The Science Behind Seismic Hazard

New Zealand’s National Seismic Hazard Model has recently been updated and the amount of research behind it is staggering.

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The Enigma of Earthquakes

Have you ever wondered how scientists put numbers on earthquake hazard? How they quantify how much higher Wellington’s hazard is over Auckland’s, for example?

We know a lot about earthquakes in Aotearoa New Zealand: we measure tens of thousands of them on seismographs every year, we’ve mapped active faults and deciphered their earthquake histories, we’re constantly measuring how fast the tectonic plates are moving with respect to each other, building up stress to be released in future earthquakes.

And yet, we don’t know what size of earthquake will happen next or where or when. Given the nearly infinite number of possibilities, that is something we must approximate, or model.

Deriving practical benefits from science often involves supplementing observable, measurable data with data that is inferred from theoretical extrapolation or modelling. The updating of Te Tauira Matapae Pūmate Rū i Aotearoa, New Zealand’s National Seismic Hazard Model (NSHM 2022), is no exception.

Te Tauira Matapae Pūmate Rū i Aotearoa - New Zealand National Seismic Hazard Model Website: https://nshm.gns.cri.nz

Models within the Model

Models enable us to produce estimates. The NSHM 2022 provides the best information about how intensely, where, and how often the ground is likely to shake in future earthquakes around New Zealand. But it’s more than one big model: it’s an expertly designed mosaic of multiple models and datasets.

With funding from the Ministry of Business, Innovation, and Employment and Toka Tū Ake EQC, and leadership from Te Pū Ao GNS Science, scientists from around the motu collaborated and innovated to determine how to update the previous NSHM with several decades of new science and technology.

A brief skim over the science reports associated with the NSHM reveals the phenomenal extent of the research. There are models for how the crust deforms, models for how the ground moves, a model of faults in 3D, seismicity rates, strain rates, an updated earthquake catalogue, a database of paleoseismic sites, a site characterisation database, and the list goes on…

Robust Results

Somehow, these diverse components have been brought together in an organised manner like the tesserae in a mosaic – many differently coloured pieces arranged in a particular pattern to create a cohesive bigger picture. In this case, the picture is a series of maps of New Zealand showing ground shaking forecasts for the next 100 years.

Logic and expert judgement are like the adhesive of the mosaic. Logic trees were used to weight the contribution of different components, international and industry peer review was embedded throughout the process, and specific approaches were used to minimise bias while making the most of expert judgement.

Director of Te Hiranga Rū QuakeCoRE, Brendon Bradley, explains,

“when working with data that’s ultimately going to influence engineering design and construction standards, land-use planning, insurance policies, risk assessments, and community resilience strategies, it’s crucial to have model results that robustly approximate the truth”.

Brendon Bradley

Recognition

On winning a Science New Zealand Team Award in 2023, earthquake geologist Russ Van Dissen reflected on behalf of the NSHM team,

“It was a hugely invigorating project. We were drawing on the greater expertise of New Zealand and the world to build this state-of-the-art model, the results of which are going to impact everybody in New Zealand.”

Russ Van Dissen

This map show the strength of shaking and how likely it is that this level of shaking will occur within the next 50 years. The dark purple colours are weak shaking and the light orange and yellow colours are strong shaking. The scale bar shows peak ground acceleration (PGA) measured in g (1g is equal to gravitational acceleration on Earth).

There is a 2% chance of experiencing this level of shaking in the next 50 years. Image supplied.

In February 2024 a joint Bulletin of the Seismological Society of America Special Issue and Seismological Research Letters Focus Section was published showcasing much of the science behind New Zealand’s NSHM update. There are now over 30 peer-reviewed articles in the academic literature – testimony to the scientific effort that’s gone into this model.

“The resulting update has not only caught up with international best practice but has also leap-frogged ahead in several areas” says Brendon.

New Zealanders may not feel like celebrating the implications of higher hazard estimates, but they can certainly celebrate the excellent science and robust process that has gone into NSHM 2022. The widespread collaboration, consultation, and testing has resulted in a model that people can trust. And the improved understanding of our seismic future sets the way for planning and design that will make New Zealand more resilient to earthquakes.

Our annual Request for Proposals (RfP) supports eighteen-month, Associate Investigator led research projects that complement the Coordinated Research Projects within the Disciplinary Themes (DT) and Inter-disciplinary Projects (IP) of QuakeCoRE's Research Programme.

The RfP includes Proposal Development Grants which enable early career researchers to develop strong contestable external research proposals.

The annual call for RfP Projects and Proposal Development Grants is held in September / October and is announced on the QuakeCoRE website and in our newsletter.