Marc Kham

Seismic Site–City Interaction: Main Governing Phenomena through Simplified Numerical Models

This work focuses on the analysis of the multiple interactions between soil layers and civil-engineering structures in dense urban areas submitted to a seismic wave. To investigate such phenomena, called site–city interaction (sci) herein, two simplified site–city configurations are considered: a homogeneous, periodically spaced city and a heterogeneous, nonperiodically spaced city, both on a constant- depth basin model. These 2D boundary-element method models are subjected to a vertically incident plane SH Ricker wavelet. A parametric study of the city parameters (density of buildings and their natural frequencies) and the thickness of the basin is carried out to characterize the sci and to investigate its sensitivity to some governing parameters. The following parameters are analyzed: building vibrations, induced ground motion, ground-motion perturbations inside and outside the city, spatial coherency, and kinetic energy of the “urban wave field.” A so-called site–city resonance is reached when the soil fundamental frequency and structure eigenfrequencies coincide; building vibrations and ground motion are then significantly decreased and the spatial coherency of the urban field is also strongly modified. Building density and city configuration play a crucial role in the energy distribution inside the city.

Seismic-Wave Propagation in Alluvial Basins and Influence of Site-City Interaction

The geometrical and mechanical features of alluvial deposits have a major influence on seismic-wave propagation and amplification. However, for alluvial basins located in densely urbanized areas, the surface structures such as buildings could influence seismic-wave propagation near the free surface. In this article, the influence of surface structures on seismic-wave propagation is analyzed numerically in the case of an actual two-dimensional (2D) shallow basin. At a local scale, the vibration of a surface structure can induce a seismic wave field in the surficial soil layers. At the scale of an alluvial basin, the site-city models considered herein show that the city effect can lead to a significant seismic wave-field modification when compared to the free-field case. The coincidence between the fundamental frequencies of the soil layers and eigenfrequencies of the surface structures is a key parameter to investigate site-city interaction. When comparing simplified sitecity models (Kham et al., 2006) to the basin-city model, the influence of the lateral heterogeneities on the site-city interaction is found to be significant. Indeed, the seismic wave field radiated by the city appears to be trapped within the alluvial basin, and specific directivity features are found for this wave field. The influence of site-city interaction on the free-field seismic hazard may then be significant. The effects of the site-city interaction are beneficial in some parts of the city or detrimental in other parts (especially city boundaries). These effects strongly depend on the urban configuration (city heterogeneity, building density, etc.). Finally, the full characterization of the seismic wave field in densely urbanized areas could often raise the need for investigating site-city interaction and consider such parameters as basin and city fundamental frequencies, building density and city arrangement, as well as basin effects combined with the seismic wave field radiated by the city.

Amplification of seismic ground motion in the Tunis basin: Numerical BEM simulations vs experimental evidences

This paper aims at the analysis of seismic wave amplification in a deep alluvial basin in the city of Tunis in Tunisia. This sedimentary basin is 3000m wide and 350m deep. Since the seismic hazard is significant in this area, the depth of the basin and the strong impedance ratio raise the need for an accurate estimation of seismic motion amplification. Various experimental investigations were performed in previous studies to characterize site effects. The Boundary Element Method is considered herein to assess the parameter sensitivity of the amplification process and analyse the prevailing phenomena. The various frequencies of maximum amplification are correctly estimated by the BEM simulations. The maximum amplification level observed in the field is also well retrieved by the numerical simulations but, due to the sensitivity of the location of maximum amplification in space, the overall maximum amplification has to be considered. The influence of the wave-field incidence and material damping is also discussed.