Auckland has a 'new' active fault: should we be worried?

Research has identified that Auckland has a ‘new’ active fault. Anthony Doesburg asks the scientists behind the findings, should we be worried?

When it was revealed in May that New Zealand’s biggest city had a previously unknown active fault, the news rumbled around the world.

A paper published in the New Zealand Journal of Geology and Geophysics detailed fieldwork, led by a former University of Auckland masters student in Earth sciences, which showed for the first time that a fault in south-east Auckland’s Hūnua Ranges is alive and a genuine hazard.

The paper detailed how, using radiocarbon dating, the researchers identified that the 18km fracture, known as the Mangatangi Fault, has moved in the past 10,000 years. That’s well inside the 125,000-year period in which any movement qualifies a fault as active.

However, Hannah Martin – that masters student who was also the paper’s lead author – says there are no signs of imminent activity.

“This fault may not rupture again for tens of thousands of years,” says Hannah, who is now doing a PhD in earthquake geology at the University of Nevada in the US.

Even so, she and her study advisers and co-authors – Auckland School of Environment senior lecturers Dr James Muirhead and Dr Jennifer Eccles, and Professor Mark Stirling of Otago University’s geology department – warn the fault’s quake potential should be taken seriously.

They write: “These results indicate that the Mangatangi Fault may pose greater seismic hazard to the Auckland area than previously considered, which, given high exposure and vulnerability, could translate into significant risk.” 

Living on shaky ground

The findings, which were under wraps between Hannah completing her masters in February 2024 and their May publication, are another indicator we’re indeed living on ‘the shaky isles’.

More than 20,000 earthquakes are detected in and around the country each year according to online encyclopaedia Te Ara – as many as 250 of them strong enough to be felt. Between 1840 and 2016, Te Ara notes that earthquakes caused 501 deaths directly or indirectly.

The most lethal of those, in Hannah’s home province Hawke’s Bay in 1931, killed 258 people. Christchurch’s February 2011 magnitude 6.2 shake, thought to have been an aftershock of one the previous September, claimed 185 lives.

In the space of less than a month this year the country was shaken by two sizeable quakes: one of 5.9 magnitude centred near Te Anau and the other magnitude 5.7 near Taumarunui. Fortunately, neither resulted in casualties.

Hannah and her co-authors have calculated that a Mangatangi Fault rupture has the potential to be about 6.9 magnitude, equating to nearly five times the shaking and 11 times the energy of the fatal Christchurch quake. Not that the researchers are losing sleep over it.

“I believe we should not be worried about the things we can’t control,” says James, “and instead continually work towards building greater resilience to natural hazards in Tāmaki Makaurau by supporting science, education and policy.”

Hannah is on the same page.

“I completely agree, but what we can control is how well prepared we are. All we can really do is try to understand as much as possible – things like how faults behave, how often they rupture, how much uncertainty there still is – so communities and decision makers have the best information available to reduce risk.”

Studying Earth sciences offers a unique perspective of the world, says Jennifer.

“Thinking about how much the Earth changes over geological time makes you realise nothing is static or ‘normal’.”

New Zealand’s seismic hyperactivity is down to friction between two pieces of the Earth’s crust, the Pacific Plate and the Australian Plate, which are colliding beneath us.

“New Zealand is well known internationally as a ‘natural geological laboratory’,” says Jennifer, who supervises a number of postgraduate students who come to the country for that reason. “When it comes to active geological processes, we do seem to have most of them as we straddle a tectonic plate boundary and have built up expertise based on this.” 

Generating research with impact

The media interest in the paper, which received coverage everywhere from the Franklin Times to the Guardian, took Hannah by surprise.

“I keep telling people I think I’ve already peaked in my career with my first first-author paper making international news.”

She began her study of the Mangatangi Fault aged 22 thinking she was merely engaged in adding to a database of existing fault measurements.

“I had no idea of the full impact it would end up having. That said, I already felt like I’d struck gold regardless of the outcome because I got to spend so much time outside collecting physical data, and my advisers were amazing.”

She first came under Mark’s tutelage while doing a BSc in geology at Otago University. “I had a lot of fun in my undergraduate independent geology project working with Mark, mapping the Taieri Ridge Fault in Otago.

“He showed me how to identify features of faulting in the landscape such as fault scarps and offset sediment and then sent me off to map half a range front on my own – just a small one!

“I had a blast hiking around, chatting to the local farmers and mapping the different geologic units and faulting features I found along the way.”

Mark, a former principal scientist at GNS Science, has an ongoing role maintaining the National Seismic Hazard Model and expects the new finding on the Mangatangi Fault to make its way into the model.

Also looking to the future, James advocates for more investigation of the region’s known faults and an ongoing search for others. “I think Auckland city would benefit from offshore fault studies in the Waitematā and Manukau harbours,” he says.

He envisages this as part of a wider policy centred on improving resilience to natural hazards, as life on our shaky isles rumbles on.