Was the Kaikoura earthquake as complex as quakes get? Project aims to find out

Researchers are trying to work out whether the 2016 Kaikōura Earthquake, which ruptured more than 20 faults, was about as complex as quakes get – or could some be even more complicated.

The 7.8-magnitude Kaikōura event is thought to have been the most complex earthquake ever recorded anywhere in world.

But it also happened after technical advances meant researchers were better able to work out which faults were involved than they would have been in the past.

Now, researchers funded by the Earthquake Commission (EQC) are feeding data about faults and earthquakes in North Canterbury and Marlborough into computer models to work out the probability of future complex earthquakes. The aim is to develop models that can be applied anywhere.

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Part of the reason it’s important is because, if a rupture on one fault spreads to other faults, the earthquake doesn’t just cover a wider area, it can also be stronger – with more ground shaking – than it would have been if restricted to just the first fault.

The Kaikōura Earthquake caused widespread damage to the road and rail routes north of Kaikōura.

John Kirk-Anderson/Stuff

The Kaikōura Earthquake caused widespread damage to the road and rail routes north of Kaikōura.

Lead researcher Dr Tim Stahl, from the University of Canterbury, said that if the Kaikōura quake had been restricted just to the Humps Fault – the first fault ruptured – it would have been a magnitude-7.0 earthquake.

But because more than 20 other faults became involved, the ruptures combined into a magnitude-7.8 event. That was around 16 times stronger than it would have been if just the first fault was involved.

Two other large earthquakes that ruptured multiple faults were the September 2010 magnitude-7.1 Darfield Earthquake and the March 1987 magnitude-6.5 Edgecumbe Earthquake.

“We have a basic idea of the different factors that govern whether an earthquake will involve multiple faults,” Stahl said.

“One key one is just simply the distance between faults.” But there were a range of other factors, some particularly complex.

Potentially, the properties of the bedrock in New Zealand could allow multiple faults to rupture, and another EQC-funded project was researching that area.

The earthquake-damaged northern facade of the Christ Church Cathedral in Christchurch.

John Kirk-Anderson/Stuff

The earthquake-damaged northern facade of the Christ Church Cathedral in Christchurch.

For now the best model for predicting the likelihood that several faults would rupture together was the Union California Earthquake Rupture Forecast, which was developed using data from around the world, but more work taking into account New Zealand conditions was needed.

“New Zealand earthquakes are inherently complex,” Stahl said. Scientists did not have a clear idea why that was, although they did have some leads.

Among outcomes of the project researchers were looking for was an indication of whether earthquakes could get more complex than the Kaikōura event, as well as how frequently they were much simpler.

“If the Kaikōura Earthquake is just an average output, that’s very useful. That increases the usual magnitude of earthquakes in the North Canterbury to Marlborough region,” Stahl said.

But if Kaikōura was about as complex as earthquakes get, he thought that would be comforting. “I think most people are hoping it can’t get much more complex,” he said.

“We want to create a suite of different rupture scenarios and rank them by different likelihoods.”

Drone footage from different sources reveals the extensive damage caused by the devastating 7.8 magnitude North Canterbury earthquake in November 2016.

Computer modelling was the crux of the two-year project, incorporating many years of fieldwork mapping faults and the fieldwork done after the Kaikōura Earthquake measuring such things as fault ruptures, Stahl said.

The work would give an indication of how frequently certain levels of ground shaking could be expected in various places. It was one piece of the puzzle that would lead in time to improvements in the building code.

Testing the computer modelling against real data was a critical part of the project, he said.

“We have a huge amount of data from Kaikōura, Darfield and even Edgecumbe. This gives us a great basis to test our modelling and make sure it is delivering good results for New Zealand conditions.”

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