Experimental Uncertainty on the Vs(z) Profile and Seismic Soil Classification

The shear wave velocity profile in the shallow subsoil plays a predominant role in quantitative seismic site characterization. In particular, the average velocity of shear waves in the first 30 meters (Vs30) is central to the present seismic soil classification of most western countries (NEHRP 1997; Eurocode 8 2003). The accuracy with which the velocity profile is intrinsically limited to less than 20% by both the technical detail of the various methods and by the intrinsic variability of natural soil. Since the accuracy with which the Vs(z) profile can be measured is inherently limited, the ensuing ambiguity in soil seismic classification based on averaged Vs must be resolved on the basis of reliable independent evidence