Spectral ratios between sediment sites and rock sites for strong and weak ground motion are compared to infer the extent of nonlinear sediment response in the Los Angeles area during the 1994 Northridge earthquake. Northridge mainshock records form the strong-motion database and aftershock records form the weak-motion database. The degree of nonlinearity is evaluated at 25 locations in and around the San Fernando and Los Angeles Basins based on the ratio of weak-to-strong spectral amplification factors (WSAF). Values of WSAF vary greatly from site to site and, for a given site, from one frequency band to another over the range from 0.75 to 20 Hz. This complexity is attributed to differences in several factors: material properties under the sites and the amplitude and frequency content of the input ground motions. Given these observations, it is not possible to assign an acceleration or velocity level at which nonlinearity initiates for all the stiff soil sites analyzed. However, no nonlinearity is seen below a peak acceleration of about 200 to 300 cm/sec2 or peak velocity of 20 to 30 cm/sec. Nonlinearity is seen at all sites in or near the San Fernando Valley. Nonlinearity in the Los Angeles Basin is limited to a few isolated sites. For the Northridge earthquake, even sites with strong nonlinearity still show amplification levels greater than 1.0 relative to rock sites for frequencies below 10 Hz.
