Determination of mean shear wave velocity to 30m depth for site classification using shallow depth shear wave velocity profile in Korea

Abstract

Abstract The mean shear wave velocity to a depth of 30 m (VS30), established in the western United States, is the current site classification criterion for determining the seismic design ground motion, taking site amplification potential account into. To evaluate VS30 at a site, a shear wave velocity (VS) profile extending to a depth of at least 30 m must be acquired using in situ seismic tests. In many cases, however, the obtained VS profile does not extend to a depth of 30 m due to unfavorable field conditions and limitations of testing techniques. In this study, VS30 and the mean shear wave velocity to depths less than 30 m (VSDs) were calculated using VS profiles of more than 30 m obtained by seismic tests at 72 sites in Korea, and the correlation between VS30 and VSDs was drawn based on the computed mean VS data. Additionally, a method for extrapolating the VS profile from shallow depths to 30 m and bedrock was developed by building a shape curve based on the average data of all VS profiles. These two methods of extrapolating VS30 from shallow VS profiles, using VSDs and a shape curve, resulted in less bias than the simple method in which the lowermost VS value obtained is extended to the depth of 30 m. These two extrapolation methods are useful for VS profiles extending to depths of at least 10 m. Furthermore, the shape curve method developed in this study may be useful in the western United States as well as in Korea.