de régions différents. Parmi ces roches, les xénolithes de Sibérie ont les compositions isotopiques attendues. Un fractionnement des REE dans la croûte profonde pourrait expliquer le découplage des systématiques 138La-138Ce et 147Sm-143Nd. Les compositions isotopiques en Sr, Nd et Pb des basaltes d’îles océaniques reflètent l’existence de pôles de composition dans le manteau, produits par le recyclage de matériaux différents. Les compositions isotopiques en Ce-Nd de certains groupes d’échantillons, caractéristiques d’un manteau enrichi correspondant aux pôles EM-1 et EM-2, ont des compositions distinctes. Les échantillons de signature EM-2 restent strictement alignés sur la tendance mantellique en Ce-Nd et ceux de signature EM-1 en dévient. Les premiers semblent particulièrement bien refléter l’effet du recyclage ancien (> 2 Ga) de croûte continentale supérieure ou de sédiments océaniques anciens, tandis que les seconds nécessitent, en plus, le recyclage ancien de croûte océanique, ou de croûte continentale inférieure.

Lowermost mantle ( D ″ layer) structure beneath the Indian Ocean: Insights from modeling of ScS-S and PcP-P residuals

Article

Dec 2021
J ASIAN EARTH SCI

In this study, we investigate the seismic structure of the D′′ layer beneath the Indian Ocean by modeling the ScS-S and PcP-P differential travel time residuals corrected for the velocity structure above it. These times, representative of the anomalies in the D′′ layer, vary from -9.58 to 5.06 s for the shear wave (ScS-S) and -4.54 to 3.14 s for the compressional wave (PcP-P). Modeling of the residuals using a grid search approach reveals velocity perturbations in the range of -3.06% to 5.72% for the shear and -4.81% to 5.47% for the compressional waves, in the D′′ layer. Interestingly, the perturbations are positive below the Indian Ocean Geoid Low (IOGL) and negative below the adjoining region. The results clearly reveal presence of high velocity material atop the Core Mantle Boundary (CMB) beneath the IOGL, representing dehydrated Tethyan subducted slabs. Further, the low RSP values in the lowermost mantle beneath the western part of IOGL, calculated using the P and S velocity estimates from this study, mostly lie between 0.01 and 2.7. This implies that the anomalies may be thermal in origin, owing to the heterogeneities resulting from cold lithospheric slabs at the CMB.

An analysis of core-mantle boundary Stoneley mode sensitivity and sources of uncertainty

Article

Full-text available

Oct 2021

Stoneley modes are a special subset of normal modes whose energy is confined along the core-mantle boundary. As such, they offer a unique glimpse into Earth structure at the base of the mantle. They are often observed through coupling with mantle modes due to rotation, ellipticity and lateral heterogeneity, though they can be detected without such coupling. In this study, we explore the relative sensitivities of seismic spectra of two low-frequency Stoneley modes to several factors, taking as reference the fully coupled computation up to 3 mHz in model S20RTS. The factors considered are (i) theoretical, by exploring the extent to which various coupling approximations can accurately reproduce reference spectra; and (ii) model-based, by exploring how various Earth parameters such as core-mantle boundary topography, attenuation, and S-wave and P-wave structures, and the seismic source solution may influence the spectra. We find that mode-pair coupling is insufficiently accurate, but coupling modes within a range of ±0.1 mHz produces acceptable spectra, compared to full coupling. This has important implications for splitting function measurements, which are computed under the assumption of isolated modes or at best, mode-pair or group coupling. We find that uncertainties in the P-wave velocity mantle model dominate compared to other model parameters. In addition, we also test several hypothetical models of mantle density structure against real data. These tests indicate that, with the low-frequency Stoneley mode spectral data considered here, it is difficult to make any firm statement on whether the large-low-shear-velocity-provinces are denser or lighter than their surroundings. We conclude that better constraints on long wavelength elastic mantle structure, particularly P-wave velocity, need to be obtained, before making further statements on deep mantle density heterogeneity. In particular, a dense anomaly confined to a thin layer at the base of the mantle (less than ∼100-200 km) may not be resolvable using the two Stoneley modes tested here, while the ability of higher frequency Stoneley modes to resolve it requires further investigations.

