20 September 2016

A science story that just won’t die: the Canary Island Megatsunami scare rears its head once more

Posted by Dave Petley

The Canary Islands Megatsunami scare story

With frightening and depressing regularity the Canary Islands Megatsunami scare story rears its ugly head, to breathless headlines in the popular media.  In case you need reminding, this is a scenario that involves a giant landslide from the flanks of the Cumbre Vieja volcano in the Canary Islands, which then generates a vast tsunami that sweeps across the Atlantic Ocean, causing untold devastation across a large area.  It has been repeatedly debunked.  This story reappeared this weekend in the Daily Star, which might not be considered to be a competitor to the Wall Street Journal, if you know what I mean.  Inevitably the story is sensational:

Canary Islands Megatsunami

The Daily Star Canary Islands Megastunami headline

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Sigh!  And of course it is accompanied with a map of the extent of the tsunami, with annotation to show just how big the impacts would be:

Canary Islands Megatsunami

The impact of the supposed Canary Islands Megatsunami via the Daily Star

 

It is hard to know where to start with this as the story has been debunked so many times.  So let me try to outline some of the key issues with the Canary Islands Megatsunami scare story:

  1. The generation of a tsunami on this scale requires a very specific landslide scenario.  This is that the flank collapse occurs in a single enormous (we are talking 350 cubic kilometre) event and is extremely rapid.  Hypothetically this is possible, but it is not likely.  This is like saying that I’m worried that my house will be destroyed by a meteorite.  It could of course happen, but it is so vanishingly unlikely that it is not worth considering.  It is simply wrong of the newspaper to present an outlier scenario as one that seems likely.
  2. Studies of submarine volcanic flank collapse deposits suggest that they do not happen as a single coherent block, but instead as a series of slides.  The resultant tsunami would be much less significant;
  3. The concept of “within three years” is a nonsense.  Of course it principle could happen within three years – it could indeed happen tomorrow.  Why three years? Not five? or 50? Or 3000?
  4. Most importantly, we know that this volcano, and many others, have undergone massive flank collapse in the past on multiple occasions.  We even know that some have generated large local tsunamis.  But large tsunamis leave a very distinct sediment footprint on the coast – tsunami deposits are well characterised and mapped, and no self-respecting geologist would miss a large one.  And the megatsunami deposit from an event like that shown in the map above would be locally huge and would be present over a vast area.  There is no way that such a deposit would be missed.  But no such pervasive, recent, continent-scale tsunami deposit has ever been found.  This indicates of course that none of the previous flank collapse events has generated a megatsunami on this scale, or even close to it.  And I mean none of them – not from the Canary Islands, not from Hawaii, not from anywhere else.  So, in some way, the potential flank collapse at Cumbre Vieja has to be fundamentally different from all that have gone before.
  5. And finally, even if Cumbre Vieja did collapse as hypothesized, the tsunami model described above would have to be right.  Personally I find it hard to believe that a tsunami wave that is in effect generated by a point source and that has crossed the mid ocean ridge (and thus encountered comparatively shallow water) could generate a wave over 50 m high along a length of coastline thousands of kilometres long.

So does this mean that Cumbre Vieja can generate a large tsunami?  Yes, of course.  But is it even remotely likely beyond the local area? Absolutely not, in my view.

And of course aspects of this are  silly from a risk perspective:

“Although Florida and the Caribbean would suffer the greatest destruction, with waves up to 164 ft smashing their coastlines, a weaker, but still massively destructive wave, is likely to batter Britain.  It is thought thousands of people living in Britain’s southernmost coastal cities and towns, including Cornwall, Devon and Portsmouth, could be wiped out within hours, assuming no effective evacuation.”

And:

“I would argue that they, their political overseers, and we the general public, should worry about the potential impacts of collapse generated tsunamis on a range of coastal nuclear facilities, ranging from power stations through nuclear reprocessing plants to dockyards full of decommissioned nuclear submarines.”

Frankly if a wave 50 metres high has just struck the entirety of the Eastern Seaboard of the USA and Canada, the Caribbean and the eastern coasts of Latin America; and a wave 100 m high has struck the western coast of Africa; and a wave has just wiped out western coasts of France, Portugal, Ireland, and the UK, all within six hours, then a wave 6 m high hitting Hinckley Point is going to be a pretty small part of the overall equation.

It really is time that this event was presented for what it is, which is an absolutely extreme scenario based on a very highly unlikely combination of events that is without precedent.  There are lots of credible hazard scenarios that deserve our attention; this one does not.


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