Comparison of observed ground acceleration response spectra from four recent large earthquakes in california with the uniform building code seismic design spectra

Abstract

A total of 302 horizontal- and 151 vertical-component ground acceleration response spectra observed from four recent large earthquakes in California, namely, the 1989 Loma Prieta (surface wave magnitude, Ms 7.1), 1992 Cape Mendocino (Ms 7.1), 1992 Landers (Ms 7.6), and 1994 Northridge (Ms 6.8) earthquakes, are analyzed in terms of the normalized spectrum shape and normalizing parameter. The studies in this chapter use the conventional peak ground acceleration (PGA) as the normalizing parameter. The resultant normalized response spectrum shapes, Sa/PGA, are grouped separately for each earthquake to calculate the overall mean (geometric) horizontal and vertical spectra. The results show that the observed mean spectra, both horizontal and vertical, are remarkably similar to one another among the earthquakes, even though they are located far apart in different regions of the state. When compared with the current Uniform Building Code (UBC) site-dependent Types S1, $2, and $3 design spectrum shapes, the mean horizontal spectrum shapes are found to be conservatively enveloped by the UBC Type $2 spectrum shape. This chapter also introduces mean spectral acceleration (MSA) as a new normalizing parameter. The observed mean vertical spectrum shapes from the earthquakes are found to be less variable and peak consistently at frequencies higher than their horizontal counterparts.