Plateau Uplift, Regional Warping, and Subsidence

Chapter

Jan 2021

High Plateaus are flat areas of great extent and elevation, and the largest include the Tibetan Plateau, the Altiplano-Puna plateau, the Colorado Plateau, the Eastern Anatolian plateau, the East African and Ethiopian Plateaus and the southern African plateau. Understanding their development is complex and requires a good knowledge of their structure at depth, of the histories of deformation and mean elevation (surface uplift). We first present the physiognomic and deep characteristics of the seven most extensive plateaus on Earth. In a second part, the evidence used by the Earth Science's Community to quantify rock and/or surface uplifts are reviewed. The basics of numerous methods and resulting uncertainties are carefully identified and exemplified by data of paleoelevation and uplift rate histories. The evidence for rock/surface uplift includes geomorphological markers, paleoatlimetry from sedimentology, paleobotany, stable isotopes, paleoatmospheric pressure, cosmogenic nuclides, thermochronology and inversion schemes of river elevations. Then, the deep processes invoke for the three largest plateaus are summarized. The main mechanisms for high plateaus formation, inferred from the deep structures and the evidence for rock/surface uplift, include crustal thickening, thinning of the mantle lithosphere and deep convective mantle. Finally, we briefly present the debate between the feedbacks on plateaus uplift and global climate change, which includes lapse-rate cooling of high-elevation surfaces, perturbation of jet stream meanders, and creation and intensification of monsoon circulations. This article deals with results coming from a very dynamic branch of Earth Sciences and some of the results presented here are still under debate. Interestingly, the results obtained from the Earth-surface and deep-Earth communities may diverge in that they invoke contradictory tectonic processes for a same geological object, and conflicting uplift histories. Future research is needed to obtain more accurate records of uplift histories and more precise crust and mantle structures that support the High Plateaus. Further advances will help resolve the long-standing question of the origin and the dynamics of the High Plateaus, and their topographic impact on the Atmosphere and the Biosphere.

Recent Ph. D.'s

Article

Jan 1985

Eos periodically lists information on recently accepted doctoral dissertations in the disciplines of geophysics. Faculty members are invited to submit the following information, on institution letterhead, above the signature of the faculty advisor or department chairman: (1) the dissertation title, (2) author's name, (3) name of the degree‐granting department and institution, (4) faculty advisor, (5) month and year degree was awarded, (6) category of dissertation subject (Atmospheric Sciences, Hydrology, Ocean Sciences, Solid Earth, or Space Sciences).

The importance and difficulties of identifying mantle plumes in orogenic belts: An example based on the fragmented large igneous province (LIP) record in the Ural fold belt

Article

Jun 2021
PRECAMBRIAN RES

Mantle plumes can be recognized by their magmatic expression as large igneous provinces (LIPs). However, identification of plumes in old, structurally complicated fold belts is particularly difficult due to deformation, which obscures the LIP record. On the other hand, fold belt regions are particularly important in the search for LIPs for at least three reasons: 1) they can represent prior plate margins associated with plume-generated continental breakup and LIP magmatism; 2) the deformation may expose basement rocks (containing LIP units) covered by younger sedimentary rocks elsewhere in the continental block; and 3) they preserve deformed remnants of oceanic LIPs and hot spot chains accreted during ocean closure. Herein we provide an initial survey of the plume /LIP record of one of the world’s great orogenic belts, the Ural fold belt. The following events are identified: The 1750 Ma Navysh event is coeval with units in Sarmatia and Karelia (other parts of Baltica) and on other crustal blocks. The 1385 Ma Mashak event is associated with a range of ore deposit types, is part of Nuna supercontinent breakup, and is postulated to have had a global environmental impact linked to the Calymmian-Ectasian boundary. The ca. 720 Ma Igonino event can be approximately matched with 720 Ma LIPs in northern Laurentia, and elsewhere, which can be linked to the onset of the Sturtian glaciation (Tonian-Cryogenian boundary). The ca. 480 Ma Kidryasovo and 450 Ma Ushat events have age matches in Siberia and other crustal blocks; the ages approximately match the end-Cambrian and end-Ordovician periods, respectively. The 370 Ma Timaiz event belongs to the c. 370 Ma Kola-Dniepr LIP which is widespread in Baltica, has an age match in Siberia and collectively can be linked with the end Devonian period. An Early Carboniferous (350–320 Ma) event follows island-arc/continent collision and slab break-up in the Magnitogorsk zone. Three orogenic/postorogenic plume intraplate episodes are also described, the ca. 285 Ma Stepninsky monzogabbro-granosyenite-granite complex, the 308–304 Ma Kalymbaevsky lamproite complex and units that are coeval with the 251 Ma Siberian Traps LIP, linked to the Ural-Siberian superplume and with the end Permian mass extinction.

The Mediterranean

Chapter

May 2021

D. Papanikolaou

The Mediterranean Sea is the last evolutionary stage of the Tethys Ocean. The geological and geophysical structure of the Mediterranean Basin is highly differentiated between the Western and the Eastern Mediterranean with opening of post-collisional basins in the west versus rapid convergence in the east. The Mediterranean Ridge corresponds to a submarine mountain chain, representing an accretionary prism involving also a backstop in front of the Hellenic arc. The Eratosthenes Seamount represents an actualistic example of a drifting terrane. Characteristic gravity values for oceanic crust are found in the Mediterranean basins with a depth of more than 3,000 m. Enormous differences in sediment thickness and sedimentation rate throughout the Plio-Quaternary are observed in the Mediterranean. The seismicity is localized along the three orogenic arcs of the Eastern Mediterranean. Seismic tomographs of the Mediterranean permit the detection of the subduction zones down to the mantle at a depth of 1,400 km. The North Aegean graben lies on a microplate boundary, separating the low convergence rate of Europe to the south (10 mm/year) from the high rate of the Aegean microplate (40–50 mm/year) to the south-southwest, considering the African plate fixed. The Aegean microplate is bordered by the Central Hellenic Shear zone towards Europe and by the West Anatolian Shear Zone towards the Anatolian microplate. The history of the Mediterranean begins with the isolation of the oceanic remnant of the former Tethys Ocean in the eastern Mediterranean basin 13 Ma ago, following the Arabia- Eurasia collision. Paleomagnetic studies supported a 40–50° clockwise rotation of the Aegean microplate since the Middle Miocene. Collision and strike-slip motions in the east are transformed to convergence and back-arc extension in the center and opening of the Tyrrhenian and Balearic oceanic basins in the west of the Mediterranean.

