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Why is ILL required to perform specific seismic hazard studies ?

Vendredi 14 mars 2014 14:00 - Duree : 1 heure
Lieu : Seminar Room, ILL - 6 rue Jules Horowitz - Grenoble

Orateur : Prof. Pierre-Yves BARD (ISTerre/IFSTTAR, Grenoble, France)

ILL Colloquium Compared to other areas in the world, French Alps are in a moderate seismicity area. Yet, ILL has been requested over the last years to update seismic hazard studies and to carry out heavy civil engineering work for seismic retrofitting. This presentation will outline the seismological background behind these requests and a few ongoing studies to address several scientific issues presently under debate. After a few generalities on seismic hazard in general and the alpine seismicity in particular, the presentation will focus on the issues faced when trying to assess the seismic hazard in the Grenoble area. The first one is the choice of the methodology to be used for such an assessment : the probabilistic approach providing estimates of annual exceedance probabilities as a function of ground motion level, or a deterministic approach based on the determination of scenario earthquakes. Another one is the selection of the maximum possible (or "credible") earthquakes in the surrounding area, which is indeed more difficult in moderate seismicity areas with very low strain rates and very long return periods extending far beyond the historical period. The estimation of the site-specific ground motion associated to a given earthquake constitutes in itself a huge endeavor, linked to the physics of the rupture dynamics on the causative fault controlling the emission of elastic waves, and their subsequent propagation in the Earth’s crust. The associated large variability results in highly uncertain deterministic predictions, which can be accounted for only on a statistical basis A topic of special concern for the Grenoble area deals with the wave propagation effects in the quaternary deposits, with very specific 3D diffraction and resonance effects resulting in very significant local amplification effects. Their a priori estimation requires in-situ instrumentation and/or careful site surveys to identify the characteristics of the shallow and deep subsoil, to feed sophisticated numerical simulation codes accounting for the 3D geometry and the non-linear sil behavior. These topics are indeed the focus of several R&D projects - some of them being co-funded by ILL and CEA and involving specific measurements at the ILL site -, which will be briefly presented. The presentation will conclude with a short discussion on the safety margins in both hazard assessment and engineering design in the light of a few recent events.

Contact : sultan@ill.fr

Discipline évènement : (Physique)
Entité organisatrice : (ILL)
Nature évènement : (Séminaire)
Evènement répétitif : (ILL Colloquium)
Site de l'évènement : Polygone scientifique

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