Multi-fault quakes - it's complicated

The black lines are known fault in the area. Pink lines are faults that ruptured in the Kaikōura quake.

If the 2016 Kaikōura quake had only ruptured the Humps fault, it would have been a magnitude 7 quake.

But because the Humps was the first of 20-odd other faults that ruptured, it combined into a 7.8 earthquake - which is about 16 times stronger.

Dr Tim Stahl says the Kaikōura quake was the most complex multi-fault earthquake recorded, although 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 Stahl.

"So it's important to understand more about multi-fault earthquakes.

"We 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."

Stahl is the lead researcher in the EQC-funded research at the University of Canterbury 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.

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," Horrocks says.

Stahl says that the Uniform California Earthquake Rupture Forecast is the current "gold standard" for understanding multi-fault earthquakes internationally, but 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'."

He said 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 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."

The project will focus on an area that has not been investigated in detail so far, between Waiau and Blenheim, where known faults may rupture in future multi-fault earthquakes.