Are you an architect, property developer, or building portfolio manager? Then the recent changes to the National Seismic Hazard Model (NSHM) may affect you, your work, and your portfolio. Read on to see how. Any new updates will be posted at the end
At a glance
Earthquake hazards have increased by an average of 50%
For new builds, engineers have been instructed to use good detailing practice which involves:
Buildings should be regular and straightforward
Provide redundant load paths
Use stiff seismic resisting elements, but detail them so that increased displacement demands can be accommodated
%NBS ratings will not change because engineers will continue to assess buildings against the existing seismic demands
What is the National Seismic Hazard Model?
NSHM is a model that calculates the likelihood and strength of earthquake shaking that might occur at any given site in Aotearoa New Zealand.
The model is used by government and industry bodies alike to manage safety risks, improve national resilience, and help minimise the economic impact of seismic events.
GNS is the custodian of the model and is periodically engaged to update it with the latest knowledge in geophysics, and earthquake science.
NSHM Hazard map, retrieved from https://nshm.gns.cri.nz/HazardMaps The model was originally created back in 1980, and the most widely referenced version was developed in 2002. The 2002 version helped create seismic loading requirements in NZS1170.5:2004 which is referenced in the Building Code. Another update, in 2010, had minimal science revision and is not adopted into the Building Code, but is still used by insurers and in the design of special buildings.
Needless to say, the flow-on effects of what the NSHM calculates have far-reaching consequences for anyone involved in the construction and property industry.
For more information on the NSHM model, its history and the current up to date model, follow these links: https://www.gns.cri.nz/research-projects/national-seismic-hazard-model/
The Current Update
Back in 2020, MBIE and Toka Tū Ake EQC engaged with GNS to review the NSHM and provide an updated version. This culminated in the most recent update, released two weeks ago on 4th October 2022.
Since the last update in 2010, there have been significant improvements in scientific modelling capability and data collection. The newest model has also been influenced by new learnings in the Canturbury Earthquake Sequence (2010-2011) and Kaikoura Earthquake (2016).
What does the update say?
Since the update, we have reviewed all the guidance from the New Zealand Society for Earthquake Engineering (NZSEE), Structural Engineering Society of New Zealand (SEESOC), and New Zealand Geotechnical Society (NZGS). This is what they say:
The latest knowledge shows that earthquake hazards around the country have increased by an average of 50%.
This is due to an increased understanding of earthquake behavior and sources, in particular the Hikurangi Subduction Zone, a fault affecting the majority of New Zealand. Fault ruptures and ground shaking are the two primary factors making up the NSHM and are based on both real-life earthquake data as well as extensive modelling.
Hikurangi Subduction Zone Graphic, taken from https://www.gns.cri.nz/our-science/land-and-marine-geoscience/our-plate-boundary/hikurangi-subduction-zone/
While this science provides us with data on likely shaking for future earthquakes, it is important to remember that Building Codes are based on more than just hazard data – they also account for risk and impact on society, which the NSHM does not do. The NSHM will be used to inform decisions around updating the Building Code requirements and earthquake loading standards, however, there is no direct correlation between the two.
Resulting Design Requirement Changes
There are currently no legal changes in design requirements, as the NSHM is not a design standard.
The current Building Code and referenced seismic loading standards still need to be followed for compliance. What we can do when designing for uncertain earthquake loads is to follow the advice released alongside the NSHM and (continue) to adopt good detailing practices to ensure building resilience.
For example:
The building structure should be regular with straightforward, redundant load paths
The performance of the building should not rely heavily on one or two key elements
Structures should be designed to be stiff but detailed so that increased movement and displacement demands can be accommodated if the building is pushed beyond the current design level earthquakes.
Resulting Assessment Requirement Changes
Building assessments are risk-based assessments and aim to identify and reduce the use of poor detailing practices that could be a life safety risk.
As noted previously, the new NSHM data released does not directly correspond to a change in risk.
It is important to keep assessing buildings against the current seismic demands required by the Building Code, otherwise %NBS ratings will be inconsistent and will have no real base data or demands to be compared against.
What's Next?
The updated NSHM does not currently change design or assessment requirements. However, the Ministry of Business, Innovation, and Employment (MBIE) has begun the process of comparing the 2010 and 2022 models to assess whether any changes to the Building Code are required.
It is expected that any updates to the Building Code will occur in stages, with the first stage released as a draft for public consultation in mid-2023. A more substantial update is expected to be released for consultation in mid-2025.
What can you do?
It can be frustrating for architects, building owners or property developers when there is a level of uncertainty regarding the seismic design and assessment of the structures they are responsible for. This, unfortunately, is the nature of seismic engineering and the earthquake science that helps govern it.
Rest assured, experienced structural and geotechnical engineers already have guidance on accounting for this uncertainty. NZSEE recently updated their guidance on "Earthquake Design for Uncertainty" which explicitly describes how to mitigate the uncertainty risks inherent in structural or geotechnical earthquake design.
We have helped hundreds of Architects, Building Owners and Property Developers navigate the intricate complexities of earthquake design, and changes to the building code. If you have any worries or need some help with a building you are in charge of, contact us and we can help guide you through any changes or uncertainties.
This information in this guidance is irrespective of whether the assessment is being used for Earthquake Prone Building legislation purposes or for client use only.