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Estimating _s(30) (or NEHRP Site Classes) from Shallow Velocity Models (Depths < 30 m)

U.S. Geological Survey
345 Middlefield Rd., MS 977
Menlo Park, California 94025
boore{at}usgs.gov

Manuscript received 29 May 2003.

The average velocity to 30 m [_s(30)] is a widely used parameter for classifying sites to predict their potential to amplify seismic shaking. In many cases, however, models of shallow shear-wave velocities, from which _s(30) can be computed, do not extend to 30 m. If the data for these cases are to be used, some method of extrapolating the velocities must be devised. Four methods for doing this are described here and are illustrated using data from 135 boreholes in California for which the velocity model extends to at least 30 m. Methods using correlations between shallow velocity and _s(30) result in significantly less bias for shallow models than the simplest method of assuming that the lowermost velocity extends to 30 m. In addition, for all methods the percent of sites misclassified is generally less than 10% and falls to negligible values for velocity models extending to at least 25 m. Although the methods using correlations do a better job on average of estimating _s(30), the simplest method will generally result in a lower value of _s(30) and thus yield a more conservative estimate of ground motion [which generally increases as _s(30) decreases].

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