ABSTRACT
This study addresses the main issue that whether a soil site is advantageous in amplifying or in damping earthquake shaking transmitted from the bedrock levels. Many observations made during earthquake show that a thick layer of soil may amplify the ground shaking significantly. Mexico City earthquake in 1984 is a good example, but there are some isolated cases that soft soil site did provide a kind of cushioning and absorbing the seismic waves before it can be transmitted to the surface. For example, the liquefaction of a particular layer in a thick layer may in fact act as an isolator. This is precisely the idea for base isolator, which has becoming more popular recently in earthquake resistance design. There seems no parametric study yet to address this subtle phenomenon of amplification or isolation. A parametric study will be given to examine the geological setting which is most conducive to amplifying, or what geological soil profile may act as isolator instead of as amplifier. Our results show that a layer of very soft clay within a thick layered soil mass does act as an isolator. In addition, 12 selected sites in Hong Kong have been examined by adopting various empirical formulas to estimate the wave speed of soil. It turns out that the results are extremely sensitive to the type of empirical formulas that we use for the modeling. The verdict is that local soil must be tested for the shear wave speed as well as the damping characteristics. The amplification factors for some locations in Hong Kong may be up to 5.2 under the strike of strong but rare earthquakes and to 7.5 under the strike of small but more frequent earthquakes.
