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  • Articles  (34)
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  • 1
    Publication Date: 2010-04-17
    Description: Earth's solid inner core is surrounded by a convecting liquid outer core, creating the geodynamo driving the planet's magnetic field. Seismic studies using compressional body waves suggest hemispherical variation in the anisotropic structure of the inner core, but are poorly constrained because of limited earthquake and receiver distribution. Here, using normal mode splitting function measurements from large earthquakes, based on extended cross-coupling theory, we observe both regional variations and eastern versus western hemispherical anisotropy in the inner core. The similarity of this pattern with Earth's magnetic field suggests freezing-in of crystal alignment during solidification or texturing by Maxwell stress as origins of the anisotropy. These observations limit the amount of inner core super rotation, but would be consistent with oscillation.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Deuss, Arwen -- Irving, Jessica C E -- Woodhouse, John H -- New York, N.Y. -- Science. 2010 May 21;328(5981):1018-20. doi: 10.1126/science.1188596. Epub 2010 Apr 15.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Department of Earth Sciences, University of Cambridge, Cambridge CB3 0EZ, UK. afd28@cam.ac.uk〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/20395476" target="_blank"〉PubMed〈/a〉
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
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  • 2
    Publication Date: 2001-05-12
    Description: Analysis of broadband teleseismic data shows that the 18 June 2000 Wharton Basin earthquake, a moment magnitude 7.8 intraplate event in the region of diffuse deformation separating the Indian and Australian plates, consisted of two subevents that simultaneously ruptured two near-conjugate planes. This mode of rupture accommodates shortening by a mechanism different from that previously known elsewhere in the region. The larger subevent occurred on a fossil fracture zone, with a relatively high stress drop of about 20 megapascals, showing that large stresses can accumulate in regions of distributed deformation.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Robinson, D P -- Henry, C -- Das, S -- Woodhouse, J H -- New York, N.Y. -- Science. 2001 May 11;292(5519):1145-8.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/11349145" target="_blank"〉PubMed〈/a〉
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
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  • 3
    Publication Date: 2006-01-18
    Description: The 660-kilometer discontinuity, which separates Earth's upper and lower mantle, has been detected routinely on a global scale in underside reflections of precursors to SS shear waves. Here, we report observations of this discontinuity in many different regions, using precursors to compressional PP waves. The apparent absence of such precursors in previous studies had posed major problems for models of mantle composition. We find a complicated structure, showing single and double reflections ranging in depth from 640 to 720 kilometers, that requires the existence of multiple phase transitions at the base of the transition zone. The results are consistent with a pyrolite mantle composition.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Deuss, Arwen -- Redfern, Simon A T -- Chambers, Kit -- Woodhouse, John H -- New York, N.Y. -- Science. 2006 Jan 13;311(5758):198-201.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Department of Earth Sciences, University of Cambridge, Cambridge CB3 0EZ, UK. deuss@esc.cam.ac.uk〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/16410518" target="_blank"〉PubMed〈/a〉
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
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  • 4
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    American Association for the Advancement of Science (AAAS)
    Publication Date: 1999-12-03
    Description: A model of three-dimensional shear wave velocity variations in the mantle reveals a tilted low velocity anomaly extending from the core-mantle boundary (CMB) region beneath the southeastern Atlantic Ocean into the upper mantle beneath eastern Africa. This anomaly suggests that Cenozoic flood basalt volcanism in the Afar region and active rifting beneath the East African Rift is linked to an extensive thermal anomaly at the CMB more than 45 degrees away. In contrast, a low velocity anomaly beneath Iceland is confined to the upper mantle.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Ritsema -- van Heijst HJ -- Woodhouse -- New York, N.Y. -- Science. 1999 Dec 3;286(5446):1925-1928.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Seismological Laboratory, California Institute of Technology, Pasadena, CA 91125, USA. Department of Earth Sciences, Oxford University, Oxford OX1 3PR, UK.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/10583949" target="_blank"〉PubMed〈/a〉
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
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  • 5
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    American Association for the Advancement of Science (AAAS)
    Publication Date: 1987-04-03
    Description: The three-dimensional maps of the earth's interior now span regions from the bottom of the crust to the inner core of the earth. Although a wealth of new information on the dynamics of the earth has been discovered, the inner core offers the greatest surprise: it appears to be anisotropic with the axis of symmetry aligned with the axis of rotation.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Dziewonski, A M -- Woodhouse, J H -- New York, N.Y. -- Science. 1987 Apr 3;236(4797):37-48.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/17759204" target="_blank"〉PubMed〈/a〉
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
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  • 6
    Electronic Resource
    Electronic Resource
    [s.l.] : Nature Publishing Group
    Nature 319 (1986), S. 551-555 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] In the Tonga subduction zone, a systematic shear deformation, in which deep material moves relatively to the south, is superimposed on the mode of down-dip shortening which is characteristic of the region. We interpret this observation, together with a tectonic reconstruction, in terms of the ...
