The stochastic finite-fault method was used to simulate ground motions against strong-motion data from the M 6.7 1994 Northridge, California, earthquake. The results follows from an analysis that combines sections of two independent lines of investigation. At first, we apply the stochastic finite-fault simulation method, which calibrated with 28 rock-site recordings of the Northridge mainshock, to the simulation of the input motions 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 frequency range from 0.1 to 12.5 Hz. The results is similar, whether we used the realistic slip model and hypocentral location or not.
The soil sites selected are those where there is colocation of strong-motion accelerographs and temporary instruments from the Northridge aftershockObservation network. At these sites, weak-motion amplification functionsbased on numerous aftershock records have been empirically determined.These empirical weak-motion amplification factors can be applied to the simulated input rock motions, at each soil site, to determine the expected motions during the mainshock.
The simulated mainshock recordings significantly overpredict the realistic motions at soil sites on average, if weak-motion amplifications are used. So we can suggest that the deamplification occurred during the Northridge mainshock. A reduction in soil amplification for strong motions relative to weak motions, caused by an increase in damping at high levels of strain, is a justly consequence. Nonlinear response at those soil stations for which the input peak acceleration exceeded 150 to 200
contributes most to this observed average difference. Therefore we can give a conclusion that the actual deamplification occurred during the Northridge mainshock were, on average ,significantly influenced by nonlinearity.