Numerical modeling of the craton destruction process, with a type-example from the North China Craton

Thesis

Apr 2018

Liang Liu

The phenomenon of cratonic destruction has attracted attention among numerical modelers. However, existing modeling work appears to lack enough constraints from geological and geophysical observations. To better combine the observations with the modeling approach, and to further understand the destruction of the North China Craton (NCC), I firstly reviewed current knowledge about the evolution of the North China Craton, and then outlined the strength and weakness associated with classic delamination and convective erosion models. By integrating new observational data and 2-D numerical modeling results, I put forward a two-stage model to account for the destruction of the NCC, that is, cratonic keel delamination followed by localized convective erosion and lithospheric extension. As revealed in the evolution of the NCC, its destruction appears to be accompanied by the inversion of a pre-existing North China Cratonic Basin (NCCB), which is partly preserved in southeastern Ordos. This >300 Myr subsidence of the NCCB can most probably be explained by assuming a pre-existing dense keel for the eastern NCC (ENCC), consistent with the petrological and geochemical observations from mantle xenoliths/xenocrysts hosted in kimberlite/basalt in the ENCC. As the bottom was dense, cratonic keel stability would have only been sustained by the buoyancy of the overlying lithospheric portion. Hereafter, if some weak intra-cratonic regions had existed above the dense bottom and were reactivated, intra-cratonic decoupling may finally occur, leading to keel delamination. Coincidentally, some seismically visible Mid-Lithospheric Discontinuity Layers (MLDLs) exist at depths of ~80-100 km within modern cratons, including the remaining portions of the NCC and the Wyoming Craton (WC). Interestingly, these depths are also where the relic lithospheric bottom appears to remain beneath the keelless sub-regions of the eastern NCC and the WC. Because the MLDLs are suggested to be regions of preferential accumulation of metasomatic minerals and/or anomalously wet (>1000 ppm) peridotites (both of which would lead to a relatively weak rheology), we accordingly propose that the weak MLDLs may have been the intra-cratonic weak zone, which allowed delamination of the dense cratonic keels of the NCC and the WC. This delamination model is studied in detail using 2-D analytical and numerical modeling, and the modeling results may help constrain the MLDL rheology needed for explaining the keel delamination rate estimated on the NCC and the WC. One can speculate that the delaminated cratonic keels may still exist or stagnate within the upper mantle. This suggestion may explain some, if not all, of the high velocity bodies beneath North China and Iowa, USA (which was the location caped by the Wyoming Craton before the Laramide Orogeny). After keel delamination along the MLDLs, a protracted (ca. 50-100 Myr) period of small-scale convective erosion and/or lithospheric extension thinned the remaining ~90 km thick lithosphere and continuously reworked the former cratonic lithospheric mantle beneath the Moho. As indicated in the modeling results, this second stage evolution may explain the episodic magmatism and crustal deformation associated with the destruction of the NCC.

Multiple-grid model parametrization for seismic tomography with application to the San Jacinto fault zone

Article

Jul 2019

We discuss the performance of the multiple-grid model parametrization in seismic tomographic inversion. Rather than mapping the velocity perturbation Δc(x) = c(x) − c0(x) on only one regular/collocated grid as many previous studies did, we obtain individual Δc(x) models on multiple grids and generate several updated velocity models during one iteration. The average of all the updated velocity models is considered to be the input model of the next iteration. Different grids should partially/fully shift from each other and/or have different grid spacings to form the multiple-grid model parametrization. The efficacy of the multiple-grid model parametrization is demonstrated through the practical example of imaging the P-wave velocity structure along the San Jacinto fault, which is one of the most seismically active faults in California. A series of synthetic recovery examples shows that the multiple-grid model parametrization generally has a better or comparable performance in capturing the heterogeneous subsurface structures than a collocated grid. The root mean square values of the traveltime residuals in the final tomographic models obtained with the multiple-grid model parametrization are smaller than those with collocated grids. Tomographic results reveal strong heterogeneities in the crust along the San Jacinto fault. Significant velocity contrasts are observable across the fault at shallow depths. A low-velocity anomaly dominates the trifurcation area of the San Jacinto fault from the middle crust to the lower crust. Relatively large earthquakes occurred at the boundaries of low-velocity structures but with high-velocity anomalies nearby. All the results suggest that the multiple-grid model parametrization can be a reliable approach in future seismic tomography studies.

