We present a new method to estimate site amplification, in which regional seismographic data are used to constrain the input motions to the soil profile. The “empirical-inferred” amplification is determined as the ratio of the Fourier spectrum of the recorded motion on soil to the spectrum of the inferred input motion at the underlying bedrock interface. The method is demonstrated for sites on the Fraser River Delta, British Columbia (B.C.), that recorded the 1996 M 5.1 Duvall, Washington, and 1997 M 4.3 Georgia Strait, B.C., earthquakes. Regional seismographic recordings on rock sites are used to determine the apparent source spectra of these events. These spectra are attenuated to sites in the Fraser River Delta, then input to a specified soil profile. We compare the empirical-inferred amplification to alternative estimates of the soil amplification based on (1) the quarter-wavelength approach and (2) the assumption that the amplification is approximately equal to the observed H/V ratio. The empirical-inferred amplification estimates are grossly consistent with the theoretical amplification estimates, but have larger peaks at specific frequencies, probably due to resonant effects not accounted for in the quarter-wavelength approach. The H/V ratio appears to be a simple and useful first approximation to the site response.
The Fraser River Delta amplifies weak motions by factors of three to six over a broad frequency range, from 0.2 to 4 Hz. High-frequency motions (f > 10 Hz) are attenuated. The long-period amplifications pose a serious concern in the event of strong earthquake shaking from large earthquakes.
A useful by-product of the analysis is the determination of apparent source spectra for the events. The apparent source spectrum of the Georgia Strait earthquake is in good agreement with the Brune point-source model, with a stress drop of approximately 45 bars. The apparent source spectrum of the slightly larger Duvall earthquake is in reasonable agreement with the Brune model, with a stress drop of about 70 bars; however, for the Duvall earthquake there is evidence of a sag in spectral amplitudes, relative to the Brune model, at intermediate frequencies (0.3 to 1 Hz). This intermediate-frequency sag is an expected result of source complexity; it is well demonstrated by the broadband seismic data.