PGA adjustment factors for TS1170.5 to account for nonlinear site response on soft soils

C.A. de la Torre, M. Cubrinovski & B.A. Bradley
Department of Civil and Environmental Engineering, University of Canterbury, Christchurch, New Zealand.
S.S. Bora
GNS Science, Avalon, New Zealand.

ABSTRACT
This paper summarises the development and implementation of an adjustment factor for PGA from the 2022 update of the New Zealand (NZ) National Seismic Hazard Model (NSHM2022) for adoption into the NZ technical specifications TS1170.5:2024 (TS1170). The study focuses on soft soil sites within TS1170 Site Classes IV, V, and VI (i.e., with VS30 ≤ 300 m/s). The adjustment factor is based on nonlinear site-response analyses 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. (2025a), and the parametrisation of the proposed adjustment factor for implementation into TS1170 is described in de la Torre et al. (2025b). We compare the adjusted PGAs to the PGAs from NSHM2022, and the PGAs from the 2004 NZ seismic loading standard NZS1170.5:2004. The adjustment factors reduce the PGA for all three site classes, and the amount of reduction increases as the PGA hazard increases. For example, the reductions for 500-year and 2500-year hazard levels for the highest hazard cities of New Zealand are approximately 10-20 % and 15-30 %, respectively. Despite this adjustment, 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.