Auckland earthquakes: Previously unknown faultlines discovered in Auckland

Dozens of previously unknown faultlines have been discovered under Auckland, prompting further investigation.

By Rowan Quinn of RNZ

Dozens of previously unknown faultlines have been discovered under Auckland – and new work is set to find out if they are likely to cause earthquakes.

Scientists will soon start to dig trenches across some of the faultlines to see if they are active.

So far, the discovery has not increased the known risk to the city.

One of the scientists, Auckland University senior lecturer James Muirhead, said any fault that had moved in the past 125,000 years was considered active.

Some of the faults could have been still for six million years – which meant they were technically not active. Others were suspected faults.

Muirhead’s team would begin by looking at some of the most likely – near Pukekohe or Drury in the city’s rural south.

Diggers would carve out 4m-deep trenches across the fault, which would then be analysed to see how recently they had moved.

“So, hopefully two years from now, we have a really good answer to that question – whether these faultlines in Auckland are active and whether they represent a significant seismic hazard,” Muirhead said.

There was a common perception that Auckland was not very seismic but that was probably because that was compared with other places in New Zealand, which was a very seismic country, he said.

Auckland had actually experienced some significant events, like the 1891 Port Waikato quake, which was strong enough to shatter windows in the city, he said.

The new potential faults were discovered when researchers analysed borehole samples.

“Basically, holes that have been drilled around Auckland to look at how the subsurface has moved both up and down below our feet through time,” Muirhead said.

The team divided the lines into likely faults, possible faults and possible structures – lines that showed signs of ground movement but it was unclear why.

Two of the most prominent probable faults ran from the Waitematā Harbour to the Manukau Harbour, one through Avondale, the other from Glendowie to Blockhouse Bay.

They had already been suspected but the latest research had shored them up and more accurately measured them, Muirhead said.

Some of the possible faults were around Northcote, Birkenhead and Wiri.

There were many more in the “possible structure” category which meant they needed a lot more investigation, such as in Sandringham, Newmarket and Ōtāhuhu.

The research said it was possible intense urbanisation could mask faults.

The report also revealed Auckland experienced more small earthquakes than previously thought, but Muirhead said that was likely because there was better monitoring.

The work has been partly funded by the Natural Hazards Commission Toka Tū Ake, formerly the Earthquake Commission, which has highlighted them in its latest Resilience Highlights Report, which looked into the impact of natural hazards.

The commission said even though major earthquakes remained unlikely, the findings could help decide how land is used as the city grew.