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  • 1
    Publication Date: 2017-03-23
    Description: We compare different methods to estimate frequency-domain amplification and duration lengthening of earthquake ground motion in the Mygdonian basin (Greece). Amplification is measured by standard spectral ratios (SSRs) of horizontal component or by single-station earthquake horizontal-to-vertical ratios (EHVRs). Duration lengthening is measured either by the group delay method ( Beauval et al. , 2003 ) and labeled GDDL, or based on the significant duration ( Trifunac and Brady, 1975 ) and labeled TBDL. The methods are applied both to high-quality recordings of the European experimental site EUROSEISTEST array and to a large set of 3D synthetics computed in a new basin model for 1260 sources regularly distributed in depth, distance, and azimuth from the center of the array. The analysis of the recordings in the center of the basin shows an anticorrelation between amplification and duration lengthening, that is, maxima (resp. minima) of GDDL correspond to minima (resp. maxima) of SSR. The maxima of GDDL are also found to coincide with those of SSR variability. This is confirmed by the analysis of the synthetics, which also reveals a pronounced north–south asymmetry of both amplification and duration lengthening caused by nonisotropic excitation of surface waves at the basin edges. We find that all estimates of site response depend on source location and that EHVR is also strongly sensitive to energy partitioning in the analyzed wavefield. We quantify the source-related variability of each estimate, discuss the biases in site response estimation using incomplete source catalogs, and investigate whether the azimuthal dependence of site response can be identified in the recordings. Electronic Supplement: Movies of simulated wave propagation, figures of surface-to-downhole standard spectral ratio (SSR), group delay duration lengthening (GDDL), earthquake horizontal-to-vertical ratio (EHVR), and synthetic waveforms.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
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  • 2
    Publication Date: 2012-04-01
    Description: The 3D wave-propagation characteristics of the 4 July 1978 aftershock (M 5.1) of the 20 June 1978 strong mainshock (M 6.5) that struck the city of Thessaloniki are studied using a 3D finite-difference approach. Synthetics are estimated for a dense grid of receivers and compared with available accelerograms from soft-soil sites in the city of Thessaloniki, exhibiting a good agreement both in time and frequency domain for the frequency band studied (0.7–3 Hz). Moreover, the spatial distribution of various measures of ground motion (peak values, spectral values) is used for the quantitative study of site effects in the broader city area. Comparisons show that the coastal zone, including the city harbor and large areas of the eastern parts of the city, exhibit high values of ground motion (and significant site amplifications), in good qualitative correlation with the observed damage distribution of the mainshock of the 1978 seismic sequence. Finally, the 3D synthetics are compared with available 2D simulations, as well as amplifications derived from macroseismic information for three typical cross sections spanning the urban area of the city. The comparisons confirm the strong spatial variability of ground motion throughout the Thessaloniki area, as well as the superiority of 3D modeling of actual recordings against previous modeling attempts. These results verify the practical usefulness of 3D wave-propagation tools for hazard mitigation, especially of specific target events, in complex geometry sedimentary basins such as the Thessaloniki area.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
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  • 3
    Publication Date: 2012-12-19
    Description: The knowledge of the local soil structure is important for the assessment of seismic hazards. A widespread, but time-consuming technique to retrieve the parameters of the local underground is the drilling of boreholes. Another way to obtain the shear wave velocity profile at a given location is the inversion of surface wave dispersion curves. To ensure a good resolution for both superficial and deeper layers, the used dispersion curves need to cover a wide frequency range. This wide frequency range can be obtained using several arrays of seismic sensors or a single array comprising a large number of sensors. Consequently, these measurements are time-consuming. A simpler alternative is provided by the use of the ellipticity of Rayleigh waves. The frequency dependence of the ellipticity is tightly linked to the shear wave velocity profile. Furthermore, it can be measured using a single seismic sensor. As soil structures obtained by scaling of a given model exhibit the same ellipticity curve, any inversion of the ellipticity curve alone will be ambiguous. Therefore, additional measurements which fix the absolute value of the shear wave velocity profile at some points have to be included in the inversion process. Small-scale spatial autocorrelation measurements or MASW measurements can provide the needed data. Using a theoretical soil structure, we show which parts of the ellipticity curve have to be included in the inversion process to get a reliable result and which parts can be omitted. Furthermore, the use of autocorrelation or high-frequency dispersion curves will be highlighted. The resulting guidelines for inversions including ellipticity data are then applied to real data measurements collected at 14 different sites during the European NERIES project. It is found that the results are in good agreement with dispersion curve measurements. Furthermore, the method can help in identifying the mode of Rayleigh waves in dispersion curve measurements.