    Type of Medium: Electronic Resource
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  • 7
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: We use new and pre-existing seismological data to study the neotectonics and the state of stress in the seismic zones of eastern Papua New Guinea and the western Solomon Sea regions. Earthquake hypocentres for the period 1964–90 were relocated, using the Joint Hypocentre Determination (JHD) technique. We show that the T axes of earthquakes in the 100–250 km depth range beneath eastern Papua New Guinea are aligned parallel to the seismic zone, with no seismicity below ≈250 km. This intermediate-depth seismic zone forms an inverted U-shaped zone, with limbs dipping to the north and south, and is interpreted to be seismicity along the subducted Solomon Sea plate. The northern and southern limbs of the seismic zone are connected by a flattened zone of seismicity which is marked by predominantly strike-slip, and a few normal, fault mechanisms. We suggest that the Solomon Sea plate at depth beneath the Finisterre mountains is no longer influenced by the tectonic forces acting at the surface, but breaking up and sinking under its own gravitational forces.A significant north-dipping seismic zone is imaged above the deeper seismic zone. This feature, characterized by thrust mechanisms, extrapolates to the surface along the Ramu Markham Valley and is attributed to the Ramu Markham Fault (RMF). The RMF has an average dip of about 40° to the north-east but its detailed geometry would appear to be complex. The RMF is interpreted to ramp steeply in the uppermost 10–20 km, flattening out sharply to a dip of ≈15° at a depth of about 20 km. The RMF marks the suture between the Finisterre Terrane and the Australia-New Guinea plate and may extend to a depth of 90 km beneath the western limit of the Finisterre mountains. There is a clear gap in seismicity between the RMF and the deeper Solomon Sea plate seismic zone beneath the Finisterre mountains, which, together with the alignment of the T axes of earthquakes along the seismic zone at depths 〉100 km, indicates that these are indeed two distinct features.The seismicity in the depth range 50–80 km, along the New Britain Arc, follows the trend of the 2000 m bathymetric contour of the New Britain Trench and the Finsch Deep far more closely than it does the trend of the surface plate boundary, indicating that the increase in the distance between the Solomon Sea/South Bismarck plate boundary and the active volcanos along the New Britain Arc, west of (6.8°S, 150.0°E) is only a shallow phenomenon, and does not reflect the geometry of the Solomon Sea plate at depths greater than 50 km.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: To date, most shear-velocity heterogeneity models in the lower mantle have been derived using long-period data. Comparatively little use has been made of the vast International Seismological Centre (ISC) data base of shear-wave arrival times. the aim of this study is to use the ISC P and S arrival times to construct global models of P and S heterogeneity in the lower mantle, and then to compare them in order to investigate whether, within the limitations of the data distributions, they might be proportional. the advantages of constructing both compressional- and shear-wave models from the one data set is that they share similar resolution properties, and hence are most reliable in the same areas.We use data from over 21000 events to derive a data set of P and S summary rays whose residuals we invert jointly along with hypocentral parameters of over 600 summary events. Particular attention is paid to data weighting so that outliers are not given undue influence. Furthermore, summary rays with high internal variances are downweighted.In order to diminish the effect of model parametrization on our conclusion, we derive three sets of P and S models expanded in terms of Legendre polynomials for their depth variation, and spherical harmonics for their lateral dependence. Comprehensive resolution and error analysis is performed.Correlation coefficients between our P and S models are highly significant, averaging approximately 0.7 for our lowest parametrization (245 model coefficients), and 0.5 for our more highly parametrized models. Visual comparisons show strong similarities in areas where resolution is high and error is small.We also conduct an experiment in which we derive compressional- and shear-wave heterogeneity models from data sets which contain P and S arrivals from the same seismograms. These data sets sample the mantle almost identically. the resultant models compare well but correlation drops towards the core-mantle boundary, indicating that there are genuine physical differences in the lowermost mantle.Our models indicate that the ratio of relative S to P heterogeneities is close to 2. This value is based on both our complete and restricted data set models and hence is not highly dependent on data weighting.
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  • 9
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Splitting functions retrieved from spectra of the free oscillations are sensitive to the lateral variations in P velocity (α), S velocity (β), and density (ρ) simultaneously. In this study they are used to constrain the values of the ratios d ln α/d ln β and d ln ρ/d ln α for the lower mantle. Assuming that the upper mantle structure is obtainable from model M84A (this is not a crucial assumption as experiments indicate), the optimal value of d ln α/d ln β inferred from the modal data is 0.44 and d ln α/d ln β lies in the interval (0.39, 0.60) with 75 per cent confidence, strongly discriminating against the value (0.8) often used. The constraints on density structure of the current data are insufficient to yield new definitive results. The analysis demonstrates, however, that the value of d ln ρ/d ln α could be estimated from a larger set of modal data.
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  • 10
    Publication Date: 1985-08-01
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences
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