Faults near Wellington found to be more active than scientists thought

GNS Science earthquake geologist Dr Genevieve Coffey overlooks a trench dug near Levin, where her team have been investigating the seismic history of the Tokomaru fault. Photo / GNS Science

Scientists have discovered two faults north of Wellington to be more active than they first thought – and potentially capable of kicking off with others in big earthquakes.

Their just-published findings offer fascinating new insights into the region’s incredibly complex seismic environment, which scientists have been painstakingly mapping for the past two decades.

The faults at the centre of the new study – the recently-discovered Tokomaru Fault and its neighbouring Northern Ohariu Fault – are two of the largest known in the lower North Island, and stretch from Horowhenua to Palmerston North.

“We had evidence that they were active and had experienced earthquakes in the last few thousand years, as well as some broad estimates on how fast they move over time,” said the study’s lead author, GNS Science earthquake geologist Dr Genevieve Coffey.

“However, what we knew was very approximate, because it was based only upon how these faults are expressed in the landscape.”

Their closeness to population centres in the growing Levin-to-Wellington corridor made their quake-making potential crucial to understand – as was their relationship with the region’s wider fault network.

The power of 2016′s 7.8 Kaikōura earthquake was enough to set off as many as 25 faults – still considered a world record for a single observed event – and Coffey and her team were eager to learn whether that could also play out around Wellington.

Damage to the Wellington waterfront following 2016's 7.8 Kaikoura earthquake. Photo / University of Canterbury

At a small swamp east of Levin, near where the Tokomaru Fault branched off the Northern Ōhāriu Fault, the scientists dug two 10m-long trenches to expose layers that had been deformed in past events.

Using radio-carbon dating, they then calculated when and how often those earthquakes had occurred, before comparing the fresh data with evidence gleaned from other local faults.

The results dramatically changed what they knew about the Tokomaru Fault’s seismic history, revealing one quake big enough to have ruptured the ground’s surface around 100 to 200 years ago, and another about 1000 years ago.

“These ages mean that we’d expect an earthquake to occur on the Tokomaru and Northern Ōhāriu faults every 470 to 2300 years,” Coffey said.

“Before this work, it was thought that earthquakes on the Tokomaru Fault occurred every 6000 to 12,000 years, so our results show that it is considerably more active than we previously thought.”

The timing of the events was found to align with that in evidence from other faults, including the Wellington and Ōhāriu Faults.

Coffey said it was possible that a quake on one fault could trigger ruptures on others; that they could all rupture at once, like in 2016; or that all formed part of a larger system.

“While we cannot identify which of these options is the most likely, our results show that fault interactions likely play an important role in the timing of earthquakes on faults in southern North Island.”

Other possible implications were that quakes on the faults could be larger in magnitude, lead to more areas affected by shaking, or occur in clusters – but more research was needed.

The study, published in the New Zealand Journal of Geology and Geophysics, comes after scientists recently published data mapping nearly 900 faults capable of generating moderate to large quakes.

This wealth of information helped inform a newly-updated national seismic hazard model which, compared with previous estimates, showed an increased risk of ground-shaking from future quakes in places such as Blenheim, Wellington, Napier and Gisborne.

“With the paleoseismic data we collect, we’re getting more and more of an understanding about the range of earthquake behaviours on faults across New Zealand,” Coffey said.

Around the Wellington region in particular, there were still many faults for which scientists still had no or little data – including several in Wairarapa only just mapped with 3D scanning technology.

Coffey said her team’s new findings might also prove useful for a new $12.6m, Victoria University-led research programme investigating whether an enormous event – like a major rupture of the Alpine Fault - could leave a decades-long legacy of large shakes for Wellington and central New Zealand.