Stochastic finite-fault modeling of ground motions from the 1994 Northridge, California, earthquake. II. Widespread nonlinear response at soil sites

On average, soil sites behaved nonlinearly during the M 6.7 1994 Northridge , California, earthquake. This conclusion follows from an analysis that com bines elements of two independent lines of investigation. First, we apply t he stochastic finite-fault simulation method, calibrated with 28 rock-site recordings of the Northridge mainshock, to the simulation of the input moti ons to the soil sites that recorded this event. The calibrated model has a near-zero average bias in reproducing ground motions at rock sites in the f requency range from 0.1 to 12.5 Hz. The soil sites selected are those where there is colocation of strong-motio n accelerographs and temporary instruments from the Northridge aftershock o bservation network. At these sites, weak-motion amplification functions bas ed on numerous aftershock records have been empirically determined, in thre e separate investigations reported in the literature. These empirical weak- motion amplification factors can be applied to the simulated input rock mot ions, at each soil site, to determine the expected motions during the mains hock (i.e., neglecting nonlinearity). These expected motions can then be co mpared to the actual recordings during the mainshock. This analysis shows that the recorded strong-motion spectra are significant ly overestimated if weak-motion amplifications are used. The null hypothesi s, stating that the inferred differences between weak- and strong-motion am plifications are statistically insignificant, is rejected with 95% confiden ce in the frequency range from approximately 2.2 to 10 Hz. On average, the difference between weak- and strong-motion amplifications is a factor of 2. Nonlinear response at those soil stations for which the input peak acceler ation exceeded 150 to 200 cm/sec(2) contributes most to this observed avera ge difference. These findings suggest a significant nonlinear response at s oil stations in the Los Angeles urban area during the Northridge mainshock. The effect is consistent with the increase in damping of shear waves at hi gh levels of strain, which is well known from geotechnical studies of soil properties.