A numerical code for calculating the time-domain response of a nonlinear soil-to-base excitation is used to examine the characteristics of strong-motion accelerograms recorded in soil. The results verify several of the effects that are usually cited as evidence of nonlinearity: decreased spectral ratios of surface-to-input motion near the dominant frequency of the soil; decreased statistical uncertainty in prediction of peak acceleration; and increased effective period of surface motion. When examined in the Fourier-transform domain, the results show that the soil response can be separated into three frequency bands. In the lowest frequency range, the spectral amplitudes are not affected by the nonlinearity. In the central band, the spectral amplitudes are decreased. The increase in the dominant period is caused primarily by a strong decrease in the amplitude of shorter-period waves, rather than by amplification of low-period motion. Above a cross-over frequency, however, the spectral amplitudes at the free surface are increased relative to linear soil response calculations. This is a consequence of the sudden change in the stiffness of the soil at reversals in the stress-strain curve. This increase in spectral amplitudes at high frequencies causes the spectral decay parameter κ to decrease for the soil model that was used. The transition frequencies separating these three types of behavior shift to lower frequencies as the thickness of the soil layer is increased.