Auckland’s earthquake history rewritten by University research

A faultline running alongside the Hunua Ranges in South Auckland is now identified as active and has the potential to cause a major earthquake with serious consequences, University of Auckland researchers say.

The Mangatangi Fault poses the risk of a magnitude 6.8 earthquake and has ruptured within the past 10,000 years, according to a study published in the New Zealand Journal of Geology and Geophysics.

“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,” says geologist Dr James Muirhead, a co-author of the research.

The research marks the first time a faultline in Auckland or the Hunua Ranges has been radiocarbon dated, highlighting how little is known of the region’s seismic history, Muirhead says.

Any fault that has moved in the past 125,000 years is considered active.

“Auckland’s level of 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,” he says.

Raising Auckland’s hazard classification to medium from low would impose tougher requirements under the Building Code.

Auckland’s continued expansion to the southwest is putting more people and critical infrastructure in closer proximity to the Mangatangi Fault, which lies 50 kilometres south, the scientists say.

“This fault may not rupture again for tens of thousands of years,” says Hannah Martin, a former Masters student in Earth Science at the University of Auckland who was lead author of the research. “However, this is an active fault with the potential to generate a large earthquake in a region that doesn’t expect one.”

Last year, the government exempted Auckland from rules for earthquake-prone buildings because of the low seismic hazard. Auckland Mayor Wayne Brown cited an absence of earthquakes in the past 100,000 years when lobbying for the exemption.

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, says Muirhead.

Muirhead and 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.

The team are 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,” says Eccles.

In 1891, an earthquake at Port Waikato rocked Auckland City, causing minor damage such as falling plaster and broken glass.

“Nearly every building in town vibrated more or less with the shock and the oscillation in some of the larger brick buildings in the city seemed to have rather alarmed the occupants, who ran out in the streets,” reported the Press Association.

Hannah Martin, who is now studying at the Nevada Seismological Laboratory at the University of Nevada in Reno, was funded by the Scientia Trust Planet Earth Fund.