Volcanoes, tsunamis, underwater landslides: New Zealand's 'near-shore' threats

An aerial view of Whaakari / White Island after the deadly eruption. Photo: Supplied
Scientists are studying the risk posed to the mainland by two of New Zealand's offshore volcanoes: Tuhua/Mayor Island and Whakaari/White Island (pictured). Photo: Supplied
Scientists are taking a fresh look at New Zealand's risk from island volcanoes and underwater landslides, in a series of new projects focused on some of our lesser-known tsunami-makers.

A programme led by GNS Science volcano geophysicist Dr Craig Miller will focus on the threat and histories of two volcanoes lying just off the Bay of Plenty: Tuhua/Mayor Island and Whakaari/White Island.

Geological evidence has pointed to eruptions at Tuhua in the past 5000 to 10,000 years large enough to have spread ash across Waikato, Auckland and the East Cape.

"Ultimately, we want to fully understand the breadth of hazards posed by these islands, how often they may occur and what impact they may have on the mainland, so that mitigation measures can be taken ahead of time reducing the harm done should such an event occur," Miller said.

Along with eruption effects like ash, his team also wanted to explore how big blows might trigger large tsunamis.

This type of hazard has been catastrophically illustrated by Indonesia's Krakatoa, whose 1883 and 2018 eruptions caused tsunamis that killed 36,417 and 426 people respectively.

"The catastrophic flank collapse at Krakatoa was part of the motivation to look at our own volcanoes in more detail, but also the recognition that even small eruptions from these islands can have devastating consequences."

Miller explained the effect as like dropping a brick in a bathtub.

"If a large chunk of the volcano flanks falls into water, then the water is displaced rapidly in the form of a surge that radiates out from the source," he said.

"We don't know if the flank would collapse all in one go, or piecemeal, so our large scale experiments will uncover what's required to generate tsunami."

In their five-year study – granted $13.3m through the Ministry of Business Innovation and Employment's Endeavour Fund - the team planned to assess areas of the volcanoes most likely to collapse, and then model the impact to the mainland under different scenarios.

"Preliminary modelling shows they will be more localised than large underwater earthquakes and are likely to be smaller scale, yet we don't know much about the frequency or magnitude relationships," he said.

The moment that White Island erupted. Photo: Alessandro Kauffmann
Whakaari, pictured as it erupted at about 2.15pm on December 9, 2019. Photo: Alessandro Kauffmann
"Also the fact that these islands are close to shore means that there is likely to be little warning or reaction time, so understanding the risks and thinking how to mitigate them is important."

Instruments capable of detecting the magmatic and hydrothermal systems in the crust will be deployed beneath the volcanoes to help the scientists build "seafloor to summit" models.

"We will also sample the sediments on the seafloor around the volcanoes for evidence of past eruptions and flank collapses to build up the complete geologic history."

Another focus was the danger of pyroclastic flows – or fast-moving surges of hot ash, gas and rock – which had similarly been observed to start tsunamis, and also travel long distances over water.

The study team will also draw on a range of other fascinating technology, such as drones and underwater robotic vehicles to map the volcanoes' surface and sub-surface, a Massey University model that simulates pyroclastic surges, and what Miller described as "serious games" for children to learn about hazards.

Tasman tsunamis

In a separate study, scientists will investigate the tsunami threat from another source: landslides on the ocean floor.

"Underwater landslides are the second-most cause of all tsunami after earthquakes, and we do know of some mysterious tsunami events from the past, prior instrumental records, that could be attributed to them," study leader and GNS geophysicist Dr Suzanne Bull said.

When a large area of the seafloor was suddenly displaced, this caused a "drag down" effect that created a trough at the ocean surface.

Water then rushed into the trough, producing a swell, followed by multiple oscillations that propagated outward.

"We have not identified any underwater landslide-tsunamis recently in New Zealand, but there have been recent global occurrences, such as the 1998 tsunami in Papua New Guinea."

And further back in time, there was plenty of evidence to show these dramatic episodes had unfolded near our shores.

There's evidence to show that past eruptions at Tuhua/Mayor Island have generated enough ash to...
There's evidence to show that past eruptions at Tuhua/Mayor Island have generated enough ash to travel across Waikato, Auckland the East Cape. Photo: Jim Eagles/NZME
In mapping the geology beneath our feet and the seafloor to better understand our largely submerged continent, GNS scientists discovered the remains of gigantic underwater landslides off the North Island's west coast.

"The biggest is about 4000 cubic kilometres in volume – that's the equivalent of 40 Mt Ruapehus, and is the biggest underwater landslide ever discovered in the waters around New Zealand," Bull said.

"The obvious questions we have are: What caused these events? How frequent are they? Is another one likely to happen in the future? And, if so, how can we best prepare?"

The study will build off existing "subsurface" information from the Tasman Sea – largely seismic reflection and drillhole data – to help describe the landslides.

