<p>The shear wave velocity (<i>V</i>_S) profile is one of the critical inputs for site-specific response analysis. The shear wave velocity profile for computer simulation is typically developed based on limited measurements, which usually results in significant shear wave velocity contrast between adjacent layers. Studies show that the velocity contrast, particularly near the surface, significantly alters the computed surface responses, but there is no guideline on the allowable maximum shear wave velocity contrast. This study investigated the effect of shear wave velocity contrast at shallow interfaces by varying interface thicknesses (transition layer thickness) and dividing the interface layer into several microlayers to reduce the contrast. The reference shear wave velocity profile and ground motions specific to Charleston, SC, used for developing seismic site factors in the past, were considered in this study. The site response analysis results of smoothened interfaces were compared with those of the reference profile. The results show that smoothened profiles predicted smaller shear strains and higher surface accelerations than the reference profile with high contrast. Seismic site factors calculated for the total (smoothened first three interfaces) and single (only the shallowest) interface analysis displayed higher values in the intermediate spectral periods. Additionally, for the shear wave velocity gradient of less than 100&#xa0;m/s, the computed surface response and shear strain do not significantly change. This indicates that smoothening at least the shear wave velocity contrast at the shallowest interface should produce better results.</p>