3D Ground-Motion Simulation-Based Site Amplification Considering Multiple Basin Geometries: A Wellington, New Zealand, Case Study

Abstract

The Wellington central business district (CBD) in New Zealand overlies a complex sedimentary basin that significantly amplifies surface ground motions, as observed during events like the 2016 Mw 7.8 Kaikōura earthquake. This article presents a study on site amplification in Wellington predicted through 3D physics-based ground-motion simulations. Eight alternative models of the Wellington basin geometry are used to quantify predicted site amplification and the between-model epistemic uncertainty associated with basin representation. Response spectral amplification factors (for periods 0.5–10 s) are shown at several locations across the Wellington CBD, along 2D transects, and in spatial maps. When compared with observed site amplification, the simulations capture broad spectral amplification peaks and provide comparable levels of site amplification at some deeper basin sites. At shallower basin sites, the simulations have discrepancies in the predicted level and period dependence of site amplification, partially due to model spatial resolution and minimum S-wave velocity (500 m/s for this study) limitations. Linear simulated spectral amplification factors are as high as 8 in some locations. These results illustrate future opportunities for improving the simulations, which include better constrained basin-depth geometry and velocity characterization of the sediments within the basin; higher spatial-resolution simulations; decreasing the minimum S-wave velocity in the simulations; and near-surface nonlinear site response modeling using physics-based approaches.