We develop empirical relationships to predict nonlinear (i.e., amplitudedependant) 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 Ͻ180 m/s) but decreases rapidly with V s-30 , and is small for V s-30 Ͼϳ300 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 .

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Empirical relationships are developed to predict amplification factors for 5% damped response spectral acceleration that incorporate basin response effects. The parameters considered are depth to the 1.5 km/sec shear-wave isosurface (z 1.5 ) as well as the location of the source beneath or outside the perimeter of the basin in which the site is located. Sites located in a basin overlying the source are denoted as having coincident source and site basin locations (CBL) and are differentiated from distinct source and site basin locations (DBL). Amplification factors for CBL and DBL sites are evaluated from simulated data (developed by others) and strongmotion data. Amplification factors derived from strong-motion recordings are taken as residuals of rock attenuation relations coupled with amplification factors for shallow-site conditions. Models relating amplification to z 1.5 were developed separately for the CBL and DBL data groups. The results indicate that the use of basin models is generally worthwhile for periods T Ն 0.75 sec. At those long periods, residuals are significantly sensitive to z 1.5 for CBL but not for DBL. The standard deviation is also reduced for long periods to an extent that the standard deviations for long and short periods are similar.

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