Integrated use of seismograph and strong-motion data to determine soil amplification: Response of the Fraser River Delta to the Duvall and Georgia Strait earthquakes

We present a new method to estimate site amplification, in which regional s eismographic data are used to constrain the input motions to the soil profi le. The "empirical-inferred" amplification is determined as the ratio of th e Fourier spectrum of the recorded motion on soil to the spectrum of the in ferred input motion at the underlying bedrock interface. The method is demo nstrated 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 use d to determine the apparent source spectra of these events. These spectra a re attenuated to sites in the Fraser River Delta, then input to a specified soil profile. We compare the empirical-inferred amplification to alternati ve estimates of the soil amplification based on (1) the quarter-wavelength approach and (2) the assumption that the amplification is approximately equ al to the observed HN ratio. The empirical-inferred amplification estimates are grossly consistent with the theoretical amplification estimates, but h ave larger peaks at specific frequencies, probably due to resonant effects not accounted for in the quarter-wavelength approach. The HN 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 ov er a broad frequency range, from 0.2 to 4 Hz. High-frequency motions (f > 1 0 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 s lightly larger Duvall earthquake is in reasonable agreement with the Brune model, with a stress drop of about 70 bars; however, for the Duvall earthqu ake there is evidence of a sag in spectral amplitudes, relative to the Brun e model, at intermediate frequencies (0.3 to 1 Hz). This intermediate-frequ ency sag is an expected result of source complexity; it is well demonstrate d by the broadband seismic data.