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Sarah Touhami will publicly defend her PhD thesis entitled :

“Numerical modeling of seismic field and soil interaction: application to the sedimentary basin of Argostoli (Greece)”

Tuesday 18 February 2020 at 2pm

Amphitheater I, Ground floor of the Eiffel Building, CentraleSupélec

8 Rue Joliot Curie, 91190 Gif-sur-Yvette

Abstract

A prediction of the seismic response of civil engineering structures that requires a high level of safety (i.e. nuclear power plants or dams) faces several major difficulties given the complexity of the problem being treated. To this end, the source simulation, seismic wave propagation and site effects have been studied by different approaches over the last two decades. Recently, numerical methods, such as the spectral element, combined with massively parallel computers have proved a good efficiency in modelling the seismic wave propagation from source to site in complex three-dimensional geological environments. However, the accuracy of these predictions remains limited due to the multi-scale nature of the problem and the large uncertainties in the data to be introduced into the model (i.e. the geometric and kinematic characterization of the seismic source, the detailed geological and numerical model of the source-to-site propagation path). Therefore, the use of a regional numerical model able to simulate the seismic phenomenon from source to site would allow a better analysis and classification of the origin of the associated uncertainties.

This work aims to study numerically the effect of local and regional geology on the seismic response of a basin and precisely the Argostoli site located on the island of Kefalonia (Greece). Firstly, the numerical code used in this work (SEM3D) is verified using three canonical cases. The simulated results showed a good agreement with the reference solutions. In the cases of a numerical model with different scales or with complex geologies, one of the important difficulties is the conformity of the numerical meshes with the geological interfaces, that will result an increase in the numerical cost. Because of the characteristics of the used numerical method, one possible approach was to use the “non-conforming” or “not-honoring” meshes to overcome this difficulty. A parametric study on the applicability of this approach was then carried out in order to highlight the influence of some numerical parameters on the obtained results.

Thereafter, parametric studies on several seismic scenarios in the Argostoli site were conducted. Concerning seismic loading, two types of source have been studied: point sources and extended faults. The study with point sources clearly revealed a phenomenon of amplification and trapping of waves in the basin, leading to a complex and elongated signal, with significant energy compared to a study with a simplified geology. For the second type of source, the modeled fault is close to the surface. It allows to study, in addition to the influence of the basin, the effect of the near-field on the seismic response of the site. Indeed, near-fault ground motion can be significantly different from ground motion observed far from the seismic source. Based on the results obtained, the basin effect is even more pronounced but with higher amplification and different resonance frequencies. In addition, the near-field effect has been highlighted, marked by strong velocity pulses at some locations in the basin. The order of magnitude of the obtained response spectra is comparable to the ones obtained during the earthquake sequence of similar magnitude that took place in 2014.

In the last part, a parametric study (allowed by the development of computing power) on the numerical aspects related to the computational accuracy was carried out. With this study, it is possible to increase the frequency resolution from 7 Hz to 10 Hz with soft soil mechanical characteristics while keeping the same domain size. These simulations open even more questions on the interdependence of the fineness of resolution of physical data and meshes for numerical simulations.

In conclusion, this thesis corresponds to a first step in the numerical characterization of the seismic response of the Argostoli basin and the effects due to the basin, the type of source and their interactions.

Composition of the jury

Luis-Fabian BONILLA HIDALGO, Directeur de recherche, IFSTTAR – Rapporteur
Jean-François SEMBLAT, Professeur, ENSTA – Rapporteur
Catherine BERGE-THIERRY, Ingénieure de recherche, CEA – Examinatrice
Roberto PAOLUCCI, Professeur, Politecnico di Milano – Examinateur
Pierre Yves BARD, Directeur de recherche, IFSTTAR – Examinateur
Didier CLOUTEAU, Professeur, CentraleSupélec – Directeur de thèse
Fernando LOPEZ-CABALLERO, Maître de Conférences, CentraleSupélec – Co-endadrant