MAJ : march 2009

Version française

jean-francois.semblat_at_lcpc.fr (at=@)

Jean-François Semblat

Research and teaching activities of Jean-François Semblat

Laboratoire Central des Ponts et Chaussées, Paris, France, jean-francois.semblat_at_lcpc.fr (at=@)

Teaching  :

• Ecole Polytechnique (Mechanics of continua, Seismology and tectonics)
• ENTPE (Numerical methods)
• University Pierre & Marie Curie (Dynamics of solids)
• Rose School (Advanced Computational Methods)

Publications :

New book “Waves and Vibrations in Soils: Earthquakes, Traffic, Shocks, Construction Works” (IUSS Press, 2009) Short list Recent publications (author version): http://hal.archives-ouvertes.fr/aut/semblat/

Researches  :

Propagation and amplification of seismic waves. Mechanical and numerical modeling.

1. Amplification of seismic waves in alluvial basins

    The amplification of seismic waves at the free surface, namely « site effects » [1], may strengthen the impact of an earthquake in specific areas (e.g. Mexico 1985). Indeed, when seismic waves propagate through alluvial layers or scatter on strong topographic irregularities, refraction/scattering phenomena may strongly increase the amplitude of the ground motion. It is then possible to observe stronger motions further away from the epicenter !
    At the scale of an alluvial basin, the analysis of seismic wave propagation is a complex problem involving as various phenomena as : resonance of the whole basin [2], propagation in heterogeneous media [3], generation of surface waves on the basin edges [5], nonlinear behaviour of geomaterials [6].

1.1 Simplified modal approach: characterization of the fundamental frequency

Site effects can then be analyzed as a global resonance of alluvial basins. Simplified modal approaches, based on the Rayleigh approximation, have been considered [2]. Starting from admissible 2D or 3D eigenmodes, these approaches allow a fast and reliable estimation of the fundamental frequency of geological structures [2]. Basin fundamental frequency by modal approach >

1.2 Propagative approaches through the Boundary Element Method

   Previous approaches do not allow a quantitative estimation of the amplification level of seismic waves and it is then necessary to simulate numerically their propagation.

Various numerical methods are available: finite differences, finite elements [6], boundary elements [5,7], spectral elements, etc. At the scale of an alluvial basin, the amplification of seismic waves in surficial layers is analyzed using the Boundary Element Method for the site of Volvi, Nice and Caracas [4,5,7]. The geometrical irregularities (topography) and the velocity heterogeneities (lithology) have a significant influence on the amplification process. Comparisons between numerical and experimental results, measured for weak earthquakes, are obvious evidences of such phenomena [5,8]. A new "fast multipole" formulation for boundary integral equations in elastodynamics has recently allowed an important reduction of the computational cost as well as the memory requirements [9,10].	Seismic wave amplification in Nice [5]
Seismic wave amplification in Volvi [4].
Seismic wave amplification in Caracas [8]	Fast Multipole Method: semi-spherical canyon [9]
Fast Multipole Method : semi-spherical basin (SV-wave) [10]

1.3 Amplification of strong earthquakes: simplified nonlinear model

    In the case of strong seismic motion, the influence of the constitutive nonlinearities in surficial layers is significant. A simplified nonlinear model (nonlinear viscoelasticity) has been developed to take simultaneously into account, for increasing shear strain, the shear modulus reduction and the increase of the energy dissipation [6]. The one-dimensional simulations performed with this simplified model lead to lower amplitudes, larger propagation delays and the generation of odd order harmonics. These results are in good agreement with the experimental observations (e.g. comparisons with surface and in-depth recordings for the Kushiro-Oki earthquake [6]).

Nonlinear model for strong motion simulations [6]

Simulations for Kushiro-Oki earthquake [6]

2. Interaction of seismic waves with a dense building array

    The surface structures may act as secondary seismic sources and modify the seismic "free-field" [11]. Considering experimental results obtained from the Volvi European test site, the numerical modeling of structure-soil-structure interaction allows the determination of the parameters governing these interactions. At large scales, the interaction between an alluvial basin and a building network – or site-city interaction - is analyzed numerically [12]. The coincidence between the eigen frequencies of the structures and the fundamental frequency of the basin strongly influences the site-city interaction. The coherency of the wavefield, the effects of the urban density and the building heterogeneity have also been studied.

Structure-soil interaction (Volvi EuroSeisTest) [11]

Site-city interaction in an alluvial basin [12,13]

References

[1] Semblat J.F., B.Gatmiri, P.Y.Bard, P.Delage (2006). Séisme en ville, La Recherche, 398 : 29-31.
[2] Semblat J.F., R.Paolucci, A.M.Duval (2003). Simplified vibratory characterization of alluvial basins, C. R. Geoscience, 33(4): 365-370.
[3] Abraham O., R.Chammas, P.Cote, H.Pedersen, J.F.Semblat (2004). Mechanical characterization of heterogeneous soils with surface waves, Near Surface Geophysics, 2: 249-258.
[4] Semblat J.F., Duval A.M., Dangla P. (2000). Numerical analysis of seismic wave amplification in Nice (France) and comparisons with experiments, Soil Dynamics and Earthquake Engineering, 19(5), pp.347-362.
[5] Semblat J.F., M.Kham, E.Parara, P.Y.Bard, K.Pitilakis, K.Makra, D.Raptakis (2005). Site effects : basin geometry vs soil layering, Soil Dynamics and Earthquake Eng., 25(7-10): 529-538.
[6] Delépine N., Bonnet G., Semblat J-F, Lenti L. (2007). A simplified non linear model to analyze site effects in alluvial deposits, 4th Int. Conf. on Earthquake Geotechnical Eng., Thessaloniki, Greece, June 25-28.
[7] Dangla P., J.F.Semblat, H.Xiao, N.Delépine (2005). A simple and efficient regularization method for 3D BEM: application to frequency-domain elastodynamics, Bull. of the Seismological Soc. of America, 95(5): 1916-1927.
[8] Semblat J.F., Duval A.M., Dangla P. (2002). Seismic Site Effects in a Deep Alluvial Basin: Numerical Analysis by the boundary element method, Computers and Geotechnics, 29(7): 573-585.
[9] Chaillat S., Bonnet M., Semblat J-F (2008). A multi-level Fast Multipole BEM for 3-D elastodynamics in the frequency domain, Computer Methods in Applied Mechanics and Engineering, 197(49-50), pp.4233-4249.
[10] Chaillat S., Bonnet M., Semblat J-F (2009). A new fast multi-domain BEM to model seismic wave propagation and amplification in 3D geological structures, Geophysical Journal International (to appear).
[11] Bard P.Y., J.L.Chazelas, P. Guéguen, M. Kham, J.F. Semblat (2005). Assessing and managing earthquake risk - Chap.5 : Site-city interaction, Eds: C.S. Oliveira, A. Roca and X. Goula, Springer, 375 pages.
[12] Kham M., J.F. Semblat, P.Y.Bard, P.Dangla (2006). Site-City Interaction: main governing phenomena through simplified numerical models, Bulletin of the Seismological Society of America, 96(5): 1934-1951.
[13] Semblat J.F., Kham M., Bard P.Y. (2008). Seismic Site Effects in Alluvial Basins and Influence of Site-City Interaction, Bulletin of the Seismological Society of America, 98(6), pp.2665-2678, 2008.
[14] Semblat J.F., Pecker A. (2009). Waves and Vibrations in Soils: Earthquakes, Traffic, Shocks, Construction Works, IUSS Press, 500 pages.

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