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  • 1
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    3-D numerical simulations of earthquake ground motion in sedimentary basins: testing accuracy through stringent models (2015)
    Oxford University Press
    Details
    Publication Date: 2015-02-12
    Description: Differences between 3-D numerical predictions of earthquake ground motion in the Mygdonian basin near Thessaloniki, Greece, led us to define four canonical stringent models derived from the complex realistic 3-D model of the Mygdonian basin. Sediments atop an elastic bedrock are modelled in the 1D-sharp and 1D-smooth models using three homogeneous layers and smooth velocity distribution, respectively. The 2D-sharp and 2D-smooth models are extensions of the 1-D models to an asymmetric sedimentary valley. In all cases, 3-D wavefields include strongly dispersive surface waves in the sediments. We compared simulations by the Fourier pseudo-spectral method (FPSM), the Legendre spectral-element method (SEM) and two formulations of the finite-difference method (FDM-S and FDM-C) up to 4 Hz. The accuracy of individual solutions and level of agreement between solutions vary with type of seismic waves and depend on the smoothness of the velocity model. The level of accuracy is high for the body waves in all solutions. However, it strongly depends on the discrete representation of the material interfaces (at which material parameters change discontinuously) for the surface waves in the sharp models. An improper discrete representation of the interfaces can cause inaccurate numerical modelling of surface waves. For all the numerical methods considered, except SEM with mesh of elements following the interfaces, a proper implementation of interfaces requires definition of an effective medium consistent with the interface boundary conditions. An orthorhombic effective medium is shown to significantly improve accuracy and preserve the computational efficiency of modelling. The conclusions drawn from the analysis of the results of the canonical cases greatly help to explain differences between numerical predictions of ground motion in realistic models of the Mygdonian basin. We recommend that any numerical method and code that is intended for numerical prediction of earthquake ground motion should be verified through stringent models that would make it possible to test the most important aspects of accuracy.
    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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  • 2
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    Site-city interaction: theoretical, numerical and experimental crossed-analysis (2016)
    Oxford University Press
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    Publication Date: 2016-03-24
    Description: The collective excitation of city structures by a seismic wavefield and the subsequent multiple Structure-Soil-Structure Interactions (SSSIs) between the buildings are usually disregarded in conventional seismology and earthquake engineering practice. The objective here is to qualify and quantify these complex multiple SSSIs through the design of an elementary study case, which serves as a benchmark for theoretical, numerical and experimental crossed-analysis. The experimental specimen consists of an idealized site-city setup with up to 37 anisotropic resonant structures arranged at the top surface of an elastic layer and in co-resonance with it. The experimental data from shaking table measurements is compared with the theoretical and numerical results provided respectively by an equivalent city-impedance model derived analytically from homogenization in the long-wavelength approximation and a model based on boundary elements. The signatures of the site-city interactions are identified in the frequency, time and space domain, and in particular consist of a frequency-dependent free/rigid switch in the surface condition at the city resonance, beatings in the records and the depolarization of the wavefield. A parametric study on the city density shows that multiple SSSIs among the city structures (five are sufficient) can have significant effects on both the seismic response of its implantation site and that of the buildings. Key parameters are provided to assess site-city interactions in the low seismic frequency range: They involve the mass and rigidity of the city compared to those of the soil and the damping of the building.
    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
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    Analysis of seismic waves crossing the Santa Clara Valley using the three-component MUSIQUE array algorithm (2016)
    Oxford University Press
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    Publication Date: 2016-09-02
    Description: We introduce the MUSIQUE algorithm and apply it to seismic wavefield recordings in California. The algorithm is designed to analyse seismic signals recorded by arrays of three-component seismic sensors. It is based on the MUSIC and the quaternion-MUSIC algorithms. In a first step, the MUSIC algorithm is applied in order to estimate the backazimuth and velocity of incident seismic waves and to discriminate between Love and possible Rayleigh waves. In a second step, the polarization parameters of possible Rayleigh waves are analysed using quaternion-MUSIC, distinguishing retrograde and prograde Rayleigh waves and determining their ellipticity. In this study, we apply the MUSIQUE algorithm to seismic wavefield recordings of the San Jose Dense Seismic Array. This array has been installed in 1999 in the Evergreen Basin, a sedimentary basin in the Eastern Santa Clara Valley. The analysis includes 22 regional earthquakes with epicentres between 40 and 600 km distant from the array and covering different backazimuths with respect to the array. The azimuthal distribution and the energy partition of the different surface wave types are analysed. Love waves dominate the wavefield for the vast majority of the events. For close events in the north, the wavefield is dominated by the first harmonic mode of Love waves, for farther events, the fundamental mode dominates. The energy distribution is different for earthquakes occurring northwest and southeast of the array. In both cases, the waves crossing the array are mostly arriving from the respective hemicycle. However, scattered Love waves arriving from the south can be seen for all earthquakes. Combining the information of all events, it is possible to retrieve the Love wave dispersion curves of the fundamental and the first harmonic mode. The particle motion of the fundamental mode of Rayleigh waves is retrograde and for the first harmonic mode, it is prograde. For both modes, we can also retrieve dispersion and ellipticity curves. Wave motion simulations for two earthquakes are in good agreement with the real data results and confirm the identification of the wave scattering formations to the south of the array, which generate the scattered Love waves visible for all earthquakes.
