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  • 1990-1994  (437)
  • 1
    Electronic Resource
    Electronic Resource
    Springer
    Pure and applied geophysics 132 (1990), S. 1-19 
    ISSN: 1420-9136
    Keywords: Regional seismograms ; attenuation ; coda ; crustal structure ; surface waves
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract TheLg wave consists of the superposition ofS waves supercritically reflected, and thus trapped, in the crust. This mode of propagation explains the strong amplitude of this phase and the large distance range in which it is observed. The numerical simulation leads to successful comparison between observed seismograms in stable continental areas and synthetics computed for simple standard crustal models. In regions with strong lateral variations, the influence of large-scale heterogeneities on theLg amplitude is not yet clearly established in terms of the geometrical characteristics of the crustal structure. The analysis of the decay of amplitude ofLg with epicentral distance allows the evaluation of the quality factor ofS waves in the crust. The results obtained show the same trends as codaQ: a clear correlation with the tectonic activity of the region considered, both for the value ofQ at 1 Hz and for its frequency dependence, suggesting that scattering plays a prominent part among the processes that cause the attenuation. The coda ofLg is made up of scatteredS waves. The study of the spatial attenuation of the coda indicated that a large part of the arrivals that compose the coda propagate asLg. The relative amplitude of the coda is larger at sites located on sediments because, in these conditions, a part ofLg energy can be converted locally into lower order surface modes.
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    Pure and applied geophysics 132 (1990), S. 67-91 
    ISSN: 1420-9136
    Keywords: Shear-wave splitting ; extensive-dilatancy anisotropy ; EDA ; stress-aligned cracks ; surface interactions ; localSP-wave
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The two major sources of scattering for shear-waves in the crust, interactions with the topography at the surface and the effective anisotropy of aligned cracks throughout the rockmass, introduce first-order changes to the shear-wave particle-motion. At the surface, shear-waves are scattered by the topography within a wavelength or two of the recording site so that, unless the effective incidence angle is less than the critical angle sin−1 V S/V P, the recorded waveforms may bear little relationship to the waveforms of the incident wave. Within the rockmass, shear-waves are scattered by extensive-dilatancy anisotropy (EDA), the distribution of stress-aligned fluid-filled cracks, microcracks, and preferentially oriented pore-space pervading most rocks in the crust. Analysis of this shear-wave splitting yields new information about the internal structure of thein situ rockmass which is not otherwise available.
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  • 3
    ISSN: 1420-9136
    Keywords: Scattering ; wave propagation ; seismic waves ; core-mantle topography ; inhomogeneities
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Statistical properties of small-scale inhomogeneities (wavelengths between 20 and 70 km) near the core-mantle boundary are inferred from scattered core waves. Observations of scattered core waves at large seismic arrays and worldwide networks indicate that the inhomogeneities have a global nature with similar characteristics. However, there may exist a few regions having markedly stronger or weaker strengths. Scattering by volumetric inhomogeneities of about 1% inP-wave velocity in the lower mantle or by about 300 m of topographic relief of the core-mantle boundary can explain the observations. At present it is not possible to rule out either of these two alternatives, or a combination of both.
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  • 4
    ISSN: 1420-9136
    Keywords: Scattering ; attenuation ; coda ; Q ; heterogeneity
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract A unified model is proposed for explaining the frequency dependent amplitude attenuation and the coda wave excitation on the basis of the single scattering process in the randomly inhomogeneous lithosphere. Adopting Birch's law and a direct proportion between density and wave velocity, we statistically describe the inhomogeneous medium by one random function characterized by the von Karman autocorrelation function. We calculate the amplitude attenuation from the solid angle integral of scattered wave energy on the basis of the Born approxiimation after subtracting the travel-time fluctuation effect caused by slowly varying velocity inhomogeneities. This subtraction is equivalent to neglect energy loss by scattering within a cone around the forward direction. The random inhomogeneity of the von Karman autocorrelation function of order 0.35 with the mean square fractional fluctuation of 7.2×10−3 ≈1.3×10−2 and the correlation distance of 2.1≈5.1 km well explains observed backward scattering coefficientg π and the ratioQ P −1 /Q S −1 , and observed and partially conjecturedQ S −1 for frequencies between 0.5 Hz and 30 Hz.
