Seismic site effects in a deep alluvial basin: numerical analysis by the boundary element method

The main purpose of the paper is the numerical analysis of seismic site effects in Caracas (Venezuela). The analysis is performed considering the Boundary Element Method in the frequency domain. A numerical model including a part of the local topography is considered, it involves a deep alluvial deposit on an elastic bedrock. The amplification of seismic motion (SH-waves, weak motion) is analyzed in terms of level, occuring frequency and location. In this specific site of Caracas, the amplification factor is found to reach a maximum value of 25. Site effects occur in the thickest part of the basin for low frequencies (below 1.0 Hz) and in two intermediate thinner areas for frequencies above 1.0 Hz. The influence of both incidence and shear wave velocities is also investigated. A comparison with microtremor recordings is presented afterwards. The results of both numerical and experimental approaches are in good agreement in terms of fundamental frequencies in the deepest part of the basin. The boundary element method appears to be a reliable and efficient approach for the analysis of seismic site effects. 1. SOME DIFFERENT ASPECTS OF EARTHQUAKE GEOTECHNICAL ENGINEERING Different types of analysis can be performed to investigate earthquake engineering problems. Depending on the purpose of the analysis, one can stress on the only structural aspect of the problem, the geotechnical point of view or, in between, various types of interactions (soil-structure, site-city...). Forgetting the far geological scale including seismic source excitations, there are different scales for the analysis of earthquake geotechnical engineering problems :  local geology : the local topography and sub-surface soil layers are considered, seismic wave propagation in a large (or semi-infinite) domain is the main point to analyze (Bard 1985, ChávezGarcía 2000, Semblat, 1999, 2000b, Somerville 2000),  site-city interaction : in highly builded areas, some strong interaction can occur between the structures and the subsurface soil layers at the scale of an entire city (Guéguen 2000),  soil-structure interaction : at the scale of several structures, the influence of the soil is mainly involved through energy radiation process, limit conditions, liquefaction... (Anandarajah 1995, Stokoe 1999) In the following, we will only consider seismic wave propagation and amplification at the local geological scale. 2. SITE EFFECTS : EXPERIMENTAL INVESTIGATIONS IN CARACAS 2.1 Amplification of seismic motion The analysis of site effects is very important since the amplification of seismic waves in some specific sites is sometimes strong (Bard, 1985). Reflections and scattering of seismic waves near the surface strengthen the effects of earthquakes (Moeen-Vaziri 1988, Pedersen 1995). The main goal is to avoid, in the design of structures, buildings involving resonant features close to that of the soil surface 1 Res.Eng., La boratoire Central des Ponts et Chaussées (LCPC), Eng. Modeling Dept, 58 bd Lefebvre, 75732 PARIS Cedex 15, France, E-mail : [email protected] 2 Res.Eng., C.E.T.E Méditerranée, Nice, France 3 Res.Eng., La boratoire des Matériaux et Structures de Génie Civil (LCPC/CNRS), Marne-la-Vallée, France ha l-0 03 55 60 5, v er si on 1 23 J an 2 00 9 Author manuscript, published in "Computers and Geotechnics 29, 7 (2002) 573-585" DOI : 10.1016/S0266-352X(02)00017-4