By Michael Daly of 
This seismic reflection profile shows the modern seafloor and million-year-old underwater landslide deposit buried by covering sediments. Photo: GNS Science
A massive landslide under the Tasman Sea a million years ago may have caused a 70m tsunami along parts of the North Island.
It was the most recent of six giant submarine landslides thought to have happened in the past 5 million years in the eastern Tasman Sea off the North Island's west coast.
Scientists are working to learn more about the landslide and other similar events, and to understand the risk of something like it happening again. They emphasise such events are "extremely rare".
As part of the research, they modelled a landslide from a million years ago in the Deepwater Taranaki Basin. About 100km offshore from the Auckland and Waikato west coast, it is estimated to have involved about 3700cu km of material and covered more than 22,000sq km.
The modelling showed that if such an event were to happen again it would pose "an extreme tsunami" hazard, the researchers from GNS Science and the University of Newcastle, Australia said. They noted such submarine landslides could potentially be triggered by earthquakes.
The researchers modelled three scenarios for the landslide.
The model came up with wave heights directly above the landslides ranging from 230m for the smallest to 250m for the largest.
The highest waves were north of the Kaipara Harbour entrance.
But the paper noted the wave heights modelled along the affected area of coastline were zero in some places, and said that highlighted the variability of the impact from tsunami generated by underwater landslides.
Water velocity ranged from 71-80m per second above the landslide, and was as high as 51m per second at the coast. It took as little as 18 minutes for the waves to reach the directly adjacent coastline.
Rocks formed in an ancient submarine landslide along the north Taranaki coast. This area shows a cross section of the ancient continental slope and basin floor as it was about 10 million years ago. Photo: GNS Science
High waves and water velocities were also modelled in Cook Strait. Maximum water velocities through the Strait, as well as along the open coastline, were an order of magnitude greater than those that had been demonstrated to cause significant damage to maritime and coastal infrastructure.
Parts of the east coast of Australia were also affected, with maximum wave heights ranging from 1.8m for the smallest scenario to 4.5m for the largest, four hours after the landslide.
The paper said the modern-day continental shelf - the edge of which was at a depth of about 200m - and continental slope of the Deepwater Taranaki Basin were major sediment storage areas.
Photo: GNS Science
Photo: GNS Science
The basin floor, which was at depths of 1500m or more, and continental slope were blanketed mostly with mud made out of material eroded from the land. Most of the sediment came from rivers along the west coast of the South Island, with a lesser amount from the North Island.
Researchers have limited information about the amount of sediment building up on the seafloor now.
In an article last June, some of the scientists who worked on the modelling study, including GNS Science marine geohazard researcher Suzanne Bull, said it wasn't known "how much, how fast, and exactly where sediment was accumulating" in the Tasman Sea region.
It wasn't clear whether a "conveyor belt" of sediment from the South Island was still moving north, or what could trigger a future landslide event, the article said.
It noted the amount of sediment being moved to the deep ocean was thought to decrease at times, such as now, when sea levels rose above the edge of the continental shelf.
*This story was first published by Stuff
