This study investigates the conditions for which one-dimensional (1-D) nonlinear (NL) site response analysis results are distinct from equivalent-linear (EL) results and provides guidance for predicting when differences are large enough to be of practical significance. Relative differences in spectral accelerations and Fourier amplitudes computed from NL and EL analyses are assessed for a range of site conditions and for suites of input motions appropriate for active crustal and stable continental regions. Among several considered parameters, EL/NL differences are most clearly dependent on shear strain index (Iγ), defined as the ratio of input motion peak velocity to time-averaged shear-wave velocity in the top 30 m of the soil profile. For small Iγ (generally under 0.03%), EL and NL results are practically identical, whereas at larger strains, differences can be significant for frequencies >0.3 Hz. Frequency-dependent Iγ values are recommended for conditions above which NL analyses are preferred to EL.

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