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  • 1
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    Validity domain of the Born approximation for seismic waveform modelling in realistic 3-D Earth structure (2014)
    Oxford University Press
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    Publication Date: 2014-12-19
    Description: While the first-order Born approximation is increasingly being used in many seismic tomography efforts, its domain of validity to forward model seismic waveforms has not been quantified in the context of current 3-D earth models yet. We here address this issue by comparing teleseismic synthetic surface waveforms calculated using the Born approximation with spectral element method solutions for a variety of realistic global 3-D earth models. We find that the Born approximation has a very limited domain of validity when applied to seismic waveforms. Specifically, it can only accurately model the phase (amplitude) of surface waveforms for source–receiver paths leading to time shifts smaller than about 15 per cent (5 per cent) of the wave period considered. These conditions usually occur in earth models as S20RTS or S40RTS combined with a homogeneous crust for periods longer than T ~ 80–90 s. For models with stronger heterogeneity and/or realistic 3-D crustal structure, only the phase of waveforms with periods longer than ~120–130 s can be accurately modelled with the Born approximation.
    Keywords: Express Letters, Seismology
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 2
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    The traveltime holographic principle (2014)
    Oxford University Press
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    Publication Date: 2014-11-07
    Description: Fermat's interferometric principle is used to compute interior transmission traveltimes pq from exterior transmission traveltimes sp and sq . Here, the exterior traveltimes are computed for sources s on a boundary B that encloses a volume V of interior points p and q . Once the exterior traveltimes are computed, no further ray tracing is needed to calculate the interior times pq . Therefore this interferometric approach can be more efficient than explicitly computing interior traveltimes pq by ray tracing. Moreover, the memory requirement of the traveltimes is reduced by one dimension, because the boundary B is of one fewer dimension than the volume V . An application of this approach is demonstrated with interbed multiple (IM) elimination. Here, the IMs in the observed data are predicted from the migration image and are subsequently removed by adaptive subtraction. This prediction is enabled by the knowledge of interior transmission traveltimes pq computed according to Fermat's interferometric principle. We denote this principle as the ‘traveltime holographic principle’, by analogy with the holographic principle in cosmology where information in a volume is encoded on the region's boundary.
    Keywords: Express Letters, Seismology
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 3
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    The Ricker wavelet and the Lambert W function (2014)
    Oxford University Press
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    Publication Date: 2014-11-07
    Description: The Ricker wavelet has been widely used in the analysis of seismic data, as its asymmetrical amplitude spectrum can represent the attenuation feature of seismic wave propagation through viscoelastic homogeneous media. However, the frequency band of the Ricker wavelet is not analytically determined yet. The determination of the frequency band leads to an inverse exponential equation. To solve this equation analytically a special function, the Lambert W function, is needed. The latter provides a closed and elegant expression of the frequency band of the Ricker wavelet, which is a sample application of the Lambert W function in geophysics and there have been other applications in various scientific and engineering fields in the past decade. Moreover, the Lambert W function is a variation of the Ricker wavelet amplitude spectrum. Since the Ricker wavelet is the second derivative of a Gaussian function and its spectrum is a single-valued smooth curve, numerical evaluation of the Lambert W function can be implemented by a stable interpolation procedure, followed by a recursive computation for high precision.
    Keywords: Express Letters, Seismology
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 4
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    Rupture velocity inferred from near-field shear strain analysis (2014)
    Oxford University Press
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    Publication Date: 2014-10-19
    Description: We propose a new technique to determine the rupture velocity of large strike slip earthquakes. By means of simple numerical ground motion simulations, we show that when the rupture penetrates a shallow layer of sediment or fractured rock, shock waves propagate along the surface fault trace in the forward rupture direction. Such shock waves, which are insensitive to the complexity of slip over the fault plane, propagate at a phase velocity equal to the rupture speed. We show that those shock waves can be easily isolated in the frequency domain, and that phase velocity can then be simply obtained from shear strain.
    Keywords: Express Letters, Seismology
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 5
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    Seismic attenuation: effects of interfacial impedance on wave-induced pressure diffusion (2014)
    Oxford University Press
    Details
    Publication Date: 2014-10-17
    Description: Seismic attenuation and dispersion in layered sedimentary structures are often interpreted in terms of the classical White model for wave-induced pressure diffusion across the layers. However, this interlayer flow is severely dependent on the properties of the interface separating two layers. This interface behaviour can be described by a pressure jump boundary condition involving a non-vanishing interfacial impedance. In this paper, we incorporate the interfacial impedance into the White model by solving a boundary value problem in the framework of quasi-static poroelasticity. We show that the White model predictions for attenuation and dispersion substantially change. These changes can be attributed to petrophysically plausible scenarios such as imperfect hydraulic contacts or the presence of capillarity.
    Keywords: Express Letters, Seismology
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 6
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    Evidences of a lithospheric fault zone in the Sicily Channel continental rift (southern Italy) from instrumental seismicity data (2014)
    Oxford University Press
    Details
    Publication Date: 2014-08-07
    Description: Sicily Channel is a portion of Mediterranean Sea, between Sicily (Southern Italy) and Tunisia, representing a part of the foreland Apennine-Maghrebian thrust belt. The seismicity of the region is commonly associated with the normal faulting related to the rifting process and volcanic activity of the region. However, certain seismic patterns suggest the existence of some mechanism coexisting with the rifting process. In this work, we present the results of a statistical analysis of the instrumental seismicity and a reliable relocalization of the events recorded in the last 30 yr in the Sicily Channel and western Sicily using the Double Difference method and 3-D Vp and Vs tomographic models. Our procedure allows us to discern the seismic regime of the Sicily sea from the Tyrrhenian one and to describe the main features of an active fault zone in the study area that could not be related to the rifting process. We report that most of the events are highly clustered in the region between 12.5°–13.5°E and 35.5°–37°N with hypocentral depth of 5–40 km, and reaching 70 km depth in the southernmost sector. The alignment of the seismic clusters, the distribution of volcanic and geothermal regions and the location of some large events occurred in the last century suggest the existence of a subvertical shear zone extending for least 250 km and oriented approximately NNE–SSW. The spatial distribution of the seismic moment suggests that this transfer fault zone is seismically discontinuous showing large seismic gaps in proximity of the Ferdinandea Island, and Graham and Nameless Bank.
    Keywords: Express Letters, Seismology
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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