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  • 1
    Electronic Resource
    Electronic Resource
    About the Efficiency of Numerical 1-D and 2-D Modelling of Site Effects in Basin Structures (2000)
    Springer
    Pure and applied geophysics 157 (2000), S. 319-342 
    Details
    ISSN: 1420-9136
    Keywords: Key Words: Site effects, seismic hazard analysis, numerical 1-D and 2-D modelling, reflectivity method, Aki-Larner method, finite difference modelling.
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract —In the present study we compare results obtained from experimental estimates of local site amplification effects with those from numerical modelling using four different techniques. We benefit from an extremely precise knowledge of the near-surface structure and experimental estimates of the local amplification factors which are determined from seismic weak-motion data recorded by a dense array across a sedimentary basin at a European test-site in Northern Greece. The possibilities and limitations of the different modelling techniques (a 1-D technique, and three 2-D techniques) to model the effects of local amplification effects are tested. Amplifications calculated by the numerical techniques are only qualitatively compared with observed data from experimental studies in the time domain and directly in the frequency domain.¶As a result we conclude that, in the case of a complex subsurface geometry, 1-D modelling underestimates the amplification patterns in terms of absolute amplification level, and cannot correctly account for resonant frequencies, at least for modes higher than the fundamental mode. If a more realistic incident wave field than just a plane wave is taken into account, 2-D modelling reveals the fundamental frequency and shows adequate amplifications not only at the fundamental frequency but also at higher frequencies. The general trend of the amplifications at the lowest frequencies is well determined by the 2-D numerical modelling, and can therefore supply information for seismic risk analysis.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s000240050002
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  • 2
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    Earthquake Ground Motion in the Mygdonian Basin, Greece: The E2VP Verification and Validation of 3D Numerical Simulation up to 4 Hz (2015)
    Seismological Society of America (SSA)
    Details
    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
    Published by Seismological Society of America (SSA)
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  • 3
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    Empirical Quantification of the Impact of Nonlinear Soil Behavior on Site Response (2016)
    Seismological Society of America (SSA)
    Details
    Publication Date: 2016-07-26
    Description: We present an extensive analysis of the quantitative impact of the nonlinear soil behavior on site response at 174 sites of the Japanese Kiban–Kyoshin (KiK-net) network. The nonlinear to linear site-response ratio (RSR NL–L ) is calculated by comparing the surface/downhole Fourier spectral ratio for strong events and for weak events. Three thresholds of surface peak ground acceleration (PGA) are tested to characterize the strong events: 100, 200, and 300 cm/s 2 , whereas weak events correspond to surface PGA in the 0.1–25 cm/s 2 range. This ratio exhibits a typical shape; with a low-frequency part above 1 and a high-frequency part generally below 1, separated by a transition zone around a site-dependent frequency labeled f NL (characterized by RSR NL–L =1). The average maximum amplitudes of RSR NL–L are 1.4, 1.5, and 1.6, and the minimums are 0.6, 0.5, and 0.5 for PGA thresholds 100, 200, and 300 cm/s 2 , respectively, showing that nonlinear soil behavior results in significant site-response modifications even for moderate PGA values of 100 cm/s 2 . The f NL value exhibits a satisfactory correlation with site classifications based on either V S 30 (travel-time averaged shear-wave velocity over the top 30 m) or f 0 (site fundamental frequency): f NL decreases when either V S 30 or f 0 decreases. In addition, the amount of the low-frequency amplification increase depends on V S 30 and reaches a maximum of 1.6 for high V S 30 soil classes associated with shallow thin soft-soil layer underlain by stiff substratum. The average high-frequency decrease is about 0.5 for all soil classes defined from either V S 30 or f 0 ; for a few sites, however, this decrease is replaced by an increase as reported in previous studies, in relation with water contents and pore-pressure issues. The increase of amplification below f NL is found to be a quasi-systematic consequence of nonlinear soil behavior, which should be emphasized, because it can reach up to 1.6 for high V S 30 sites. Online Material: Figures of site-response ratio (RSR) curves and tables of site characteristics.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
    Published by Seismological Society of America (SSA)
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  • 4
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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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  • 5
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    The Analysis of Long-Term Frequency and Damping Wandering in Buildings Using the Random Decrement Technique (2013)
    Seismological Society of America (SSA)
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    Publication Date: 2013-02-07
    Description: The characterization and monitoring of buildings is an issue that has attracted the interest of many sectors over the last two decades. With the increasing use of permanent, continuous, and real-time networks, ambient vibrations can provide a simple tool for the identification of dynamic building parameters. This study is focused on the long-term variation of frequency and damping in several buildings, using the Random Decrement Technique (RDT). RDT provides a fast, robust, and accurate long-term analysis and improves the reliability of frequency and damping measurements for structural health monitoring. This reveals particularly useful information for finding out precisely how far changes in modal parameters can be related to changes in physical properties. This paper highlights the reversible changes of the structure’s dynamic parameters, correlated with external forces, such as temperature and exposure to the sun. Contrasting behaviors are observed, including correlation and anticorrelation with temperature variations.
