This article aims to provide an overview of successive seismic zonations in mainland France and probabilistic seismic hazard studies performed over the previous \({\approx }\)30 years. A short ...presentation is given on the engineering and regulatory framework that shapes the background of seismic zonation and hazard studies, followed by a scientific overview of published probabilistic seismic hazard estimations. The components of these past PSHA studies are summarized (seismic source models, ground-motion models, management of uncertainties). The evolution of hazard estimates with time is displayed for a set of representative cities. Finally, three generations of official seismic zonation maps in France are presented (1967, 1986, 2004), and the evolution of methods and results is highlighted. This review shows that the academic community has previously had little involvement in this topic. As current zonation relies on outdated models, there is an opportunity for scientific and engineering communities to efficiently work together to build the next generation of probabilistic seismic hazard models for France.
Many applications related to ground-motion studies and engineering seismology benefit from the opportunity to easily download large dataset of earthquake recordings with different magnitudes. In such ...applications, it is important to have a reliable seismic characterization of the stations to introduce appropriate correction factors for including site amplification. Generally, seismic networks in Europe describe the site properties of a station through geophysical or geological reports, but often ad-hoc field surveys are missing and the characterization is done using indirect proxy. It is then necessary to evaluate the quality of a seismic characterization, accounting for the available site information, the measurements procedure and the reliability of the applied methods to obtain the site parameters.In this paper, we propose a strategy to evaluate the quality of site characterization, to be included in the station metadata. The idea is that a station with a good site characterization should have a larger ranking with respect to one with poor or incomplete information. The proposed quality metric includes the computation of three indices, which take into account the reliability of the available site indicators, their number and importance, together with their consistency defined through scatter plots for each single pair of indicators. For this purpose, we consider the seven indicators identified as most relevant in a companion paper (Cultrera et al.
2021
): fundamental resonance frequency, shear-wave velocity profile, time-averaged shear-wave velocity over the first 30 m, depth of both seismological and engineering bedrock, surface geology and soil class.
Site-effect assessments performed through earthquake-based approaches, such as the standard spectral ratio (SSR), require good quality records of numerous earthquakes. In contrast, the use of ambient ...noise appears to be an attractive solution for ease and rapid computation of site responses with sufficient spatial resolution (microzonation), especially in low seismicity areas. Two main approaches are tested here: the horizontal-to-vertical spectral ratio (HVSR) and the noise-based SSR (SSRn). The HVSR uses the relative amplitude of the horizontal and vertical components of the ambient noise. Instead, the SSRn defines the spectral ratio between the seismic noise recorded simultaneously at a site and at a rock reference station, similar to earthquake-based SSR. While the HVSR is currently used in hundreds of site-specific studies, the SSRn approach has been gradually abandoned since the 1990s. In this study, we compare the results obtain from these two approaches with those of earthquake-based SSR. This comparison is carried out for two sedimentary basins, in Provence (southeastern France) and in Argostoli (western Greece). In agreement with the literature, the HVSR does not provide more than the fundamental resonance frequency of the site (f0). The SSRn leads to overestimation of the SSR amplification factors for frequencies higher than the minimal f0 of the basin (f0min). This discrepancy between SSRn and SSR is discussed, and appears to be mainly dependent on the local geological configuration. We thus introduce the hybrid standard spectral ratio (SSRh) approach, which aims to improve upon the SSRn by adding an intermediate station inside the basin for which the SSR is known. This station is used in turn as a local reference inside the basin for the SSRn computation. The SSRh provides site transfer functions very similar to those of the SSR, in a broad frequency range. Based on these results, the SSRn (or SSRh) should be further tested and should receive renewed attention for microzonation inside sedimentary basins.
The impact of non-linear soil behavior on site response may be described by the non-linear to linear site response ratio RSRNL introduced in Régnier et al. (2013). This ratio most often exhibits a ...typical shape with an amplitude above one below a site-specific frequency fNL, and an amplitude below one beyond fNL. This paper presents an investigation of the correlation between this RSRNL ratio and various parameters used to characterize the site (Site Condition Proxies: SCPs) and the seismic loading level (Ground Motion Intensity Measures: GMIMs).
