Shaky Isles: Why NZ's seismic hazard has just increased

The risk of ground shaking in future earthquakes has doubled in many places, under a newly updated seismic hazard model.

But that doesn't mean our shaky isles have become any shakier, with experts putting the new calculations down to what they've learned from a decade of quakes and new science.

The latest estimates under the revised National Seismic Hazard Model (NSHM), released today, show the shaking hazard has grown, on average, by 50 per cent or more in most locations.

"It's important to note there's a lot of variability here – while there's been no change in some places, in others the shaking hazard could be more than double," said the model's project lead, Dr Matt Gerstenberger of GNS Science.

The model estimated the likelihood and potential strength of shaking across the country, where the hazard varied because of factors like ground conditions and local earthquake risk.

That risk was much higher in areas along the plate boundary, marked off the North Island's East Coast by the Hikurangi Subduction Zone, and over land in the south by the sprawling Alpine Fault.

One indicator in the model is what's called peak ground acceleration, or PGA, measuring earthquake shaking at its strongest – and often by g, or the force of gravity.

In a quake with a PGA of more than 0.4g, it became hard to stand.

At more than 0.6 or toward 1.0, the shaking might be intense enough to throw people about, cause landslides and crack the ground open.

The 6.2 Christchurch Earthquake reached a PGA of about 1.2g when it erupted beneath the city on February 22, 2011, causing 185 deaths and some $45 billion in damage.

The new model estimated that, within the next 50 years, there was a 2 per cent probability of PGA exceeding 1.42g in the Wellington area, 1.2g around Napier, 1.2g around Blenheim and 1.22g around Gisborne.

That was more than double the level of shaking that had been estimated for those centres - 0.52g, 0.49g, 0.43g and 0.48g respectively – under the 2010 model.

Estimates in that same scenario had also risen in Nelson (0.28g to 0.85g), Christchurch (0.27g to 0.73g), Greymouth (0.37g to 0.76g) Whanganui (0.29g to 0.87g), and generally everywhere else.

The trend was similar under PGA calculations that instead looked at 10 per cent probabilities of various PGA levels being exceeded within 50 years – or where the likelihood was higher, but the shaking itself was less intense.

Around Wellington, for instance, that PGA value here climbed from 0.32g to 0.82g, compared with 0.29g to 0.64g in Napier, 0.27g to 0.68g in Blenheim and 0.31g to 0.65g in Gisborne.

"When interpreting how the hazard results have changed from previous models, we need to consider the relative value of the shaking forecast," Gerstenberger said.

"For example, if there has been a doubling in hazard in a region with a previously very low estimated hazard, then the updated hazard – though greater – may still be low.

"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," says Dr Matt Gerstenberger of GNS Science. Image / Supplied

"Northland is a good example of this, where the ground shaking hazard is now estimated at twice its previous known level, but it can still be considered a very low hazard risk region."

It was important to stress that increased hazard in an area didn't necessarily mean a bigger impact.

"The impact on society includes many additional factors that are not part of the science model - like building aspects and structure design – and the National Seismic Hazard Model doesn't address those."

Generally, Gerstenberger said the increases weren't unexpected.

"We know a lot more about earthquake behaviour now due to better global understanding, more sophisticated science and more than a decade of advancements in technical computing," he said.

"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."

Each year, GeoNet locates about 20,000 quakes in and around the country, of which about 250 are large enough to be felt.

Since the NSHM was last updated, there have been 45 quakes around the country measuring over 6.0, while the Government has introduced new legislation aimed at managing and identifying quake-prone buildings.

The model, funded by the Ministry of Business, Employment and Innovation and Toka Tū Ake EQC, was revised by a large team of local and international scientists and engineers across government agencies, research institutes and universities.

By the Numbers

50%: The shaking hazard from future earthquakes has increased by 50 per cent or more across the country, under a just-revised seismic hazard model.

1.42g: In Wellington, it's now estimated that, within 50 years, there's a 2 per cent probability of shaking strong enough to exceed a peak ground acceleration (PGA) of 1.42g - compared with 0.52g in the previous model. The Christchurch Earthquake had a PGA of about 1.2g.

250: GeoNet detects around 20,000 earthquakes a year in New Zealand, of which 250 are large enough to be felt by people.