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  • 1995-1999  (275)
  • 1935-1939
  • 1995  (275)
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  • 1995-1999  (275)
  • 1935-1939
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  • 1
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: We evaluate the common practice of utilizing elastic models of outer-rise flexure to constrain lithospheric parameters, such as mechanical thickness and ambient in-plane force. We numerically compute ‘synthetic’ flexural profiles consistent with empirically determined constraints on lithospheric rheology and representative trench-type boundary conditions, and misfit minimization is utilized to determine the analytical solution to elastic-plate flexure that most closely resembles each synthetic profile. We then determine if it is possible to use the best-fitting elastic solutions to recover the lithospheric mechanical thickness and level of in-plane force that were assumed during the numerical computation of the synthetic profiles. This methodology is analogous to the common practice of estimating lithospheric parameters by modelling bathymetric profiles with analytical descriptions of elastic-plate flexure. Our results unequivocally indicate that in-plane force cannot be reliably constrained in this manner. Such an approach does not even allow the qualitative nature of in-plane force to be distinguished (i.e. compressional versus tensional). Although, in principle, elastic-plate models may provide reliable constraints on the mechanical thickness of oceanic lithosphere, in practice uncertainties associated with bathymetric noise and the level of in-plane force may, in some instances, render such constraints unreliable.
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  • 2
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: An accurate and efficient 3-D finite-difference forward algorithm for DC resistivity modelling is developed. The governing differential equations of the resistivity problem are discretized using central finite differences that are derived by a second-order Taylor series expansion. Electrical conductivity values may be arbitrarily distributed within the half-space. Conductivities at the grid points are calculated by a volume-weighted arithmetic average from conductivities assigned to grid cells. Variable grid spacing is incorporated. The algorithm does not limit the number and configuration of the sources, although all illustrative examples are computed using two current electrodes at the surface.In general, the linear set of equations resulting from this kind of discretization is non-symmetric and requires generalized numerical equation solvers. However, after symmetrizing the matrix equations, the ordinary conjugate gradient method becomes applicable. It takes advantage of the matrix symmetry and, thus, is superior to the generalized methods. An efficient SSOR-preconditioner (SSOR symmetric successive overrelaxation) provides fast convergence by decreasing the spectral condition number of the matrix without using additional memory. Furthermore, a compact storage scheme reduces memory requirements and accelerates mathematical matrix operations.The performance of five different equation solvers is investigated in terms of cpu time. The preconditioned conjugate gradient method (CGPC) is shown to be the most efficient matrix solver and is able to solve large equation systems in moderate times (approximately 21/2 minutes on a DEC alpha workstation for a grid with 50 000 nodes, and 48 minutes for 200000 nodes). The importance of the tolerance value in the stopping criterion for the iteration process is pointed out. In order to investigate the accuracy, the numerical results are compared with analytical or other solutions for three different model classes, yielding maximum deviations of 3.5 per cent or much less for most of the computed values of the apparent resistivity.In conclusion, the presented algorithm provides a powerful and flexible tool for practical application in resistivity modelling.
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  • 3
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Results from archaeomagnetic research on a Roman pottery kiln are presented. In a previous paper (Pares et al. 1992), a collection of samples from the kiln's heating chamber was studied and palaeodirection results were obtained. In the same paper, dating was performed on the basis of the secular variation reference curve of the inclination at Paris. In the present study, the palaeointensity was evaluated on the basis of the same collection of samples. The mean value obtained is 60.9 μT. A comparison of this value with other available archaeointensity values, obtained from neighbouring territories, confirms the date estimated by Pares et al. (1992k—the middle of the 1st Century AD.
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  • 4
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: An automatic analyser which characterizes, as well as detects, seismic phases can be a significant help in the interpretation of seismograms. Such an analyser has been constructed using concepts from work in artificial intelligence. A seismogram is recognized as a hierarchical structure of subunits and the features characterizing a phase are extracted via a structural analysis. The automatic analyser detects the onsets of phases, separates the relevant waveform segments, fits phase structures to standard wavelets, and generates a set of five parameters (‘a quintuple’) to characterize a detected phase. The whole analysis system requires just a few input parameters and can be accomplished swiftly enough to allow continuous real-time operation.The analysis procedure is adaptive, including updating of the estimate of the local frequency so that it can be applied to broad-band records with a wide range of frequency content. By coupling the analysis procedure with real-time filtering, very good results for phase detection and characterization can be achieved for weak distant events. The characterization of phase segments also provides useful information for the comparison of seismograms.
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  • 5
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: In all media in which the elastic stiffness tensor and density vary in space, an implicit, frequency-dependent anisotropy is present, even if the medium is locally isotropic. This predicts a non-linear, frequency-dependent polarization of body waves, and explains shear-wave splitting in heterogeneous media. Examples are presented for compressional and shear waves in a medium in which the elastic properties vary exponentially; this specific form of heterogeneity allows solutions in terms of plane waves.
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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
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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: The location of seismic events can be improved if accurate picks can be assigned for later seismic phases, which requires both the detection of an arrival and the recognition of its character. Such phase identifications are particularly valuable if they can be provided in real-time as the seismic disturbance passes across a broad-band seismic recording station.A simple but promising scheme for characterizing arrivals can be constructed by analysing the energy content of the seismic trace as a function of time. Such an approach can be used to detect arrivals by using a method comparing the short-term average energy to a long-term average, with averaging windows that are adaptive to the local frequency of the seismic disturbance. The phase detector can be tuned to different classes of arrivals by utilizing three-component records. By comparing the energy on the vertical component of motion to that in the horizontal plane, it is possible to start to separate P and S arrivals. Phase assignments can be refined by the use of adaptive filtering and by including polarization information.With an estimate of the azimuth of propagation it is possible to use approximate projection methods which attempt to compensate for the influence of the free surface, since the surface corrections are not a strong function of slowness for teleseismic arrivals. By this means, an instantaneous estimate can be made of the relative contributions of P, SV and SH arrivals which can be very helpful in determining the phase assignment for a particular arrival.