They'll then be translated into models to simulate their tsunami-making potential.

"To better understand the modern day conditions at the site of the landslides - and if there is likely to be another landslide in the future - we will be carrying out a research cruise to collect and analyse the sediments making up the modern day seafloor."

Bull pointed out that the Tasman wasn't a place that most Kiwis would expect to see a massive tsunami come from.

"We do expect, and are increasingly well prepared for, tsunami impacting the East Coast of New Zealand but not 'the other side'," she said.

"We want to make sure that we understand all we need to about this particular hazard, to maximise New Zealand's resilience."

A third GNS-led project, also granted a million dollars through the Endeavour Fund, meanwhile aimed to simulate the decisions and movements of individual people during a tsunami evacuation – right down to what vehicles they use and what barriers might lie in their path.

Study leader Dr William Power said the project was inspired by work he had helped carry out to define evacuation zones around the country.

"When we discussed our maps with Civil Defence, it became clear that there were many challenges to be overcome to achieve quick and safe evacuations when there are large numbers of people in the zones."

That was where modelling the problem could make a difference, he said.

"They may help by identifying 'bottlenecks' where too many people may attempt to pass, which might be alleviated in some cases by widening tracks and in others by signing alternative routes," he said.

"We also hope to identify communities that have specific problems such as being far from high ground, possible solutions might include building tsunami evacuation towers.

"We also hope to be able to advise on when it may be appropriate to use other forms of transport besides walking."

Power and colleagues have already built models in which simulated evacuees – or "agents" - attempt to take the fastest route to safety.

"Our existing model only simulates pedestrians," he said.

"In our new project we hope to build enhanced models that will help us to understand what may happen if some agents use other modes of transport such as bicycles and cars, we also hope to incorporate how people might behave if they encounter obstacles while evacuating, such as areas of liquefaction."

Reconstructing Auckland's prehistoric tsunamis

Meanwhile, a just-published stocktake of evidence has shown Auckland is far from sheltered from tsunami risk.

A new science review, featured in the New Zealand Journal of Geology and Geophysics, analysed decades of research into local "paleotsunamis", or events that took place hundreds to thousands of years ago.

Lead author Dr Kate Clark, a paleoecologist and earthquake geologist at GNS Science, said about 30 tsunamis had been observed in the region over the past 150 years.

All had been relatively minor – yet their impacts could have been different if they had swept into the more developed and populated coasts of today.

"For example, a tsunami in 1868 from an earthquake in Peru caused a tsunami up to 2.5m off Great Barrier Island. If that occurred tomorrow, it's likely to have a greater impact than it did in 1868."

Some parts of the region appeared to be more vulnerable than others.

"In particular, the West Coast has a low hazard of tsunamis because that coastline faces a less tectonically active area," she said.

The coastal settlement of Mokau. The Tasman Sea off the North Island's wild west coast isn't...
The coastal settlement of Mokau. The Tasman Sea off the North Island's wild west coast isn't known as a source of near-shore tsunamis. Photo: Alan Gibson/NZME
"The east coast of Great Barrier Island and the northern east coast of Auckland have a higher risk of damaging tsunami because they are more exposed."

In their review, a project funded by the Earthquake Commission, she and colleagues sought to get a better picture of whether large tsunamis could have been triggered by earthquakes in the Kermadec subduction zone, which is part of a wider system that spanned from the East Cape area to Tonga.

If this zone did generate them, models showed that the areas most affected would be the coasts of Northland, Auckland and Coromandel.

"There is not a lot of land out near the Kermadec trench, so it doesn't have many instruments on it, such as GPS and seismic stations, and we have a pretty weak understanding of how big the largest earthquakes on the Kermadec subduction zone could be," Clark said.

"Using the geological record of past tsunamis could help improve that understanding."

The paper cited evidence of these prehistoric surges from 18 sites – all discovered only in the past two decades – and concluded that just three of them bore convincing signs of inundation.

One of the strongest records was found at Aotea/Great Barrier Island, where an extensive layer of pebbles was found within sand dunes.

"The type of pebble found means they could only have been transported from relatively deep offshore which requires a fairly large tsunami."

More rich evidence was found elsewhere on the island, and also at Tāwharanui.

"Importantly, we also found that some tsunami evidence, for example, from the inner Hauraki Gulf area, including Orewa and Waiheke Island, is very weak and just not compelling or reliable," Clark said.

"This points toward lower tsunami hazard in the inner Hauraki Gulf - consistent with all the latest tsunami hazard models."

Still, Clark said there were major knowledge gaps around the timing of some of the events – and whether it was 400 or 4000 years ago that they struck the coast.

"We'd like to do more research to understand where the tsunamis came from, specifically whether the tsunamis were generated at the Kermadec subduction zone, or some somewhere further afield," she said.

"This is of interest, because we'd like to know how much warning time we are likely to get of large tsunamis."

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