Assessing seismic response of soft soil sites in Hong Kong using microtremor records

Microtremor records have been used to assess seismic amplification of soil sites in Hong Kong. The method is based on ratios of horizontal and vertical components of microtremor (ambient noise) records, and the ratios are often referred to as Nakamura's ratios. The peak frequency of the Nakamura's ratio correlates well with the fundamental modal frequency estimated from the site shear wave velocities which were derived by a vertical seismic profiling method or were converted from SPT data. The amplitudes of the ratios are very stable and the standard deviation of the ratios derived from a series of measurements is reasonably small. Results from rock sites show that the ratios take a value of unity at a wide frequency range when the site has no large topographical effect. The ratios are generally larger than unity if the site has topographical effect but no dominant peak appears in the ratios. Results from soil sites show that a dominant peak appears at the fundamental frequency of the site and the amplitudes of the dominant peak are between 6 and 12 for mean ratios. It is plausible to take the maximum value of the ratios as the peak amplification of the soil layers (Fourier spectral ratio) at small strain motions. Results from soil sites also show that many sites on reclaimed land and land fills are likely to be susceptible to seismic amplification. The seismic amplification has possible implications for those buildings with about 20 storeys or less founded on soil sites. Site fundamental modal frequencies estimated by microtremor records have been obtained from extremely small strain motions and they are likely to change during a moderate or strong earthquake shaking. However, overseas experience has shown that this method can be used to identify soil sites susceptible to seismic amplification.Department of Civil and Environmental Engineerin