Thesis (Ph.D.)--Boston University / PLEASE NOTE: Boston University Libraries did not receive an Authorization To Manage form for this thesis or dissertation. It is therefore not openly accessible, though it may be available by request. If you are the author or principal advisor of this work and would like to request open access for it, please contact us at open-help@bu.edu. Thank you. / The Earth's magnetic cusps are the regions with the most direct transfer of energy, mass, and momentum from the flowing solar wind to the Earth's magnetosphere. Spacecraft observations in the cusp have revealed a high energy component to the thermal particle distribution. This has raised the question as to whether significant plasma heating may also be occurring in this region. Since the cusp is magnetically connected to a number of other regions in geospace, plasma heating in this region could be a significant contributor to magnetospheric dynamics. The goal of this thesis is to answer the question, what is the source of the energetic particle population in the cusp? Since the initial observations measuring the energetic component were made, the source of the energetic population has been open to conjecture. A number of sources have been proposed: (1) the terrestrial bow shock, (2) the Earth's high-latitude trapping region, and (3) heating of plasma locally in the cusp. Depending on which source is the dominant provider of the energetic particles, the particle population will exhibit different properties. Particle flow direction, intensity, spectral characteristics, and species/charge state are all properties that can change depending on the dominant source. In-situ measurements by the ISEE, Polar, and Cluster spacecraft are used to derive the particle properties. These properties are compared with predictions for each of the proposed sources to determine which is most consistent with the observations. Case studies show that, under different conditions, the high-latitude trapping region and local heating can both be the dominant source of the energetic particle population up to energies of hundreds of keV. Results from a large scale statistical study, however, are more consistent with local heating indicating that this is the dominant source the majority of the time. / 2031-01-01

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A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfillment of the requirements for the degree of Doctor of Philosophy. October 2017. / Since the arc assembly and continental collision of the East African Orogen some 640 million years ago, Madagascar has gone through several geodynamic and tectonic episodes that have formed and subsequently modified its lithosphere. This thesis aims to investigate the structure of the crust, the uppermost mantle, and the mantle transition zone beneath Madagascar to gain insights into the relationship between present-day lithosphere structure and tectonic evolution, and to evaluate candidate models for the origin of the Cenozoic intraplate volcanism. To address these issues, local, regional, and teleseismic events recorded by several temporary seismic networks; the MAdagascar-COmoros-MOzambique (MACOMO), the SEismological signatures in the Lithosphere/Asthenosphere system of SOuthern MAdagascar (SELASOMA), and the Réunion Hotspot and Upper Mantle – Réunions Unterer Mantel (RHUM-RUM) were used to complement the seismic events recorded by the permanent seismic stations in Madagascar. The different methods used and the primary results of this study are explained in each section of this thesis. In the first part of this thesis, crustal and uppermost mantle structure beneath Madagascar was studied by analyzing receiver functions using an H-κ stacking technique and a joint inversion with Rayleigh-wave phase-velocity measurements. Results reflect the eastward and northward progressive development of the western sedimentary basins of Madagascar. The thickness of the Malagasy crust ranges between 18 km and 46 km. The thinnest crust (18-36 km thick) is located beneath the western basins and it is due to the Mesozoic rifting of Madagascar from eastern Africa. The slight thinning of the crust (31-36 km thick) along the east coast may have been caused by crustal uplift and erosion when Madagascar moved over the Marion hotspot and India broke away from it. The parameters describing the crustal structure of Archean and Proterozoic terranes, including thickness, Poisson’s ratio, average shear-wave velocity, thickness of mafic lower crust, show little evidence of secular variation. Slow shear-wave velocity of the uppermost mantle (4.2-4.3 km/s) are observed beneath the northern tip, central part and southwestern region of the island, which encompass major Cenozoic volcanic provinces in Madagascar. The second part of the thesis describes a seismic tomography study that determines the lateral variation of Pn-wave velocity and anisotropy within the uppermost mantle beneath Madagascar. Results show an average uppermost mantle Pn-velocity of 8.1 km/s. However, zones of relatively low-Pn-velocity (~7.9 km/s) are found beneath the Cenozoic volcanic provinces in the northern, central, and southwestern region of the island. These low-Pn-velocity zones are attributed to thermal anomalies that are associated with upwelling of hot mantle materials that gave rise to the Cenozoic volcanism. The direction of Pn anisotropy shows a dominant NW-SE direction of fast-polarization in the northern region and around the Ranostara shear zone, in the south-central Madagascar. The anisotropy in the uppermost mantle beneath these regions aligns with the existing geological framework, e.g. volcanic complex and shear zones, and can be attributed to a fossil anisotropy. The Pn anisotropy in the southwestern region, around the Morondava basin, is E-W to NE-SW-oriented. It can be attributed either to the mantle flow from plate motion, the African superplume, or the Mesozoic rifting from Africa. Results from this study do not show any substantial evidence of the formation of a diffuse boundary of the Lwandle plate, cutting through the central region of Madagascar. Station static delays reflect the significant variation in the Moho depth beneath the island. In the third part of the thesis, the thickness of the mantle transition zone beneath Madagascar, which is sensitive to the surrounding temperature variation, has been estimated by stacking receiver functions. Single-station and common-conversionpoint stacking procedures show no detectable thinning of the mantle transition zone and thus no evidence for a thermal anomaly in the mantle under Madagascar that extends as deep as the mantle transition zone. Therefore, this study supports an upper mantle origin for the Cenozoic volcanism. However, the resolution of the study is not sufficient to rule out the presence of a narrow thermal anomaly as might arise from a plume tail. Overall, the findings in this research are broadly consistent with the crustal and upper mantle structure of Madagascar determined by previous studies, but provides significantly greater detail with regard to the crustal and uppermost mantle structure as more seismic stations were used. / LG2018

