Friday, February 28, 2020

Slipping and stressed Wairarapa fault line could produce 'giant earthquakes' - report

The researchers say it's likely the 2016 Kaikōura earthquake has placed more stress on the Wairarapa fault.

Cracks beneath Wairarapa have been put under new pressure by the 2016 Kaikōura earthquake, increasing the potential for "giant earthquakes" that could damage large swathes of the country.
New research has found the 2016 Kaikōura quake had "loaded" the Wairarapa fault line to the north, which may now be at breaking point.
However, Geonet has warned that while the research was "well-founded", there remained only a "very low" chance of a giant Wairarapa earthquake in our lifetime.
The Wairarapa fault line is responsible for New Zealand's most severe earthquake since colonisation. In 1855, a 8.2 magnitude earthquake killed nine people, causing severe damage from Whanganui to Kaikoura and generating a tsunami.That quake was so severe that it remade much of the Wellington and Wairarapa coastlines, rasing the ground by as much as 2.7 metres. Wellington's Basin Reserve cricket ground is built on land lifted by the quake.

The report from European researchers, published in Scientific Reports this month, says the key to predicting the size and damage of potential earthquakes is to know the size of previous large earthquakes on that fault. 
It found the Wairarapa fault line had "repeatedly produced giant earthquakes and is likely able to produce a similarly strong forthcoming event". 
"Past earthquakes were dramatically large. Beyond the high seismic hazard these large earthquakes pose in New Zealand, their extreme larger size questions our understanding of fault and earthquake physics.

The 2016 magnitude 7.8 Kaikōura earthquake could have "stress loaded" on to its counterpart, the Wairarapa fault.

The new report also described the Wairarapa fault line as "fast slipping".
This means that "stress loading" caused by the Kaikōura earthquake, and the deformations observed at the Wairarapa fault line, may be bringing it "closer to failure", the report said.  
"If a similar earthquake were to occur today, it would initiate where stresses have been amplified by the 2016 Kaikōura earthquake." 
The possibility of an earthquake on the Wairarapa fault line in the near future needed to be considered, it said. 
Cracks observed near Kaikōura after the 7.5 magnitude in 2016

 Jeremy Holmes, regional manager at the Wellington Region Emergency Management Office, said an earthquake from the Wairarapa fault would affect Wellington greatly. 
Links between faults, like Kaikōura and the Wairarapa, were being discovered all the time, he said.
"But earthquake prediction is not an exact science ... we just have to be mindful that it could put pressures on [the Wairarapa fault] and it could cause an earthquake at any time," Holmes said. 
An aerial photograph of work road and railway line on SH1, south of Kaikōura, after the 7.8 quake damaged the road. 

GNS Science natural hazards and risks leader Kelvin Berryman said the paper was "well founded" and reinforced previous research by GNS Science and Victoria University. 
While the Kaikōura earthquake would have created additional pressure on Wairarapa faults, a major earthquake could still be decades away, he said.
Giant earthquakes on the Wairarapa fault, like the 1855 quake, had an average repeat time of about 1300 years. This meant there was about a 1 to 7 in 100 chance of "giant earthquake" in the next century, he said. 
 Damage to Wellington near Statistics House after the Kaikōura earthquake.

 The conclusion in the paper that the fault "may be prone to break" went beyond what the data supported, he said. 
Otago University's Caroline Orchiston, who specialises in resilience to natural disasters, said the Wairarapa fault was a major risk to Wellington, among many other faults. 
"Wellington as we all know has a number of sources of earthquake risk," she said.

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