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  • 1
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    In:  Phys. Earth Plan. Int., Warszawa, Elsevier, vol. 92, no. 12, pp. 39-44, pp. L11309, (ISBN: 0-12-018847-3)
    Publication Date: 1995
    Description: Mode conversions and reflections at upper-mantle seismic discontinuities may be contained in earthquake seismograms as weak secondary phases that often become visible only after special signal processing techniques are applied to the data. To extract fully the information these secondary phases carry about the three-dimensional structure of the Earth, new observational and interpretational methods have to be developed. However, new sources of possible systematic errors may lead to conflicting results. Studies carried out by various research groups on the thickness of the upper-mantle transition zone, the sharpness of upper-mantle discontinuities and the global existence of a 520 km discontinuity are examples where such discrepancies did arise. Although there is a general consensus that the depths to the 410 km and 660 km discontinuities vary by a few tens of kilometres at most, the question of wither the depth variations of the 410 km and 660 km discontinuities are correlated or anticorrelated is still unresolved. Similarly, different data sets and methods yielded different answers on the sharpness of the upper-mantle discontinuities at 410 km and 660 km depth. Finally, data apparently supporting the global existence of a seismic discontinuity at 520 km depth can be equally well explained by models that do not contain this discontinuity.
    Keywords: Seismology ; earth mantle ; Gossler ; PEPI
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  • 2
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    In:  Geophys. Res. Lett., Houston, Akademie-Verlag, vol. 19, no. 1-2, pp. 1563-1566, pp. L08310, (ISSN: 1340-4202)
    Publication Date: 1992
    Keywords: Seismology ; Earth model, also for more shallow analyses ! ; earth mantle ; GRL
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  • 3
    Publication Date: 2019
    Description: Abstract We investigate the upper mantle discontinuities in the Central Mediterranean region by applying the P and S receiver function techniques on waveforms recorded at broadband stations located around the Tyrrhenian basin. P and S‐wave velocity profiles (down to 300km depth) are calculated with joint inversion of P and S receiver functions. We could identify the Moho, Lithosphere‐Asthenosphere Boundary and an underlying low‐velocity layer between ~60 and ~200km depth. The low‐velocity layer is interpreted as asthenospheric material, and its lower boundary is identified below the western Ionian and Tyrrhenian basins as a sharp Lehmann discontinuity. Although the stations are located on different lithospheric domains we find a strong correlation between Moho and the Lithosphere‐Asthenosphere Boundary depths, which suggests ubiquitous coupling of the crust and lithospheric mantle, consistently with the southward opening of the Tyrrhenian basin. The Tyrrhenian and western Ionian basins present thinning of the transition zone of ~14km, as inferred from a reduced P660s‐P410s differential time. Below the southern Apennines we observe a standard differential time that implies an average mantle transition zone thickness. We explain these mantle transition zone thickness variations as due to temperature heterogeneity linked to the area's subduction history. Finally, under central Europe (the location of the deep S‐to‐p conversion points) two strong signals from non‐standard discontinuities within the mantle transition zone are observed. These signals can be explained as being generated at the boundaries of high seismic velocity layers that are spatially correlated with stagnant slabs in the transition zone detected by seismic tomography.
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 4
    Publication Date: 2012-04-17
    Description: P receiver functions from 23 stations of the SASE experiment in southern Africa are inverted simultaneously with SKS waveforms for azimuthal anisotropy in the upper mantle. Our analysis resolves the long-standing issue of depth dependence and origins of anisotropy beneath southern Africa. In the uppermost mantle we observe anisotropy with a nearly E-W fast direction, parallel to the trend of the Limpopo belt. This anisotropy may be frozen since the Archean. At a depth of 160 km the fast direction of anisotropy changes to 40° and becomes close to the recent plate motion direction. This transition is nearly coincident in depth with activation of dominant glide systems in olivine and with a pronounced change in other properties of the upper mantle. Another large change in the fast direction of anisotropy corresponds to the previously found low-S-velocity layer atop the 410-km discontinuity.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 5
    Electronic Resource
    Electronic Resource
    Springer
    Pure and applied geophysics 103 (1973), S. 282-289 
    ISSN: 1420-9136
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Summary Origin ofP waves detected earlier in microseisms of very quiet locations in the USSR is discussed in detail. It appears that the most pronounced sources ofP waves are tropical cyclones over the Pacific. The amplitude of the force in the source for a medium power typhoon is found to be of the order of 1016 dynes. The effective source area is estimated as 104–105 km2 approximately. The shape of the amplitude spectrum ofP wave corrected for the absorption in the mantle does not contradict with the standing wave theory of microseisms generation. Results of observations at various epicentral distances give strong evidences of the predominant attenuation of the fundamental Rayleigh mode as compared with higher Rayleigh modes andP waves in the frequency band of 0.3–0.15 cps.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    [s.l.] : Nature Publishing Group
    Nature 358 (1992), S. 144-147 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Azimuthal anisotropy is a useful indicator of deformation in the mantle because the principal constituent of the mantle, olivine, is highly anisotropic and its lattice-preferred orientation is sensitive to the direction of flow1"6. Many recent studies of azimuthal anisotropy are based on ...
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    [s.l.] : Nature Publishing Group
    Nature 375 (1995), S. 50-52 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Shear-wave splitting in the mantle is related to the lattice preferred orientation of olivine, which depends on finite strain. Axes [100], [010] and [001] in olivine tend to become aligned with the longest, shortest and intermediate axes, respectively, of the strain ...
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    [s.l.] : Nature Publishing Group
    Nature 318 (1985), S. 358-361 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] The method6 was published in Russian, so we will summarize it briefly. The idea is to analyse the transverse component T of converted shear waves like SKS or SmKS. These phases have travelled part of their path as P-waves and are therefore, in a laterally homogeneous and isotropic medium, strictly ...
    Type of Medium: Electronic Resource
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  • 9
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: The delay-and-sum method by Vinnik (1977) to detect weak PS conversions in the P coda is applied to German, Czechoslovakian and Chinese broadband data. Clear phases (a factor 4-5 above the noise level) are observed in the 3-11 s period band from the 670 and 400 km discontinuities after summing several hundred traces. The decisive step to obtain such a good signal-to-noise ratio was the summation of records from stations in different regions. The summation of a large number (many hundreds) of traces of a single station does not result in similar improvements because of coherent signal-generated noise due to scattering below the station. There are weak indications of a 520 km discontinuity in our data. The delay times relative to P of the converted phases are stable within 1 s between central Europe and China.
    Type of Medium: Electronic Resource
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  • 10
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: The paper gives a detailed description of the technique for measuring azimuthal anisotropy which was suggested earlier by Vinnik, Kosarev & Makeyeva (1984) and used by Kind et al. (1985). The technique is based on the observations of long-period converted phases like SKS. Additionally, we describe a generalization of this approach which makes use of long-period S-waves of arbitrary polarization. Both modifications were applied to records of the GRF array in southern Germany. The results of this analysis, if combined with the data on Pn velocities (Bamford 1977), suggest that the direction of the fast velocity in the lithosphere of the region varies with depth.
    Type of Medium: Electronic Resource
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