Wellington basin amplification: research goals and update from the NSHM programme

Many New Zealand urban centres are situated on sedimentary basins within which seismic waves can become trapped and amplified. In Wellington, 3D basin amplification effects observed at 1 – 2 second spectral periods were identified as one factor likely to have exacerbated damage to mid-rise structures during the 2016 Mw 7.8 Kaikōura earthquake (Bradley et al. 2018; MBIE, 2017). These local basin-specific ground-motion effects are not typically fully captured in traditional seismic hazard models. 

A component of the newly launched National Seismic Hazard Model (NSHM) programme aims to quantify spatial patterns and event-to-event variability of ground-motion amplification effects in central Wellington. This project will investigate the advances provided by both new empirical ground motion models available since the previous NSHM (Stirling et al. 2012) and also by physics-based ground motion simulation methods. The geometry and sediment fill of the Wellington basin under Wellington’s CBD is relatively well-characterised (Kaiser et al. 2019; 2020), allowing a detailed case study to be undertaken.

We ultimately aim to provide a summary of the state-of-knowledge of Wellington amplification effects and an initial roadmap towards capturing these effects in seismic hazard nationwide guided by international best practice. 

Here we present an introduction to the NSHM Wellington basin project and its goals. We also provide an update on first steps, including a central Wellington Vs30 map, construction of a regional basin velocity model and first Hikurangi earthquake ground-motion simulations.