The data used in this analysis come from sites of the Japanese Kiban–Kyoshin (KiK-net) network, for which the nonlinear to linear site-response ratio (RSRNL) is obtained by comparing the surface/down-hole Fourier spectral ratio for strong events and for weak events. The five SCPs are VS30, the minimum velocity of the soil profile (Vsmin), an index of the velocity gradient over the top 30 m (B30), the fundamental frequency f0HV, as measured from the H/V earthquake ratio, and the corresponding amplitude A0HV. The seven GMIMs are PGA, PGV, PGV/VS30 (peak strain proxy), IA (Arias Intensity), CAV (Cumulative Absolute Velocity), arms (Root Mean Square Acceleration) and Trifunac-Brady Duration (DT). The original data set consists of a total of 2927 RSRNL derived from KiK-net recordings at 132 sites. To assign an equal weight to each site, and to avoid any bias linked to sites with many recordings, for each GMIM, this original data set is grouped in 15 different intervals corresponding to fixed fractiles of the statistical distribution of the considered GMIM (every 10% from F10 to F50, and every 5% from F55 to F100). In each group, the average RSRNL-GM for each site is computed. For each of these seven advanced data sets, a neural network approach is used to predict the behavior of RSRNL-GM as a function of the corresponding GMIM, and one or two SCPs. The performance of each model is quantified through the average variance reduction coefficient μ(Rc) in a fixed frequency range. This sensitivity study is performed in the normalized frequency (f/fNL) domain to identify the best combinations (GMIM, SCPs) providing the largest variance reduction, and then in the absolute frequency domain for the final optimal combination. The optimal combinations GMIM, two-SCPs are triplets PGV/VS30, VS30-f0HV; μ(Rc) = 18.6%, PGV/VS30, VS30-A0HV; μ(Rc) = 18.16%, PGV, VS30-f0HV; μ(Rc) = 17.3% and PGA, B30-A0HV; μ(Rc) = 17.2%. The final absolute frequency model with the best triplet makes it possible to predict the non-linear response of a given site knowing its linear, weak-motion response, and two site proxy parameters, for wide ranges of the considered ground motion parameters.
Road-map to obtain non-linear to linear site response ratio (RSRNL-GM). In step 1 the initial dataset of RSRNL, SCPs and GMIM are presented (Régnier et al., 2016a). Step 2: the RSRNL are normalized by fNL. Step 3: Seven different “advanced data sets” (RSRNL-GM) are produced. Step 4 illustrates the sensitivity study to choose the best SCPs and GMIMs in the normalized frequency domain. Step 5: the best model of RSRNL-GM in the absolute frequency domain is established using ANN approach. Display omitted
Performance of site conditions proxies (SCP) and ground motion intensity measures (GMIM) for predicting the nonlinear modulation of site response: •The best SCPs are VS30 and the fundamental frequency f0HV•The worst performing SCPs are the minimum velocity VSmin and the fundamental peak amplitude A0HV.•The best performing GMIM is the peak strain proxy PGV/VS30, but only when associated with at least two SCPs•The worst performing GMIM is the Trifunac-Brady duration.•A satisfactory estimate of non-linear modulation of linear site response can be obtained from VS30, f0HV and PGV/VS30
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.
During the past two decades, the use of ambient vibrations for modal analysis of structures has increased as compared to the traditional techniques (forced vibrations). The frequency domain ...decomposition (FDD) method is nowadays widely used in modal analysis because of its accuracy and simplicity. In this paper, we first present the physical meaning of the FDD method to estimate the modal parameters. We discuss then the process used for the evaluation of the building stiffness deduced from the modal shapes. The models considered here are 1D lumped-mass beams and especially the shear beam. The analytical solution of the equations of motion makes it possible to simulate the motion due to a weak to moderate earthquake and then the inter-storey drift knowing only the modal parameters (modal model). This process is finally applied to a nine-storey reinforced concrete (RC) dwelling in Grenoble (France). We successfully compared the building motion for an artificial ground motion deduced from the model estimated using ambient vibrations and recorded in the building. The stiffness of each storey and the inter-storey drift were also calculated.