    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
    Publication Date: 2014-12-05
    Description: The time-averaged shear-wave velocity in the upper 30 m of a site ( V S 30 ) is commonly used for ground-motion prediction. When measured velocities are unavailable, V S 30 is estimated from proxy-based relationships developed for application on global or local scales. We describe the development of a local relationship for Greece, which begins with compilation of a profile database (PDB) from published sources and engineering reports. The PDB contains 314 sites; 238 have profile depths≥30 m and 59 are within 100 m of accelerographs. We find existing relations for extrapolating a time-averaged velocity for depths less than 30 m to V S 30 to overpredict V S 30 . We present equations for these extrapolations. We then compile proxies for PDB sites, including terrain type, surface geology, and surface gradients at 30 and 3 arcsec resolution (from radar-derived digital elevation models [DEMs]). When checked against ground survey data, we find ground elevations from 3 arcsec DEMs to be more accurate relative to survey data than alternative 30, 9, and 1 arcsec DEMs. Drawing upon expert opinion, we develop geologic categories based on age, gradation, and depositional environment and assign such categories to PDB sites. We find an existing 30 arcsec gradient-based global model to be biased relative to local V S 30 data for gradients 〉~0.05 m/m. Bias relative to a California model is also found for four of the eight well-populated geomorphic categories, and new (local) values are provided. We find statistically significant effects of the 3 arcsec gradient on V S 30 for Quaternary and Tertiary materials but no gradient effect for those from the Mesozoic. Among Quaternary sediments, Holocene, mapped Quaternary (age unspecified), and mixed/fine-gradation materials exhibit consistent V S 30 -gradient trends, whereas Pleistocene and coarse-gradation sediments have faster velocities. For the study region, we recommend use of the modified terrain- and geology-based methods in combination for proxy-based V S 30 estimation. Online Material: Profile database (spreadsheet) and figures of elevation residuals.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
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  • 5
    Publication Date: 2015-06-09
    Description: In a low-seismicity context, the use of numerical simulations becomes essential due to the lack of representative earthquakes for empirical approaches. The goals of the EUROSEISTEST Verification and Validation Project (E2VP) are to provide (1) a quantitative analysis of accuracy of the current, most advanced numerical methods applied to realistic 3D models of sedimentary basins (verification) and (2) a quantitative comparison of the recorded ground motions with their numerical predictions (validation). The target is the EUROSEISTEST site located within the Mygdonian basin, Greece. The site is instrumented with surface and borehole accelerometers, and a 3D model of the medium is available. The simulations are performed up to 4 Hz, beyond the 0.7 Hz fundamental frequency, thus covering a frequency range at which ground motion undergoes significant amplification. The discrete representation of material heterogeneities, the attenuation model, the approximation of the free surface, and nonreflecting boundaries are identified as the main sources of differences among the numerical predictions. The predictions well reproduce some, but not all, features of the actual site effect. The differences between real and predicted ground motions have multiple origins: the accuracy of source parameters (location, hypocentral depth, and focal mechanism), the uncertainties in the description of the geological medium (damping, internal sediment layering structure, and shape of the sediment-basement interface). Overall, the agreement reached among synthetics up to 4 Hz despite the complexity of the basin model, with code-to-code differences much smaller than predictions-to-observations differences, makes it possible to include the numerical simulations in site-specific analysis in the 3D linear case and low-to-intermediate frequency range.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
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  • 6
    Publication Date: 2020-12-14
    Description: Within the scope of the EC-projects NERIES and ITSAK-GR we have applied a procedure able to combine a multi-model space parameterization and an information theoretic approach in analysis of dispersion curve inversion. In detail we considered the dispersion curve assessed at 14 strong motion European sites. At each site we investigated the model space through four different parameterization groups within the wavelength range estimated by actual dispersion curves. In order to explore the influence of model space we increased progressively the number of layers for each parameterization. We therefore addressed the model evaluation among a set of competing models obtained by inversion following the corrected Akaike’s Information Criterion(AICc). By using such information-theoretic approach, we found an acceptable agreement between the inverted shear-velocity profiles of the best models and the available borehole results.
