Associate Professor Tom Wilson of the University of Canterbury, whose area of ​​specialization is disaster risk and resilience, talks about the impacts of an earthquake in the Alpine Fault. Source: Youtube / @EMSouthland

The high-risk Alpine fault in New Zealand has a 75% chance of causing a major earthquake – possibly a powerful magnitude 8 event – within the next 50 years.

That’s according to a new study which found that the probability of an event in the coming decades is much higher than the 30% previously estimated.

Scientists have also calculated an 82% chance that the earthquake will be greater than 8.0.

One of New Zealand’s greatest natural threats is the Alpine Rift, which stretches about 600 km on the west side of the South Island between Milford Sound and Marlborough.

It has a clear geologic record of breaking roughly every three centuries – and 2017 marked the 300th anniversary of what is believed to be a magnitude 8 earthquake that displaced one side of the fault by about 8m in seconds. .

Recent research has suggested that a large earthquake could block South Island highways in more than 120 places, leave 10,000 people cut off and cost the economy an estimated $ 10 billion.

The new study analyzed evidence from 20 previous Alpine Fault earthquakes recorded in sediments from four west coast lakes and two swamps over the past 4,000 years, to form one of the most comprehensive seismic records in this guy to the world.

Source / GNS Science
Source / GNS Science

“Detailed analysis and careful dating of sediments in lakes adjacent to the Alpine Fault has led to new knowledge about the distribution of earthquakes along the fault,” said study co-author the Professor Sean Fitzsimons of the University of Otago.

The research highlighted a curious “earthquake gate” on the fault south of Jackson Bay near the Martyr River, which appeared to determine the magnitude of an Alpine fault earthquake.

Some ruptures stopped at the gate, producing “major” magnitude 7 earthquakes; but the ruptures left by the door became even greater.

These transient or shutdown breaks tended to unfold in sequence, producing phases of major or significant earthquakes over time.

Lead author of the study, Dr Jamie Howarth of Victoria University, said the gate could help predict the size of future Alpine Fault earthquakes.

“From the toll of past earthquakes, we can determine that the probability of an event of magnitude 7 or greater is around 75 percent over the next 50 years,” he said.

“So now we know that the chances of seeing a big earthquake in the Alpine Rift in our lifetime are better than a toss.

“This is a really significant result, but we cannot predict the magnitude of the next event from these data alone.”

The recording showed that the last three earthquakes broke through the gate, producing more than 8 earthquakes.

A visitor places a finger in the rock in the middle of the South Island Alpine Fault, at the point where two tectonic plates divide.  Photo / Supplied, Alpine Fault Tours
A visitor places a finger in the rock in the middle of the South Island Alpine Fault, at the point where two tectonic plates divide. Photo / Supplied, Alpine Fault Tours

“Our modeling shows that if you’ve had a series of three transient ruptures, the next will likely go through the door as well,” he said.

“We therefore expect the next earthquake to be similar to the last one in 1717, with an estimated magnitude of 8.1, which broke about 380 kilometers from the fault.”

The best prediction was that there would be an 82% chance that the next such earthquake would pass through the gate and cause an earthquake of magnitude 8 or greater.

“This finding does not change the fact that the Alpine fault has always been dangerous,” said Howarth.

“But now we can say that the next earthquake will probably happen in most of our lives.

“We need to go beyond planning the immediate response to the next event, which was done well under the AF8 program, to think about how we make decisions about future investments to improve our infrastructure and community preparation.

Franz Josef, on the west coast of the South Island, is particularly threatened by the major fault.  Photo / Supplied
Franz Josef, on the west coast of the South Island, is particularly threatened by the major fault. Photo / Supplied

Seen from space, the fault appears as a straight line on the western side of the Southern Alps.

But, in reality, it exhibits variations in geometry and slip rate and is divided into different segments, said study co-author Assistant Professor Nicolas Barth of the University of California at Riverside.

The Martyr River Gate appears to be due to a slight change in direction of the fault and the angle of its dip below the surface, he said.

The researchers took a physics-based model of earthquake behavior and applied it to the Alpine Fault, testing it against paleoseismic data.

“If you just run the model on a fault with no geometric complexity, then you only experience earthquakes, so they’re only magnitude 8 all the time,” Barth said.

“But when you actually include all of the geometric complexity that is happening here, then you get the behavior that we have observed, with transient and terminal breaking phases.”

Howarth found it fascinating that scientists are approaching the point where physics-based models can be used for predictions.

“This is the first time that there is a paleoseismic dataset covering several large earthquakes and seismic cycles of sufficient quality to allow us to assess the behavior of these models.”

More about this article: Read More
Source: www.nzherald.co.nz
This notice was published: 2021-04-19 15:00:00

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