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  • 1995-1999  (578)
  • 1940-1944  (12)
  • 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
    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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  • 6
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Electromagnetic inverse problems can be formulated using exact non-linear integral equations similar to the traditional linearized equations. It has been suggested by other workers that, while these exact equations are correct, they are not an appropriate basis for general inversion algorithms, in particular for magnetotelluric soundings. In this paper we present a genera! inversion algorithm based on the exact equation for magnetotelluric soundings. The integral equation is derived directly from the 1-D electromagnetic induction equation using elementary operations, and it is solved numerically by iterations using linear programming techniques. At each iteration the equation is approximated by a linear relationship similar to that of traditional linearization, except that the data and the conductivity profile are not referred to as perturbations, although a reference model is still needed for computing derivatives. It is argued that the method is a natural extension of the Niblett-Bostick transformation for magnetotelluric soundings. The performance of the method is illustrated using numerical experiments and applications to field data.
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  • 7
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: A striking feature of Russian long-range seismic refraction data from Peaceful Nuclear Explosions is the observation of a high-frequency teleseismic Pn, phase, which travels with a group velocity of 8.0 km s-1 out to distances of several thousands of kilometres. Modelling using the reflectivity method shows that this phase can be understood as the response of an upper mantle that contains random RMS velocity fluctuations of about ±4 per cent superimposed on a positive velocity gradient. This class of model explains the existence of the teleseismic Pn, its high-frequency content and its coda length. A teleseismic Pn can only be generated if velocity flucluations are strong enough to cause multiple scattering and occur on a subwavelength scale. Cross-correlation properties of P- and S-wave velocity fluctuations exert a substantial influence on the wavefield. A completely unexpected phase can be observed if the fluctuations are imposed on a negative gradient
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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: We present a uniformly valid ray theory for body-wave propagation in laterally heterogeneous earth models. This is accomplished by implementing Maslov theory, which is a 3-D analogue of the widely used WKBJ seismogram method for spherically symmetric earth models. Away from caustics, complete seismic waveforms can be calculated by solving a system of 14 coupled first-order ordinary differential equations: four equations determine the ray geometry, eight additional equations determine the amplitude, and two further equations determine traveltime and attenuation. In the vicinity of a caustic, neighbouring rays cross, and asymptotic ray theory breaks down. Rather than considering the contribution to the wavefield of one single ray, our strategy is to express the wavefield in the vicinity of a caustic as a summation over neighbouring, non-Fermat rays based upon Maslov theory. Away from caustics, Maslov theory reduces to asymptotic ray theory. We present examples of the ray geometry in the 3-D model SKS12WM13, and demonstrate that small-scale triplications in the traveltime curve associated with large-scale heterogeneities in the lowermost mantle are ubiquitous. The theory is applicable to direct, turning and reflected waves, may to a limited extent be advanced to include head waves, but does not describe waves that are diffracted into the deep shadow.The determination of the geometric ray that connects a given source and receiver is based upon a ‘shooting’ method. Initial guesses for the take-off angles are determined based upon perturbation theory, which substantially reduces the number of iterations required to hit a receiver. Perturbation theory also provides predictions for arrival angles and amplitude anomalies. These predictions incorporate the effects of long-wavelength topography on internal boundaries and the free surface, and may be used as a basis for tomographic inversions. Generally, predictions based upon perturbation theory agree very well with exact 3-D ray tracing. Just as traveltime measurements provide constraints on velocity, arrival angles constrain velocity gradients, and amplitude anomalies put constraints on second derivatives in velocity. In SKS12WM13, traveltime anomalies can be as much as ±10 s, arrival-angle anomalies can be larger than ±5°, and 3-D amplitudes can differ by more than 100 per cent from PREM amplitudes.
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  • 9
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Magnetobiostratigraphic data are presented from three Early/Middle Triassic Han-Bulog Limestone successions from Kçira, northern Albania. A total of 206 standard palaeomagnetic samples were obtained for thermal demagnetization and statistical analysis from the 42, 10 and 5 m thick sections. The reversal-bearing characteristic component, carried by haematite and magnetite, defines a composite sequence of six main polarity intervals (Kçln to Kç3r) in which are embedded four short polarity intervals, one at the base of Kçln and three towards the top of Kçlr. The early acquisition of the characteristic remanence is supported by the lateral correlation of magnetozones between sections. The Early/Middle Triassic boundary, approximated by the first occurrence of the conodont Chiosella timorensis, falls close to the Kçlr/Kç2n polarity transition. This is in good agreement with recently published magnetobiostratigraphic data from the coeval Chios (Greece) sections. The palaeomagnetic pole calculated from the Kçira characteristic directions lies close to the Triassic portion of the apparent polar wander path for Laurussia (in European coordinates). However, a 40-45° clockwise rotation of the external zone of the Albano-Hellenic Belt to the south of the Scutari-Pec Line is thought to have occurred since the Early-Middle Miocene. The Kçira pole acquires a West Gondwana affinity when restored for the Neogene clockwise rotation. If the clockwise rotation was entirely related to Neogene tectonics, the Kçira area was evidently associated with West Gondwana and located at 12-16°N of the western Tethys margin.
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  • 10
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: We determine the isotropic component of large deep earthquakes by inversion for their full six-component moment tensors using the normal-mode data. We show that it is possible to reduce the correlation between the isotropic component and other components and to determine the isotropic component independently by analyses of normal-mode data at periods longer than 500 s. We find no significant isotropic component for the earthquakes we studied; the magnitudes of the estimated isotropic components are comparable to the uncertainty due to the mislocation of the centroid. The magnitude of the isotropic component is at most 5 per cent of the deviatoric seismic moment if it exists. This suggests that, if a rapid phase change of mantle minerals occurs during the rupture of deep earthquakes, the mantle material that transformed would be confined within an extremely thin layer.We also conduct moment tensor inversion using body waves and surface waves in the period band between 20 and 500 s. We find that there is a relatively consistent non-double-couple component of the deviatoric moment tensor for the Japan Sea earthquake (1994 July 21). This is likely to be caused by source complexity
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