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  • 1
    Publication Date: 2020-12-14
    Description: The objective of this work is to perform a purely empirical assessment of the actual capabilities of the horizontal-to-vertical (H/V) spectral ratio technique to provide reliable and relevant information concerning site conditions and/or site amplification. This objective has been tackled through the homogeneous (re)processing of a large volume of earthquakes and ambient noise data recorded by different research teams in more than 200 sites located mainly in Europe, but also in the Caribbean and in Tehran. The original recordings were first gathered in a specific database with information on both the sites and recorded events. Then, for all sites close to an instrumented reference, average site-to-reference spectral ratios (“spectral ratio method” (SSR)) were derived in a homogeneous way (window selection, smoothing, signal-to-noise ratio threshold, averaging), as well as H/V ratios (“HVSRE–RF”) on earthquake recordings. H/V ratios were also obtained from noise recordings at each site (either specific measurements, or extracted from pre- or post-event noise windows). The spectral curves resulting from these three techniques were estimated reliable for a subset of 104 sites, and were thus compared in terms of fundamental frequency, amplitude and amplification bandwidth, exhibiting agreements and disagreements, for which interpretations are looked for in relation with characteristics of site conditions. The first important result consists in the very good agreement between fundamental frequencies obtained with either technique, observed for 81% of the analyzed sites. A significant part of the disagreements correspond to thick, low frequency, continental sites where natural noise level is often very low and H/V noise ratios do not exhibit any clear peak. The second important result is the absence of correlation between H/V peak amplitude and the actual site amplification measured on site-to-reference spectral ratios. There are, however, two statistically significant results about the amplitude of the H/V curve: the peak amplitude may be considered as a lower bound estimate of the actual amplification indicated by SSR (it is smaller for 79% of the 104 investigated sites), and, from another point of view, the difference in amplitude exhibits a questioning correlation with the geometrical characteristics of the sediment/basement interface: large SSR/HV differences might thus help to detect the existence of significant 2D or 3D effects.
    Description: Published
    Description: 75-108
    Description: 4.1. Metodologie sismologiche per l'ingegneria sismica
    Description: JCR Journal
    Description: reserved
    Keywords: Microtremor ; H/V ; Site effects ; SESAME ; 04. Solid Earth::04.02. Exploration geophysics::04.02.06. Seismic methods ; 04. Solid Earth::04.06. Seismology::04.06.09. Waves and wave analysis
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Type: article
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  • 2
    Publication Date: 2020-12-14
    Description: We analyze the dispersion characteristics of ambient noise vibrations. For this purpose, two-dimensional (2D) seismic array data were acquired in four different sites in the Colfiorito plain, an alluvial intramountain basin that exhibits strong site effects. Assuming seismic noise being mainly composed of surface waves, we derive one-dimensional (1D) shallow shear-velocity profiles through the inversion of dispersion curves measured by frequency–wavenumber (f-k) methods. The inverted shear-wave velocity profiles are consistent with a priori information for those sites that can be approximated by 1D simple models. In these cases, the use of passive records of seismic vibrations can be a valuable tool for determining the shallow velocity profile if a detailed depiction of velocity structure is not required. The theoretical dispersion curves for Rayleigh and Love waves were compared with the measured dispersion curves for vertical and horizontal components, respectively. This allows us to discuss qualitatively the composition of ambient vibrations (outlining a large proportion of Love waves in the noise wave field) and the effects of higher modes. We also use the single-station method for investigating the origin of the horizontal-to-vertical (H/V) peak in the plain of Colfiorito in terms of ellipticity of the fundamental Rayleigh mode.
    Description: Published
    Description: 1915-1933
    Description: reserved
    Keywords: surface waves, microtremor, f-k 2D analysis ; 04. Solid Earth::04.06. Seismology::04.06.04. Ground motion ; 04. Solid Earth::04.06. Seismology::04.06.09. Waves and wave analysis
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Type: article
    Format: 1456132 bytes
    Format: application/pdf
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