Uncertainty in Shear-Wave Velocity Profiles

Abstract

This paper considers the uncertainty in the shear wave velocity (Vs) of soil and rock profiles for use in earthquake site response calculations. This uncertainty is an important contributor to uncertainty in site response, which in turn is an important contributor to uncertainty in earthquake ground motions and in seismic hazard. The paper begins with a discussion of the different types of uncertainty and how they are characterized in probabilistic seismic hazard analysis, and how this differentiation is particularly ambiguous in the case of soil properties. This is followed by a description of the probabilistic models of Vs that are most commonly used in engineering practice, for both generic and site-specific applications. In site-specific applications, the uncertainty in Vs (which is measured by the logarithmic standard deviation or by the coefficient of variation of Vs) is lower than in generic applications, but other elements of the profile model are also different. Next, the paper discusses the issues that arise in characterizing the uncertainty in Vs in site-specific applications using non-invasive surface wave methods and summarizes the insights obtained by comparing the results from multiple blind studies in which the same surface-wave data (and no other site-specific data) were provided to multiple teams of analysts. Finally, the paper provides recommendations on how to characterize uncertainty in Vs for both generic and site-specific applications.