Earthquakes and Ground Motions - Earthquake Engineering for Structural Design - page 240

In the last decades there was a clear gap between seismologists and
geotechnical engineers regarding the effects of soft soils, considering the nonlinear
behavior of such soil type (Bard, 1995).
The presence of soft sediments produces a decrease of site dominant
frequencies. For low and moderate earthquakes, the modifications refer only to the
increase of the natural period. Contrary, for large earthquakes, this modification is
accompanied also by the increase of site acceleration (Trifunac, 1990). Opposite
effects, expressed by the decrease of amplification factors for peak acceleration
exceeding 0.4g, are referred by Bard (1995), Liam and Matsunga (1995), Dobry et
al (2000), Rodriguez-Marek et al (2001) (Fig. 7.19). For low rock accelerations on
the order of 0.05g to 0.20g, corresponding to Mexico City and Loma Prieta
earthquakes, the soft soil accelerations are 1.5 to 4.0 times greater than the rock
accelerations. The amplification factor decreases as the rock acceleration increases,
this phenomenon being directly related to the nonlinear behavior of soil (Faccioli,
1996).

Field of strong amplification Field of de-amplification

due to soft sediments

Figure 7.19 Maximum rock accelerations versus soil site accelerations
(after Idriss, 1990)