Nonlinear Site Amplification as Function of 30 m Shear Wave Velocity

• DOI: 10.1193/1.1856535
• Publication Date: 2005-02-25T23:01:08Z
• Authors: Yoojoong Choi; Jonathan P. Stewart
• Keywords: 13. Climate action; ground motion; soil conditions; 0211 other engineering and technologies; probabilistic seismic hazard analysis; site effects; building codes; response spectra; 02 engineering and technology; 01 natural sciences; attenuation; 0105 earth and related environmental sciences

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, Vs-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 Vs-30 and to exhibit nonlinearity that is dependent on Vs-30. The degree of nonlinearity is large for NEHRP Category E (Vs-30<180 m/s) but decreases rapidly with Vs-30, and is small for Vs-30>∼300 m/s. The results can be used as Vs-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 Vs-30.