Analysis of Site-Response Residuals from Empirical Ground-Motion Models to Account for Observed Sedimentary Basin Effects in Wellington, New Zealand

Abstract

Analysis of prediction-observation residuals from the empirical ground motion models (GMMs) used in the 2022 New Zealand National Seismic Hazard Model (NZ NSHM) update indicates a general underprediction of ground motions in the period range of 0.5-2 seconds for soft sedimentary basin sites in Wellington, the capital city of New Zealand. This study uses residual analysis to quantify this underprediction, understand the spatial distribution of these site-response residuals and the specific conditions that cause them, and investigate options for non-ergodic adjustments to the GMMs. All GMMs used in the NZ NSHM were evaluated, and the variability in site-response residuals between different models and different tectonic types of earthquake sources are quantified. Site are regionalised into different geomorphic features, such as individual basins and valley. For example, average site terms are calculated for Te Aro, Thorndon, Miramar, Lower Hutt, and Upper Hutt. The period at which maximum underprediction occurs at these sedimentary basin and valley sites was found to correlate well with the fundamental site period of the soil profile (T₀). This suggests that improvements can be made to regionalised GMMs by incorporating site period into the site-response prediction for sedimentary basin sites.