Catastrophic landslides can trigger tsunamis like this one in Japan JIJI PRESS/AFP/Getty

How is it that one underwater landslide leads to a devastating tsunami, while another of similar size barely causes a ripple? The answer may lie in the way the sediments slide. Cascading landslides are the ones to watch, step by step slippage is usually more benign.

Just over 8000 years ago one of the largest underwater landslides on record occurred off the coast of Norway. Dubbed the Storegga slide, it covered an area of sea floor larger than Scotland, and generated a massive tsunami in the north Atlantic Ocean. That left debris perched 20 metres above sea level on the Shetland islands, and possibly caused a Stone-age population collapse in the coastal region of what is now the north-east UK. Curiously, though, the Trænadjupet slide, another giant underwater landslide in the same area 4500 years ago failed to generate a destructive wave.

To find out why the two landslides led to such different outcomes, Finn Løvholt from the Norwegian Geotechnical Institute in Oslo, and his colleagues, produced computer models of two types of underwater landslides and compared their tsunami-generating potential.


The first model, known as a retrogressive slide, starts at the bottom of the slope and releases its energy block by block, in a staggered series of smaller landslides. The second model, known as the debris slide, begins as a retrogressive slide, but then spreads much more quickly up the slope, leading to multiple blocks of sediment failing in one go.

Debris on the slide

Løvholt and his colleagues found that Storegga could only have produced a tsunami capable of the damage observed today if the slope developed into a fast-moving debris slide, carrying all of its staggering 3000 cubic kilometres of material simultaneously. Meanwhile, Trænadjupet’s lack of tsunami is best explained by a sequence of slope failures, ultimately releasing a similar amount of sediment, but doing so in batches – none of which could generate a tsunami on its own.

“Studies like this are very valuable and highlight how important it is to get the landslide mechanism correct to understand the tsunami hazard,” says Joshu Mountjoy at the National Institute of Water and Atmospheric Research in Wellington, New Zealand – who, later this year, will be drilling into a large active landslide that is believed to be flowing slowly on New Zealand’s Hikurangi Margin, to investigate its movements.

Until now tsunamis were assumed to be almost inevitable in the wake of massive submarine landslides, but the new findings suggests that perhaps we don’t need to fret so much. “Many giant landslide deposits across the world’s continental margins display retrogressive development, and our results suggest that some landslides, even when exhibiting giant volumes, may be less tsunamigenic than previously thought,” says Løvholt.

Journal reference: Geophysical Research Letters, DOI: 10.1002/2017GL074062