7,355 research outputs found
Radial profiles of seismic attenuation in the upper mantle based on physical models
No full textThermally activated, viscoelastic relaxation of the Earth's materials is responsible for intrinsic attenuation of seismic waves. Seismic observations have been used to define layered radially symmetric attenuation models, independent of any constraints on temperature and composition. Here, we interpret free-oscillation and surface wave attenuation measurements in terms of physical structures, by using the available knowledge on the physical mechanisms that govern attenuation at upper-mantle (<400 km) conditions. We find that observations can be explained by relatively simple thermal and grain-size structures. The 1-D attenuation models obtained do not have any sharp gradients below 100km, but fit the data equally well as the seismic models. The sharp gradients which characterize these models are therefore not required by the data. In spite of the large sensitivity of seismic observations to temperature, a definitive interpretation is limited by the unknown effects of pressure on anelasticity. Frequency dependence of anelasticity, as well as trade-offs with deeper attenuation structure and dependence on the elastic background model, are less important. Effects of water and dislocations can play an important role as well and further complicate the interpretation. Independent constraints on temperature and grain size expected around 100km depth, help to constrain better the thermal and grain-size profiles at greater depth. For example, starting from a temperature of 1550 K at 100 km and assuming that the seismic attenuation is governed by the Faul & Jackson's (2005) mechanism, we found that negative thermal gradients associated with several cm grain sizes (assuming low activation volume) or an adiabatic gradient associated with ∼1 cm grain size, can explain the data. A full waveform analysis, combining the effects on phase and amplitude of, respectively, elasticity and anelasticity, holds promise for further improving our knowledge on the average composition and thermal structure of the upper mantle. © 2008 The Authors Journal compilation © 2008 RAS
Insights into the nature of the transition zone from physically constrained inversion of long-period seismic data
No full textImposing a thermal and compositional significance to the outcome of the inversion of seismic data facilitates their interpretation. Using long-period seismic waveforms and an inversion approach that includes constraints from mineral physics, we find that lateral variations of temperature can explain a large part of the data in the upper mantle. The additional compositional signature of cratons emerges in the global model as well. Above 300 km, we obtain seismic geotherms that span the range of expected temperatures in various tectonic regions. Absolute velocities and gradients with depth are well constrained by the seismic data throughout the upper mantle, except near discontinuities. The seismic data are consistent with a slower transition zone and an overall faster shallow upper mantle, which is not compatible with a homogenous dry pyrolite composition. A gradual enrichment with depth in a garnet-rich component helps to reduce the observed discrepancies. A hydrated transition zone would help to lower the velocities in the transition zone, but it does not explain the seismic structure above it. © 2007 by The National Academy of Sciences of the USA
On the computation of long period seismograms in a 3-D earth using normal mode based approximations
No full textPhysique de l’intérieur de la Terre
No full textSéismes, tsunamis, éruptions volcaniques : l’ampleur de certaines catastrophes naturelles récentes et leurs conséquences dramatiques nous ont rappelé la puissance des phénomènes terrestres et la nécessité de la recherche scientifique pour mieux comprendre la dynamique du Globe. L’étude de l’intérieur de la Terre connaît depuis vingt ans une véritable révolution, notamment grâce au développement de techniques toujours plus performantes de tomographie sismique et à la puissance de calcul rendue possible par l’informatique. Barbara Romanowicz nous présente les progrès et les défis actuels de la sismologie globale.Earthquakes, tsunamis, volcanic eruptions: the extent of certain recent natural disasters and their dramatic consequences have reminded us of the power of terrestrial phenomena and of the need for scientific research to understand the Earth’s dynamics more fully. Over the last twenty years the study of the Earth’s interior has witnessed a real revolution, owing in particular to the development of increasingly sophisticated seismic tomography techniques and the powerful computations made possible by progress in numerical methods and computer technology. Barbara Romanowicz presents us with the current progress and challenges of global seismology
Physics of the Earth’s Interior
No full textEarthquakes, tsunamis, volcanic eruptions: the extent of certain recent natural disasters and their dramatic consequences have reminded us of the power of terrestrial phenomena and of the need for scientific research to understand the Earth’s dynamics more fully. Over the last twenty years the study of the Earth’s interior has witnessed a real revolution, owing in particular to the development of increasingly sophisticated seismic tomography techniques and the powerful computations made possible by progress in numerical methods and computer technology. Barbara Romanowicz presents us with the current progress and challenges of global seismology
'A date with Barbara': paracosms of the self in biographies of Barbara Newhall Follett
No full textIn 1927, 13-year-old Barbara Newhall Follett published her first book, the critically acclaimed novel, The House Without Windows and Eepersip's Life There.
