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  • 1
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    Structure of the crust in the Black Sea and adjoining regions from surface wave data (1998)
    Springer
    Details
    Publication Date: 1998-01-01
    Description: Group velocities of Rayleigh and Love waves along the paths across the Black Sea and partly Asia Minor and the Balkan Peninsula are used to estimate lateral variations of the crustal structure in the region. As a first step, lateral variations of group velocities for periods in the range 10-20 s are determined using a 2D tomography method. Since the paths are oriented predominantly in NE-SW or N-S direction, the resolution is estimated as a function of azimuth. The 'local' dispersion curves are actually averaged over the extended areas stretched in the predominant direction of the paths. The size of the averaging area in the direction of the best resolution is approximately 200 km. As a second step, the local averaged dispersion curves are inverted to vertical sections of S-wave velocities. Since the dispersion curves in the 10-20 s period range are mostly affected by the upper crustal structure, the velocities are estimated to a depth of approximately 25 km. Velocity sections along 43° N latitude are determined separately from Rayleigh and Love wave data. It is shown that the crust under the sea contains a low-velocity sedimentary layer of 2-3 km thickness, localized in the eastern and western deeps, as found earlier from DSS data. Beneath the sedimentary layer, two layers are present with velocity values lying between those of granite and consolidated sediments. Velocities in these layers are slightly lower in the deeps, and the boundaries of the layers are lowered. S-wave velocities obtained from Love wave data are found to be larger than those from Rayleigh wave data, the difference being most pronounced in the basaltic layer. If this difference is attributed to anisotropy, the anisotropy coefficient χ = (SH - SV)/Smean is reasonable (2-3%) in the upper layers, and exceeds 9% in the basaltic layer.
    Print ISSN: 1383-4649
    Electronic ISSN: 1573-157X
    Topics: Geosciences , Physics
    Published by Springer
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  • 2
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    Verification and refinement of spectral magnitude calibrating functions for P-waves (1998)
    Springer
    Details
    Publication Date: 1998-01-01
    Description: Calibrating functions for determination of P-wave spectral magnitudes calculated by Duda and Yanovskaya (1994) on the basis of the IASP91 velocity model and the PREM Q-model are verified empirically. For this purpose IRIS broadband records for 120 earthquakes are used, each earthquake having been recorded at about 100 stations. The discrepancies indicate that anelastic absorption assumed in PREM is too high. New calibrating functions are calculated on the basis of the AK135 velocity and anelasticity models (Kennet et al., 1995; Montagner and Kennet, 1996), in which Q in the mantle is higher than in PREM. The verification of the new calibrating functions based on the same observations yields magnitude figures less depending on the epicentral distance. In addition the parameter τ2in the Liu-Anderson Q(T) model is estimated, proceeding from the assumption that on the average the radiated spectra comply with the ω2-model. The value of τ2 was assumed to be 0.1 s in the analysis and its use resulted in the verification of the ω2-model for the source spectrum.
    Print ISSN: 1383-4649
    Electronic ISSN: 1573-157X
    Topics: Geosciences , Physics
    Published by Springer
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  • 3
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    Ray tomography based on azimuthal anomalies (1996)
    Springer
    Details
    Publication Date: 1996-01-01
    Print ISSN: 0033-4553
    Electronic ISSN: 1420-9136
    Topics: Geosciences , Physics
    Published by Springer
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  • 4
    Electronic Resource
    Electronic Resource
    Ray tomography based on azimuthal anomalies (1996)
    Springer
    Pure and applied geophysics 148 (1996), S. 319-336 
    Details
    ISSN: 1420-9136
    Keywords: Surface waves ; phase velocities ; azimuthal anomalies ; seismic tomography
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract A method of estimating the lateral velocity variations in the 2D case using the data on deviations of wave paths from straight lines (or great circle paths in the spherical case) is proposed. The method is designed for interpretation of azimuthal anomalies of surface waves which contain information on lateral variations of phase velocities supplementary to that obtained from travel-time data in traditional surface wave tomography. In the particular 2D case, when the starting velocity is constant (c 0) and velocity perturbations δc(x,y) are sufficiently smooth, a relationship between azimuthal anomaly δα and velocity perturbations δc(x,y) can be obtained by approximate integration of the ray tracing system, which leads to the following functional: $$\delta \alpha = \int_0^L {\frac{{s(\nabla m,n_0 )}}{L}} ds,$$ wherem(x,y)=δc(x,y)/c 0,L is the length of the ray,n 0 is a unit vector perpendicular to the ray in the starting model, integration being performed from the source to the receiver. This formula is valid for both plane and spherical cases. Numerical testing proves that for a velocity perturbation which does not exceed 10%, this approximation is fairly good. Lateral variations of surface wave velocities satisfy these assumptions. Therefore this functional may be used in surface wave tomography. For the determination ofm(x,y) from a set ofδα k corresponding to different wave paths, the solution is represented as a series in basis functions, which are constructed using the criterion of smoothness of the solution proposed byTarantola andNersessian (1984) for time-delay tomography problems. Numerical testing demonstrates the efficiency of the tomography method. The method is applied to the reconstruction of lateral variations of Rayleigh wave phase velocities in the Carpathian-Balkan region. The variations of phase velocities obtained from data on azimuthal anomalies are found to be correlated with group-velocity variations obtained from travel-time data.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00882065
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  • 5
    Electronic Resource
    Electronic Resource
    Verification and refinement of spectral magnitude calibrating functions for P-waves (1998)
    Springer
    Journal of seismology 2 (1998), S. 117-124 
    Details
    ISSN: 1573-157X
    Keywords: P-wave spectra ; Magnitude ; Anelastic attenuation ; Calibrating functions
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Calibrating functions for determination of P-wave spectral magnitudes calculated by Duda and Yanovskaya (1994) on the basis of the IASP91 velocity model and the PREM Q-model are verified empirically. For this purpose IRIS broadband records for 120 earthquakes are used, each earthquake having been recorded at about 100 stations. The discrepancies indicate that anelastic absorption assumed in PREM is too high. New calibrating functions are calculated on the basis of the AK135 velocity and anelasticity models (Kennet et al., 1995; Montagner and Kennet, 1996), in which Q in the mantle is higher than in PREM. The verification of the new calibrating functions based on the same observations yields magnitude figures less depending on the epicentral distance. In addition the parameter τ2in the Liu-Anderson Q(T) model is estimated, proceeding from the assumption that on the average the radiated spectra comply with the ω2-model. The value of τ2 was assumed to be 0.1 s in the analysis and its use resulted in the verification of the ω2-model for the source spectrum.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1008054913617
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  • 6
    Electronic Resource
    Electronic Resource
    Structure of the crust in the Black Sea and adjoining regions from surface wave data (1998)
    Springer
    Journal of seismology 2 (1998), S. 303-316 
    Details
    ISSN: 1573-157X
    Keywords: Black Sea ; crustal structure ; group velocity ; surface waves ; tomography
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Group velocities of Rayleigh and Love waves along the paths across the Black Sea and partly Asia Minor and the Balkan Peninsula are used to estimate lateral variations of the crustal structure in the region. As a first step, lateral variations of group velocities for periods in the range 10–20 s are determined using a 2D tomography method. Since the paths are oriented predominantly in NE–SW or N–S direction, the resolution is estimated as a function of azimuth. The ‘local’ dispersion curves are actually averaged over the extended areas stretched in the predominant direction of the paths. The size of the averaging area in the direction of the best resolution is approximately 200 km. As a second step, the local averaged dispersion curves are inverted to vertical sections of S-wave velocities. Since the dispersion curves in the 10–20 s period range are mostly affected by the upper crustal structure, the velocities are estimated to a depth of approximately 25 km. Velocity sections along 43° N latitude are determined separately from Rayleigh and Love wave data. It is shown that the crust under the sea contains a low-velocity sedimentary layer of 2–3 km thickness, localized in the eastern and western deeps, as found earlier from DSS data. Beneath the sedimentary layer, two layers are present with velocity values lying between those of granite and consolidated sediments. Velocities in these layers are slightly lower in the deeps, and the boundaries of the layers are lowered. S-wave velocities obtained from Love wave data are found to be larger than those from Rayleigh wave data, the difference being most pronounced in the basaltic layer. If this difference is attributed to anisotropy, the anisotropy coefficient χ = (SH - SV)/Smean is reasonable (2–3%) in the upper layers, and exceeds 9% in the basaltic layer.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1009716017960
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