Examination of Very High Peak Ground Acceleration and Short‐Period Spectral Accelerations on Soft‐Soil Sites from the New Zealand National Seismic Hazard Model

This article investigates the very large ground‐motion amplitudes of peak ground acceleration (PGA) and short vibration periods on soft soil sites in high‐seismicity regions from the 2022 New Zealand National Seismic Hazard Model (NZ NSHM). For example, a PGA of 1.36 g is predicted for the 2% in 50 year exceedance probability at a site with V S 30 = 225 m/s in Wellington, New Zealand. Such values are in the realm of the largest globally‐recorded ground motions on soft‐soil sites. In this context, the article examines the amplitudes of these NSHM‐based ground‐motion amplitudes against historical observations, and scrutinizes the treatment of nonlinear site amplification adopted in common ground‐motion models (GMMs). Through such an examination, we form the opinion that, while the design ground motions can physically occur, there are multiple aspects that likely lead to overestimations, particularly for PGA. The most notable is the use of the equivalent‐linear method to constrain nonlinear site response for high‐intensity ground motions and associated shear strains, well beyond those for which this method is suitable. We propose adjusted nonlinear functions for PGA, for soft‐soil sites with V S 30 < 300 m/s, based on nonlinear simulations for New Zealand sites, which more rigorously consider effects of nonlinear soil behavior. The adopted nonlinear functions were subsequently implemented into GMMs used in the NZ NSHM, and it was illustrated that they result in a decrease in PGA hazard, which becomes more pronounced with increasing ground‐motion intensity. For example, a reduction in PGA of approximately 25% is predicted for the highest‐hazard regions of New Zealand, such as Wellington, at the 2% in 50 year exceedance probability. This study highlights the important impacts of soil nonlinearity on PGA for soft‐soil sites in high seismic hazard regions, and the need to more rigorously consider this modeling aspect in future GMM development.