Four of the country's leading scientists have shared a few earthquake secrets but could offer little reassurance about the potentially lethal Hikurangi subduction zone, and almost inevitable resulting tsunami.

They spoke to about 600 people at the Napier Conference Centre on Wednesday night, conceding that much mystery remains about the zone off the East Coast.

But they have been working tirelessly to seek patterns, occurrences, movements and possible clues from the past, to hopefully one day get a clearer picture of what is happening "out there".

As Dr Kate Clark, of the New Zealand Geoscience Society (GNS), said, extensive research thus far had not revealed any very large subduction earthquakes occurring along the Hikurangi zone "that we know about" in relatively recent historic times.

Advertisement

But she then added "it is building up for something in the future" — which drew some audible intakes of breath.

As she and her colleagues explained, it is inevitable as the great Pacific and Australian plates off the eastern seaboard are moving. They always have been and always will be.

There have, however, been large earthquakes along the coast, as well as evidence of tsunamis having reached shore, but nothing like the great subduction events which devastated Chile in 1960 (a 9.3 on the Richter scale) and the 9.4 which hammered Sumatra in 2004.

Clark said the 7.8 Hawke's Bay earthquake of 1931 was not a subduction earthquake — it was an "upper plate"-driven earthquake.

Subduction is a geological process that takes place at convergent boundaries of tectonic plates where one plate moves under another and is forced or sinks due to gravity into the mantle.

Regions where this process occurs are known as subduction zones.

HBT18219801: Dr Kate Clark (speaking) and her colleagues (L-R) Dr Laura Wallace, Dr Dan Bassett and Dr Alan Orpin. Photograph Warren Buckland
HBT18219801: Dr Kate Clark (speaking) and her colleagues (L-R) Dr Laura Wallace, Dr Dan Bassett and Dr Alan Orpin. Photograph Warren Buckland
Huge slips, caused by the 7.5 earthquake in 2016, are seen blocking State Highway 1 north of Kaikoura. Photo / Getty Images
Huge slips, caused by the 7.5 earthquake in 2016, are seen blocking State Highway 1 north of Kaikoura. Photo / Getty Images

There had been some major earthquakes along the stretch though — like what Clark described as a "megathrust" off Gisborne in 1947 which measured 7.2.

There was evidence earthquakes had affected the regional coastline, with evidence of coastal uplift found at Mahia, Ocean Beach, Waimarama and Ahuriri.

Extreme subduction events are still pretty much off the radar, although research has shown there have been about eight major seismic events, potentially subduction earthquakes, along the Hikurangi stretch over the past 7000 years — the last about 500 or 600 years ago.

On paper, on average, that meant they could be occurring about every 800.

"But this is not a clockwork system," Clark pointed out.

"There could be intervals of just 200 years."

That led one member of the audience to quietly turn to his partner and whisper "crikey we could be overdue then".

The unpredictably of the earth's plates, especially through the Pacific rim "of fire" which includes the Hikurangi zone, is what the scientists here and in other affected countries are trying to unravel.

"We can't predict when the next one will be," Clark said.

"But research says there will be another one."

As her colleague, Dr Laura Wallace, pointed out, there is a lot of friction going on out there.

"You can't stop the plates."

It was a full house for the geoscience presentation about the Hikurangi subduction zone in Napier on Wednesday. Photo / Warren Buckland
It was a full house for the geoscience presentation about the Hikurangi subduction zone in Napier on Wednesday. Photo / Warren Buckland

At this time, one zone of plates "out there" are stuck.

"When a stuck area ruptures it releases energy in an earthquake."

Wallace brought some sense of ease to the gathering when she began talking about "slow-slip events".

These are small and relatively constant movements where two pieces of tectonic plates slide past each other.

The Hikurangi zone off Hawke's Bay and Gisborne are in a slow-slip region.

"It is steadily creeping — there is not so much evidence of stress build-up."

The zone off Wairarapa and Wellington is currently stuck — no movement detected.

It it were to rupture, the result would be an earthquake event up to 8.5 on the scale.

And we would receive much of that as these events spread along such zones.

The Kaikoura earthquake in February 2013 sent seismic waves and disturbances up the east coast.

Wallace said since 2002 scientists had observed at least 30 distant slow-slip events off the North Island, with two GPS monitored sites moving 2cm to 3cm over 10 days.

"Slow-slip events may give us important insights into where great subduction earthquakes are most likely," she said.

"In the future, once we better understand the influence of slow-slip events on earthquake rates we can use them to greatly improve our short-term earthquake forecasting."

Dr Alan Orpin, of the National Institute of Water and Atmospheric Research (Niwa), brought a light touch to the presentation when he paid an entertaining tribute to US President Donald Trump who had recently issued some "wise words".

"It's not often I can agree with him, but he did say the world is a dangerous place."

He then showed an image of Star Trek skipper Captain James Kirk being threatened by a weapon-wielding reptilian alien and said "we are facing a monster, too".

Orpin explained the scale of seismic-driven sediment flows in the ocean, and while at sea on research in one of the Hikurangi subduction zones the Kaikoura earthquake struck, and detectors showed ocean floor material "flooded" up the coast.

They are turbidity flows, and there is evidence "out there" that the basins along the Hikurangi margin have a turbidite rich history.

"It suggests a high frequency of triggers over many thousand of years."

The challenge in collecting historic samples of turbidite flows was to work to understand what happened during the Kaikoura event and others in the past.

GNS scientist Dan Bassett said, in terms of research, the Hikurangi zone was "our best natural laboratory" and there was a huge amount of data collecting going on.

There were gasps when he said the 49 seimolographs set into the zone had taken about 170,000 measurements and what was happening in the wake of that was to try and work out what was creating the differences in the north and south zones of the Hikurangi.

"We have some work to do."

The second stage of their research, dubbed Shire II, will start next year and entail the use of 900 seismolographs on the sea floor which can potentially map out earthquake and tsunami models.

The tsunami ingredient sparked a couple of questions from the audience which were later fielded by Hawke's Bay Civil Defence emergency management adviser Jim Tetlow.

One was if a major subduction event occurred "out there", how big could the resulting tsunami be and how far would it surge into Hawke's Bay.

"Some of the worst-case scenarios is up to around 10m but that is the very, worst scenario ... it would have to be a massive, massive earthquake."

It had the potential to surge up to 6km inland.

"So Hastings and Havelock North, you're okay," he said, before adding with a smile " but hey, I live in Whirinaki."

In closing, Wallace said the ongoing research into the region meant increasing the amount of knowledge, and she hoped "we can come back in a few years and give you some more information — to better understand what is happening out there".

Tetlow advised those living in potentially geologically violent zones to "be prepared".

BE PREPARED

Long or strong get gone: If you are in a tsunami evacuation zone and feel a strong earthquake that makes it hard to stand up, or a weak rolling earthquake that lasts a minute or more, move away from the coast immediately.

Emergency supplies and getaway bags: In an emergency you could be stuck at home for three days or more. You probably have what you will need already, and you don't have to have everything all in one place, but you might have to find things in a hurry and/or in the dark. It is a good idea to have a pre-packed getaway bag if you live or work in a flood or tsunami zone.

Making a plan: Talk with your family and friends, and work out what you'll do in emergency situations and how you'll communicate. When developing your plan, think about not being able to get home, having to evacuate, being stuck at home and having no water, no power and no communication.

Connecting in an emergency: In an emergency people generally need two things – contact with others and information. In an emergency people will check on each other and share resources. Knowing a variety of people in your local and wider community will make you able to manage better in an emergency.