The effect of soil nonlinearity on high-frequency spectral decay and implications for site response analysis

Abstract

This study uses recorded ground motions at soil sites over a range of shaking intensities to investigate the effects of soil nonlinearity on the high-frequency spectral decay, as quantified by the parameter $\kappa$ . Equivalent-linear site response analyses indicate that $\kappa$ should increase significantly with increasing shear strain and ground motion intensity due to increases in soil damping. However, using more than 2500 motions from 32 sites, this study shows that $\kappa$ does not vary systematically with the induced shear strain but instead remains at its small-strain value. This observation indicates that high-frequency components of motion are consistent with small-strain damping, rather than the strain-compatible damping used in site response analysis. It is demonstrated that equivalent-linear site response analyses for large strains can be modified to generate surface motions with more realistic high-frequency content by scaling the predicted surface motion to fit the small-strain $\kappa$ or by employing frequency-dependent soil properties that account for the frequency dependence of the induced strains.