Finite-frequency traveltime tomography using the Generalized Rytov approximation

Article

May 2020

Wave-equation-based traveltime tomography has been extensively applied in both global tomography and seismic exploration. Typically, the traveltime Fréchet derivative is obtained using the first-order Born approximation, which is only satisfied for weak velocity perturbations and small phase shifts (i.e. the weak-scattering assumption). Although the small phase-shift restriction can be handled with the Rytov approximation, the weak velocity-perturbation assumption is still a major limitation. The recently developed generalized Rytov approximation (GRA) method can achieve an improved phase accuracy of the forward-scattered wavefield, in the presence of large-scale and strong velocity perturbations. In this paper, we combine GRA with the classical finite-frequency theory and propose a GRA-based traveltime sensitivity kernel (GRA-TSK), which overcomes the weak-scattering limitation of the conventional finite-frequency methods. Numerical examples demonstrate that the accumulated time delay of forward-scattered waves caused by large-scale smooth perturbations can be correctly handled by the GRA-TSK, regardless of the magnitude of the velocity perturbations. Then, we apply the new sensitivity kernel to solve the traveltime inverse problem, and we propose a matrix-free Gauss–Newton method that has a faster convergence rate compared with the gradient-based method. Numerical tests show that, compared with the conventional adjoint traveltime tomography, the proposed GRA-based traveltime tomography can obtain a more accurate model with a faster convergence rate, making it more suited for recovering the large-intermediate scale of the velocity model, even for strong-perturbation and complex subsurface structures.

Increased density of large low-velocity provinces recovered by seismologically constrained gravity inversion

Article

Full-text available

Aug 2020
SE

The nature and origin of the two large low-velocity provinces (LLVPs) in the lowest part of the mantle remain controversial. These structures have been interpreted as a purely thermal feature, accumulation of subducted oceanic lithosphere or a primordial zone of iron enrichment. Information regarding the density of the LLVPs would help to constrain a possible explanation. In this work, we perform a density inversion for the entire mantle, by constraining the geometry of potential density anomalies using tomographic vote maps. Vote maps describe the geometry of potential density anomalies according to their agreement with multiple seismic tomographies, hence not depending on a single representation. We use linear inversion and determine the regularization parameters using cross-validation. Two different input fields are used to study the sensitivity of the mantle density results to the treatment of the lithosphere. We find the best data fit is achieved if we assume that the lithosphere is in isostatic balance. The estimated densities obtained for the LLVPs are systematically positive density anomalies for the LLVPs in the lower 800–1000 km of the mantle, which would indicate a chemical component for the origin of the LLVPs. Both iron-enrichment and a mid-oceanic ridge basalt (MORB) contribution are in accordance with our data, but the required superadiabatic temperature anomalies for MORB would be close to 1000 K.

Travel times and station corrections of P waves at teleseismic distances

Article

Full-text available

Apr 1983

Approximately 3300 shallow focus earthquakes and 1000 seismic stations have been used in a study of P wave travel times and station residuals, including azimuthal effects. The events were selected from a catalog containing 160,000 earthquakes, and those having uniform distance and azimuthal coverage were systematically relocated and used to refine P wave travel times and station corrections. Station corrections are provided for 994 seismic stations. The station corrections involve three terms: the static effect and two cosine terms with appropriate phase shifts. They exhibit general consistency over broad geographic areas and, where coverage is dense, often show abrupt changes from one geological province to another. The cos2theta terms appear to be due to upper mantle anisotropy, and they correlate with the stress direction in the crust.

Aspherical Earth structure from fundamental speroidal-mode data

Article

Full-text available

Jul 1982

Free-oscillation data reveal heterogeneity in the earth's mantle whose geographical pattern is dominated by spherical harmonics of angular degree two and correlates well with the hydrostatically referenced geoid. The heterogeneity can be modelled as localized in the transition zone (420-670 km depth) and may be related to a large-scale component of mantle convection.

Subducted Slabs and the Geoid: Constraints on Mantle Rheology and Flow

Article

Full-text available

Aug 1984
J GEOPHYS RES

Bradford H. Hager

The total geoid anomaly which is the result of a given density contrast in a convecting viscous earth is affected by the mass anomalies associated with the flow induced deformation of the upper surface and internal compositional boundaries, as well as by the density contrast itself is discussed. If the internal density contrasts can be estimated, the depth and variation of viscosity with depth of the convecting system can be constrained. The observed long wavelength geoid is highly correlated with that predicted by a density model for seismically active subducted slabs. The amplitude of the correlation is explained if the density contrasts associated with subduction extend into the lower mantle or if subducted slabs exceeding 350 km in thickness are piled up over horizontal distances of thousands of km at the base of the upper mantle. Mantle wide convection in a mantle that has a viscosity increasing with depth provides the explanation of the long-wavelength geoid anomalies over subduction zones. Previously announced in STAR as N83-22874

Composition of the Earth's Mantle

Article

Dec 1961

Francis Birch

Measurements of the velocity of compressional waves in silicates and oxides having a range of density from 2.6 to 5g/cm³ suggest a simple dependence of velocity upon density and mean atomic weight. Consequences of the assumption that this relation remains valid at high pressures are examined with reference to the composition of the mantle, especially of the transition layer. It appears probable that the abnormally high rate of increase of velocity with depth in the transition layer may be accounted for principally in terms of phase change, with little change of composition. Recent studies with strong shock waves are examined in connection with the composition of the core; the evidence is unfavourable to the hypothesis of a metallic core of light elements, but is consistent with a core of iron alloy.

