Wellington's sedimentary basin and its role in amplifying earthquake ground motions: new CBD 3D model and maps

Abstract

During the Mw7.8 Kaikōura earthquake, ground motions in central Wellington were clearly influenced by both the nature of the earthquake source and local amplification effects due to the sedimentary basin. These effects led to high spectral accelerations at periods of 1 – 2 seconds in deeper parts of the basin that exacerbated damage to midrise structures. Following these observations, GNS Science and collaborators from the University of Auckland undertook a Natural Hazards Research Platform project to model the 3D basin structure and update the Semmens et al. (2010) geotechnical maps for Wellington CBD. The results contribute to our understanding of the role of the basin structure in amplifying earthquake ground motion. Our new 3D model utilises the wealth of geological data in the region, modern computing software, a suite of new block-by-block geophysical measurements, surface and subsurface data such as Light Detection and Ranging (LiDAR) and a new borehole database. We model five different geological deposits above the basement rocks and provide new maps of, fundamental site period and NZS1170.5 subsoil classification. These maps can be used as a starting point to guide engineering design. They also provide new insights into the local structure of the Thorndon and Te Aro basins, the newly discovered Aotea Fault, as well as the role of 3D basin effects. One key observation is that the traditional treatment of site response as a 1D problem may lead to underestimation of amplification effects at the steep-sided basin margins.