The relation between site amplification factor and surficial geology in central California

Abstract

Abstract Site amplification factors at frequencies of 1.5, 3, 6, and 12 Hz were determined for 132 stations of the USGS seismic network in central California from coda waves of 185 local earthquakes in this area using a recursive stochastic inversion method. We found that the site amplification at a station is systematically related to the geology underlying that station. The site amplification is high for young, Quaternary sediments and decreases with increasing geologic age at all frequencies between 1.5 and 12 Hz. The rate of decrease varies with frequency where site amplification at low frequencies shows a faster rate of decrease with age than at higher frequencies. To quantify the relation between site amplification factor and site geology, the surface geology of station sites was classified into five groups, namely, (1) Quaternary sediments, (2) Pliocene sediments, (3) Miocene through Cretaceous sediments, (4) Franciscan formation and Mesozoic granitic rocks, and (5) Pre-Cretaceous metamorphic rocks. The station site amplification factors for each group were logarithmically averaged and the mean value was assigned to the median geologic age of that group. A smooth power law relation was found between the mean site amplification and the median geologic age. This relation provides a simple way of estimating site effect at a specific site with known surficial geology. Our site amplification factors were compared with magnitude station residuals determined by Eaton (1990) for the same network. A remarkable linear correlation was found between the logarithmic coda amplification factor and the magnitude station residual value, confirming that the coda method provides an effective means of estimating site amplification factor. Comparison of our site amplification factor with strong-motion results obtained from the Loma Prieta earthquake (Boore et al. , 1989; Maley et al. , 1989; Shakal et al. , 1989; Chin and Aki, 1991) suggests that weak and strong-motion site amplification factors correlate well in the region outside the epicentral source region beyond epicentral distances of about 50 km. Within the epicentral source region, however, the weak motion amplification factors estimated from coda waves do not agree with the observed for strong ground motion, suggesting a nonlinear behavior at sediment sites. These results together with studies on the correlation between weak-motion site amplification and earthquake intensity (Borcherdt, 1970; Borcherdt and Gibbs, 1976; Chavez-Garcia et al. , 1990; King et al. , 1990) demonstrate the importance of in situ determination of weak-motion site amplification for seismic zonation.