Geographical interpretation of measurements of average phase velocities of surface waves over great circular and great semi-circular paths

Article

Apr 1964

George E. Backus

If the averages of the reciprocal phase velocity c−1 of a given Rayleigh or Love mode over various great circular or great semicircular paths are known, information can be extracted about how c−1 varies with geographical position. Assuming that geometrical optics is applicable, it is shown that if c−1 is isotropic its great circular averages determine only the sum of the values of c−1 at antipodal points and not their difference. The great semicircular averages determine the difference as well. If c−1 is anisotropic through any cause other than the earth's rotation, even great semicircular averages do not determine c−1 completely. Rotation has negligible effect on Love waves, and if it is the only anisotropy present its effect on Rayleigh waves can be measured and removed by comparing the averages of c−1 for the two directions of travel around any great circle not intersecting the poles of rotation. Only great circular and great semicircular paths are considered because every earthquake produces two averages of c−1 over such paths for each seismic station. No other paths permit such rapid accumulation of data when the azimuthal variations of the earthquakes' radiation patterns are unknown. Expansion of the data in generalized spherical harmonics circumvents the fact that the explicit formulas for c−1 in terms of its great circular or great semicircular integrals require differentiation of the data. Formulas are given for calculating the generalized spherical harmonics numerically.

The joint inversion of seismic waveforms for lateral variations in earth structure and earthquake source parameters

Article

Jan 1983

J.H. Woodhouse

Estimation of surface focus P travel times

Article

Aug 1968

An iterative technique was used to locate some 400 earthquakes, estimate corrections to the Jeffreys-Bullen P travel times and estimate azimuthally dependent station adjustments (station corrections). An oversimplified model for the upper 700 km of the mantle was adopted in order to provide a standard for the investigation of regional variations in travel times. In the estimation procedure only data for epicentral distances in the range 20° to 105° were used, and the data were truncated to remove gross errors. The new P travel times are generally 2 to 3 sec less than those given in the Jeffreys-Bullen Tables.

Travel times, apparent velocities and amplitudes of body waves

Article

Feb 1968

Surface wave studies have shown that the transition region of the upper mantle, Bullen's Region C, is not spread uniformly over some 600 km but contains two relatively thin zones in which the velocity gradient is extremely high. In addition to these transition regions which start at depths near 350 and 650 km, there is another region of high velocity gradient which terminates the lowvelocity zone near 160 km. Theoretical body wave travel time and amplitude calculations for the surface wave model CIT11GB predict two prominent regions of triplication in the travel-time curves between about 15° and 40° for both P and S waves, with large amplitude later arrivals. These large later arivals provide an explanation for the scatter of travel time data in this region, as well as the varied interpretations of the “20° discontinuity.” Travel times, apparent velocities and amplitudes of P waves are calculated for the Earth models of Gutenberg, Lehmann, Jeffreys and Lukk and Nersesov. These quantities are calculated for both P and S waves for model CIT11GB. Although the first arrival travel times are similar for all the models except that of Lukk and Nersesov, the times of the later arrivals differ greatly. The neglect of later arrivals is one reason for the discrepancies among the body wave models and between the surface wave and body wave models. The amplitude calculations take into account both geometric spreading and anelasticity. Geometric spreading produces large variations in the amplitude with distance, and is an extremely sensitive function of the model parameters, providing a potentially powerful tool for studying details of the Earth's structure. The effect of attenuation on the amplitudes varies much less with distance than does the geometric spreading effect. Its main effect is to reduce the amplitude at higher frequencies, particularly for S waves, which may accunt for their observed low frequency character. Data along a profile to the northeast of the Nevada Test Site clearly show a later branch similar to the one predicted for model CIT11GB, beginning at about 12° with very large amplitudes and becoming a first arrival at about 18°. Strong later arrivals occur in the entire distance range of the data shown, Formula. to 21°. Two models are presented which fit these data. They differ only slightly and confirm the existence of discontinuities near 400 and 600 kilometers. A method is described for predicting the effect on travel times of small changes in the Earth structure.

Quadrupole convection in the lower mantle

Article

Apr 1983

Friedrich Busse

Global gravity data and seismic inferences of lateral inhomogeneities in the density distribution of the Earth's mantle provide information on convection in the lower mantle. The data are interpreted in terms of a model of mantle convection with two layers separated by the interface corresponding to the 670 km seismic discontinuity. It is shown that the observations are in approximate agreement with theoretical deductions for a l=2 convection mode in the lower mantle.

