Scientists probe giant undersea landslides to learn about NZ’s big tsunami risk

GNS Science computational geophysicist Dr Christof Mueller and Niwa marine geoscientist Dr Joshu Mountjoy are leading a month-long expedition aboard German research vessel RV Sonne, surveying ancient landslide sites off New Zealand's east coast. Photo / Niwa

• A new international collaboration is surveying ancient landslide sites in two deep undersea canyons to better understand New Zealand’s tsunami risk
• If some of the documented landslides occurred today, they could cause multimetre-high tsunamis, says one of the scientists leading the expedition
• A separate project is turning to the trench that unleashed Japan’s devastating 2011 earthquake and tsunami, to better gauge the threat posed by our own Hikurangi Subduction Zone

Scientists are probing giant landslide sites off New Zealand’s east coast — and the undersea trench that unleashed Japan’s 2011 Tohoku disaster — to learn more about our tsunami danger.

In a new month-long expedition, an international team is surveying remnants of ancient landslides within vast underwater canyons off the coasts of Canterbury and Wairarapa.

“If these landslides happened again, we know they could cause devastating tsunamis,” Niwa marine geoscientist Dr Joshu Mountjoy said.

“What we are trying to understand is where and when they might occur in the future.”

From German research vessel RV Sonne, the team is gathering samples from the seafloor to reveal when the landslides occurred, while using multi-beam sonar to create detailed new maps.

GNS Science computational geophysicist Christof Mueller likened this mapping — which also involves deep-penetrating seismic surveys — to a “Cat scan of the Earth”.

“Sediment cores and geophysical data will be analysed to reveal the layers, because we are interested in the mechanical strength of the sediments and rocks and how they respond to earthquake motions.”

Mountjoy said the seafloor’s depth, a lack of light, cold temperatures and high pressure all made the deep canyons challenging to explore.

“From these extensive surveys, and analysis of the sediment cores, we hope to uncover the secrets of underwater landslides — how and where they form, when they last occurred and their frequency and magnitude, and their potential to trigger tsunamis.”

While the two survey areas were less than 200km apart, they had markedly contrasting geology the team would directly compare.

The Palliser Canyon, south of Wairarapa’s Cape Palliser, was dominated by rock and produced earthquakes regularly, while the Pegasus Canyon, northeast of Banks Peninsula, was characterised by softer sediments and experienced fewer earthquakes.

“We often don’t know what causes individual undersea landslides, but we do know that some of these are vast, greater than 5 cu km in size, and can potentially generate tsunami waves up to 5 metres high,” Mountjoy said.

“What we don’t know is how often and what controls these landslides. So, the outstanding science challenge is to identify what causes the big ones.”

The collaboration comes as GNS scientists are partnering with Japanese colleagues on two new major hazards-focused projects.

One of those will 3D-scan part of the Japan Trench, which unleashed 2011’s magnitude 9.0 earthquake and tsunami, to better understand our own Hikurangi Subduction Margin.

“The Hikurangi subduction zone is New Zealand’s largest source of earthquake and tsunami hazard,” GNS geophysicist Dan Bassett said.

“However, the lack of large earthquakes in historical records means we have key uncertainties on both the magnitude and location of future earthquakes — as well as the size and impact of resulting tsunami.”

Drawing on Japan’s rich data would ultimately allow scientists here to refine and test earthquake and tsunamis along the Hikurangi margin — estimated to have one-in-four odds of generating an 8.0 event within the next 50 years.

Jamie Morton is a specialist in science and environmental reporting. He joined the Herald in 2011 and writes about everything from conservation and climate change to natural hazards and new technology.

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