Site effects: From observation and modelling to accounting for them in building codes

Abstract

Site effects are defined as the modifications that seismic motion undergoes when travelling through the topmost layers before hitting the ground surface or a structure built on it. The definition of topmost layers is frequency dependent. This paper compiles results regarding the estimation of site effects and their use to decrease seismic risk. Site effects may be estimated either by direct measurements or indirectly, meaning that subsoil mechanical properties and geometry are evaluated and from them an estimate of local amplification is computed. The first approach has the advantage of its directness. It has been shown that this approach is most effective when local amplification is significant and brought about by a simple configuration (e.g., one soft layer overlying a more rigid basement). However, this first approach lacks flexibility and when it fails the invested effort is practically lost. The second approach, although more tortuous, offers larger flexibility. It is very likely that efforts directed to the estimation of the subsoil structure, even if not completely successful, will allow to build some model and to estimate an order of magnitude of the amplification. Current methods used to estimate site effects based on earthquake or ambient noise data are discussed. In addition, the main techniques that are used to estimate the subsoil structure are reviewed, in particular, the new proposals regarding the use of the correlation of noise measurements to determine the properties of the subsoil layers. Finally, some comments are offered on the current approaches available to take into account site effects in the framework of building codes. © 2007 Springer.