Large-scale heterogeneities in the lower mantle

Article

Jan 1977

Coefficients of a spherical harmonic expansion, up to angular order 3, of velocity anomalies in five shells within the earth's mantle were obtained from an analysis of nearly 700,000 P wave travel time residuals. The results for depths less than 1100 km are unreliable; on the basis of tests and numerical experiments we infer that lateral heterogeneities in this depth interval are dominated by velocity perturbations of lateral dimensions less than 5000 km. Relatively large wavelength features were resolved below 1500-km depth; the average perturbation level increases in the lowermost mantle. The region betwen 1100 and 1500 km may represent a transition zone with respect to the dimensions of anomalies. We present statistical evidence for a negative correlation between the long wavelength gravity anomalies observed at the surface and those computed from velocity anomalies at depths greater than 1100 km under the assumption of proportionality between velocity and density perturbations. The proportionality coefficient Deltav/Deltarho has been determined by using two different methods: the values are -4.45 and -6.02 (km/s)/(g/cm3). Only minor changes in the velocity distribution are needed to satisfy the long wavelength gravity field exactly. Possible origins of the correlation are (1) sinking of eclogite-rich material into the lower mantle from regions of lithospheric subduction, (2) chemical plumes of light high-velocity material originating near the core-mantle boundary, (3) temperature differences and perturbations of the core-mantle boundary and the earth's surface associated with mantle-wide convection, or (4) static chemical heterogencities in a nonconvecting mantle. The first three alternatives, all involving flow in the lower mantle, may be complementary but act on a different time scale. There appears to be a westward or northwestward translation of some anomalies with respect to the pattern obtained for the innermost shell. In particular, the direction of translation of large negative anomaly under the Pacific is in agreement with the sense of motion of the Pacific plate. We must caution the reader, however, that this is a highly speculative interpretation. If correct, it would favor the convective hypotheses.

Seismic Travel Time Evidence for Lateral Inhomogeneity in the Deep Mantle

Article

Apr 1973
NATURE

We present evidence from seismic travel time data of lateral variations in the properties of the lower mantle. The size of some anomalies is about 1,000 km.

Measurement of Mantle Wave Velocities and Inversion for Lateral Heterogeneity and Anisotropy.1. Analysis Of Great Circle Phase Velocities

Article

Dec 1983

The global lateral heterogeneity of the upper mantle is investigated using the classical Fourier-transform method of Sato(1958) and IDA/GDSN data from 25 1980 earthquakes. The great-circle phase velocities of 200 Love and 250 Rayleigh 100-330-sec-period fundamental-mode wave paths are determined and interpreted in terms of regional phase-velocity variation, using additional data on surface tectonics to extrapolate odd-harmonic information from the even-harmonic data. The results are presented in extensive tables, maps, and graphs. Regionalized inversion using the seven-region model of Okal (1977) is found to give maximum variance reductions of 65 percent for Love waves and 85 percent for Rayleigh waves, compared to 60 and 90 percent for l(max) = 2 inversion. Significant interregion differences are found in the regionalized Love-wave phase velocities.

Mapping the Upper Mantle: Three-Dimensional Modeling of Earth Structure by Inversion of Seismic Waveforms

Article

Jul 1984

A method is presented for the inversion of waveform data for the 3-D distribution of seismic wave velocities. The method is applied to data from the global digital networks: the selected data set consists of some 2000 seismograms corresponding to 53 events and 870 paths. The moment tensors of the earthquakes are determined through an iterative procedure which minimizes the corrupting influence of lateral heterogeneity. A global model is constructed for shear wave velocity, expanded up to degree and order 8 in spherical harmonics, and described by a cubic polynomial in depth for the upper 670km of the Earth's mantle. The model predictions reproduce much of what is known about the dispersion of mantle waves. Since the method makes use of complete waveforms, overtone data are also included. It is shown that the model is reproducible in that substantially the same model can be constructed from each half of the total data set considered independently. The model is discussed.-after Authors

Three dimensional model of seismic velocity variation in the Earth's mantle

Article

Feb 1976

Three dimensional velocity models for the earth's mantle were obtained satisfying P, S and some PcP and ScS travel time anomalies from deep focus earthquakes. A method of successive approximation was used to compute the uniform relative velocity perturbation over three dimensional blocks of size 10°×10° in longitude and latitude, and 500 km in thickness. Features of these velocity perturbations indicated that lateral heterogeneity is the most pronounced in the upper mantle and near the core-mantle boundary. The upper mantle anomalies are correlated with surface tectonic features, but these are not correlated with deep mantle anomalies.

Phase Velocities of Long-Period Surface Waves and Structure of the Upper Mantle, 1, Great-Circle Love and Rayleigh Wave Data

Article

Mar 1966

New long-period dispersion data are obtained from the surface waves generated by the Alaska earthquake of March 28, 1964, and recorded at Isabella, Kipapa, and Stutt- gart. Digital techniques were used to isolate phases and determine spectrums over the period band 80 to 670 seconds. Available phase velocity data are now accurate enough to permit us to discuss regional variations which can be attributed to heterogeneity of the upper 400 km of the mantle. Average phase velocities are markedly affected by the character of the conti- nental fraction of the path. Shield areas raise the average phase velocity; tectonic and mountainous areas have the opposite effect. The tectonic-shield distinction is as important as the more obvious continental-oceanic distinction. An average mantle structure, designated CIT 12, is determined for the Mongolia-Pasadena composite great-circle path. The major features of this new mantle model are similar to those determined for the New Guinea- Pasadena great-circle path (model CIT 11), namely, a pronounced and deep low-velocity zone and two discontinuities in the upper mantle at depths near 350 km and 700 kin. The two models differ in a way that suggests lower average shear velocities under tectonic regions than under shield areas to depths of the order of 400 kin.

