SEMINARS IN MECHANICS
Amplification of seismic waves in alluvial basins: modeling weak/strong motions and interactions with surface structures
Jean-François Semblat, Université Paris-Est,
Laboratoire Central des Ponts et Chaussées,
Dept Geotechnical Eng., Water & Risks
Paris, France
The amplification of seismic waves at the scale of an alluvial basin is generally controlled by complex phenomena: global resonance of the basin, propagation in heterogeneous media, surface waves generation at the basin edges, nonlinear behavior of surficial geomaterials.
To simulate such phenomena numerically, various approaches are available: finite differences, finite elements, boundary elements, spectral elements, etc. At the scale of an alluvial basin, the amplification of seismic waves in surficial geological layers is first investigated considering the Boundary Element Method. The topographical irregularities and the velocity constrasts are evidenced as governing parameters of the amplification process. Comparisons between numerical and experimental results, recorded from weak seismic motions, are fully satisfactory. Recent results on a fast BEM formulation (Fast Multipole BEM) for boundary integral equations in 3D elastodynamics allow a very important reduction of the computation duration and the memory requirements.
For strong seismic motions, the nonlinear behavior of the materials in the surficial layers plays a significant role. A nonlinear viscoelastic model (X-NCQ) has been recently developed to account for the reduction of the shear modulus and the increase of the dissipation for larger shear strains. The 1D simulations performed with this simplified model lead to lower amplitudes, larger propagation delays and the generation of odd harmonics. These results allow to recover experimental observations from a qualitative as well as quantitative point of view (e.g. comparisons with surface and in-depth recordings for the Kushiro-Oki earthquake).
Finally, surface structures may act as secondary seismic sources and modify the so-called seismic free-field. Considering experimental results from the Volvi european test site, the numerical modeling of structure-soil-structure interaction allows the identification of the main governing parameters. At a large scale, the interaction between an entire alluvial basin and a building network – or site-city interaction - is investigated numerically. The coincidence between the eigenfrequencies of the structures and the fundamental frequency of the basin strongly influences the site-city interaction. The coherency of the seismic wavefield and the effects of the soil nonlinearities are also discussed.
Friday, October 1, 2010
Macdonald Engineering Building
Civil Engineering Department and Applied Mechanics
Conference Room 497
13:00 – 14:00 PM
Refreshments will be served at 12:45 PM
All interested persons are welcome to attend
Inquires: luc [dot] chouinard [at] mcgill [dot] ca (Professor Luc Chouinard)
Tel: 514-398-6446