The risk of undersea landslides and their potential to cause tsunamis along New Zealand’s east coast is being investigated by scientists aboard the German research vessel RV Sonne as part of a month-long international collaboration.
Researchers from the National Institute of Water and Atmospheric Research (NIWA) and GNS Science are examining underwater canyons alongside colleagues from German institutions GEOMAR and Kiel University on the deep ocean research vessel.
By surveying, mapping and sampling two different areas off the Wairarapa and Canterbury coasts which have previously experienced huge landslides, they hope to better understand the hazard and risk potential of large underwater canyons, says NIWA marine geoscientist Dr Joshu Mountjoy.
“Future undersea landslides could trigger tsunamis as well as impact seafloor infrastructure. If these landslides happened again, we know they could cause devastating tsunamis. What we are trying to understand is where and when they might occur in the future.”
Joshu Mountjoy
To better understand what lies beneath the ocean, they are mapping the seafloor using RV Sonne’s multi-beam sonar to create contour maps and using seismic surveying, as well as collecting core samples from the seafloor to reveal the age and when landslides previously occurred.
Surveying and mapping will provide insights into the structure and geological formations below the seabed says GNS Science Computational Geophysicist Christof Mueller. “It is like a CAT scan of the Earth, with seismic surveying penetrating deep into the crust to map geological structures, while acoustic mapping maps shallower features like the seafloor depth and topography with greater detail. 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.”
While the ocean floor covers more than 70 per cent of the planet’s surface, it isn’t flat or unchanging as some people assume, he says. “Like dry land, the seafloor has rugged mountains, long valleys, flat plains, steep-sided canyons and exposed rock. Covered in layers of marine sediments, it is a dynamic place continually changing. The ocean is roughly four times deeper than land is high.”
The distance from the sea surface to the seafloor makes deep canyons difficult to explore, along with the lack of light, cold temperatures, and high pressure, says Mountjoy.
“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 we are studying two canyons less than 200km apart, they have contrasting geology, so we’ll be able to directly compare underwater canyons on active and passive continental slopes. In the Palliser Canyon study area, south of Cape Palliser in Wairarapa, the Pacific Plate moves beneath the Australian Plate, the geology is dominated by rock and earthquakes occur regularly. In the Pegasus Canyon study area, north-east of Banks Peninsula, the geology is dominated by softer sediments and earthquakes are less frequent. These factors should have a big influence on how and where landslides occur.”
He says the research aims to look at the past to understand future possibilities. “We often don’t know what causes individual undersea landslides, but we do know that some of these are vast, greater than 5 cubic kilometres in size, and can potentially generate tsunami waves up to 5 m high. 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. This improved understanding will better position New Zealand to be one step ahead, as data will allow for better community resilience and protection of our national infrastructure and assets which keep New Zealand moving.
The current 2025 voyage of the RV Sonne highlights collaboration between New Zealand research organisations, GEOMAR Helmholtz Centre for Ocean Research Kiel (GEOMAR) and Kiel University, Germany, which spans more than 30 years. The 116m-long RV Sonne has worked for much of its life as a platform for scientific research around the Pacific Ocean.
