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  • 1995-1999  (578)
  • 1955-1959  (33)
  • 1935-1939  (96)
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  • 1
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: In order to provide a quantitative analysis of real seismic records from complex regions we need to be able to calculate the wavefields in three dimensions. However. full 3-D modelling of seismic-wave propagation is still computationally intensive. An economical approach to the modelling of seismic-wave propagation which includes many important aspects of the propagation process is to examine the 3-D response of a model where the material parameters vary in two dimensions. Such a configuration, in which a 3-D wavefield is calculated for a 2-D medium, is called the “2.5-D problem”. Recently. Takenaka & Kennett (1996) proposed a 2.5-D time-domain elastodynamic equation for seismic wavefields in models with a 2-D variation in structure but obliquely incident plane waves in the absence of source. This approach is useful even for non-plane waves. In the presence of source a new 2.5-D elastodynamic equation for general anisotropic media can be derived in the time domain based on the Radon transform over slowness in the direction with constant medium properties. The approach can also be formulated in terms of velocity—stress, a representation which is well suited to the use of numerical techniques for 2-D time-domain problems such as velocity—stress finite-difference or velocity—stress pseudospectral techniques.
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  • 2
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: In some studies on glacially induced true polar wander (TPW). the tidal-effective relaxation of the fundamental mantle mode (M0) is lacking. We show that this is caused by the deletion of the Chandler wobble in an early stage of the theory development to facilitate the retrieval of the rotational relaxation modes. We derive an analytical approximation formula for the M0 rotational relaxation mode (including the Chandler wobble), which can be of practical value for TPW simulations with realistically stratified earth models. However, we point out that the contribution of the M0 rotational relaxation mode has, to a high approximation, the same effect on secular TPW as the contribution from an elastic term in models that do not have the M0 rotational relaxation mode. The two model approaches lead to the same polar wander results whenever the Chandler wobble is filtered from models in which the M0 rotational relaxation mode is retained.
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  • 3
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: The buried Chicxulub impact structure is marked by a dramatic ring of sinkholes (called cenotes if containing water), and adjacent less prominent partial rings, which have been shown to coincide with maxima in horizontal gravity gradients and a topographic depression. These observations, along with the discreteness and spacing of the features, suggest a formation mechanism involving faulting in the outer slump zone of the crater, which would thus have a diameter of approximately 180 km.An opposing view, based primarily on the interpretation of gravity data, is that (he crater is much larger than the cenote ring implies. Given the association of the known cenote ring with faults, we here examine northern Yucatan for similar rings in gravity, surface features and elevation, which we might expect to be associated with outer concentric faults in the case of a larger, possibly multiring, structure.No such outer rings have been found, although definite patterns are seen in the distribution of karst features outside the crater rim. We explain these patterns as resulting mainly from deformation related to the block fault zone that parallels tbe shelf edge of eastern Yucatan.
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  • 4
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Geodetic evidence of crustal deformation in the Ionian area detected by GPS surveys is given in this paper. A network consisting of nine geodetic sites crossing the Ionian sea from Calabria (southern Italy) to northwestern Greece was repeatedly surveyed, starting in 1991, within the framework of the TYRGEONET project. The results, obtained from processing data from three GPS campaigns performed on the same network in 1991. 1994 and recently in 1995, show significant changes in the positions of five sites. The deformations in terms of displacements (coordinate differences), are obtained by applying a rigorous statistical approach that analyses their significance by an original iterative procedure based on the classical F (Fisher) test.Although more substantial geophysical conclusions require more observations, the estimated relative displacement pattern is generally coherent with the features of the main tectonic structures identified for this area. The detected deformations for the Greek sites confirm the activity of the Kephalonia right-lateral transform fault, and a right-lateral motion of the Mattinata fault (northern Apulia) seems to have been detected by the Italian sites. Moreover, within the time span analysed, the site of Matera shows a different behaviour from the three Adriatic sites (Tremiti, M.S. Angelo and Specchia Cristi), since relative displacements among Matera and these sites were detected. This fact may indicate the weakness of the assumption, reported in some papers, that Matera could be a representative site of the motion of the whole Adriatic plate. Furthermore, the site of Specchia Cristi shows the maximum relative displacement in the network, with a vector magnitude of about 5 cm with a 3 cm confidence interval at the 95 per cent level. Therefore, even if some additional GPS observations are needed to achieve a clearer picture of the tectonic behaviour of the Ionian Sea area, the detected relative displacement pattern of the Italian sites agrees with a possible rotational behaviour of the Apulian platform with respect to the Adriatic plate, as already hypothesized by Finetti (1982).
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  • 5
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: iThe 5 per cent and 1 per cent significance points are given for a test of randomness of unit vectors in three dimensions. The test has been designed for use in the analysis of palaeomagnetic data.
