Kaiser AE, Manea EF, Wotherspoon LM, Hill MP, Lee RL, de la Torre CA, Stolte AC, Bora S, Bradley BA, Hulsey A, Gerstenberger MC. 2022. 2022 Revision of the National Seismic Hazard Model for New Zealand: overview of site/basin effects, including a case study of the Wellington Basin. Lower Hutt (NZ): GNS Science. 46 p. (GNS Science report; 2022/56). doi:10.21420/3XXY-T303.
Abstract
This report provides a high-level overview of the treatment of site/basin effects in New Zealand seismic hazard. We outline progress of the National Seismic Hazard Model (NSHM) working group on national site characterisation and a Wellington Basin case study. A key goal of the 2022 NSHM programme was an update of the site characterisation database (Wotherspoon et al. 2022), which provides a suite of site parameters for the full 870 national seismic network stations in New Zealand. The quality of Vs30 characterisation (the solesite parameter for the current 2022 NSHM) can be improved significantly in future studies with additional site-specific measurements, in particular, the examination of rock Vs30 in New Zealand. Further consideration and characterisation of the full soil profile and site parameters related to the basin depth is also recommended. We have also developed a new coarse-scale regional 3D geological and velocity model and a detailed 3D model of the Wellington City area (Hill et al. 2022). This model provides estimates of Quaternary sediment thicknesses overlying greywacke basement rock. The model allows the application and testing of basin-scale advanced ground-motion modelling methods. The detailed central Wellington section of the model has also been used to develop a new map of Vs30 for the Wellington central business district (CBD). The mapping of NZS1170.5 Site Class to Vs30 is complex, such that a range of hazard changes (compared to previous versions of the NSHM) are possible for a given urban location. For Wellington, we use the new Vs30 map to provide a first analysis of the 2022 NSHM hazard changes in Wellington CBD with respect to the previous loading standard (NZS1170.5, based on the 2002 NSHM). From this analysis, the greatest changes are seen at Site Class C locations with low (<300 m/s). We also present a preliminary analysis of Wellington site terms with respect to the 2022 NSHM and discuss alternative site parameters (e.g. Z1.0) that are not currently adopted in the 2022 NSHM. Physics-based ground-motion simulations have been used to model complex 3D wave propagation for a regional-scale amplification study of the Wellington Basin (Lee et al. 2022). This study uses a finer basin model realisation and provides predictions down to shorter periods (approximately 0.5 s) than previously attempted, allowing more meaningful comparisons with observations. The study highlights the role of 3D effects arising from the basin geometry. The consideration of nonlinear effects is also important at the spectral periods of interest in Wellington. Site-specific 1D models of nonlinear effects in the shallow subsurface have been developed for nine Wellington sites, highlighting the significance of nonlinear soil behaviour during high-strain shaking (de la Torre et al. 2022). A novel methodology that adjusts empirical amplification observations at these sites for nonlinear effects expected at higher levels of shaking is also presented in this study. In summary, site/basin effects show complex spectral and spatial patterns that are highly specific to a particular basin. The generic site models adopted in traditional seismic hazard calculations provide a useful means to accommodate these first-order effects, but more advanced treatment of amplification is a promising avenue to reduce uncertainties in the future. Ongoing work will include the development and comparison of alternative site models for application in Wellington. To advance the treatment of local site/basin amplification nationally, improvements in both site/basin characterisation, coupled with the further exploration and testing of advanced modelling methods, is required. (The authors)
