Estimation of Local Site Effects on Strong Ground Motion

Remarkably consistent results have changed from empirical studies of strong ground motion data about the effect of local geology on peak ground acceleration, peak ground velocity and response spectra, both in U.S. and Japan. Aki (1988) summarized these results in the following observations: the site amplification factor on response spectra depends on the frequency of ground motion. Soil sites show higher amplification than rock sites by a factor of 2 to 3 for periods longer
than about 0.2 seconds, which the relation is reversed for periods shorter than about 0.2 seconds. This frequency dependence is reflected in the site dependence of peak ground motions. Peak ground velocity and displacement as well as the Arias intensity show high amplifications for soil sites than rock sites, which peak ground acceleration is independent of the site classification.

Aki (1988) also recognized a similar reversal in the weak motion amplification factor determined by Phillips and Aki (1986) between the granite site and the fault-zone sediment site, but not between the Franciscan rock site and non-faultzone sediment. In order to establish more firmly the dependence of amplification on site geology at various frequencies, we extend the work of Phillips and Aki to a greater amount of better calibrated data from the USGS central California network using a new efficient inversion method. We inverted for the relative site amplification factors in central California from coda waves of local earthquakes with magnitudes ranging between 1.8 and 3.5 recorded by the USGS seismographic network. A 90 percent variance reduction was achieved after inversion. We found that the site amplification of a station is controlled by the surface geology underlying that station. In general, the site amplification is high for young, Quarterary and Tertiary Pliocene sediments. The amplification decreases with increasing geology age. The decreasing rate is different for different frequency bands. The low frequencies show sharper decreasing rates than those of high frequencies.