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Shaking Islands: Why New Zealand’s Earthquake Risk Just Increased

Shaking Islands: Why New Zealand’s Earthquake Risk Just Increased

 


The research program led by GNS Science explains the National Seismic Hazard Model, or NSHM for short. Video / GNS Science

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

But that doesn’t mean our shaky islands are getting more shaken, as experts put together the new calculations based on what they’ve learned from a decade of earthquakes and new science.

The latest estimates under the revised National Seismic Hazard Model (NSHM), released today, show that the shaking risk has grown, on average, by 50 percent or more at most locations.

“It is important to note that there is a lot of variance here – while there has been no change in some places, the risk of vibration in others may be more than doubled,” said model project leader Dr Matt Gerstenberger from GNS Science.

The model estimated the probability and potential strength of shaking across the country, as the risk varied due to factors such as ground conditions and local seismic risk.

This risk was much higher in areas along the plate boundary, marked off the east coast of the North Island by the Hikurangi subduction zone, and on land in the south by the sprawling Alpine fault.

One indicator in the model is the so-called Earth-peak acceleration, or PGA, which measures the shaking of an earthquake at its strongest—often by g, or the force of gravity.

In an earthquake with a PGA value of more than 0.4 g, it became difficult to stand.

At more than 0.6 or about 1.0, the shaking could be severe enough to throw people around, cause landslides and crack the ground.

Christchurch’s 6.2-magnitude earthquake reached a PGA of about 1.2 grams when it erupted under the city on February 22, 2011, causing 185 deaths and an estimated $45 billion in losses.

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

This was more than twice the level of vibration estimated for those centers – 0.52 g, 0.49 g, 0.43 g, and 0.48 g, respectively – according to the 2010 model.

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

The trend was similar under PGA calculations that instead looked at 10 percent odds of different PGA levels being exceeded within 50 years—or where the probability was higher, but the vibration itself was less severe.

Around Wellington, for example, the PGA value here increased from 0.32g to 0.82g, compared to 0.29g to 0.64g in Napier, 0.27g to 0.68g in Blenheim and 0.31g to 0.65g at Gisborne.

“When interpreting how the risk results change from previous models, we need to consider the relative value of the vibration forecast,” Gerstenberger said.

“For example, if there was a doubling of risk in an area with a previously very low estimated risk, the induced risk—although greater—may still be low.

“It is critical to have an up-to-date understanding of the seismic risks facing New Zealand to help us manage earthquake risks,” says Dr. Matt Gerstenberger of GNS Science. The attached photo

“Northland is a good example of this, with the Earth shaking risk now estimated to be twice its previous known level, but it can still be considered a very low risk area.”

It was important to stress that an increased risk in an area does not necessarily mean a greater impact.

“The impact on society includes many additional factors that are not part of the scientific model – such as aspects of construction and structure design – and the national seismic hazard model does not address these factors.”

Overall, Gerstenberger said the increases were not unexpected.

“We know a lot about earthquake behavior now because of better global understanding, more advanced science, and more than a decade of advances in technical computing,” he said.

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

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

Since the last NSHM update, there have been 45 earthquakes across the country with a magnitude of more than 6.0, while the government has introduced new legislation aimed at managing and identifying earthquake-prone buildings.

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

in numbers

50%: The risk of vibration from future earthquakes increased by 50 percent or more across the country, under the just-revised seismic hazard model.

1.42 g: At Wellington, it is now estimated that in 50 years, there is a 2 percent chance of shaking with enough force to exceed the peak of Earth’s acceleration (PGA) of 1.42 g – compared to 0.52 g in the previous model. The Christchurch PGA earthquake had about 1.2 grams.

250: GeoNet detects about 20,000 earthquakes annually in New Zealand, of which 250 are large enough for people to feel.

Sources

1/ https://Google.com/

2/ https://www.nzherald.co.nz/nz/shaky-isles-why-nzs-seismic-risk-has-just-increased/T3E7EOHJYAVMSKRVWWOENOP2MY/

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