Site-Specific Site Response Analysis and Cascadia Subduction Zone Ground Motions Evaluation for a Site Near Portland

Site-specific ground motions, site-specific site response analysis, and development of site-specific acceleration ground spectra at the ground surface was developed per ASCE 7-10 for the Kirkland Renaissance Boardwalk, a site in Vancouver, Washington. Synthetic broadband ground motions developed specifically for Cascadia Subduction Zone (CSZ) megathrust earthquake were used for site-response analysis. The results of site-specific site response analyses using the strong motions were compared with the results from the 5 motions considered for the ASCE 7-10 procedure. The effect of motion duration and other ground motion intensity measures on the results of the site-specific site response analysis was investigated. Similar trends of resulting amplification factors were observed when considering long duration CSZ motions and the 5 historical motions utilized for the ASCE 7-10 site-specific site response analysis procedure. Maximum amplification from the CSZ motions was consistent with the 5 historical motions, although maximum amplification occurred at shorter periods. In addition, the amplification factors at PGA from the CSZ motions were generally higher. It was found that amplification factors developed using 5 historic motions or CSZ-specific synthetic motions did not show a strong correlation with strong motion duration and other ground motion intensity measures. For the CSZ motions, amplification at PGA tended to decrease as intensity parameters increased, but increased as significant duration increased. The trends between maximum amplification and intensity measures were not as apparent for the 5 historical motions. The results of this study highlighted the advantages of performing site-specific site response analysis in reducing uncertainty in design accelerations. This study also indicated the applicability of broadband synthetic CSZ ground motions for the purpose of performing site response analysis