(From 'Charnia' Newsletter, Winter 2005)

There’s a picture, a C19th engraving (possibly by Gustave Dore – I can’t remember) of a sailing ship ploughing through a sea choked with human corpses. Until the December 26th tsunami I though such visions highly improbable.  Not that the Sunda Arc is a stranger to cataclysmic seisms. The engraving referred to was, I think, made after the 1883 Krakatoa event. Following that eruption the surrounding seas were covered with a thick carpet of pumice. Months later, some of these floating islands, complete with human and animal skeletons clinging to their upper surfaces, washed up on East African shores.

Tsunamis are the result of huge displacements of water and there is plenty of evidence supporting the case that such phenomena, whether the result of shifting plates or littoral/underwater landslides or impacts by large objects from space, have occurred on a regular basis over a long period of the Earth’s history. Four years ago BBC2’s ‘Horizon’ screened an edition called ‘Wave of Destruction’. This documentary focused not only on tsunamis but ‘megatsunamis’ and in particular the potential for a sizeable chunk of La Palma to slip into the Atlantic and devastate the eastern seaboard of the United States. On the western side of this volcanic island, a vent known as the Cumbre Vieja, did in fact experience a flank slip of four metres in 1949, though no one knew at the time of the potential for large-scale disaster. Come the day when this event takes place, South America, the Iberian Peninsula, France, the Netherlands and the south and east of England will be affected too. In fact, the ‘Horizon’ programme was rather simplistic and I would refer you to a four-page paper on this particular case at http://www.benfieldhrc.org/SiteRoot/tsunamis/WardandDay.pdf (site no longer available as of Mar 2011).

Much has been made of the fact that the US and other countries have the Pacific covered by a pressure sensor network linked to satellite with downlinks to communications infrastructures. That there is a case for such protection for countries around the Indian Ocean is now beyond doubt, though the risk of frequent tsunamis is somewhat less than the Pacific. Does the Atlantic Ocean likewise merit the same warning technology? Any tsunami originating in the western Canary Islands will travel at around 700 km per hour, giving New York around eight hours to prepare, if indeed any preparations at all can be made. I recall from my schooldays a geography teacher telling us about a huge mudslide on the Newfoundland Grand Banks/Laurentian slope breaking the Transatlantic communication cable. Looking into this it turns out that there was a Magnitude 7.2 earthquake on November 18th, 1929, 250km south of Newfoundland and the tremors were felt in Ottawa and New York. The resulting rotation and slump of around 200 cubic kilometres of sediment off the continental shelf actually broke twelve communications cables in multiple places. More significantly, this mass movement generated a tsunami that was detected as far south as Carolina and across the Atlantic in Portugal. The tsunami’s greatest effect was in the Burin Peninsula in south Newfoundland. The tsunami arrived here approximately 150 minutes after the earthquake, claiming 29 lives; the sea was raised 27 metres along that coastline. Many buildings were pulverised by the weight of water smashing into them, though several simply floated off their foundations. A grocery store was moved 60 metres inland in this manner, being set down in a meadow with its stock intact. Perhaps the most famous Atlantic tsunami was the inundation following the November 1st 1755 earthquake in Lisbon (contemporary drawing below). This particular tsunami was caused by the slumping of marine sediments off the Portuguese coast, generating three large waves.

Around 80% of tsunamis occur in the Pacific, 10% in the Indian Ocean and 5% in the Mediterranean. Mediterranean tsunamis tend to be weaker though this is not to say the potential for widespread devastation does not exist. Greece experiences more than 50% of Europe’s seismicity and the last big tsunami recorded there was in July 1956. This was the result of a Richter 7.7 earthquake that threw up a 25 metre high wall of water on the island of Amorgos in the Cyclades, killing four. With the huge build up of tourism such an event would result in a great number of fatalities today. With this in mind, Greek and Japanese seismologists have been collaborating for the past decade to produce maps of high-risk areas. These are the Aegean, the Tunisian coast, the Algerian coast, Italy and Sicily (a tsunami in the Strait of Messina killed more than a thousand people in 1908) and the Turkish coast. There is a fear that Rhodes could one day be Greece’s Phuket. As recently as May 2003 a small tsunami reached the Balearic Islands and the French Riviera, resulting from an earthquake at Boumerdes in Algeria.

