Re-evaluation of New Zealand seismic hazard for geotechnical assessment and design

This paper scrutinises the seismic hazard of New Zealand (NZ) from a geotechnical engineering perspective. The two codified versions of the seismic hazard in NZS1170.5 (structural loading standard) and NZTA Bridge Manual (NZTA-BM) are shown to yield consistently different peak ground acceleration (PGA) hazards throughout NZ. Results from site-specific PSHA for 24 locations in NZ are used to examine key hazard characteristics, including earthquake magnitude and the effects of site conditions (classes) on the PGA hazard. The comparative evaluations show that for most of the locations considered, NZS1170.5 and NZTA-BM overestimate the PGA hazard. However, NZS1170.5, and NZTA-BM in particular, significantly underestimate the PGA hazard for locations that are at a short source-to-site distance from the Hikurangi Subduction Zone (HSZ), and for which HSZ significantly contributes to their hazard. Using the results from this study, an interim PGA hazard is recommended for geotechnical assessment and design in support of the NZ guidelines for geotechnical earthquake engineering practice. The recommended interim PGA hazard is applicable to all site classes without any modification or use of site amplification factors.