PGA adjustment factors for nonlinear site-response effects on soft soil sites: Application to TS1170.5
DOI:
https://doi.org/10.5459/bnzsee.1727Abstract
This paper discusses the development of an adjustment factor for PGA from the 2022 update of the New Zealand (NZ) National Seismic Hazard Model (NSHM2022) and its implementation in the NZ technical specifications TS1170.5:2024 (TS1170). The study focuses on soft soil sites with VS30 ≤ 300 m/s (i.e., Site Classes IV, V, and VI in TS1170). The adjustment factor is based on nonlinear site-response simulations of NZ characteristic soft soil sites and an examination of observations from extensive national and global ground-motion databases. These simulations treat soil nonlinearity more rigorously than the approximations used in the empirical ground-motion models employed in NSHM2022. The scientific background and details of the analyses used to develop the PGA adjustment factors are documented in de la Torre et al. [1], while the focus of this paper is the parametrisation of the proposed adjustment factor for implementation into TS1170. The adjusted PGAs are compared to the PGAs obtained directly from NSHM2022, and the PGAs from the 2004 NZ seismic loading standard NZS1170.5:2004 for many cities. Application of the adjustment factor results in a reduction to PGA for all three site classes. The amount of reduction increases with increasing intensity of the NSHM2022 predicted ground motion (i.e., PGA), resulting in approximately 15-25 % reduction to the 2500-year return period PGA in the highest hazard regions of NZ. However, even with the proposed reduction factors, compared with NZS1170.5:2004, the adjusted PGAs in these high-hazard regions are still 40-50 % higher for the 500-year return-period ground motion.
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