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  • 5
    ISSN: 1420-9136
    Keywords: Wave propagation ; seismic waves ; heterogeneities ; lithosphere
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Adopting the spectral approach, we derive the formulation of angular coherence and transverse coherence of transmission fluctuations. Our derivation and results provide new insight on transmission fluctuation analysis. A review of research work on fluctuation analysis using observations at large seismic arrays such as LASA and NORSAR-follows. We point out that the model of a single-layer Gaussian medium cannot explain the angular coherence of NORSAR data and a more general model of a non-Gaussian, multi-scale, vertically inhomogeneous random media is needed. The model of a two-layer power-law medium proposed by Flatté and Wu is among the simplest of such models.
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  • 6
    ISSN: 1420-9136
    Keywords: P wave amplitudes ; focussing ; three-dimensional wave propagation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Short-period teleseismicP waves from the Nevada Test Site (NTS) show systematic variations in amplitudes and travel times, with low amplitudes corresponding to fast travel times, suggesting elastic focussing-defocussing effects. Also, the azimuthal amplitude and travel time patterns for events at the Pahute Mesa subsite are systematically different from those at the Yucca Flat subsite, indicating the presence of a near-source component in both the amplitude and travel-time variations. This component is isolated by removing the mean station pattern for all of NTS from the observations. A very-near-source component in the Pahute Mesa observations is also isolated by removing subsite station means from the measurements, whereas the Yucca Flat observations exhibited no coherent very-near-source component. These anomalies are back-projected through laterally homogeneous structure to form thin lens models at various depths. Travel-time delays are predicted from the amplitude variations using the equation for wavefront curvature. The long-wavelength components of the predicted and observed time delays correlate well, at depths of 25 km for the very-near-source component under Pahute Mesa and 160 km for the regional component under NTS. The time delay surfaces predicted by the amplitudes at these depths are mapped into warped velocity discontinuities suitable for the calculation of synthetic seismograms using the Kirchhoff-Helmholtz integral formulation. Both the intersite (near-source) and intrasite (very-near-source) differences in amplitudes are qualitatively predicted very well, although the range of variation is somewhat underpredicted. This deficiency is likely due to the destructive interference of anomalies inherent in back-projection to a single layer.
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  • 7
    ISSN: 1420-9136
    Keywords: Strong scattering ; spatial fluctuation ; scattering attenuation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract To study the effects of strong scattering on elastic waves, spatial fluctuation and scattering attenuation ofP waves were examined by laboratory experiments for 2-D models of random media approximately characterized by a triangular correlation function in the range of 2〈ka〈33, wherek is the wave number anda is the correlation distance of the heterogeneities, i.e., the heterogeneity size. The results obtained are as follows: (1) Forka〉10, both the intensity and the correlation distance of the amplitude fluctuation are approximate for any phase of theP-wave train. The correlation distance nearly agrees with the heterogeneity size. These fluctuation properties are quite consistent with the theoretical prediction by the forward-scattering approximation. (2) For 3〈ka〈6, the fluctuation intensity becomes stronger in later phases of theP-wave train. This shows that scattering is approximately isotropic, and therefore, the scattered energy increases with time within theP-wave train. The correlation distance of the amplitude fluctuation disagrees with the heterogeneity size, and it shows a frequency-dependent property decreasing from 7a to 4a with the increase ofka from 3 to 6. These properties for 3〈ka〈6 have not yet been predicted theoretically. (3) Forka〈3, though the fluctuation is considerably smaller compared with that ofka〉10 and 3〈ka〈6, the fluctuation property is considered similar to that of 3〈ka〈6. (4) The observed scattering attenuation,Q −1, increases withka forka〈3, has a peak aroundka=3∼5, and then decreases withka. (5) When θmin = 15° and σ = 0.075, the theoreticalQ −1 curve, predicted by the approximate theory of Wu, roughly matches the observedQ −1 values, where θmin is the minimum scattering angle measured from the propagation direction of theP waves and σ is the rms of fractional velocity fluctuation. This suggests that the energy scattered in the range of θ〉15° is lost from theP waves, while the energy scattered in the range of θ〈15° is retained; and that the approximate theory overestimates by about three times the σ value of the model media used owing to the neglect of multiple scattering. (6) When the size of velocity heterogeneities responsible for forward scattering at 3〈ka〈6 is estimated from the θmin value of 15° on the basis of Wu's theory, it nearly agrees with the correlation distance for the initial phase of theP-wave train.