Twelve years later, on December 7, 1939, 25-year-old Barbara quarrelled with her husband and left her apartment in Boston with $30 in her pocket, and a notebook. She was never seen again.
The House Without Windows is set in a paracosm (Farksolia) she invented, and ends with the metamorphosis of the titular character into a 'fairy-a wood nymph … invisible for ever to all mortals, save those few who have minds to believe, eyes to see'.
In Barbara's (auto)biography, The Unconscious Autobiography of a Child Genius (1966), written by Harold Grier McCurdy 'in collaboration with Helen Follett' (Barbara's mother), the authors wonder: 'Can we be far wrong in substituting Barbara's name for Eepersip's in the closing scenes of [House Without Windows]?
In this paper, I grapple with the formal and ethical challenges of writing about Barbara Newhall Follett, and the ways her family and others have approached the problem of writing her unresolved life story: a child raised and educated in solitude, a celebrated 'natural' child author, a young woman whose disappearance remains unsolved. The paper will explore the ways in which adults write the stories of children's lives, as nostalgia and fable, as fairytale and paracosmic narrative, and the ways in which Barbara's biographers have, consciously and unconsciously, created biographical concordances, or paracosms of the self, in seeking to make meaning of her life's story
Barbara James
No full textDate:1943Barbara was born in Holdredge, Nebraska in the United States of America in 1943. In 1960 she arrived in Darwin working in a variety of occupations such as a journalist, historian, author, activist, advocate and editor. Barbara wrote 13 books including "No Man's Land" which explored the contributions of women in the Northern Territory. She also received a number of awards including 2001 NT Heritage Award, the 2000 NT Literary Essay Awards and the Chief Minister's Women's Achievement Award in 1999.JournalistHistorianAuthorActivistEditorAmerica
Designer: from author to creative commons
No full textThe essay explores the transformations that have occurred in the role of design, from authorship to networking, sharing and opensource modes
Modeling seismic wave propagation through the Earth for imaging localized structures
No full textInternational audienceDeveloping fast and accurate methods for modeling seismic wave propagation is important to image the Earth's interior and thus better understand its internal structure and its history. The most recent global tomographic models (e.g. French and Romanowicz, 2015, Bozdag et al., 2016) have been obtained by modeling wave propagation using the spectral element method and exhibit key structures for our understanding of mantle dynamics, for example, vertically oriented broad low velocity plume conduits extend throughout the lower mantle beneath major hotspots. Improving the resolution of our global models, to further investigate key structures, is difficult because it is limited by the computational power currently available. To overcome this limitation, a novel imaging method called box-tomography have been introduced (Masson and Romanowicz, 2017), it allows to efficiently image localized structures of interest buried at arbitrary depth inside the Earth. In box-tomography wave propagation modeling, for the most part, is carried out in the vicinity of the structure of interest using a local solver. The wavefield recorded at the surface of the local modeling domain is then convolved with pre-computed Green's functions to obtain synthetic seismograms at arbitrary position at the surface of the Earth (Masson and Romanowicz, 2016). Clouzet et al. (2108) successfully implemented box-tomography using a regional spectral element solver (Cupillard et al., 2012) and obtained a tomographic model of North America. In this work, we investigate numerical schemes alternative to the spectral element method for modeling wave propagation locally in the box-tomography context. We aim at constructing a numerical scheme which combines the efficiency and the relative simplicity of the finite difference method together with an accuracy that compares to that of the finite/spectral element method when introducing sharp interfaces and complex geometries
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