Scientists reveal more information about our most active fault, the Hikurangi subduction zone

Scientists are well underway finding new information about the risk one of our most active faults poses to New Zealand.

The Hikurangi subduction zone is off the East Coast of the North Island. It is where the Pacific tectonic plate subducts, or dives underneath, the Australian tectonic plate.

Four of the country's leading scientists presented the latest findings of the zone at Te Papa on Wednesday night.

Dr Laura Wallace is a geophysicist at GNS Science and involved in the research.

She said because the fault is so large, it can produce the biggest earthquakes and tsunamis compared with any other fault in New Zealand.

"We really don't understand the hazard posed by the Hikurangi subduction zone ... We don't know how often it ruptures in big earthquakes and we don't know how big the earthquakes are when it does rupture,

"We also don't have a good understanding of how often tsunamis are generated by the subduction zone."

Since 2016, a large team of national and international scientists have been studying the plate boundary to help forecast the likelihood of damaging earthquakes or tsunamis.

It involved looking back at past earthquakes and tsunamis, although there had not been any major events on the subduction zone since European settlement.

Dr Kate Clark, an earthquake geologist, said they compiled data from lots of sites along the boundary looking at uplift and carbon dating samples.

"We have come up with a record of 10 earthquakes across approximately the past 7000 years."

Looking at past events could contribute to forecasting the likelihood of earthquakes and tsunamis in the future.

Wallace was also doing work with GPS data to track how the fault was moving, even down to the millimetre level.

She said the Pacific and Australian plates can sometimes get stuck together along the fault and continue to move in other places.

It was currently stuck together, with stress building up, under the southern part of the North Island.

"That can really highlight the pressure points, the places where we expect more frequent, large earthquakes."

Scientists were also excited about the slow-slip event currently occurring near Gisborne.

A slow-slip event is a silent, slow-moving earthquake that happens over a long of time. They have only been able to be detected recently thanks to new equipment.

They have been recorded in New Zealand since 2002, but the current one is the largest observed in the country yet.

Wallace said the movements that were happening offshore were about 10 – 20 cm - a few years of plate motion in a few weeks.

"This is really exciting because we've been deploying a lot of instruments on the seafloor to try and better study these events that are happening offshore."

The research will help to re-evaluate and change how the regions react to a large earthquake or tsunami.

At present the fault could produce at least a magnitude 7 earthquake. In a worst-case tsunami scenario, it could leave Wellington residents with only 10 minutes to get to high ground.

The region's emergency management office general manager, Jeremy Holmes, said the research was helping them to update their understanding of the risks the region faces.

"We're looking to use that information to inform the way we evacuate people safely and as quickly as possible,

"It's informing the understanding of the evacuation zones and where people need to go to be safe following a worst-case scenario."

The research project will continue through to 2021.