EMPIRICAL EVIDENCE FROM THE NORTHRIDGE EARTHQUAKE FOR SITE-SPECIFIC AMPLIFICATION FACTORS USED IN US BUILDING CODES

摘要

Site-specific amplification factors, F_a and F_v used in current US building codes are dependent on amplitude of the input motion. The Northridge earthquake of January 17,1994 provided a large set of in-situ recordings of input "base" motions up to about 0.5g. Extensive sets of borehole geotechnical data, collected at many of these sites since the earthquake, provide an improved basis to reexamine the dependency of site specific amplification factors on input base shaking level. This paper summarizes recent results for this dependency as implied by empirical amplification factors inferred with respect to nearby "rock" stations underlain by granite. Preliminary results suggest that the site factors are in good agreement with those suggested for the code provisions at the 0.1 g base acceleration level. At successively higher input ground-motion levels the preliminary empirical estimates do not show a well-defined tendency to decrease with increasing amplitude. The detailed geotechnical data recently available for the Northridge recording sites and further study is expected to better define the dependency of site-specific amplification factors on amplitude, distance and azimuth with respect to the earthquake source.