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    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
    Published by Seismological Society of America (SSA)
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  • 6
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    A Nonlinear Site-Amplification Model for the Next Pan-European Ground-Motion Prediction Equations (2013)
    Seismological Society of America (SSA)
    Details
    Publication Date: 2013-02-07
    Description: A site-amplification model for shallow crustal regions that considers both linear and nonlinear soil effects is proposed. The original functional form of the model was developed by Walling et al. (2008) (WAS08) using stochastic simulations and site-response analysis. The major difference between the proposed model and WAS08 is that our site-amplification expression is entirely based on empirical data. To comply with this objective, a database with the most recent V S 30 information from the pan-European region has been compiled. This feature of the model encourages its use for the future ground-motion prediction equations (GMPEs) that will be devised particularly for this region. Worldwide accelerograms are also considered to have a better representation of the soil behavior under strong-motion excitations. As an auxiliary tool a GMPE for reference-rock sites is also developed to calculate the site-amplification factors. The coefficients of the site-amplification model as well as the reference-rock model are computed by applying the random-effects regression technique proposed by Abrahamson and Youngs (1992) . Preliminary results of this article suggest a more comprehensive study for the revision of site factors in Eurocode 8 ( European Committee for Standardization [CEN], 2004 ). Online Material: Tables of database statistics, regression coefficients, and standard deviations.
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    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
    Published by Seismological Society of America (SSA)
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  • 7
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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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  • 8
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    Adapting the Neural Network Approach to PGA Prediction: An Example Based on the KiK-net Data (2012)
    Seismological Society of America (SSA)
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    Publication Date: 2012-08-01
    Description: We have investigated the artificial neural network method for the derivation of physically sound, easy-to-handle, predictive ground-motion models. Avoiding the specification of any a priori functional form, artificial neural networks (ANNs) provide fully data-driven predictive models and allow the testing of the relative importance of the effects of independent variables on seismic ground motion. This approach is applied here as an illustrative example, using a large subset of the KiK-net seismic database, which includes 3891 records from 398 sites and 335 earthquakes. The independent variables tested are the moment magnitude ( M w ), the focal depth, the epicentral distance ( R ), the site resonance frequency ( f 0 ), and the time-averaged shear-wave velocity down to 30 m ( V S 30 ). The neural model output is the horizontal peak ground acceleration (PGA). The standard deviation obtained for the model of 0.34 is comparable to, or slightly lower than, conventional ground-motion prediction equations (GMPEs). Although not imposed a priori , these results have a number of physically sound features: clear magnitude and depth dependency of the decay of the ground motion with distance, near-fault saturation for large magnitudes, and indications of nonlinear effects in softer soils. In addition, the ANN method also allows the ranking of the importance of explanatory input parameters: while M w and R represent the key control parameters, depth is shown to mainly affect moderate magnitude events in the epicentral area. Considering the two site parameters V S 30 and f 0 , the latter is shown to be more efficient in fitting the data than V S 30 . The ability to implement this model using Microsoft Excel or another simple script is demonstrated, which opens a vast field for its use. Online Material: MATLAB script and parameters used for ANN model implementation.
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    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
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  • 9
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    International Benchmark on Numerical Simulations for 1D, Nonlinear Site Response (PRENOLIN): Verification Phase Based on Canonical Cases (2016)
    Seismological Society of America (SSA)
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    Publication Date: 2016-10-08
    Description: PREdiction of NOn-LINear soil behavior (PRENOLIN) is an international benchmark aiming to test multiple numerical simulation codes that are capable of predicting nonlinear seismic site response with various constitutive models. One of the objectives of this project is the assessment of the uncertainties associated with nonlinear simulation of 1D site effects. A first verification phase (i.e., comparison between numerical codes on simple idealistic cases) will be followed by a validation phase, comparing the predictions of such numerical estimations with actual strong-motion recordings obtained at well-known sites. The benchmark presently involves 21 teams and 23 different computational codes. We present here the main results of the verification phase dealing with simple cases. Three different idealized soil profiles were tested over a wide range of shear strains with different input motions and different boundary conditions at the sediment/bedrock interface. A first iteration focusing on the elastic and viscoelastic cases was proved to be useful to ensure a common understanding and to identify numerical issues before pursuing the nonlinear modeling. Besides minor mistakes in the implementation of input parameters and output units, the initial discrepancies between the numerical results can be attributed to (1) different understanding of the expression "input motion" in different communities, and (2) different implementations of material damping and possible numerical energy dissipation. The second round of computations thus allowed a convergence of all teams to the Haskell–Thomson analytical solution in elastic and viscoelastic cases. For nonlinear computations, we investigate the epistemic uncertainties related only to wave propagation modeling using different nonlinear constitutive models. Such epistemic uncertainties are shown to increase with the strain level and to reach values around 0.2 (log 10 scale) for a peak ground acceleration of 5 m/s 2 at the base of the soil column, which may be reduced by almost 50% when the various constitutive models used the same shear strength and damping implementation.
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    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
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  • 10
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    Ground structure imaging by inversions of Rayleigh wave ellipticity: sensitivity analysis and application to European strong-motion sites (2012)
    Oxford University Press
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    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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