Monday , June 21 2021

Our most complicated earthquake: Kaikōura two years later



Scientists are still getting rich lessons from the Kaikōura Earthquake, two years after the 7.8 shock shook the country with energy releases equivalent to 400 atomic bombs.

GNS Earthquake Geologist Dr. Rob Langridge said the event, which occurred right after midnight on November 14, 2016, was one of the most complex earthquakes observed anywhere in the world.

That is mostly due to the number of errors that broke out in just one incident.

More than 20 faults were activated – 14 of them were hard enough to replace more than one meter of land.

None of these effects may be more dramatic than along the Kekerengu Fault in Marlborough, where 12m of land is balanced.

In some places the error was seen with folds of earth stretching across the countryside – dubbed by some as the Waiau Wall.

"Another lesson is that this complex earthquake spread from one region to another – an earthquake starting at a fault in North Canterbury and jumping across the north, which triggered a big mistake in the Marlborough region," Langridge said.

"Broken faults form a complex network of faults that attack the northeast that are interrupted with striking northern errors, which have different modes of movement."

More broadly, the incident has prompted scientists to consider the role of the Pacific Plate which is subducted below Marlborough.

Previously, this part of the plate was considered to be fully "locked", or attached to the crust, but now it appears that it plays a large seismic role in this part of the South Island.

Immediately after the incident, Langridge and his colleagues began collecting scientific data on fault faults, landslides, landslides, tsunamis and disbursements to help communities and provide advice to the government and the council.

"After a lot of data collection, we began to publish a series of scientific papers in several foreign and local journals, so that the knowledge gained has a lasting place," he said.

"At present, we are trying to understand the history of previous errors in some of these errors.

"So, we have the funds to go back to Papatea, Kekerengu, Hundalee, Humps and Leader faults and beach stories, to understand how this error worked in the previous earthquake cycle."

There is still a lot of work to be done in various scientific fields, from seismology to GPS-based geodesy.

In the Kekerengu Fault in Marlborough, near Waiau, land was raised in several places as far as 12m. Photo / Kate Pedley
In the Kekerengu Fault in Marlborough, near Waiau, land was raised in several places as far as 12m. Photo / Kate Pedley

This summer, his team will dig a trench in the Papatea Fault to understand when moves in the past and how many have moved for thousands of years.

This fracture unleashes a remarkable amount of slipping in the 2016 earthquake – scientists have since recorded up to 9m vertical movements and 6m horizontal movements.

"It really is on the world stage as a large displacement on land," Langridge said.

"Understanding how long it takes to accumulate this amount of pressure is the key to unlocking this error."

What is still unclear is when the next big earthquake will strike.

GeoNet recorded more than 20,000 aftershocks in the year after the earthquake, with several thousand more recorded since then.

However, only a small percentage of them were greater than 3.0 in size, and the number of aftershocks continued to decline.

GeoNet's latest earthquake shock – a statistical based measure that calculates probabilities – gives a 15% chance of aftershocks between 6.0 and 6.9 in the next three months, and 46% the likelihood of this occurring at several points above the 12-month period.

Langridge saw the potential of earthquakes in the future as a case of human time span compared to geological ones.

"From the way I see it, we live on plate boundaries which accumulate around 4m of tension every century," he said.

"In our distant memories we speak of the Awatere and Wairarapa earthquakes in the mid-19th century which solved several important plate boundary faults.

"We have now seen several other errors in central New Zealand that were involved in this earthquake.

"We just have to be prepared individually and as a community for the prospect of future earthquakes – they are part of our make-up."


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