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  • 1
    Publication Date: 2016-07-26
    Description: The distribution of damage due to recent earthquakes has shown that the effects of shallow geological structures on the level of ground shaking represent an important factor in engineering seismology. Whereas many previous studies have estimated site amplification factors in the frequency domain, their application to the real-time modeling of ground motion is not yet fully established. In this article, a method for the real-time correction of frequency-dependent site-response factors is proposed, which accounts not only for the modulus, but also for the changes in the signal phase related to local site conditions. The transformation of the complex standard spectral ratios to a causal recursive filter in the time domain allows for the forecasting of the waveforms for soft-soil sites almost in real time when the signal is recorded earlier at a reference site. When considering travel-time differences of the various seismic phases between the hypocenter and the studied sites, the level of ground motion at soft-soil sites with respect to arrival time, energy, duration, and frequency content can be well constrained, even in cases of a high spatial variability of the amplification patterns.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
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  • 2
    Publication Date: 2014-08-13
    Description: Seismic noise is generally considered as a reproducible and temporarily stationary natural source of energy. We present a study on the statistical features of the soil motion due to the seismic noise wavefield and the dependencies on the near-surface geology. We have investigated the variations of the 3-D average squared soil displacement over different timescales. The results clearly indicate ballistic behaviour for short timescales being indicative for the properties of the shallow material. Differences in the structural heterogeneity of the subsoil produce different scattering properties, changing the character of motion from nearly ballistic to diffusive on frequency-dependent timescales for all materials. Although in a strict sense the seismic noise wavefield is not completely isotropic, an ultimate pre-condition for a diffusive wavefield, the deviations compared to a uniform distribution are rather small. This means that the emergence of the Green's function is effective for all network sites after a sufficient self-averaging process that is provided by the scattering and the random spatial-temporal noise source distribution.
    Keywords: 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
    Publication Date: 2013-08-13
    Description: Nowadays, an increasing number of seismological imaging studies are published taking advantage of the increasing popularity of analysing empirical Green's functions obtained from high-frequency ambient seismic noise. However, especially on a local scale results could potentially be biased in regions where topography is not small compared to the wavelength and the penetration depth of the considered waves. Current 2-D seismic techniques are often inadequate when solving such 3-D geophysical problems, which include the complication of seismic imaging for cases where there are pronounced relief effects. For example, information about the geologic subsurface structure and deformational patterns is necessary for accurate site characterization and seismic hazard assessment. Here we show that an ad hoc passive seismic tomography approach can identify and describe complex 3-D structures, which can help to accurately and efficiently map the shear-wave velocities of the surficial soil layers, even in cases of significant topography relief. We test our technique by using simulations of seismic noise for a simple realistic site and show for a real data set across the Issyk-Ata fault, Kyrgyzstan, which is located at southern border of the capital, Bishkek, this novel approach has identified two different small fault branches and a clear shear-wave velocity contrast across the fault.
    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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