| Abstract: | To study the characteristic features of the in-plane free-field response, two actual sites of nuclear power plants, a soft and a rock site, are analysed, by varying the location of the control point and the nature of the wave pattern. Harmonic and transient seismic excitations are examined. The conclusions reached in the vast parametric study of Reference 6 are confirmed for the actual sites. These apply to the range of possible apparent velocities and the associated motions, the spatial variations with depth and in the horizontal direction. If only one component of the control motion, e.g. the horizontal, is matched, then it can be associated either with a body wave or with a surface wave. In the latter case, a specific mode is used up to the frequency at which the next higher mode starts, since for a given frequency, the higher modes attenuate less. The other component of the motion follows. If both components are prescribed, the motion can be interpreted as arising from a combination of a P- and an SV-wave (with a common apparent velocity). Surface waves alone cannot be used to match both components. A body wave has to be included, at least up to the frequency at which the second mode starts. For the soft site, the surface waves decay significantly, especially in the range of higher frequencies, where the apparent velocity is considerably smaller than the shear-wave velocity of the rock. For this site, it seems sufficient to examine only (extremely shallow) body waves. For a rock site, however, Rayleigh-waves exist which attenuate little, leading to smaller apparent velocities than extremely shallow body waves. |
