Are NZ building code requirements for seismic resilience of existing buildings fit-for-purpose?
The occurrence of the 2011-2012 Canterbury and 2016 Kaikōura earthquakes prompted a revision of the national seismic hazard map. This revision, as described by Risk Frontiers Briefing Note 471 (2022), was led by GNS (Gerstenberger et al., 2022; GNS, 2022) and is called the National Seismic Hazard Model 2022 (NSHM22), The public consultation draft of the revised building code, prepared by NZSEE et al. (2022), uses a version of Structural Design Actions Part 5: Earthquake Actions, New Zealand, denoted DZ TS 1170.5:2024, which is intended to supersede the 2004 version (NZS1170.5:2004).
The NSHM22 revision posits an average countrywide increase of 50% in the probabilistic seismic hazard level, as shown in the right panel of Figure 1. For Auckland, Wellington, and Christchurch, the estimated hazard doubles!
The changes are particularly severe for Auckland and Northland, regions, which along with Central Otago, face the lowest seismic risks in the country. Many buildings in these regions now lie derelict because the cost to owners of bringing them up to code is prohibitive. Whether this can be justified from a risk perspective remains a moot point.
The impacts of these changes in seismic hazard levels for the design of new buildings were anticipated in an Advisory prepared by a group of organisations led by NZSEE (2022) and MBIE (2022). These guidelines basically adopt the NSHM22 hazard levels for the design of new buildings, the New Building Standard (NBS).
For existing buildings, it is a different story. In practice, an earthquake-prone building is defined as one with a seismic rating less than 34% of what would be required for a new building on the same site. While explicitly stating that the NSHM22 hazard levels are NOT intended for the assessment of existing buildings, which are subject to the 34% of the New Building Standard (NBS) hazard levels derived from NZS1170.5:2004, these revised levels present a daunting challenge to such buildings.
In Christchurch, this has allowed analyses of existing buildings to retain use of 34% of a Z factor of 0.22 (representing a peak acceleration of 0.22g on rock) in NZS1170.5:2004, which comes to 0.075g, even though the Z factor for Christchurch was raised from 0.22 to 0.30 in 2012 (McVerry et al., 2012), and from 0.30 to 0.33 in 2022 in NSHM22 and DZ TS 1170.5:2024.
In Auckland, the Z factor of 0.13 (0.13g) from NZS1170.5:2004 was not derived probabilistically, but instead from a scenario earthquake, yielding a Z factor of 0.13. It represents a conservative lower bound ground motion, designed by a committee to be the 84th percentile ground motions from a magnitude 6.5 earthquake at a closest distance of 20 km. The probabilistic analysis, not adopted in the code because it was thought to be too low, yielded a value of about 0.08g (Bradley et al., 2012; Risk Frontiers, 2012; Stirling et al., 2012), about half the scenario-based result used in the 2004 code.
The intentional conservatism of this scenario-based code value, with a peak acceleration of about 0.13g, is demonstrated by the fact that it had an AEP of about 1:2,500 (according to Bradley et al., 2012; Risk Frontiers, 2012; Stirling et al., 2012), not the 1:500 AEP intended by the code and applied in the rest of the country. The fact that this scenario ground motion was nearly twice the probabilistic value derived from NSHM10 meant that the code value was already elevated in NSZ1170.5:2004, so analyses of existing buildings in Auckland were still facing the use of 34% of this high base value, bringing no relief in Auckland like that available in Christchurch.
Coincidentally, the scenario-based Z factor of 0.13 for Auckland from NSHM10 and NZS1170.5:2004 is identical to the probabilistically derived Z factor from NSHM22 and DZ TS 1170.5:2024. Consequently, the above argument that use of the scenario-based method inflated the estimated Z factor in Auckland no longer holds if NSHM10 is incorrect and NSHM22 is correct. The proposed new code requirement for new buildings in DZ TS 1170.5:2024 applies a probabilistic approach uniformly across all of New Zealand, but there is still a legacy of different requirements for existing buildings as noted above in relation to Auckland and Christchurch.
In the aftermath of the Canterbury and Kaikōura earthquakes, the owners of earthquake-prone buildings around New Zealand were ordered to strengthen or demolish them, a task which would cost billions of dollars. The existing seismic-strengthening regime was set in place by the John Key Government, in response to the tragic Canterbury earthquake of 22 February, 2011, that killed 185 people. The Building (Earthquake-prone Buildings) Amendment Act 2016 was passed in May 2016, just a few months before the Kaikōura earthquake that killed two people and seriously damaged hundreds of buildings in Marlborough, Wellington and the lower North Island.
As a result, many buildings in Auckland have been boarded up and left derelict. Under the current law, owners of earthquake prone buildings in Auckland have 35 years to strengthen buildings above 34 per cent of the 2004 new building standard. The situation was described in Risk Frontiers Briefing Note 252 (2013).
As reported by Jonathan Milne (2025), the Auckland Council is among owners that have been forced by seismic strengthening rules to shut down public facilities including the Ponsonby library and gymnasium housed in the historic Leys Institute in St. Mary’s Bay. The 120-year-old twin buildings were shut down in 2019 after a seismic assessment found structural issues that would make the buildings unsafe to occupy in the unlikely event of an earthquake. The council hopes to get building consents to strengthen the building later this year, and to reopen it in 2028 after nine years’ closure, and millions of dollars of ratepayer money spent. The Domain Wintergardens have already been strengthened and reopened this year.
