The site response to strong and weak ground motion depends largely on the subsurface conditions at the soil site for the two rock-soil station pairs studied. The first station pair consists of a soft-soil site (Treasure Island) and a sandstone and shale site (Yerba Buena Island). These stations recorded strong ground shaking from the Loma Prieta mainshock and weak ground motion from four aftershocks. The range of peak ground acceleration is from approximately 0.00006 to 0.07 g at the rock site. Compared to the rock site, the strong ground motion at the soft-soil site is amplified by a factor of about 3 over a frequency range from 0.5 to 2.0 Hz. The amplification is much higher for weak motion and suggests a dependence on signal amplitude. For example, near 1 Hz, the site response shows an increasing amplification as magnitude (and the peak velocity at the rock site) decreases. For events of local magnitude 7.0, 4.3, 4.1, 3.5, and 3.3, the maximum soil-site amplifications are 4, 12, 17, 19, and 25, respectively.
A second station pair consisting of a stiff-soil site (Gilroy #2) and a sandstone site (Gilroy #1) was also studied with contrasting results. These two stations recorded strong ground shaking from the 1979 Coyote Lake, 1984 Morgan Hill, and 1989 Loma Prieta mainshocks. Weak ground motion was recorded at these stations after the Loma Prieta mainshock. The range of peak ground acceleration is from 0.006 to 0.43 g at the rock site. Unlike the results for the soft-soil study above, the estimated stiff-soil site responses are not significantly different for strong and weak motion from 0.5 to 2.0 Hz. Near 0.7 Hz, the stiff-soil site responses range from 2.5 to 4.5 for strong ground shaking from three mainshocks and from 1.5 to 4.0 for weak ground shaking from thirteen aftershocks.