Most modern seismic codes account for site effects using an amplification factor (AF) that modifies the rock acceleration response spectra in relation to a “site condition proxy,” i.e., a parameter ...related to the velocity profile at the site under consideration. Therefore, for practical purposes, it is interesting to identify the site parameters that best control the frequency-dependent shape of the AF. The goal of the present study is to provide a quantitative assessment of the performance of various site condition proxies to predict the main AF features, including the often used short- and mid-period amplification factors,
F
a
and
F
v
, proposed by Borcherdt (in Earthq Spectra 10:617–653,
1994
). In this context, the linear, viscoelastic responses of a set of 858 actual soil columns from Japan, the USA, and Europe are computed for a set of 14 real accelerograms with varying frequency contents. The correlation between the corresponding site-specific average amplification factors and several site proxies (considered alone or as multiple combinations) is analyzed using the generalized regression neural network (GRNN). The performance of each site proxy combination is assessed through the variance reduction with respect to the initial amplification factor variability of the 858 profiles. Both the whole period range and specific short- and mid-period ranges associated with the Borcherdt factors
F
a
and
F
v
are considered. The actual amplification factor of an arbitrary soil profile is found to be satisfactorily approximated with a limited number of site proxies (4–6). As the usual code practice implies a lower number of site proxies (generally one, sometimes two), a sensitivity analysis is conducted to identify the “best performing” site parameters. The best one is the overall velocity contrast between underlying bedrock and minimum velocity in the soil column. Because these are the most difficult and expensive parameters to measure, especially for thick deposits, other more convenient parameters are preferred, especially the couple
V
s
30
,
f
0
that leads to a variance reduction in at least 60%. From a code perspective, equations and plots are provided describing the dependence of the short- and mid-period amplification factors
F
a
and
F
v
on these two parameters. The robustness of the results is analyzed by performing a similar analysis for two alternative sets of velocity profiles, for which the bedrock velocity is constrained to have the same value for all velocity profiles, which is not the case in the original set.
Graphical abstract
Performance of various site proxies (velocity contrast
C
v
, fundamental frequency
f
0
, harmonic velocity average over the top 30 m
V
S30
, total sediment thickness Depth, average corresponding velocity
V
Sm
) to predict the short-period (top,
F
a
) and mid-period (bottom,
F
v
) amplification factors. Proxies may be considered alone, or in combination with several other proxies.
Alluvial valleys generate strong effects on earthquake ground motion (EGM). These effects are rarely accounted for even in site-specific studies because of (a) the cost of the required geophysical ...surveys to constrain the site model, (b) lack of data for empirical prediction, and (c) poor knowledge of the key controlling parameters. We performed 3D, 2D and 1D simulations for six typical sedimentary valleys of various width and depth, and for a variety of modifications of these 6 “nominal models” to investigate sensitivity of EGM characteristics to impedance contrast, attenuation, velocity gradient and geometry. We calculated amplification factors, and 2D/1D and 3D/2D aggravation factors for 10 EGM characteristics, using a representative set of recorded accelerograms to account for input motion variability. The largest values of the amplification and aggravation factors are found for the Arias intensity and cumulative absolute velocity, the lowest for the root-mean-square acceleration. The aggravation factors are largest for the vertical component. For each model, at least one EGM characteristic exhibits a significant 2D/1D aggravation factor, while all EGM characteristics exhibit significant 2D/1D aggravation factor on the vertical component. For all investigated sites, there is always an area in the valley for which 1D estimates are not sufficient. 2D estimates are insufficient at several sites. The key structural parameters are the shape ratio and overall geometry of the sediment-bedrock interface, impedance contrast at the sediment-bedrock interface, and attenuation in sediments. The amplification factors may largely exceed the values that are usually considered in GMPEs between soft soils and rock sites.
This work is a multidisciplinary approach from geological and geophysical surveys to build a 3D geological model of Argostoli Basin (Cephalonia Island, Greece) aiming to be used for computational 3D ...simulation of seismic motion. Cephalonia Island is located at the north-western end of the Aegean subduction frontal thrust that is linked to the dextral Cephalonia Transform Fault (west of Cephalonia) where the seismic hazard is high in terms of earthquake frequency and magnitude. The Plio-Quaternary Koutavos-Argostoli basin site was selected within the French Research Agency PIA SINAPS@ project (www.institut-seism.fr/projets/sinaps/ - last accessed on November 25th 2019) to host a vertical accelerometer array. The long-term goal is to validate three-dimensional nonlinear numerical simulation codes to assess the site-specific amplification and nonlinearity. Herein the geological and geophysical surveys carried out from 2011 to 2017 are presented and in particular the complementary investigations that led to the identification of the main stratigraphic units and their structures. In addition, coral debris sampled from the vertical array deep borehole cores were used for 230Th/234U measurements, which confirmed the Pleistocene age of the Koutavos basin. The characterization of the three-dimensional structure of the stratigraphic units was achieved by coupling geological cross-sections (i.e., depth geometry) and geophysical surveys based of surface wave analysis.
•Geological survey details the stratigraphy of the Argostoli basin.•Surface waves analysis allows to establish the geometries of units at the basin scale.•From an updated geological map, geophysics and cross sections, a 3D model is built.•The geological model will be useful to assess the seismic amplification of this basin.
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