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  • 8
    Electronic Resource
    Electronic Resource
    Springer
    Pure and applied geophysics 132 (1990), S. 221-244 
    ISSN: 1420-9136
    Keywords: Coda ; arrays ; three-component ; scattering ; phase velocity ; azimuth
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The application of standard array processing techniques to the study of coda presents difficulties due to the design criteria of these techniques. Typically the techniques are designed to analyze isolated, short arrivals with definite phase velocity and azimuth and have been useful in the frequency range around 1 Hz. Coda is long in time and may contain waves of different types, phase velocities and azimuths. Nonetheless, it has proved possible to use or adapt array methods to answer two questions: what types of waves are present in coda and where are they scattered? Most work has been carried out on teleseismicP coda; work on local coda has lagged due to lack of suitable data and the difficulties of dealing with high frequencies. The time domain methods of beamforming and Vespagram analysis have shown that there is coherent energy with a high phase velocity comparable toP orPP in teleseismicP coda. These methods can detect this “coherent” coda because it has a fairly definite phase velocity and the same, or close to, azimuth as firstP orPP. This component must consist ofP waves and is either scattered near the source, or reflected in the mantle path as apdpP or precursorPP reflection. The Fourier transform method of the frequency-wavenumber spectrum has been adapted by integrating around circles of constant phase velocity (constant total wavenumber) to produce the wavenumber spectrum, which shows power as a function of wavenumber, or phase velocity. For teleseismicP coda, wavenumber spectra demonstrate that there is a “diffuse” coda of shear,Lg or surface waves scattered from teleseismicP near the receiver. Wavenumber spectra also suggest that the coherent coda is produced by near-source scattering in the crust, not mantle reflection, since it is absent or weak for deep-focus events. Crustal earthquakes have a very strong coherent component of teleseismic coda, suggesting scattering from shear to teleseismicP near the source. Three-component analysis of single-station data has shown the presence of off-azimuth arrivals and may lead to the identification of waves scattered from a single scatterer.
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  • 9
    Electronic Resource
    Electronic Resource
    Springer
    Pure and applied geophysics 132 (1990), S. 269-288 
    ISSN: 1420-9136
    Keywords: Complexity ; scattering ; crust ; refraction ; wide-angle reflection ; coda
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract ThePmP wide-angle reflected signal from the Moho shows a wide variation in its characteristics when cornpared from region to region. If the earth's crust is simple and homogeneous and the Moho is a sharp discontinuity, supercriticalPmP wavelets are large and isolated in the 100 km to 300 km distance range when compared to the preceding signals on the seismic traces. If the crust itself has numerous short reflectors and the Moho transition zone is thick or badly disrupted from past tectonic disturbances, thenPmP is often poorly defined and difficult to identify in the coda of earlier arriving signals. These signals are poorly correlated because the reflectors themselves are in general not only discontinuous but tend to be distributed at various depths within the crust. The effect of the vertical velocity gradient in the crust is to make the reflective field for downgoing waves much greater than for upgoing waves. The large variations in reflection coefficients wiht angle of incidence has the effect of making the coda generated by a reflective machanism distance dependent. The reflected signals are also contaminated with scattered signals from smaller scale heterogeneities which may be distributed more uniformly within the crust. In conventional seismic inversion methods applied to crustal refraction experiments, emphasis is usually placed on obtaining a velocity model of the subsurface structure. These models are relatively simple and are limited in the amount of complexity which can be uniquely inferred from the data. The main problem arises because of the difficulties in the identification of the origin of the signals which are themselves often incoherent from trace to trace. In this paper it is shown how the conventional record section may be complemented with a normal moveout corrected intensity section which emphasizes areas of large signal complexities. Data from this intensity section is then used as input to obtain a quantitative measurement of a complexity parameter. These measurements may be used to infer or compare differences in crustal heterogeneity from one region to another. The discussion is illustrated with both numerical modeling data as well as data from recent crustal experiments which were conducted over the Canadian Shield.
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  • 10
    ISSN: 1420-9136
    Keywords: Seismicity ; codaQ ; Q variations
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Seismicity in the La Cerdanya region of the eastern Pyrenees has been accurately mapped for the first time using data from a local seismic network. The majority of earthquakes lies on or near the La Cerdanya fault or secondary faults to the south. Coda magnitudes determined for these earthquakes, using magnitude relations from other regions, range between −0.5 and 2.2. These are, however, presumed to be underdetermined values sinceQ values appear to be very low in the La Cerdanya region. CodaQ values at a frequency of 1.5 Hz range between 17 and 120, the lowest values being obtained for the most seismically active regions. CodaQ values also increase with increasing distance, a result which indicates decreasing seismic attenuation with increasing depth in the crust.
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