The use of surface waves for site characterization and seismic hazard analysis

Abstract

Standard approaches to evaluate seismic hazard rely on the evaluation of the expected ground motion with respect to a generic rock site condition. For a sediment site, such estimate has to be corrected for the local seismic response, or the relative amplification spectrum. In order to correctly compute the seismic response, an adequate level of knowledge of the structural characteristics for the site area is necessary. As a matter of fact, the final ground motion is highly controlled by the geophysical properties of the first tens to hundreds of meters of the soil. The available level of knowledge, however, is often insufficient because of the considerable investments required for measurements. Cost efficient methods to estimate soil parameters mainly shear wave velocity as a function of depth are therefore the major issues in local seismic response evaluation and site-specific seismic hazard assessment. The main goal of this thesis is the development and optimization of new and existing techniques to reduce the uncertainty level in the definition of the local seismic response. In particular, this study focuses on surface wave analysis techniques, whose dispersion and polarization features represent a formidable constraint in obtaining the geophysical parameters using non-linear inversion procedures. New methods for analyzing ambient vibration and artificially generated wave-fields are introduced. Combining source types has the advantage of exciting surface waves in a broad range of frequencies, both for the fundamental and the higher modes. In the first section, a new method to analyze Rayleigh wave ellipticity is presented. The method is based on the frequency-wavenumber transform of three-component synchronous recordings from an array of stations. Such an approach is suitable for ambient noise processing, because it allows the retrieval of dispersion functions and the corresponding polarization information simultaneously. In particular, the main advantage is the capability of separating the contribution of the different modes of propagation, in the case where they are sufficiently energetic. The proposed method has been successfully tested in a microzonation experiment in the city of Lucerne. Using the ellipticity