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By Jim Birchall
Last month, a fault line capable of producing a magnitude 6.8 earthquake was identified south of Auckland, with researchers warning that communities across south and east Auckland could experience significant impacts if it were to rupture.
The Mangatangi Fault, which runs alongside the Hunua Ranges, has been confirmed as active following new research by the University of Auckland. Scientists say the fault has ruptured within the past 10,000 years and poses a greater seismic risk than previously thought.
The findings, published in the New Zealand Journal of Geology and Geophysics, mark the first time a fault in Auckland or the Hunua Ranges has been radiocarbon dated.
The research is particularly relevant for southeastern Auckland including Hunua, Beachlands and Clevedon, where rapid population growth has increased the number of people living in areas that could be affected by a significant earthquake.
Researchers stress the risk at the Mangatangi Fault is separate from Auckland’s volcanic history.
“If the whole fault ruptured, there would likely be serious consequences for people in south Auckland and possibly further into central Auckland as well,” says University of Auckland geologist Dr James Muirhead, a co-author of the study.
The discovery suggests Auckland’s earthquake hazard may be higher than many residents and policymakers realise.
“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.”
In New Zealand, any fault that has moved within the past 125,000 years is classified as active.
Researchers say the findings come as Auckland continues to expand, placing more homes, businesses and critical infrastructure closer to known fault systems.
Lead author Hannah Martin, who completed the research as part of her Master’s degree in Earth Science at the University of Auckland, says the fault may not rupture again for many 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,” she confirms.
The study has renewed questions about Auckland’s seismic risk. Last year, the government exempted Auckland from earthquake-prone building regulations because of its low hazard classification. Auckland Mayor Wayne Brown argued for the exemption, citing the city’s apparent lack of major earthquakes during the past 100,000 years.
Auckland experiences earthquakes every year, but most are too small to be felt or originate elsewhere, Muirhead adds.
The Mangatangi research forms part of a wider effort to better understand Auckland’s earthquake hazards. Muirhead, Dr Jennifer Eccles of the University of Auckland and Professor Mark Stirling of the University of Otago are investigating other faults across the region through a project funded by the Natural Hazards Commission.
Using advanced mapping technology, ground-penetrating radar and trenching, the team hopes to build a clearer picture of seismic history.
“It’s always better to plan based on evidence rather than on our best hopes or worst fears,” Eccles says.
Auckland has experienced the effects of earthquakes before. In 1891, an earthquake centred near Port Waikato shook the city, causing minor damage including falling plaster and broken glass.
