The Detail: Faultline discovery changes perception of Auckland

Scientists have confirmed a fault-line on Auckland's doorstep as active.

Photo: Supplied

Auckland residents are told they should be "alert, but not alarmed", after the discovery of faultlines beneath the city's streets

For most Aucklanders, earthquakes aren't on our radar - they aren't something we spend much time worrying about.

Volcanoes perhaps, traffic definitely, but earthquakes? That's usually a problem for Kiwis further down the country - or so we think.

Scientists have now confirmed a faultline on Auckland's doorstep is active and, if it ruptured, it could generate an earthquake measuring up to magnitude 6.8.

The Mangatangi Fault sits at the southern end of the Hunua Ranges, southeast of Auckland city. New research has revealed it has moved within the last 10,000 years, making it active in geological terms.

For researchers, that's a significant discovery.

"The Mangatangi Fault is actually the first faultline that we've ever got radiometric data on to understand the exact timing of a movement on a fault line," University of Auckland senior lecturer James Muirhead, who worked on the study, tells The Detail.

The fault's size gives scientists clues about its potential power.

"Basically, how large a fault is will determine how much energy it can store and that will determine the maximum amount of energy it can release during an earthquake," he says.

"With the Mangatangi Fault, based on its size, we're looking at about a 6.8-magnitude earthquake, if the whole faultline were to rupture in a single earthquake event."

The Maungatangi Fault at the southern end of the Hunua Ranges.

Photo: Supplied

That doesn't mean such an earthquake is imminent, but if it does happen, parts of Auckland could feel it strongly.

"Pukekohe, Drury, places like Takanini are situated quite close to the Mangatangi Fault," says Muirhead. "They would definitely feel the effects."

The implications go beyond homes and businesses.

Critical infrastructure, including water reservoirs and dams in the Hunua Ranges, may also be vulnerable to damage in a significant event.

The discovery is also raising questions about other faultlines hidden beneath New Zealand's largest city and recently discovered.

Researchers have identified a network of faults beneath Auckland itself - potentially about 10 of them - but whether they're active remains unknown.

"Below Auckland, all we know is that we have a number of faultlines - it could be in the order of about 10 - that are closer to the city centre and they've moved in the last six million years," says Muirhead.

"The question is, are they active? The idea is for us to do the work as quickly as we can to answer those kinds of questions."

The faults were uncovered using thousands of borehole records collected during decades of construction and infrastructure projects across Auckland.

Scientists examined rock layers deep beneath the city and looked for places where those layers had been displaced - telltale signs of fault movement.

How worried should Aucklanders be?

"I like to say we should probably be alert, but not alarmed," Muirhead says.

He says the research isn't about predicting an earthquake tomorrow, next week or even next year.

Instead, it's about ensuring future generations have the best possible information, when making decisions about where and how Auckland grows.

"I personally am not concerned on a day-to-day basis about an earthquake," he says. "I am concerned when I think about the long-term and the generational legacy we leave to future generations."

The findings also come as New Zealand's earthquake-prone building rules have been revised, with Auckland classified as a low-risk area.

Muirhead says understanding the city's true earthquake history is critical to ensuring those decisions are based on evidence, rather than assumptions.

"Auckland sits less than 100km from a medium-risk zone," he says. "The question is, if we collect more data, does that understanding change?"

The answer may take years to emerge.

Researchers hope that, within the next 1-2 years, they'll know whether at least two more Auckland faults are active. Within the next decade, they hope to build a much clearer picture of how often the city experiences significant shaking.

For now, the message is simple.

Auckland remains a relatively low-risk earthquake region by New Zealand standards, but beneath the city's motorways, suburbs and shopping centres, scientists are discovering there may be more going on underground than anyone realised.

They're racing to understand it, before nature reminds us first.

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