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  • 6
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: iGravity data are used to investigate the geological structure of an area of about 160000 square miles in the light of the crustal warping hypothesis. The region is in N.W. Pakistan and India, and includes the whole of the alluvial plains of Sind and Punjab, the Salt Range and the Potwar Plateau.A new gravity anomaly is introduced and used by an original method, which leads automatically to the detailed contouring of the basement rock below the region, the basement being assumed to be the upper surface of the Earth's crust. The standard crustal section employed is a two-layer crust with a total thickness of thirty kilometres but a table permits direct comparison with other sections of a six-layer crust, two of which have increased thickness.The basement contours show a ridge about 300 miles long separating the Indus Basin from the Lahore Basin. It is hidden by alluvium except for a few outcrops near its northern end. This ridge has apparently suffered sub-aerial erosion under typical S.W. monsoon conditions, extending in places to a depth of over 3000 feet below sea level. This modification of the basement requires a revision of the contours over the ridge, and two contoured charts show firstly the simple crustal upwarp underlying the ridge and secondly the eroded surface of the basement. A deep valley with its bottom far below sea level cuts through the ridge connecting the Indus and Lahore Basins. In Sind a similar valley leads from the direction of the sea to the Indus Basin, but here interpretation is uncertain.It is concluded that the hypothesis yields results giving depths to the basement of the right order in deeply downwarped areas, but in upwarped areas the possibility of erosion, or other concealing factors, leads to uncertainty of interpretation unless the area is wide enough to include a complete section of the upwarp.
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  • 7
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: iTerrestrial heat flow has been measured in three Alpine railroad tunnels. The geothermal gradients were calculated from temperatures measured during the construction of the tunnels, and corrections for topographic irregularities were made. The thermal conductivity of 113 rock specimens from the vicinity of the tunnels was measured. The heat flow in the Gotthard tunnel was found to be 1.6 10-6 cal/cm2 sec, in the Simplon 2.2 10-6 cal/cm2 sec, and in the Loetschberg 1.9 10-6 cal/cm2 sec. Most of the flux at the surface can be attributed to radioactive decay in a thickened crust, but a non-uniform distribution of radioactive elements may be required to explain the relatively high heat flow in the Simplon and Loetschberg tunnels.
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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: iA detailed study has been made of the remanent magnetization of five Pilansberg dykes, by means of measurements on oriented specimens of the rocks. Outcrops of the dykes prove to have suffered magnetic disturbance of their original thermo-remanent magnetization. Specimens from depths of a few thousand feet, taken in Witwatersrand gold mines, show highly consistent magnetisation of the basic parts of the dykes over considerable distances between sampling sites. The mean directions given by the five dykes agree well, and the mean direction from the five dykes gives a North-seeking magnetic pole with inclination +69.3 and azimuth N 24° E. Assuming thermo-remanent magnetisation by a geocentric dipole field, this places a North magnetic pole in latitude 71/2° N, longitude 421/ E at the time of intrusion of the dykes. The age of the dykes is uncertain, but is probably about 300 to 400 million years. Some of the specimens have been subjected to alternating magnetic fields in order to test the stability of their magnetisations. The basic specimens are found to be highly stable, being only slightly affected by fields of 100 to 300 oersteds. The possible implications of the results are discussed, in terms of hypotheses of polar wandering and continental drift.
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  • 9
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Deformation of peridotite caused by mantle flow beneath an oceanic spreading centre can result in the development of seismic anisotropy. Traveltime anomalies and shearwave splitting will develop as seismic energy propagates through such an anisotropic region, thus providing a signature of the deformation field at depth. In this study we investigate the nature of deformation associated with mantle upwelling for two models of flow in the upper 100 km of the mantle. The finite-strain fields of the passive upwelling model versus the buoyancy-enhanced upwelling model are quite different. This suggests that mineral aggregates deform differently in the two models, thus developing seismic signatures that are distinguishable. Numerical estimates of the corresponding mineral textures are made using polycrystal theory for olivine with four operative slip systems. The activation of a slip system is determined for each grain on the basis of the local critical resolved shear stress. The computed grain deformation reflects a balance between stress equilibrium, for the aggregate as a whole, and strain continuity between neighbouring grains within the aggregate. This approach enables a direct link to be made between the model flow fields and the resulting texture development. Given these mineral orientation distributions, elastic parameters are calculated and wavefronts are propagated through the anisotropic structure. Traveltimes for teleseismic body waves are computed using ray theory, and amplitudes are estimated for an across-axis profile extending 100 km from the ridge axis. Relative P-wave residuals of up to 1 s are predicted for the buoyant model with on-axis arrivals being earliest, since near-vertical velocities are fastest beneath the axis. On-axis P-wave arrivals for the passive model are half a second earlier than arrivals 60 km off-axis, and relative delays continue to increase slowly as distance from the ridge increases. S-wave splitting of almost a second is predicted for the buoyant model, whereas less than a half-second of splitting is determined for the passive model.
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  • 10
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: The free (420d) nutation is known to be heavily damped, the characteristic damping time being less than ten periods. The origin of this damping is unknown, but must be due to dissipative, non-rigid-body movements of the Earth. The view, which has been widely held, that this may arise from the relative motion between the liquid core of the Earth and the mantle is shown to be based on an error. When account is taken of its small moment of inertia, the core cannot be held responsible for this damping, nor can a limit be derived for its viscosity. This confines the origin of the damping to a non-elastic behaviour of the mantle.
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