Site effects from seismic shaking: a comparison of strong and weak ground-motion in Wellington City and the Hutt Valley

Abstract

Weak- and strong-motion records have been compared at six sites in Lower Hutt and ten sites in Wellington city. The comparisons have been made in terms of Fourier spectral ratios of ground velocity and 5% damped response spectral ratios of ground acceleration. The peak accelerations for the strong-motion data range from 0.5% g to 10% g while the weak-motion range is 0.005% g to 0.4% g.

Non-linear behaviour does not appear to be significant over this ground motion range. In general the strong-motion ratios lie within one standard deviation of the weak-motion ratios at most frequencies, though the strong-motion ratios tend to lie below the mean of the weak-motion ratios. Only one site common to the weak- and strong-motion recorders was located on more than 10 m of soft soil. At this site, the lower strong-motion ratios might be due to non-linear behaviour. One site on about 70 m of alluvial gravels also had lower strong-motion ratios. One site on alluvium and thick gravel to a total depth of 275 m in the Hutt Valley had a greater strong-motion response in the longer period range. A site near the edge of the valley on alluvium had its peak at a shorter period in the weak-motion ratios.