I have developed a waveform inversion procedure to invert 3 component broadband seismic data for models of the anisotropic seismic structure of the Earth and applied the technique to an investigation of wave propagation through anisotropic media and earthquake data sampling the upper mantle beneath the East European platform. The procedure combines the conjugate-gradient and very fast simulated annealing methods and attempts to minimize a cross-correlation misfit function comparing data to synthetic seismograms. A series of inversion passes are performed over a range of frequency and time windows to progressively focus in on structural details. The intent is to obtain P and S velocity models that simultaneously match all components of the data (radial, vertical and tangential). The variables in the problem are the seismic velocities ([alpha] and [beta]) as a function of depth. When radial anisotropy is required this set is expanded to include the five variables that determine the seismic velocities in a radially anisotropic medium ([alpha subscript h, alpha subscript v, beta subscript h, beta subscript v, eta]). I investigate the propagation of seismic waves through radially anisotropic media, evaluate which elements of radial anisotropy are best resolved by seismic data and discuss strategies for identifying radial anisotropy in the Earth. S anisotropy, [beta]%, and the horizontal component of P velocity, [alpha subscript h], are typically well resolved by multicomponent seismic data. P anisotropy, [alpha]%, and [eta] are often poorly resolved and trade off with one another in terms of their effect on S[subscript V] arrivals. Erroneous structure will be mapped into models if anisotropy is neglected. The size of the erroneous structure will be proportional to the magnitude of anisotropy present and extend well below the anisotropic zone. The effects of anisotropy on P models produced with an isotropic assumption are most similar to the effects on isotropic S[subscript H] models. When comparing isotropic models, [alpha/beta subscript sh] is therefore often a better measure than [alpha/beta subscript sv] for characterizing mantle petrology. Isotropic S[subscript H], S[subscript V] and P models developed separately using the same data set can provide a good initial estimate of the presence, location and magnitude of anisotropy and those results can be used to create an initial model for an anisotropic inversion solving simultaneously for all 3 components of the data. Finally, I present models for the P and S velocity structure of the upper mantle beneath the East European platform including an analysis of radial anisotropy. The data are 3-component broadband seismograms from strike-slip earthquakes located near the edge of the platform and recorded in Russia and Europe. The timing, amplitude and interference characteristics of direct arrivals (S, P), multiply reflected arrivals (SS, PP), converted phases and surface waves provide very good radial resolution throughout the upper 400 km of the mantle. The platform is underlain by a radially anisotropic seismic mantle lid extending to a depth of 200 km with a largely isotropic mantle below. The model has a positive velocity gradient from 41 km to 100 km depth, and a relatively uniform velocity structure from 100 km to 200 km depth with high S[subscript H] and P[subscript H] velocities (4.77 km /s, 8.45 km/s). Shear anisotropy is uniform at 5% ([beta subscript H] > [beta subscript V]) from 41 to 200 km depth, drops to 2% from 200 to 250 km and is isotropic below that. The average shear velocity from 100 to 250 km is also uniform at 4.65 km/s and the drop in anisotropy is matched by a drop in [beta subscript H] to 4.70 km/s combined with an increase in [beta subscript V] to 4.60 km/s. Below 250 km there is a positive velocity gradient in both P and S velocity down to 410 km. P anisotropy is not well resolved, but P structure mimics the S[subscript H] velocity structure, suggesting that P is also anisotropic within the lid. / text

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