Probabilistic Methodology for Developing Regional and Site-Class Dependent Seismic Site Amplification Factors

NEHRP guidelines for simplified seismic design provide site amplification factors as a function of the average shear wave velocity (Vs) of the top 30 m of a site, or Vs30. These guidelines are mainly based on experience and observations from the Western United States (WUS), where regional geology is conditioned by frequent tectonic activity. However, the Vs30 value does not capture sharp Vs contrasts due to very hard rock overlain by softer soil, conditions prevalent in the Central and Eastern United States (CEUS), which lead to significant ground motion amplification. Site response analyses performed on CEUS sites have confirmed this inconsistency between NEHRP and calculated amplification factors. This paper presents the development of a model to generate Vs profiles from measurements in the WUS. Parameters such as variation, correlation, and transition frequency of Vs values were defined in order to capture geological trends for the WUS. This same model can then be applied to CEUS Vs profiles and in order to generate groups of Vs profiles representing regional geological trends. From site response analyses using generated Vs profiles, amplification factors can be calculated and compared to verify if region-specific amplification factors should be implemented.