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Multi-fault quakes – It's complicated

EQC-funded research at the University of Canterbury is trying to pinpoint where faults might set each other off, creating a major multi-fault earthquake, and to estimate what the maximum magnitude could be.

Lead researcher Dr Tim Stahl says that the Kaikōura quake was the most complex multi-fault earthquake ever recorded, while other recent earthquakes have also involved more than one fault.

“Earthquakes that rupture across multiple faults don’t just affect a bigger area, they also add to the amount of energy released, creating stronger quakes,” says Dr Stahl.

“So it’s really important to understand more about multi-fault earthquakes,” he says.

Dr Stahl explains that if the 2016 Kaikōura quake had only ruptured the first fault, the Humps fault, it would have been a Mw 7 quake. But because it was the first of 20-odd other faults, it combined into a Mw 7.8 earthquake - which is around 16 times stronger.

“We really need to get a better idea of where this could happen and the impact it could create on the ground surface, on infrastructure like water and electricity, and on buildings,” he says.

EQC’s Chief Resilience and Research Officer Jo Horrocks says now that researchers can get detailed data on earthquakes through GeoNet, earthquakes rupturing across multiple faults almost seemed to be the “new normal”. 

“Understanding more about how, when and where these types of multi-fault earthquakes might happen is critical for understanding natural hazard risk in New Zealand. Dr Stahl’s work bringing together the range of factors that lead to multi-fault earthquakes will be an extremely valuable contribution to understanding our earthquake risk and taking action to reduce the impact,” she says.

Dr Stahl says that while the Uniform California Earthquake Rupture Forecast is the current “gold standard” for understanding multi-fault earthquakes internationally, much more is needed for New Zealand conditions.

“When you’re looking at why and where an earthquake might jump across faults, you’re looking at factors like the distance between faults and whether a fault travels straight or on an angle. In New Zealand, however, we also want to be able to take into account how our rock types behave under seismic stress and what happens with faults that have been labelled ‘inactive’.”

Dr Stahl says that up until now, criteria for multi-fault earthquakes have been considered as black and white thresholds.  His research is introducing “fuzzy logic” to put all the relevant factors together, giving a range of scenarios of what could happen.

“We want to be able to understand how all these factors might combine to make some rupture scenarios more likely than others.  Ultimately the question we’re asking is “What is the maximum size earthquake we could have in a particular location?” says Dr Stahl.

Testing the computer modelling against real data is a critical part of the project.

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

“Better models mean that we can give better information on the likelihood and impact of future multi-fault earthquakes to emergency managers, councils, infrastructure providers and the public.”

Dr Stahl says the project will focus on an area that has not had detailed investigation so far, between Waiau and Blenheim, where there are known faults that may rupture in future multi-fault earthquakes.

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