SITE RESPONSE IN SEATTLE DETERMINED FROM RECORDINGS OF THE M6.8 NISQUALLY, WASHINGTON EARTHQUAKE

摘要

We analyzed strong-motion records in Seattle of the Feb. 28, 2001 M6.8 Nisqually earthquake to determine the response at 35 sites on various geologic units and to assess the importance of surface waves in the Seattle Basin. The response for each site was found from the Fourier spectral ratio of the horizontal accelerograms with respect to a soft-rock reference site. We observe strong amplification (factors of 3-7) at 1 Hz for soft-soil sites (typically NEHRP class E) on artificial fill and young alluvium, areas with higher damage and liquefaction. There are clear indications of nonlinear site response at the soft-soil sites, despite the modest peak ground accelerations of 15-22%g. Nonlinear response at the soft-soil sites during the mainshock ground motions eliminates the amplification at 2-8 Hz that is observed for a M_L3.4 aftershock. Spectral peaks in the site response shift to lower frequency for the mainshock compared to the aftershock. Three of the soft-soil sites display mainshock accelerograms with cusped waveforms after the S-wave, which is indicative of nonlinear site response. These sites show amplification at 10-20 Hz in the S-wave and later cusped arrivals that is not observed for the aftershock and is another likely symptom of nonlinear soil response. Many of these soft-soil sites had liquefaction nearby. Stiff-soil sites (NEHRP classes C and D) of Pleistocene glacial deposits exhibit moderate amplification (factors of 1.1-2.4) at 1 Hz and approximately linear site response. Amplification is inversely related to the shear-wave velocity averaged over the top 30m at each site (Vs30), as determined from shear-wave refraction. Record sections for the mainshock indicate that basin surface waves often dominate the velocity waveforms at frequencies of 1 Hz and lower, for sites in the Seattle Basin.