Nonlinear Site Amplification as Function of 30 m Shear Wave Velocity

We develop empirical relationships to predict nonlinear (i.e., amplitude‐dependant) amplification factors for 5% damped response spectral acceleration as a continuous function of average shear wave velocity in the upper 30 m, V s‐30 . We evaluate amplification factors as residuals between spectral accelerations from recordings and modified rock attenuation relationships for active regions. Amplification at low‐ and mid‐periods is shown to increase with decreasing V s‐30 and to exhibit nonlinearity that is dependent on V s‐30 . The degree of nonlinearity is large for NEHRP Category E (V s‐30 ∼300 m/s. The results can be used as V s‐30 ‐based site factors with attenuation relationships. The results also provide an independent check of site factors published in the NEHRP Provisions , and apparent bias in some of the existing NEHRP factors is identified. Moreover, the results provide evidence that data dispersion is dependent on V s‐30 .