Recorded frequency response characteristics of an adjacent pair of silty gravel sites

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Sritharan, S.; McVerry, G.H. 1993 Recorded frequency response characteristics of an adjacent pair of silty gravel sites. Lower Hutt: Institute of Geological & Nuclear Sciences Institute of Geological & Nuclear Sciences science report 93/34 25 p.

Abstract: Amplification of recorded earthquake ground motions at a pair of closely placed silty gravel sites (A and B) on the edge of the Hutt Valley with respect to a nearby rock site is demonstrated as a function of amplitude for various spectral periods. Records from site A and the rock site were obtained in 3 earthquakes between 1968 and 1977, with peak ground accelerations on rock ranging from 0.026g to 0.095g. The accelerograph was removed from site A in 1986, following the installation in 1985 of an accelerograph at site B less than 100 metres away on similar silty gravels. Since then, another nine earthquakes have produced pairs of records at site B and the rock site, but with smaller peak rock accelerations than the earlier records, between 0.005g and 0.024g. Sites A and B show moderate to strong amplification of the rock motions, with the peak of the average amplification spectrum over the 12 records of the 5% damped response spectrum accelerations having a value of 4.0 for the along valley component. At periods shorter than that of the peak amplification, the plots show that the amplification reduces as the strength of the rock motion increases, suggesting nonlinear behaviour. However, detailed investigations indicate that the apparent nonlinearity may result in part from differences between the responses of sites A and B. For periods from 0.35s up to about 1.0s, the plots of the response spectrum accelerations for sites A and B against those for the rock site suggest that the two silty gravel sites have similar amplifications and linear responses, with the amplification independent of the spectral value of the rock motion to within the scatter of the data. The spectral components above I.Os period lie near the noise level of the processing system, especially for the rock site, but there are indications of differences in the amplifications of sites A and B for long periods. Amplification of the Fourier spectra values shows similar behaviour to that of the response spectrum values, but is less well defined from the recorded motions. (auths)