The average strong- to weak-motion amplification ratios during the 1989 Loma Prieta and 1987 Whittier Narrows earthquakes in California are estimated by modeling strong ground motions at soil sites in the linear-response assumption and comparing the simulated and observed records. The linear amplification function, used to synthesize ground motions, is the generic transfer function developed for California soil. The database consists of 22 and 80 soil stations for the Loma Prieta and Whittier Narrows events, respectively. A statistically significant (at 95% confidence) reduction in amplification for the Loma Prieta event, within 40 km from the fault, is observed between approximately 1 and 3 Hz. Examining individual strong- to weak-motion amplifications as a function of peak acceleration at the base of soil, within this frequency range, shows that detectable reduction in amplification occurs at accelerations above about 200 cm/sec2. The strong-motion amplification is reduced by a factor of 1.7–2 on average, relative to a generic weak-motion response. The results of this study suggest the magnitude of the reduction in amplification, caused by soil nonlinearity at large strain, that can be expected on average at California soil sites.

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