13 December 2013

Killing off the Canary Islands landslide megatsunami scare

Posted by dr-dave

ResearchBlogging.org
For those of us working on and researching natural hazards, one topic repeatedly comes up at social events.  The conversation usually goes like this:

New Acquaintance: “So what do you do?”

Me: “I research natural hazards, and in particular landslides”

NA: “Oh – is it true that one of the Canary Islands volcanoes is going to collapse in a giant landslide?  I was told that it’ll cause a tsunami that’ll devastate the coasts of Europe and America”

Me: “Excuse me – I’m just going to bang my head on the table…” [thump!]

The question of course refers to a scare a few years ago, based on a peer-reviewed paper, that a flank collapse of Cumbre Vieja volcano on the island of La Palma in the Canary Islands could collapse, generating a megatsunami that would initially be about 1,000 metres high, and would still be about 50 metres high when it reached for example the eastern seaboard of the USA (Wikipedia has quite a nice article on megatsunamis if you are interested).  It is fair to say that in my experience there is agreement that such a flank collapse could occur (and there is lots of evidence that such events have happened in the past), but the level of support for the megatsunami hypothesis is, shall we say, somewhat lacking within the scientific community.  There is a nice evaluation of the work by Dr George Pararas-Carayannis, which concludes that:

“The threat of mega tsunami generation from collapses of oceanic island stratovolcanoes has been greatly overstated. No mega tsunamis can be expected”

A key issue here is the mechanics of the landslide.  To generate a very large tsunami, this slide would have to happen very fast and as an essentially coherent block.  Remember that this is a landslide of 500 cubic kilometres – we do not think that very, very large landslides usually behave like this. The chances are that a collapse would occur in stages over a longer time period, which would generate a much smaller wave.  Most scientists recognise that the single, intact block collapsing very fast idea is theoretically possible, but that it is the extreme end-member of a wide range of scenarios, and thus is highly unlikely.  There are other issues too (like where are the tsunami deposits from other megatsunamis given that we know that previous collapses have occurred? A tsunami on this scale should leave deposits that would be very easy to map).  Unfortunately, although most landslide scientists view the likelihood of a single coherent landslide as being very low,the actual evidence to support that view in the case of these types of landslides has not been strong..

Unfortunately, despite these rebuttals the story has had remarkable traction.  Take this page on Virgin Media for example, which has this wonderful (?) image:

http://www.virginmedia.com/science-nature/natural-world/natural-disasters-waiting-to-happen.php

http://www.virginmedia.com/science-nature/natural-world/natural-disasters-waiting-to-happen.php

And text that says:

“A colossal wave caused by a chunk of rock the size of the Isle of Man breaking off La Palma in the Canary Islands, will one day devastate Southern Europe and the entire East coast of America. Scientists believe this disaster is already way overdue. When it does come, it will cause a mega tsunami that will hit with the power of an atomic bomb travelling at the speed of a jumbo jet.  Starting off at 2000ft, the giant wave will still be 150ft high by the time it reaches the other side of the Atlantic, destroying everything in its wake and affecting up to 100 million people.”

Given the ridiculousness of this sort of statement (and there are worse examples out there), it is good to see a new paper that erodes the case for the megatsunami still further.  This paper, Hunt et al. (2013) has just been published in the journal Geochemistry, Geophysics, Geosystems (sadly the article is behind a paywall).  The paper presents a very detailed analysis of the deposits left on the sea floor by Canary Island flank collapses.  The research is meticulous and comprehensive.  The authors note that the sea floor deposits record eight volcanic flank collapse events, the largest of which was about 350 cubic kilometres.  However, the key element is that each deposit is formed from a series of subunits, each of which can be clearly differentiated from other subunits based on the geochemistry of the materials that they contain.  So, the interpretation by the authors, which sounds very sensible to me, is that each subunit represents a different phase of the collapse event.  In other words, each of these major collapses did not occur as a single, coherent block, but as a series of sections one after the other.  If you want an analogy, then what better example than the famous 1993 Pantai Remis landslide in Malaysia:

The implications are clear.  Previous flank collapses have occurred as a series of distinct events rather than as a su=ingle coherent block.  Each of these could have been able to generate a very large wave, and even a local tsunami.  However, they would not have generated a megatsunami.  There is no reason to believe that a future event will behave differently, so this scare should be consigned to the garbage can once and for all.

Reference

J.E. Hunt, R.B. Wynn, P.J. Talling, & D.G. Masson (2013). Multistage collapse of eight western Canary Island landslides in the last 1.5 Ma: Sedimentological and geochemical evidence from subunits in submarine flow deposits Geochemistry, Geophysics, Geosystems, 14 (7), 2159-2181 DOI: 10.1002/ggge.20138