Seismic characterization of an Alpine site

Site amplification of earthquake ground motion can arise from local stratigraphic and morphological conditions. In this respect, a detailed geo-mechanical 2D model is needed, in particular for peculiar situations such as the ones in Alpine valleys. This paper reports a site characterization case history for seismic response analysis, where a combination of both body and surface wave seismic techniques has been used. In particular, it is shown that synergies and mutual cross-checking of information lead to a reliable 2D model for site amplification studies. The evaluation of the deep structure and the location of the seismic bedrock have been obtained through seismic reflection and combined active-passive surface wave analysis, while the shear-wave velocity profile of the sediments has been obtained by downhole measurements and surface wave tests. Furthermore, a detailed compressional wave velocity model has been retrieved by tomographic inversion of the first arrivals picked on seismic reflection records. This model is useful for geological interpretation as well as for quality control of the other methods. Surface wave analysis has also been performed on the ground roll of the seismic reflection records. The large amount of profile oriented data has made it possible to reconstruct lateral variations of the shear-wave velocity in the soil deposit. The final pseudo-2D shear-wave velocity model is the product of an innovative inversion approach, based on the integration of the Monte Carlo and laterally constrained least-squares techniques. In the laterally constrained inversion the shear-wave velocity profiles obtained from downhole tests and data from passive surface wave measurements have been used to constrain the inversion. This has greatly improved the final solution. This case history underlines the necessity of combining different seismic techniques to derive an accurate and reliable 2D model for site amplification studies. Through several different and independent analyses of the same seismic records, the cost effectiveness of the whole survey is optimized and a full exploitation of the information contained in the reflection seismic data set is achieved.