The Mixed Discrete-Continuous Inverse Problem: Application to the Simultaneous Determination of Earthquake Hypocenter and Velocity Structure

Article

Jan 1980

Dome inverse problems are characterized by a model consisting of a piecewise continuous function and a set of discrete parameters. For linear problems of this general type, which we call mixed, we show that when the number of data d is greater than the number of parameters p, it is always possible to construct a set of a least d - p equations that are independent of the values of the discrete part of the model. These equations, which we call the annulled data set, can be used to estimate the continuous part of the model.The discrete part of the model can be estimated from a second set of p equations that relate the discrete and continuous parts of the model. The linearization of the nonlinear travel time functionals that enter in the hypocenter location problem leads to a mixed inverse problem. The splitting procedure is natural to this problem. The splitting procedure is natural to this problem if the hypocenters are estimated initially by conventional nonlinear least squares by using travel time calculated from some initial estimate of the velocity model. The annulled data are a set of linear combinations of the residuals that are unbiased by that initial location, and as a result, they can be used directly to estimate a peturbation to the velocity model by a Backus-Gilbert procedure. -Authors

The Effect of Small, Aspherical Perturbations on Travel Times and a Re‐examination of the Corrections for Ellipticity

Article

Jan 1976
GEOPHYS J INT

We use the stationarity of the Fermat ray path to develop theoretical expressions that relate small, aspherical perturbations in velocity to small perturbations in travel times. For the Earth's ellipticity of figure we derive a compact expression for the perturbation in travel time. If the ellipticity is hydrostatic and is the dominant perturbation, then anomalies in travel times are linear constraints on the radial gradient of velocity. Otherwise, the anomalies are constraints on the (unknown) aspherical perturbations. We are led to an inverse problem in either case. Using recently derived models of the Earth we present calculations of the effect of ellipticity. If the effect of focal depth is neglected in the calculations, then mislocations in both epicentre and origin time can result. Differences between our calculations and those predicted by the approximate formula δt = (h+H)f(Δ) are as large as 0–25 s for P at 90°. Nowadays, seismologists are prone to attach significance to anomalies as small as 0.10 s. Consequently, we advocate that the effect of focal depth be considered and that traditional approximations be replaced by the more accurate calculations tabulated in this report.

Travel-time inversion for simultaneous earthquake location and velocity structure in laterally varying media

Article

Oct 1980
GEOPHYS J INT

Travel times are observed at a network of seismometers from a number of earthquakes. The seismic velocity of the medium through which the waves pass is allowed to vary in all three dimensions and is represented in terms of a finite number (P) of parameters. The travel-time data is inverted for the velocity parameters and the hypocentre coordinates of the earthquakes. A linearized least squares inversion technique is used and both the stochastic and generalized inverses are considered. The method is computationally efficient and requires inversion (or decomposition) of only P × P and 4 × 4 matrices. The method is applied to a dataset of travel times reported to the I. S. C. from 56 earthquakes recorded at stations on the North Island of New Zealand. The velocity structure is characterized by only five parameters, so that only 5 × 5 matrices need be inverted. The inversion gives a 30 per cent reduction in residuals. The resulting velocity model is substantially different from that obtained when the hypocentres are kept fixed and only the velocity parameters are inverted for. A parallel study of synthetic data gives insight into the interpretation of results. In particular, the depth and origin time of some of the earthquakes, together with two of the five velocity parameters, are very poorly determined.

Preliminary reference Earth model (PREM)

Article

Jun 1981
PHYS EARTH PLANET IN

A large data set consisting of about 1000 normal mode periods, 500 summary travel time observations, 100 normal mode Q values, mass and moment of inertia have been inverted to obtain the radial distribution of elastic properties, Q values and density in the Earth's interior. The data set was supplemented with a special study of 12 years of ISC phase data which yielded an additional 1.75 × 10^6 travel time observations for P and S waves. In order to obtain satisfactory agreement with the entire data set we were required to take into account anelastic dispersion. The introduction of transverse isotropy into the outer 220 km of the mantle was required in order to satisfy the shorter period fundamental toroidal and spheroidal modes. This anisotropy also improved the fit of the larger data set. The horizontal and vertical velocities in the upper mantle differ by 2–4%, both for P and S waves. The mantle below 220 km is not required to be anisotropic. Mantle Rayleigh waves are surprisingly sensitive to compressional velocity in the upper mantle. High S_n velocities, low P_n velocities and a pronounced low-velocity zone are features of most global inversion models that are suppressed when anisotropy is allowed for in the inversion. The Preliminary Reference Earth Model, PREM, and auxiliary tables showing fits to the data are presented.

Determination of the three-dimensional seismic structure of the lithosphere

Article

Jan 1977

A new three-dimensional earth modeling is proposed as a framework to obtain more detailed and accurate information about the earth's interior. We start with a layered medium of classic seismology but divide each layer into many blocks and assign a parameter to each block which describes the velocity fluctuation from the average for the layer. Our data are the teleseismic P travel time residuals observed at an array of seismographs distributed on the surface above the earth's volume we are modeling. By isolating various sources of errors and biases we arrive at a system of equations to determine the model parameters. The solution was obtained by the use of generalized inverse and stochastic inverse methods. Our method also gives a lower limit of the true rms slowness fluctuation in the earth under the assumption of ray theory. Using P wave residual data from the Norwegian Seismic Array (Norsar), we have obtained the map of velocity anomalies at various depths up to a depth of 126 km. The rms slowness fl

Geoid anomalies in a dynamic Earth

Article

Aug 1984

Potential and surface deformation Love numbers for internal loading have been calculated in order to obtain a dynamically consistent relationship between the geoid and the earth's response to internal buoyancy forces. These quantities depend on the depth and harmonic degree of loading, and can be integrated as Green functions to obtain the dynamic response due to an arbitrary distribution of internal density contrasts. Constructing a series of spherically symmetric, self-gravitating flow models for a variety of radial Newtonian viscosity variations and flow configurations, and calculating relaxation times for spherically symmetric viscous earth models, it is demonstrated that boundary deformation due to internal loading reaches its equilibrium value on the same time scale as postglacial rebound; this is much less time than the time scale for significant change in the convective flow pattern.

