New seismic hazard model reveals updated threat level to nation’s faults

file image. Photo: 123RF

The chances of the ground shaking more strongly in an earthquake are much higher than previously thought in large parts of the country.

In some places it doubled or tripled.

A new National Seismic Hazard Model is released today that provides the most accurate predictions yet of the shaking every town and city can expect, and what to build for.

GNS put 50 scientists to work modeling hundreds of thousands of fault cracks to fix the previous 12-year-old hazard model.

The main threat from the Hikurangi subduction zone raises the risk in the lower North Island by 1.5 to 2.5 times what it was before.

GNS said a few areas in low-risk Northland and Auckland, and high-risk Wairarapa, face three times the previous risk, although this is rare.

1 equals the same risk as before, 2 represents double, and 3 triples the previous risk, while <1-2 means either the same or half the risk - while "hill" appears most of the time the risk in the middle of both ends. Photo: Introduction / GNS

GNS seismologist Matt Gerstenberger said the higher the risk was more if a greater range of vibrations occurred, than if a person had been exposed to more earthquakes.

“The vibration we would expect to occur anywhere alone in New Zealand in the coming decades is about half again or maybe double what we would have expected 10 years ago,” he said.

Incredible math has been taken to assess the type, intensity, and frequency of shaking, from the type of sharp blow in the 2011 Christchurch to the long, slow shift that moved Wellington a full 50 centimeters over a 25-second period in the 2016 earthquake.

“Overall, the earthquake numbers are about the same as what we’ve said in the past,” Gerstenberger said.

“It’s for any given earthquake, the potential range of vibration that can occur – that increases.”

In a few areas on the west coast – surprisingly Kaikoura – danger comes at the same time or less than half as before.

Mostly though the rate is going up—sometimes sharply, though three times as much, “that’s not common, that’s rare,” with a lot to do with the soil type of a particular site, Gerstenberger said.

Thirty-one reports detail the details.

The new model only captures what is happening underground, not any above-ground influence.

This graph compares the Peak Earth Acceleration (PGA) hazard maps of the 2010 and 2022 National Seismic Hazard Models. Image: Introduction/GNS

However, Hawke’s Bay Hospital, which foresaw the greater risk that there might be from Hikurangi subduction, got its start early on by working on moving the new MRI machine and examining other clinical service buildings.

The Ministry of Business, Innovation and Employment anticipates that there will be more demands on building designs.

But its chief engineer, Dr. Ken Ellwood, can’t say exactly yet.

“It tells us about the potential for vibration,” he said of the model.

“On its own, it doesn’t tell us how buildings will perform in this vibration.

“That’s why we are now working with New Zealand Engineering to find out exactly what changes need to be made in the design of new buildings.”

More details on that will come next year.

The ministry has no plans to change the earthquake-prone buildings regulation system that defines methods for building assessment, and deadlines for their repair or demolition.

It is promising that any change will take time and consultation.

The department’s director of building performance, Dave Giggins, said many existing buildings already had valid earthquake assessments.

The ministry previously said the model “will not require earthquake-prone buildings that have been repaired for rehabilitation”.

For new buildings, the ministry released an earthquake guide and engineers issued a guide titled “Seismic Design for Uncertainty.”

“We are new to this.

“We haven’t kept the results a secret,” Glings said.

“So it will take some time.

“The range of possibilities for any given site, there’s a lot of variety, so the way we put that into a regulatory framework is complex.”

For home and contents premiums, the EQC said it will also take time for the new risk ratings to be factored into insurance risk.

What the model did, Gerstenberger said, was to combine the largest volume of data ever on the latest and widest range of earthquakes, with previously unattainable computing power.

“The national model produces a wide range of predictions from several thousand potential earthquakes. Depending on the nature of the community or the user’s appetite for risk, we will look at the relevant results for that.”

The scientists looked at smaller earthquakes with a 10 percent chance of occurring once every 50 years, and larger earthquakes with a 2 percent chance of occurring.

Ground acceleration or vibration frequency is another vital factor because faster vibrations are worse for liquefaction, while longer vibrations are worse for taller buildings.

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