Etienne Bertrand

Erratum to: Site effect assessment using KiK-net data: part 2—site amplification prediction equation based on f0 and Vsz

This paper presents empirical correlations between amplification factors and simple site parameters derived from a large subset of the KiK-net data. The amplification factor is estimated from the ratios between the surface and down-hole horizontal response spectra, corrected for the varying depths and impedance of the down-hole sites (Cadet et al. in Site effect assessment using KiK-net data—part 1—a simple correction procedure for surface/downhole spectral ratios, 2011). Several site parameters are selected on the basis of their simplicity and availability at relatively low cost. They are the shallow time-average velocities VSZ, with z equal to 5, 10, 20 and 30 m, and the fundamental frequency f0. The amplification factors are then correlated with each of the individual site parameters; four other “twin-parameter”—couples (f0, VSZ)—are also considered and the correlation with amplification factors is performed through a normalization of the frequencies by each site fundamental frequency. The quality of the correlations is given by a misfit compared with the original data variance. The largest variance reduction is obtained with twin-parameter characterizations, out of which the couple (f0, VS30) proves to provide the lower misfit. The performance of single parameter correlations is relatively lower; however, the best single parameter proves to be the fundamental frequency, which provides smaller misfit than the Vsz parameters. A comparison is also performed with the amplification factors recommended in European regulations, showing that it is possible right now to significantly improve both the site characterization criteria and the associated amplification factors, for use in building codes and microzonation studies.

International Benchmark on Numerical Simulations for 1D, Nonlinear Site Response (PRENOLIN): Verification Phase Based on Canonical Cases

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.

Influence of the Vs profiles beyond 30 m depth on linear site effects: assessment from the KiK-net data

Site effects may be assessed using a standard soil classification parameter, VS30 (the harmonic average shear-wave velocity in the first 30 m); however, this index does not account for the complexity of the velocity profile, especially its variability at depth. In the present study, in addition to VS30, we propose consideration of the gradient of the VS profile from 0 to 30 m depth, denoted B30. A lower gradient value means low velocity increases with depth; a higher gradient indicates a rapid velocity increase with depth in the shallow layers. In addition, we consider the fundamental resonance frequency of the soil (f0), which has been shown to be a relevant parameter for site-effect assessment and which is obtained from the empirical site response. Using the Japanese KiK-net database, we analyze the variability of the VS profiles and the empirical borehole site responses of selected sites through the VS30, the velocity gradient B30, and f0. We select 289 sites for which the 1D linear numerical modeling is close to the empirical site response and a VS at the downhole station is greater than 1000 m=s. For a given VS30 class, B30, and f0 can be used to distinguish between two types of sites: deep sedimentary sites and sites with high velocity contrast at shallow depths. We find that, even if the gradient is calculated using shallow information, its use improves the site amplification characterization, compared to using only VS30, by reducing the intersite site-response variability. As expected, however, this improvement is limited for deep sedimentary sites. On the other hand, f0 is able to reduce the intersite response variability for deep sedimentary sites even though it is limited to specific VS30 classes. Thus, the combined use of VS30, B30, and f0 improves the assessment of linear site amplification.

The deep structure of Corsica as inferred by a broad band seismological profile

To investigate the influence of inherited inhomogeneity in a lithosphere under extension, we studied the deep structure of Corsica, an island in the Mediterranean sea, at the boundary of the extensional Tyrrhenian basin, using a temporary array of eight broad-band seismographs. Between the stations in the western Hercynian part of the island and the stations in eastern Alpine Corsica, an average static station delay of 0.4 s was observed for P waves, and 0.7 s for S waves. The crustal structure is obtained through seismic data, gravity modeling and the receiver function method. The difference in crustal structure between the west and east of Corsica can explain only about half of the anomaly. Consequently we postulate a sharp increment in the thickness of the lithosphere to account for the remaining 0.2 s of P anomaly.

PRENOLIN: International Benchmark on 1D Nonlinear Site‐Response Analysis—Validation Phase Exercise

This article presents the main results of the validation phase of the PRENOLIN project. PRENOLIN is an international benchmark on 1D nonlinear (NL) site‐response analysis. This project involved 19 teams with 23 different codes tested. It was divided into two phases; with the first phase verifying the numerical solution of these codes on idealized soil profiles using simple signals and real seismic records. The second phase described in this article referred to code validation for the analysis of real instrumented sites. This validation phase was performed on two sites (KSRH10 and Sendai) of the Japanese strong‐motion networks KiK‐net and Port and Airport Research Institute (PARI), respectively, with a pair of accelerometers at surface and depth. Extensive additional site characterizations were performed at both sites involving in situ and laboratory measurements of the soil properties. At each site, sets of input motions were selected to represent different peak ground acceleration (PGA) and frequency content. It was found that the code‐to‐code variability given by the standard deviation of the computed surface‐response spectra is around 0.1 (in log10 scale) regardless of the site and input motions. This indicates a quite large influence of the numerical methods on site‐effect assessment and more generally on seismic hazard. Besides, it was observed that site‐specific measurements are of primary importance for defining the input data in site‐response analysis. The NL parameters obtained from the laboratory measurements should be compared with curves coming from the literature. Finally, the lessons learned from this exercise are synthesized, resulting also in a few recommendations for future benchmarking studies, and the use of 1D NL, total stress site‐response analysis.

Une expérience multi-antennes à Annot pour l'analyse des effets de site en sismologie

Abstract For two months, ground motions induced by natural seismicity were recorded by 4 dense arrays, each of them including 9 seismological receivers. The target zone is a 400 km2 area and displays a characteristic topographic organization. The sites correspond to flat valleys filled with surficial soft sediments. The data recorded during this experiment will be used to characterize the wavefields through the different valleys (main energetic azimuthal contributions, apparent velocities) in the aim to quantify site effects.

Numerical and Empirical Simulation of Linear Elastic Seismic Response of a Building: The Case of Nice Prefecture

The structural motion of a tall reinforced concrete (RC) building on alluvial soil in Nice (France) is continuously recorded using accelerometers. The structural behavior of the building is studied using operational modal analysis (OMA) to identify its dynamic properties, a finite element (FE) model to reproduce the building response, and empirical Greens functions (EGFs) to generate the structural response to ground motions stronger than those registered in the analyzed seismic area. These different approaches are applied for the analysis of seismic response of the instrumented building and results are consistent. The FE model is calibrated by comparing natural frequencies and mode shapes with those obtained using OMA. Numerically-simulated time histories are qualitatively and quantitatively compared with recordings showing good agreement. Based on regional earthquakes, linear seismic response of the building is simulated for a stronger scenario earthquake using EGF. This approach allows for structural deformation analysis of existing buildings without the need of structural plans and mechanical parameter calibration in the case where the seismic response is within linear elastic regime.