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.