New assessment doubles risk of earthquake shaking hazard in Napier

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This colonnade on Marine Parade in Napier was built in 1935 following the 1931 earthquake. Photo / NZME

The risk of a future earthquake causing similar ground shaking to the 1931 Napier earthquake is higher than previously thought, scientists have found.

The revised National Seismic Hazard Model (NSHM) was released on Tuesday, showing the likelihood and potential strength of shaking in different parts of the country in the case of an earthquake.

The shaking hazard has increased at least 50 per cent for many locations across the country.

But in the Napier area, the shaking hazard has increased by roughly 100 per cent.

That does not mean the region or the city is twice as likely to experience an earthquake, but rather the intensity of ground shaking in future quakes could be twice as strong compared to previous estimates.

The last time the modeling was updated was 2010. Scientists have put the new calculations down to what they’ve learned from the past 20 years of quakes and new science.

Aftermath of the 1931 Napier earthquake. Photo / NZME

One key indicator measuring the strength of ground shaking is called peak ground acceleration (PGA).

That measures shaking at its strongest during a quake, and is measured by the G-force scale (gravitational acceleration).

According to the new modeling, during the next 50 years near Napier, there is a 10 per cent chance of a quake causing a PGA of 0.64G or greater, and a two per cent chance of a PGA of 1.2G or greater.

In 2010, the models predicted a 10 per cent chance of a 0.29G quake within 50 years and a 2 per cent chance of a 0.49G quake.

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The PGA level for the 1931 Napier earthquake was not officially recorded, but scientists estimate it was over 1G.

Where there is a quake with a PGA of more than 0.4G it becomes hard to stand.

Toward 1.0G, the shaking might be intense enough to throw people about, cause landslides and crack the ground open.

Dr Matt Gerstenberger of GNS Science says the new modeling is an important tool. Photo / NZME

The modeling does not forecast how much damage could be caused by future earthquakes.

A NSHM report stated Hawke’s Bay was above a number of faults such as the Poukawa fault – which produced the Napier 1931 earthquake which flattened much of the city – and the Mohaka fault.

“The Whakatane and Waimana faults also extend into [Hawke’s Bay]the report stated.

“The shaking is forecast to be greatest in the south of the region, and higher towards the north-east.”

Hawke’s Bay has some of the highest risks in the country when it comes to the potential for strong-shaking earthquakes, behind the likes of Wellington.

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GNS Science NSHM project lead Dr Matt Gerstenberger said the revised modeling was an important tool.

“It is critical to have an up-to-date understanding of the seismic hazards facing New Zealand to help us manage the risks from earthquakes to people, property and the environment,” he said.

“The increase in hazard estimates in the 2022 revision is not unexpected.

“We know a lot more about earthquake behavior now due to better global understanding, more sophisticated science, and more than a decade of advancements in technical computing.”

Led by a team at GNS Science, and funded by the Ministry of Business, Innovation and Employment and Toka Tū Ake EQC, the modeling revision has incorporated years of work and research.

Around 250 earthquakes are felt on average every year in New Zealand, and thousands more are measured.

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