    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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  • 4
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    Rayleigh-wave dispersion curve: a proxy for site effect estimation? (2012)
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    Publication Date: 2017-04-04
    Description: One of the open issues on the effects of surface geology regards the estimation of site response when limited resources are available. In that restrictive context, one solution is to use soil characteristics as proxy. Despite its extensive use, the most common proxy, Vs30, is presently criticized because it cannot carry alone the main physics of site response. We propose here a statistical investigation of the capabilities of another proxy, the Rayleigh-wave dispersion curve, DC. When considered over a broad enough frequency band, it can provide deeper information missing in the single Vs30 parameter. A set of shear-wave velocity profiles measured for more than 600 Japanese KiK-net stations is used to compute theoretical dispersion curves (DC) and theoretical SH transfer functions (SH), while instrumental surface/downhole spectral ratios were calculated in a previous work (Cadet et al., 2011a). Canonical correlation techniques are applied to this large data set to analyze the relationship between DC and theoretical or empirical site responses. The results indicate very encouraging qualitative statistical relationships between DC and site amplification for numerically derived SH transfer functions, showing significant canonical couples with correlations up to 0.95. Results for instrumental surface/downhole transfer functions correspond to lower correlations (up to 0.73) but still allow the development of quantitative relationships.
    Description: Published
    Description: Santa Barbara (California, USA)
    Description: 4.1. Metodologie sismologiche per l'ingegneria sismica
    Description: open
    Keywords: site effects ; dispersion curve ; 04. Solid Earth::04.06. Seismology::04.06.04. Ground motion
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Type: Conference paper
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  • 5
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    Statistical estimation of earthquake site response from noise recordings (2012)
    Details
    Publication Date: 2017-04-04
    Description: Standard spectral ratio from earthquake recordings (SSR) is considered the reference empirical method for assessing site effects as a function of frequency. However, other estimates can be easily obtained from noise measurements (i.e., Horizontal-to-Vertical Spectral Ratio, HVN), even though their reliability in terms of amplitude is controversial. In the framework of the ToK ITSAK-GR (2006-2010) EC project, Cultrera et al. (2010) analyzed recordings from 64 sites worldwide, founding that it is possible to have linear combinations of the HVN amplitudes significantly correlated to linear combinations of the SSR. In the present paper we show how to estimate the SSR spectral ratios when only noise measurements are available, using the results of the canonical correlation analysis between SSR and HVN recorded at several sites. The SSR evaluation has been tested by a cross validation procedure: the expected SSR at each validation site are in turn estimated by a weighted average of the SSR values measured at the other sites; the weights are properly set to account more for the sites with similar behavior in terms of the canonical correlation between HVN and SSR. To evaluate the goodness of the estimation, we compared all the inferred and original SSR, and we performed a critical analysis on the spectral characteristics of earthquake site response that can be easily recovered from noise measurements.
    Description: Published
    Description: Santa Barbara (California, USA)
    Description: 4.1. Metodologie sismologiche per l'ingegneria sismica
    Description: open
    Keywords: site effects ; statistical seismology ; 04. Solid Earth::04.06. Seismology::04.06.04. Ground motion
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Type: Conference paper
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  • 6
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    Statistical investigation of site effects using earthquake and ambient noise recordings (part 2): reconstruction of earthquake spectral ratio from noise (2012)
    Details
    Publication Date: 2017-04-04
    Description: Standard spectral ratio from earthquake recordings (SSR) is considered the reference empirical method for assessing site effects as a function of frequency. However, other estimates can be easily obtained from noise measurements (i.e., Horizontal-to-Vertical Spectral Ratio, HVN), even though their reliability in terms of amplitude is controversial. In the framework of the ToK ITSAK-GR (2006-2010) and NERA (2007-2013) EC projects. Recordings from 64 sites from very different geographical areas where analyzed in the companion paper Cultrera et al. (2012); The authors evidenced statistically significant correlations between SSR and HVN through an appropriate linear combination of spectral ratio amplitudes using the canonical correlation technique. In the present paper we go one step further and show how to estimate the SSR spectral ratio of specific sites when only single station noise measurements are available, using the results of the canonical correlation analysis between SSR and HVN recorded at several sites and pertaining to the same area. A sistematic SSR evaluation has been conducted and tested by a cross validation procedure: the expected SSR at each validation site are in turn estimated by a weighted average of the SSR values measured at the other sites; the weights are properly set to account more for the sites with similar behavior in terms of the canonical correlation results between HVN and SSR. To evaluate the goodness of the estimation, we compared all the inferred and the original SSR, and we performed a critical analysis on the spectral characteristics of earthquake site response as recovered from noise measurements.
    Description: Submitted
    Description: 4.1. Metodologie sismologiche per l'ingegneria sismica
    Description: JCR Journal
    Description: reserved
    Keywords: Earthquake ground motion prediction ; site effects ; statistical seismology ; 04. Solid Earth::04.06. Seismology::04.06.04. Ground motion
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Type: manuscript
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