World-wide distribution of group velocity of mantle Rayleigh waves as determined by spherical harmonic analysis

Article

Sep 1982

We have determined the worldwide distribution of group velocity of mantle Rayleigh waves for periods between 100 and 300 sec without assuming any regionalization. Group slowness 1/u(θ, φ) is expressed by spherical harmonics, and the coefficients, up to angular order 7, have been determined from travel times of Rayleigh waves by a least-squares method. From these, u(θ, φ) has been synthesized. Since we cannot obtain information about the odd terms of the expansion from one circuit measurements around the world, we have used group velocities of mainly R_2 and R_3. The overall pattern of u(θ, φ) for periods between 100 and 200 sec is consistent with results of previous pure-path and regional studies. Group velocities for tectonically active regions are low, and those of the shields and the northwestern Pacific are high.

Global gravity field to degree and order 30 from Geos 3 satellite altimetry and other data

Article

Jan 1981

E. M. Gaposchkin

A model of the geopotential field in spherical harmonics to degree and order 30 is obtained from Geos 3 satellite to sea surface altimetry data, terrestrial gravity measurements and satellite perturbation analysis. A general perturbation solution is employed for the calculation of the orbits of 10 satellites based on satellite laser ranging data, and 1 deg x 1 deg surface gravity data are used to compute 550 km x 550 km block anomalies by means of autocovariance analysis. Altimeter-determined sea-surface heights, which are taken as the geoid, are averaged for each 1 deg x 1 deg ocean surface area and treated by autocovariance analysis to obtain 550 x 550 km block undulations. Observation and normal equations are formed from the altimeter and surface gravity data, which together cover 1635 out of 1654 possible surface elements, and are combined with the available satellite-derived normal equations to obtain a solution for the spherical harmonics coefficients. In addition, a value of 6,378,138.23 + or - 1.3 m is obtained for the earth's semimajor axis.

Upper Mantle Anisotropy - Evidence from Free Oscillations

Article

Feb 1982

Isotropic earth models are unable to provide uniform fits to the gross Earth normal mode data set or, in many cases, to regional Love-and Rayleigh-wave data. Anisotropic inversion provides a good fit to the data and indicates that the upper 200km of the mantle is anisotropic. The nature and magnitude of the required anisotropy, moreover, is similar to that found in body wave studies and in studies of ultramafic samples from the upper mantle. Pronounced upper mantle low-velocity zones are characteristic of models resulting from isotropic inversion of global or regional data sets. Anisotropic models have more nearly constant velocities in the upper mantle. Normal mode partial (Frediét) derivatives are calculated for a transversely isotropic earth model with a radial axis of symmetry. For this type of anisotropy there are five elastic constant. The two shear-type moduli can be determined from the toroidal modes. Spheroidal and Rayleigh modes are sensitive to all five elastic constants but are mainly controlled by the two compressional-type moduli, one of the shear-type moduli and the remaining, mixed-mode, modulus. The lack of sensitivity of Rayleigh waves to compressional wave velocities is a characteristic only of the isotropic case. The partial derivatives of the horizontal and vertical components of the compressional velocity are nearly equal and opposite in the region of the mantle where the shear velocity sensitivity is the greatest. The net compressional wave partial derivative, at depth, is therefore very small for isotropic perturbations. Compressional wave anisotropy, however, has a significant effect on Rayleigh-wave dispersion. Once it has been established that transverse anisotropy is important it is necessary to invert for all five elastic constants. If the azimuthal effect has not been averaged out a more general anisotropy may have to be allowed for.

A tomographic analysis of mantle heterogeneities from body wave travel time data (abstract)

Jan 1983
776

Clayton R. W.

Global gravity field to degree and order 30 from GEOS 3 satellite altimetry and other data

Jan 1979
7221

Gaposhkin

An analysis of the travel times of P waves to North American stations, in the distance range of 30° to 100°

Jan 1966
467

Cleary J.

Romanowicz An exact solution to the problem of excitation of normal modes by a propagating fault Semiannual Technical Summary 88-90Appl

A M B A Dziewonski

Estimation of the radial variation of seismic velocities and density in the earth Pasadena

T H Jordan

P wave travel times from deep earthquakes

Jan 1976
1555

Sengupta M. K.

Estimation of the radial variation of seismic velocities and density in the earth Ph.D. thesis Calif. Inst. of Technol

T H Jordan

Mapping the Lower Mantle: Determination of Lateral Heterogeneity in P Velocity up to Degree and Order 6

Abstract

No full-text available

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