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  • 8
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Mantle-wide heterogeneity is largely controlled by deeply penetrating thermal convective currents. These thermal currents are likely to produce significant lateral variation in rheology, and this can profoundly influence overall material behaviour. How thermally related lateral viscosity variations impact models of glacio-isostatic and tidal deformation is largely unknown. An important step towards model improvement is to quantify, or bound, the actual viscosity variations that characterize the mantle. Simple scaling of viscosity to shear-wave velocity fluctuations yields map-views of long-wavelength viscosity variation. These give a general quantitative description and aid in estimating the depth dependence of rheological heterogeneity throughout the mantle. The upper mantle is probably characterized by two to four orders of magnitude variation (peak-to-peak). Discrepant time-scales for rebounding Holocene shorelines of Hudson Bay and southern Iceland are consistent with this characterization. Results are given in terms of a local average viscosity ratio, 〈inlineGraphic alt="inline image" href="urn:x-wiley:0956540X:GJI305:GJI_305_mu1" location="equation/GJI_305_mu1.gif"/〉, of volumetric concentration, φi. For the upper mantle deeper than 340 km the following reasonable limits are estimated for 〈inlineGraphic alt="inline image" href="urn:x-wiley:0956540X:GJI305:GJI_305_mu2" location="equation/GJI_305_mu2.gif"/〉. A spectrum of ratios 〈inlineGraphic alt="inline image" href="urn:x-wiley:0956540X:GJI305:GJI_305_mu3" location="equation/GJI_305_mu3.gif"/〉 at concentration level φi≈ 10−6−10−1 in the lower mantle implies a spectrum of shorter time-scale deformational response modes for second-degree spherical harmonic deformations of the Earth. Although highly uncertain, this spectrum of spatial variation allows a purely Maxwellian viscoelastic rheology simultaneously to explain all solid tidal dispersion phenomena and long-term rebound-related mantle viscosity. Composite theory of multiphase viscoelastic media is used to demonstrate this effect.
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  • 9
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: We derive modified matrix operators that minimize the numerical error of solutions of the discretized elastic equation of motion. The criterion for obtaining the modified matrix operators is that the net error of the discretized equation of motion must be approximately equal to zero whenever the operand is an eigenfunction and the frequency is equal to the corresponding eigenfrequency. As it is not necessary to know the explicit values of the eigensolutions, our approach can be applied to arbitrarily heterogeneous media. In this paper we primarily consider frequency domain solutions calculated using the direct solution method (DSM) (Geller et al. 1990; Hara, Tsuboi & Geller 1991; Geller & Ohminato 1994). We present explicit formulations of the modified operators and numerical examples for P-SV and SH wave propagation in laterally homogeneous, isotropic media. The numerical solutions obtained using the modified operators are about 30 times more accurate than those obtained using the unmodified operators for the same CPU time. Our methods are readily applicable to problems in spherical coordinates or involving laterally heterogeneous media, as well as to time-domain solutions. It should also be possible to apply the methods of this paper to numerical methods other than the DSM.
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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: Using the boundary integral method to simulate SH waves numerically in 2-D homogeneous full- or half-space media with randomly distributed cavities, we compare the amplitude attenuation of direct waves with the temporal decay of the coda. The boundary integral method includes the effect of any degree of multiply scattered waves for a wide frequency range, up to wavelengths smaller than the size of the cavities. We consider seismograms on the free surface so that heterogeneities exist only on one side of the receivers, a situation that resembles actual seismic observations. Seismograms are computed for a vertically incident plane wave and for an isotropic line source. In both cases, the value of Q−1 as a function of kd, where k is the wavenumber and d is the cavity diameter, peaks around kd = 2 for the direct wave, which is consistent with some single-scattering models. Coda Q−1 determined by the temporal decay of the coda envelope agrees well with Q−1 for the direct wave for models with a root-mean-square fluctuation of velocity, Q, of about 10 per cent in a half-space. On the other hand, the coda Q−1 is systematically larger than the direct wave Q−1 in full-space models, that is, without the inclusion of the reflection at the free surface. When the cavity density is doubled (σ 〉 20 per cent), the coda energy increases rapidly and its temporal decay decreases, so that coda Q−1 becomes smaller than the direct wave Q−1, even for full-space models. With a smaller value of cr (about 5 per cent), the coda decays rapidly and the relation between the two types of Q−1 is reversed: the coda Q−1 becomes larger than the direct wave Q−1. By comparing results from seismograms composed only of singly scattered waves with those that include multiply scattered waves, we can compare the relative contribution of each singly and multiply scattered wavefield to the two measures of Q. Single scattering mainly determines both the direct wave Q−1 and the coda Q−1 for the smallest value of a, while the values of both kinds of attenuation, particularly the direct wave Q−1, are strongly affected by multiple scattering when σ is large. Our results imply that a reasonable estimate of scattering attenuation can be obtained by measuring the temporal decay of the coda, if the scattering character of the Earth is similar to our models with a σ of around 10 per cent, where the single scattering is found to be dominant.
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