Nonstationary spatial correlation in New Zealand strong ground-motion data

This paper presents new techniques for quantifying nonstationary spatial variations in strong ground motion, using data from recent well-recorded earthquakes in New Zealand. The data set is unique in that many recording stations are relatively densely spaced, and multiple strong ground motions have been recorded at the same stations. This allows calculation of site-specific and region-specific correlations in ground-motion amplitude for Wellington and Christchurch, and the results are compared to a model assuming stationary correlations at all locations. Strong nonstationarity in spatial correlation is observed in the Wellington and Christchurch regions. Heterogeneous geologic conditions appear to be associated with the nonstationary spatial correlation. Several factors influencing nonstationary spatial correlation were studied: (a) site-specific residuals indicate deviation of correlation from a stationary model; (b) most earthquakes have no systematic effect on spatial correlations and there is no indication of a trend in correlations with magnitude; (c) rupture complexity is related to the variation of spatial correlations in ground-motion residuals; (d) variation of site-specific correlations cannot be resolved by using VS,30 terms in Ground-Motion Models. The nonstationary correlation approach provides an opportunity to incorporate the site-specific effects in future correlation models.