Probabilistic Seismic Hazard Analysis for Maximum Seismic Shear Stresses in Soils Using Improved Ground-Motion Parameters

Maximum seismic shear stresses (${\mathit{\tau }}_{\text{max}}$) have been recognized as one of the important parameters in design practice. This study develops ground-motion parameters for ${\mathit{\tau }}_{\text{max}}$ and implements these in probabilistic seismic hazard analysis to provide the ${\mathit{\tau }}_{\text{max}}$ distribution of deep soil layers for design purposes. The application of improved ground-motion parameters for ${\mathit{\tau }}_{\text{max}}$ is demonstrated at the Oakland International Airport, where a thick Young Bay Mud deposit exists under the artificial fill. Model biases in the predictive equations of seismic shear-stress reduction coefficients (${\mathit{r}}_{\mathit{d}}$) are evaluated by comparison with the site response analyses performed with a wide range of input ground motions. Based on these results, we introduce improved ground-motion parameters for ${\mathit{\tau }}_{\text{max}}\left({\mathit{I}}_{\mathit{tau}}\right)$ as a linear combination of spectral accelerations, implemented in probabilistic seismic hazard analysis to calculate seismic hazard curves. Conditional mean spectra are calculated, given ${\mathit{I}}_{\mathit{tau}}$ at 10% in 50 years to illustrate the variations in frequency contents with depth compared with the uniform hazard spectra. Finally, ${\mathit{\tau }}_{\text{max}}$ is calculated with depth by using hazard values of ${\mathit{I}}_{\mathit{tau}}$ and compared with the peak-ground-acceleration-based and uniform-hazard-spectra-based calculations. Analysis results show that ${\mathit{\tau }}_{\text{max}}$ will be underestimated for deep soil layers by peak-ground-acceleration-based calculation if the median value of ${\mathit{r}}_{\mathit{d}}$ is used in design practice.