Potential for 6.8m quake from faultline in Auckland's south – study

Researchers have discovered the potential for a "major earthquake" with "serious consequences" for parts of Auckland, with a possible magnitude of 6.8.

The Mangatangi Fault, which runs alongside the Hunua Ranges in southern Auckland, has now been identified as active by the researchers at University of Auckland.

In a study published today in the New Zealand Journal of Geology and Geophysics, the research marked the first time a faultline in Auckland or the Hunua Ranges had been radiocarbon dated.

Research co-author Dr James Murihead said this highlights how little is known of the region's seismic history.

"If the whole fault ruptured, there would likely be serious consequences for people living in South Auckland, and possibly further into central Auckland as well."

The fault was identified as active as it had ruptured within the last 10,000 years. Any fault which has moved in the past 125,000 is considered active.

Murihead said Auckland’s level of earthquake hazard is "obviously nothing like Wellington’s or much of the South Island", but it "may be significantly higher than the public and policymakers believe – we need more factual information to know".

If Auckland's earthquake hazard classification was raised to medium from low, this would impose tougher requirements under the Building Code.

Auckland has also continued to expand to the southwest, meaning more people and critical infrastructure is closer in proximity to the Mangatangi Fault, the researchers said.

Former masters student in earth science at University of Auckland Hannah Martin said the fault may not rupture again for "thousands of years".

"However, this is an active fault with the potential to generate a large earthquake in a region that doesn't expect one."

Muirhead said Auckland experiences earthquakes every year – including very small magnitude ones locally and larger regional earthquakes from far afield – but typically people don’t notice them.

Last year, the Government exempted Auckland from rules for earthquake-prone buildings due to a low seismic hazard.

Now, Muirhead, along with two co-authors of the Mangatangi study, Dr Jennifer Eccles, of the University of Auckland, and professor Mark Stirling, of the University of Otago, are investigating more Auckland faults to learn how active they are in a separate project funded by the Natural Hazards Commission.

Auckland University said the team is using advanced mapping, ground-penetrating radar and trenching to uncover evidence of past earthquakes.

"It’s always better to plan based on evidence rather than on our best hopes or worst fears," Eccles said.