On 13 November 2023, Auckland Mayor Wayne Brown, a qualified civil engineer, said that he wanted the new Government to review earthquake-strengthening rules. He said that earthquake strengthening costs have become a big earner for consultants writing reports, preparing estimates of building resistance, proposed strengthening, and the “dreaded earthquake resistance rating.”
The Mayor stated that Wellington bureaucrats are ignoring the risk of volcanoes in Auckland while they are imposing financial stress on the owners of old buildings. He said that “A review of Auckland’s geology shows no signs of earthquake activity in the last 100,000 years but there are 40 volcanoes here, the oldest of which is 40,000 years, and the newest, Rangitoto is around 800 years… Our emergency teams should list volcanoes as a natural hazard and do some preparation based on what has happened in places like Iceland and Hawaii.”
On 10 July 2025, Chris Penk, who is the Minister for Building and Construction in the National government, proposed that buildings in Auckland, Northland and coastal Otago be removed from earthquake-strengthening requirements, saying that the science shows those regions are less risky (Milne, 2025), as portrayed in the 2004 Building Code Act earthquake hazard map shown in the left panel of Figure 1. This is one of a set of changes that Penk is going to submit to the Cabinet in a bid to reset the balance between preserving life, preserving buildings, and saving money. He also wants changes to foreign investment and consent rules to make it more cost-effective to replace or strengthen the buildings, especially heritage buildings. Both he and the Mayor agree that Auckland is at greater risk from volcanoes and tsunamis than it is from earthquakes.
Penk commissioned a seismic risk management review, chaired by Dr Erica Seville, which reported back to him a month ago. He intends to report to the Cabinet in the coming weeks. This independent chair and steering group was appointed to provide expert oversight for the review of earthquake-prone buildings and seismic risk management and find practical solutions that provide a more certain future for building owners and the public. Dr Seville will be leading a steering group consisting of 10 nominated representatives from both public and private organisations across the building, construction and property sectors.
Penk states that ultimately, decisions on how to prioritise the spending of public funds come down to how much can be justified to save one life. The government’s ability to oblige private entities to spend money to protect against risk requires proportionality and consistency applied to different kinds of risks. He distinguishes between society’s expectations and willingness to pay to mitigate the risk of injury and death in an earthquake, and for improving the resilience of buildings over time. Building codes already distinguish between different levels of importance in relation to the function of the building. Cost-effective methods of remediation need to be found, otherwise if they are too high, then nothing will be done.
References
Bradley BA, Cubrinovski M and Wentz F (2022). “Probabilistic seismic hazard analysis of peak ground acceleration for major regional New Zealand locations”. Bulletin of the New Zealand Society for Earthquake Engineering, 55(1), 15–24. https://doi.org/10.5459/bnzsee.55.1.15-24
Gerstenberger, Matt et al. (2022). New Zealand Seismic Hazard Model 2022 Revision: model, hazard and process overview. GNS Science Report 2022/57, September 2022.
GNS (2022). New Zealand National Seismic Hazard Model 2022 Revision. https://nshm.gns.cri.nz/
NZSEE et al. (2022). Earthquake Design for Uncertainty. Advisory jointly prepared by NZSEE, SESOC and NZGS Revision 1, August 2022: Seismic design of building structures.
McVerry, G., M. Gerstenberger, D. Rhoades and M. Stirling (2012). Spectra and PGA’s for the Assessment and Reconstruction of Christchurch. NZSEE Conference, 2012, Paper No. 115.
Milne, Jonathan (2025). Govt set to quash death sentences on hundreds of quake-prone buildings. https://newsroom.co.nz/2025/07/10/govt-set-to-quash-death-sentences-on-hundreds-of-quake-prone-buildings/?utm_source=Newsroom&utm_campaign=99208bee53-Week+In+Review+13.07.2025&utm_medium=email&utm_term=0_71de5c4b35-99208bee53-576269316&mc_cid=99208bee53&mc_eid=a5fefd9213
Risk Frontiers (2013). Are there 15000 to 25000 earthquake prone buildings in New Zealand? Briefing Note 252.
Risk Frontiers (2022). New Zealand National Seismic Hazard Model 2022 Revision and NZSEE Advisory on Buildings. Briefing Note 471.
Risk Frontiers (2012). QuakeNZ: New Zealand earthquake loss model. https://riskfrontiers.com/solutions/model-data-solutions/quakenz/
Stirling M, McVerry G, Gerstenberger M, Litchfield N, Van Dissen R, Berryman K, Barnes P, Wallace L, Villamor P, Langridge R, Lamarche G, Nodder S, Reyners M, Bradley B, Rhoades D, Smith W, Nicol A, Pettinga J, Clark K and Jacobs K (2012). “National seismic hazard model for New Zealand: 2010 update. Bulletin of the Seismological Society of America, 102(4): 1514–1542. https://doi.org/10.1785/0120110170
