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The prediction of the seismic response of critical structures is highly sensitive to many aspects, among which the earthquake source and the geological setting are prominent. The related uncertainty issues must be taken into account in seismic risk mitigation studies, for example through the evaluation of several realizations of a future earthquake scenario. This aspect is crucial when addressing vulnerability studies at a regional scale. When opting for physical-based numerical simulations (PBS), however, the computational burden increases along with the expected degree of fidelity, making it difficult to evaluate more than a few dozens of alternatives. To cope with this disadvantage, in this work an alternative method is proposed, which exploits a rather low number of synthetic earthquake simulations and combines them with the empirical Green function (EGF) method, to finally generate thousands of alternative yet realistic seismic response of the site of interest. This hybrid strong motion predictions benefit of both (i) PBS high fidelity and (ii) data assimilation of strong ground motion records in the seismic area of interest, via EGF method, producing broadband synthetics at a relative cheap computational price. The power of the hybrid method is tested on a real case scenario, embodied by the ground-shaking prediction at the nuclear site of Cadarache, in the surroundings of the fault of Middle Durance, in South-Eastern France. Thousands of broadband (0–15 Hz) hybrid synthetic seismic response are generated, associated with different fault parameters (EGF method) and based upon a few key physics-based simulations, accurate up to 5 Hz.

This study investigates the sensitivity of physics-based earthquake prediction to the choice of the regional geological model. The generally poor information on the mechanical properties of the Earth’s crust induces large uncertainty margins on the synthetic seismic response. Therefore, uncertainty quantification must be associated with high-fidelity numerical simulation, especially when verging on broad-band simulations (i.e., numerical theoretical accuracy higher than the conventional 1 Hz limit). In this paper, the synthetic seismic response of the Niigata region (Japan), during the Ch u etsu offshore earthquake (MW6.6) serves as representative case study. Three plausible three-dimensional (3-D) geological structures are compared for two point-wise aftershock numerical simulations, duly validated. The framework of comparison targets the Ground Motion Prediction Equation (GMPE) for pseudo-spectral acceleration at the surface. The impact of the geology on the variability of between-event and the within-event residuals is inferred from synthetic simulations and related to common parameters representing site-specific conditions.

In this article, a novel strategy to generate broadband earthquake ground motions from the results of 3D physics-based numerical simulations (PBSs) is presented. Physics-based simulated ground motions embody a rigorous seismic-wave propagation model (i.e., including source, path, and site effects), which is however reliable only in the long-period range (typically above 0.75–1 s), owing to the limitations posed both by computational constraints and by insufficient knowledge of the medium at short wavelengths. To cope with these limitations, the proposed approach makes use of Artificial Neural Networks (ANNs), trained on a set of strong-motion records, to predict the response spectral ordinates at short periods. The essence of the procedure is, first, to use the trained ANN to estimate the short-period response spectral ordinates using as input the long-period ones obtained by the PBS, and, then, to enrich the PBS time histories at short periods by scaling iteratively their Fourier spectrum, with no phase change, until their response spectrum matches the ANN target spectrum. After several validation checks of the accuracy of the ANN predictions, the case study of the 29 May 2012 Mw 6.0 Po Plain earthquake is illustrated as a comprehensive example of application of the proposed procedure. The capability of the proposed approach to reproduce in a realistic way the engineering features of earthquake ground motion, including the peak values and their spatial correlation structure, is successfully proven.

International audience; The scope of this paper is to give an insight into the advantages of a new, all-embracing, modeling approach of a strong ground motion scenario, by carrying out a source-to-structure analysis at regional scale, accounting explicitly for the uncertainties related to the databases and the models. To this end, a suitable case-study is represented by the 2007 Mw6.6 Niigata-Ken Ch¯ uetsu-Oki seismic sequence (west Japan), that damaged the Kashiwazaki Kariwa Nuclear Power Plant. This study describes the effect of the wave propagation path within the Earth's crust on the seismic response of nuclear reactor buildings located nearby a seismogenic source. The multiscale problem is de-coupled into three steps: (1) a parallel simulation of seismic-wave propagation throughout the Earth's crust at regional scale (≈ 60 km wide, major 3-D geological interfaces found below the nuclear site), reliable up to 5.0 Hz; (2) a mid hybridization step consisting in enriching the synthetic wave-field at high frequency (up to 30 Hz), employing an Artificial Neural Network to predict the short-period (SP) spectral ordinates; (3) a high-resolution structural dynamic analysis, introducing the hybrid broad-band synthetics as input wave-motion. A simplified stress-test is performed, ✩ Fully documented templates are available in the elsarticle package on CTAN. by simulating two small point-wise aftershocks at different source-site position. The impact of the underground 3-D geology on the structural components is finally quantified, by injecting the obtained broad-band time-histories in a Soil-Structure Interaction (SSI) model of the nuclear reactor building. The good fit obtained in terms of amplification factor at different recording stations assures the high-fidelity of the holistic philosophy endorsed.

International audience; The aim of this paper is to clarify the extent of near-source effects and non-linear soil response that took place at the Japanese nuclear site of Kashiwazaki-Kariwa,during the MW 6.6 Niigata-Ken Chūetsu-Oki earthquake (July, 16th 2007). The interest ofthis case study stems from the relative small source-to-site distance and shallowhypocenter depth, coupled with the consistent seismic record database available from twodown-hole arrays of strong motion accelerometers, installed within the nuclear site.Records were processed first to highlight those features related to near-source conditions,such as directivity and polarization, and, subsequently, to identify dependence of siteresponse on the direction of motion and on the non-linear soil behaviour. Moreover,borehole interferometry was used to check the two available velocity profiles (estimated byin situ PS logging) used for 1D linear equivalent numerical simulations. In one case, animproved agreement with the empirical amplification functions was obtained using asmoothed profile. The main non-linear features highlighted by signal processing wereglobally reproduced by means of linear-equivalent analyses, that pointed out the stronginfluence of the input motion direction. Among the principal results of this research, it wasfound that the hanging-wall and directivity effects along with the non-linear site-effectsdominate the ground response during the main shock, and that the site amplificationfeatures are strongly dependent on the rotation angle of the horizontal components.

International audience

International audience; The physics-based numerical simulation is a reliable tool to build-up source-to-site strong ground motion earthquake scenarios. In this paper, we enhance those forward simulations by including (1) the hysteretic non-linear behaviour of geomaterials and (2) the heterogeneous nature of soils. Scattering-induced damping, ground motion spatial incoherence and de-amplification may therefore be reproduced. A classical case study is considered herein: a seismic wave generated by a localized double-couple earthquake propagating towards the surface through layered elastic homogeneous bedrock, underlying a heterogeneous non-linear soil stratum. 3D numerical simulations were performed by means of a Spectral Element Method (SEM) code. The latter was coupled with a highly scalable random field generator to reproduce the media heterogeneity. Moreover, an extension to J 2 elasto-plasticity numerical integration was introduced. Time-histories and lagged-coherency curves at the surface show the ground motion de-amplification (mainly related to soil non-linearity) and loss of coherency (due to the wave-scattering caused by soil heterogeneity).