We investigated the influence of depth to the half-space on evaluations of seismic response of a dry sandy soil deposit with thickness exceeding 100 m. Our investigation is based on the premise that the best estimate of surface response of such deposits is obtained using a statistically derived attenuation relationship, and the best estimate of other parameters required for geotechnical earthquake-engineering evaluations is obtained by calibrating the evaluated spectral characteristics of the site response using the best-estimate surface response. Results of our site-specific evaluations indicate that the preferred depth to the half-space does not coincide with the depth to bedrock, nor is the preferred depth uniquely defined in the seismic site-response analysis. We find that for a given accelerogram, the preferred depth to the half-space corresponds closely to the depth for which the peak horizontal ground acceleration (PHGA) predicted in the site-response analysis matches the median PHGA developed from the appropriate statistical attenuation relationship. Our results further indicate that the effect of the magnitude of the shear-wave velocity assigned to the half-space is minor in comparison with depth to the half-space.

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