Distribution of scatter in New Zealand accelerograph data
DOI:
https://doi.org/10.5459/bnzsee.18.2.151-164Abstract
This study examines the currently available set of 22 New Zealand strong motion accelerograms to determine values of the scatter parameters for use in correcting for attenuation uncertainty in seismic hazard analyses. In particular, scatter of observed acceleration response spectral ordinates about mean values predicted by the attenuation law of Katayama, Iwaski and Seaki is investigated. It is found that to a high level of statistical significancef the observed spectral ordinates are log-normally distributed about the predicted values. For the data as a whole, lumping samples from the nine natural periods studied, the standard deviation of the logarithmic residuals is σ10=0.25. However, values ofσ10 for distinct natural periods are quite scattered in an apparently random manner, suggesting that the set of accelerograms is not large enough to yield stable results on a period-by-period basis. Furthermore, since there is no recording of severely damaging ground motions amongst the accelerograms, it is not clear that the value of σ10=0.25 is characteristic of hazardous ground motions in New Zealand, especially when it is compared with the value of 0.3 obtained from the more extensive Japanese data set. For immediate practical purposes, it is proposed that scatter parameters obtained from a combined New Zealand-Japanese data set be used. These records taken together yield values of σ10 increasing from 0.28 at a natural period of 0.1s to 0.32 at 1.0 seconds and above. Employing these values rather than σ10 = 0.25, results in a 20 percent increase in the peak of the 150-year uniform risk spectrum for Wellington.
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