    Description: IZIIS
    Description: Published
    Description: Ohrid, Republic of Macedonia
    Description: 4.1. Metodologie sismologiche per l'ingegneria sismica
    Description: open
    Keywords: surface waves, inversion of dispersion curves, Akaike's information criterion ; 04. Solid Earth::04.02. Exploration geophysics::04.02.99. General or miscellaneous
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Type: Extended abstract
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  • 7
    Publication Date: 2020-12-14
    Description: The inversion of surface-wave dispersion curve to derive shear-wave velocity profile is a very delicate process dealing with a non-unique problem, which is strongly dependent on the model space parameterization. When independent and reliable information are not available, the selection of most representative models within the ensemble produced by the inversion is often difficult. We present a strategy in the inversion of dispersion curves able to investigate the influence of the parameterization of the model space, and to select a ‘’best’’ class of models. We analyze surface-wave dispersion curves measured at 14 European strong-motion sites within the EC-project NERIES. We focus on the inversion task exploring the model space by means of four distinct parameterization classes composed of layers progressively added over a half-space. The classes differ in the definition of the shear-wave velocity profile; we consider models with uniform velocity as well as models with increasing velocity with depth. At each site and for each model parameterization, we perform an extensive surface-wave inversion (200100 models for 5 seeds) using the conditional neighbourhood algorithm. We address the model evaluation following the corrected Akaike’s Information Criterion (AICc) which combines the concept of misfit to the number of degrees of freedom (dof) of the system. The misfit is computed as least-squares estimation between theoretical and observed dispersion curve. The model complexity is accounted in a penalty term by AICc. By applying such inversion strategy on 14 strong-motion sites, we find that the best parameterization of the model space is mostly 3-4 layers over a half-space; where the shear-wave velocity of the uppermost layers can follow uniform or power-law dependence with depth. The shear-wave velocity profiles derived by inversion agree with shear-wave velocity profiles provided by borehole surveys at approximately 80% of the sites.
    Description: Published
    Description: B147–B166
    Description: 4.1. Metodologie sismologiche per l'ingegneria sismica
    Description: JCR Journal
    Description: restricted
    Keywords: surface-wave dispersion inversion ; 04. Solid Earth::04.06. Seismology::04.06.04. Ground motion
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Type: article
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  • 8
    Publication Date: 2020-12-15
    Description: The Seismic Hazard Harmonization in Europe (SHARE) project, which began in June 2009, aims at establishing new standards for probabilistic seismic hazard assessment in the Euro-Mediterranean region. In this context, a logic tree for ground-motion prediction in Europe has been constructed. Ground-motion prediction equations (GMPEs) and weights have been determined so that the logic tree captures epistemic uncertainty in ground-motion prediction for six different tectonic regimes in Europe. Here we present the strategy that we adopted to build such a logic tree. This strategy has the particularity of combining two complementary and independent approaches: expert judgment and data testing. A set of six experts was asked to weight pre-selected GMPEs while the ability of these GMPEs to predict available data was evaluated with the method of Scherbaum et al. (Bull Seismol Soc Am 99:3234–3247, 2009). Results of both approaches were taken into account to commonly select the smallest set of GMPEs to capture the uncertainty in ground-motion prediction in Europe. For stable continental regions, two models, both from eastern North America, have been selected for shields, and three GMPEs from active shallow crustal regions have been added for continental crust. For subduction zones, four models, all non-European, have been chosen. Finally, for active shallow crustal regions, we selected four models, each of them from a different host region but only two of them were kept for long periods. In most cases, a common agreement has been also reached for the weights. In case of divergence, a sensitivity analysis of the weights on the seismic hazard has been conducted, showing that once the GMPEs have been selected, the associated set of weights has a smaller influence on the hazard.
