Sensitivity of ground motion parameters to local shear-wave velocity models : the case of buried low-velocity layers

Abstract

Seismic site response analysis is an important procedure used to obtain parameters such as peak ground motion values. These are needed by the engineering community for reliable seismic design, analysis and retrofitting of structures. One of the inputs for such analysis is the shear-wave velocity profile (VS) of the site. Surface wave methods are increasingly being used to obtain these profiles. However, these are characterised by uncertainties related to the measurements and the model used for interpretation (e.g. the inadequacy of 1D assumption). Moreover, the inversion procedure is also highly non-unique because numerous profiles are able to fit the experimental dispersion curve/s. Even though the uncertainties are not commonly included in site response analysis, their propagation from the dispersion curves to the inversion procedure can lead to significant differences in the site response analysis results.

In a previous study, VS profiles were obtained at 20 sites on the Maltese islands (Central Mediterranean) using the Horizontal-to-Vertical Spectral Ratio (H/V), the Extended Spatial Auto-Correlation (ESAC) technique and the Genetic Algorithm. The geology of all the sites is characterised by a layer of clay, which can be up to 75 m thick, buried under the Upper Coralline Limestone, a fossiliferous coarse grained limestone. The effect of a thick buried low VS layer, which gives rise to a velocity inversion, in ground response is not well understood and/or documented.

The above concerns raised the need for carrying out tests with the aim of better clarifying and understanding which parameters have the most influence on site response analysis and to assess the consequences of inversion non-uniqueness on ground motion amplification and response. From the first part of the study, it was observed that the thickness of the clay layer has the greatest effect on the ground response results. By obtaining ground motion parameters from equivalent profiles, the consequences of the uncertainty in the inversion process, which can be serious at times, are revealed.