Killer tidal waves...
fact or fiction?

Could the collapse of a volcanic island really generate a killer tidal wave?

Place: Canary Islands. Date: July 5th 2026. Time: 0712 hrs

Something deep in the volcano stirs and the island of La Palma grumbles quietly. On her western flank a gaping fissure appears, as a giant slab of earth and rock slowly detaches itself and begins to slide inexorably towards the sea. Gravity soon tightens its grip and just before impact the unstoppable mass is travelling at 225 mph, destroying everything in its path. The landslide enters the water with a triumphant roar, and for a brief moment a huge hole appears in the sea surface as water is displaced in all directions. Then the hole fills again as the ocean gleefully swallows its prize, while beneath the waves the dying slab disintegrates into huge kilometre-scale blocks before coming to rest on the seafloor. After just a few minutes peace and tranquillity returns again. The island is left to mourn her loss as the gaping hole in her side is exposed for all to see. But she will have her revenge for she has spawned a monster that is about to be unleashed upon the world...

The giant landslide that spilled from her western flank has generated a huge tidal wave called a mega-tsunami, and its malevolent force is now directed towards the coastlines of Europe and the US. A few minutes after its creation the tsunami hurls 100 m waves upon the tourist-drenched beaches of the eastern Canary Islands, and within a couple of hours southwest England is inundated by waves up to seven metres high. About nine hours after the landslide was triggered a series of devastating 25 m high waves crash onto the golden beaches of Florida and penetrate several hundred metres inland. Everything is destroyed: homes, cars, boats, animals, people, children. It becomes the worst natural disaster in human history, with a death toll of many thousands on both sides of the Atlantic.

So is such a disaster science fiction or science fact? Well this nightmare scenario was actually outlined in a 2001 paper published in Geophysical Research Letters by scientists Steven Ward from the University of California and Simon Day from University College, London. They suggested that “during a future volcanic eruption, La Palma may experience a catastrophic failure of its west flank” and that “waves generated by the landslide could transit the entire Atlantic Ocean and arrive on the coast of the Americas with 10-25 m height”. Predictably the media seized upon the story with relish, and the findings were subsequently aired on BBC’s Horizon programme and in several daily newspapers. There was even a knock-on effect on La Palma itself, with a noticeable reduction in tourism revenue and a rumoured abandonment of timeshare properties on the western coast.

The scientific basis for the Ward and Day study at first appeared to be sound; there was good evidence indicating recent instability on the west flank of La Palma (left), which could indeed be a precursor to a dramatic landslide. More debatable, however, was their modelling of the landslide itself and the resulting tsunami. Key factors such as the size, speed and nature of the landslide all directly influence the size of any subsequent tsunami. Ward and Day modelled what was very much a ‘worst case’ scenario, with a huge landslide block travelling at high velocity and generating a massive tidal wave.

A recent research cruise around the Canary Islands, led by geologists from Southampton Oceanography Centre, has investigated the problem from a different perspective. They looked at submarine deposits of the two most recent giant landslides to have affected the Canary Islands, and found evidence to suggest that these events may actually have occurred in several stages. They recently presented their findings at an international conference in Nice, and argued that "if a future Canary Islands landslide were also to occur in multiple stages, the resulting tsunamis would be unlikely to cause significant damage far from the source". This is because several small slides will generate several small tsunamis, whereas a single large slide will generate a large tsunami. By simple analogy, if you drop a brick into a bath you get a big splash, but if you break the brick into five piece and drop them in one after the other you get five smaller splashes.

Clearly, more research on this important issue is required, but at least the facts are now being looked at in a serious scientific context. Hopefully geologists can work together and deliver results that will allow governments to better predict and plan for any future landslide-induced tsunami, before it is too late...

This article was written by Dr Russell Wynn and won a Highly Commended honour in the 2003 Daily Telegraph BASF Science Writer Awards.

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February 2007