    Description: EC-Research Framework programme FP7, Seismic Hazard Harmonization in Europe, contract number 226967.
    Description: Published
    Description: 451-473
    Description: 4.1. Metodologie sismologiche per l'ingegneria sismica
    Description: 4.2. TTC - Modelli per la stima della pericolosità sismica a scala nazionale
    Description: JCR Journal
    Description: restricted
    Keywords: GMPE ; logic tree ; seismic hazard ; SHARE ; 04. Solid Earth::04.06. Seismology::04.06.04. Ground motion ; 04. Solid Earth::04.06. Seismology::04.06.11. Seismic risk
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Type: article
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  • 9
    Publication Date: 2020-12-14
    Description: The knowledge of the local soil structure is important for the assessment of seismic hazards. A widespread, but time-consuming technique to retrieve the parameters of the local underground is the drilling of boreholes. Another way to obtain the shear wave velocity profile at a given location is the inversion of surface wave dispersion curves. To ensure a good resolution for both superficial and deeper layers, the used dispersion curves need to cover a wide frequency range. This wide frequency range can be obtained using several arrays of seismic sensors or a single array comprising a large number of sensors. Consequently, these measurements are time-consuming. A simpler alternative is provided by the use of the ellipticity of Rayleigh waves. The frequency dependence of the ellipticity is tightly linked to the shear wave velocity profile. Furthermore, it can be measured using a single seismic sensor. As soil structures obtained by scaling of a given model exhibit the same ellipticity curve, any inversion of the ellipticity curve alone will be ambiguous. Therefore, additional measurements which fix the absolute value of the shear wave velocity profile at some points have to be included in the inversion process. Small-scale spatial autocorrelation measurements or MASW measurements can provide the needed data. Using a theoretical soil structure, we show which parts of the ellipticity curve have to be included in the inversion process to get a reliable result and which parts can be omitted. Furthermore, the use of autocorrelation or high-frequency dispersion curves will be highlighted. The resulting guidelines for inversions including ellipticity data are then applied to real data measurements collected at 14 different sites during the European NERIES project. It is found that the results are in good agreement with dispersion curve measurements. Furthermore, the method can help in identifying the mode of Rayleigh waves in dispersion curve measurements.
    Description: Published
    Description: 207-229
    Description: 3T. Pericolosità sismica e contributo alla definizione del rischio
    Description: JCR Journal
    Description: restricted
    Keywords: Inverse theory Surface waves and free oscillations Site effects Computational seismology Wave propagation ; 04. Solid Earth::04.02. Exploration geophysics::04.02.06. Seismic methods
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Type: article
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  • 10
    Publication Date: 2017-04-04
    Description: The Horizontal-to-Vertical Spectral Ratio from earthquake recordings (HVSR) and from ambient noise recordings (HVN) realistically indicate fundamental frequency but, for the majority of the worldwide examined sites, they do not provide reliable amplification curves as predicted by the earthquake Spectral Ratio to a reference site (SSR). Given the fact that HVSR and especially HVN can be easily obtained, it is challenging to search for a meaningful correlation with SSR amplification functions in the whole frequency band. To this aim we used recordings from 75 sites worldwide and we applied a multivariate statistical approach (the canonical correlation analysis) to investigate and quantify any correlation among spectral ratios and, from a subset of sites in sedimentary basins, with geometrical and geophysical parameters (S-wave velocity and the 2D geometry of the valley). Results show a correlation between SSR and HVN or HVSR that can be statistically quantified, and it is modulated in specific frequency range. Moreover, the larger are the Vs30, the bedrock depth and the valley’s shape ratio, the higher is the low-frequency amplitude in HVSR and HVN, and the lower is the high-frequency value. Despite the limitation of the database, the correlation results can be used to estimate the excepted SSR spectral ratio of specific sites in the investigated areas when only noise measurements are available(see the companion paper De Rubeis et al., 2012).
    Description: Submitted
    Description: 4.1. Metodologie sismologiche per l'ingegneria sismica
    Description: JCR Journal
    Description: reserved
    Keywords: Statistical seismology ; Earthquake ground motions ; Site effects ; Basin effects ; 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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