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 <180 m/s) but decreases rapidly with V s-30 , and is small for 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 .