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    The role of topography and lateral velocity heterogeneities on near-source scattering and ground-motion variability

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    Geophys. J. Int.-2015-Imperatori-2163-81.pdf
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    Type
    Article
    Authors
    Imperatori, W.
    Mai, Paul Martin cc
    KAUST Department
    Computational Earthquake Seismology (CES) Research Group
    Earth Science and Engineering Program
    Physical Science and Engineering (PSE) Division
    Date
    2015-07-28
    Online Publication Date
    2015-07-28
    Print Publication Date
    2015-09-01
    Permanent link to this record
    http://hdl.handle.net/10754/577020
    
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    Abstract
    The scattering of seismic waves travelling in the Earth is not only caused by random velocity heterogeneity but also by surface topography. Both factors are known to strongly affect ground-motion complexity even at relatively short distance from the source. In this study, we simulate ground motion with a 3-D finite-difference wave propagation solver in the 0–5 Hz frequency band using three topography models representative of the Swiss alpine region and realistic heterogeneous media characterized by the Von Karman correlation functions. Subsequently, we analyse and quantify the characteristics of the scattered wavefield in the near-source region. Our study shows that both topography and velocity heterogeneity scattering may excite large coda waves of comparable relative amplitude, especially at around 1 Hz, although large variability in space may occur. Using the single scattering model, we estimate average QC values in the range 20–30 at 1 Hz, 36–54 at 1.5 Hz and 62–109 at 3 Hz for constant background velocity models with no intrinsic attenuation. In principle, envelopes of topography-scattered seismic waves can be qualitatively predicted by theoretical back-scattering models, while forward- or hybrid-scattering models better reproduce the effects of random velocity heterogeneity on the wavefield. This is because continuous multiple scattering caused by small-scale velocity perturbations leads to more gentle coda decay and envelope broadening, while topography abruptly scatters the wavefield once it impinges the free surface. The large impedance contrast also results in more efficient mode mixing. However, the introduction of realistic low-velocity layers near the free surface increases the complexity of ground motion dramatically and indicates that the role of topography in elastic waves scattering can be relevant especially in proximity of the source. Long-period surface waves can form most of the late coda, especially when intrinsic attenuation is taken into account. Our simulations indicate that both topography and velocity heterogeneity scattering may result in large ground-motion variability, characterized by standard deviation values in the range 0.2–0.5 also at short distance from the source. We conclude that both topography and velocity heterogeneity should be considered to correctly assess the ground-motion variability in earthquake scenario studies even at intermediate frequency.
    Citation
    The role of topography and lateral velocity heterogeneities on near-source scattering and ground-motion variability 2015, 202 (3):2163 Geophysical Journal International
    Publisher
    Oxford University Press (OUP)
    Journal
    Geophysical Journal International
    DOI
    10.1093/gji/ggv281
    Additional Links
    http://gji.oxfordjournals.org/lookup/doi/10.1093/gji/ggv281
    ae974a485f413a2113503eed53cd6c53
    10.1093/gji/ggv281
    Scopus Count
    Collections
    Articles; Physical Science and Engineering (PSE) Division; Earth Science and Engineering Program

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