Returning to tsunami warning systems, the Pacific system did know what was about to threaten countries around the Indian Ocean, though there was no infrastructure in place to effectively issue timely warnings. Having said that, a warning was received by the US military base on Diego Garcia in the middle of the Indian Ocean two and three quarter hours after the earthquake. The island itself is British territory and is linked to the US Pacific Command, though in spite of this our MoD has insisted that no advance warning was received. For a detailed analysis of the Sumatran earthquake and tsunami see   http://iri.columbia.edu/~lareef/tsunami/ from which the data table below, showing the main shock and aftershocks, is taken. 













 6.3 2004/12/26 11:05:01 13.542 92.877   10.0 ANDAMAN ISLANDS, INDIA REGION
 6.2 2004/12/26 10:19:30 13.455 92.791   10.0 ANDAMAN ISLANDS, INDIA REGION
 6.5 2004/12/26 09:20:01 8.867 92.382   10.0 NICOBAR ISLANDS, INDIA REGION
 5.8 2004/12/26 07:38:25 13.119 93.051   10.0 ANDAMAN ISLANDS, INDIA REGION
 5.7 2004/12/26 07:07:10 10.336 93.756   10.0 ANDAMAN ISLANDS, INDIA REGION
 5.7 2004/12/26 06:21:58 10.623 92.323   10.0 ANDAMAN ISLANDS, INDIA REGION
 7.3 2004/12/26 04:21:26 6.901 92.952   10.0 NICOBAR ISLANDS, INDIA REGION
 6.1 2004/12/26 03:08:42 13.808 92.974   10.0 ANDAMAN ISLANDS, INDIA REGION
 5.9 2004/12/26 02:59:12 3.177 94.259   10.0 W. COAST OF NORTHERN SUMATRA
 6.0 2004/12/26 02:51:59 12.511 92.592   10.0 ANDAMAN ISLANDS, INDIA REGION
 5.8 2004/12/26 02:36:06 12.139 93.011   10.0 ANDAMAN ISLANDS, INDIA REGION
 5.8 2004/12/26 02:34:50 4.104 94.184   10.0 W. COAST OF NORTHERN SUMATRA
 6.0 2004/12/26 02:22:02 8.838 92.532   10.0 NICOBAR ISLANDS, INDIA REGION
 5.8 2004/12/26 02:15:58 12.375 92.509   10.0 ANDAMAN ISLANDS, INDIA REGION
 5.9 2004/12/26 01:48:47 5.393 94.423   10.0 NORTHERN SUMATRA, INDONESIA
 8.9 2004/12/26 00:58:51 3.298 95.779   10.0 W. COAST OF NORTHERN SUMATRA

The tide gauge graph below shows the effect of the tsunami on sea level before and after the arrival of the main run-up:

This data was obtained from the website of the Indian National Institute of Oceanography at www.nio.org/jsp/tsunami.jsp. The Columbia University website, cited above, also has the animated tsunami sequence that gives a better understanding of the chatter and noise superimposed on the normal diurnal tidal rhythm, as a result of interference from multiple tsunami waves and their refractions and reflections.

The area of subduction that gave rise to the Indian Ocean tsunami is complex; the Indian plate descends as the Burma plate overrides and the line of fusion is marked by the Sunda Trench. For a detailed map, refer to the Columbia University website. The Sumatran Arc is itself the result of plumes of molten material rising from the friction-fed melt zone as kinetic energy is converted to heat at depth in the mantle. Will the acceleration in movement of these plates generate new volcanic activity in northern Sumatra and in the ocean to the east of the Andaman and Nicobar Islands, where most of the aftershocks occurred?

Which brings us, I suppose, to the rather perilous state of affairs in North America. Mount St. Helens appears to be a warning shot across the bows in that a considerable area of the Rockies is perhaps building up to cataclysmic vulcanicity dwarfing anything witnessed in history. The whole chain from Mt. Lassen in the south, through Mts. Shasta, Jefferson, Hood, Adams, St. Helens, Rainier and Baker in the north, up into the Canadian Rockies seem to be far from being anything other than a considerable threat. The Yellowstone caldera ejected 247 cubic miles of material when it ‘blew’ 600,000 years ago and the caldera chain could well perform several encores as the North American plate passes over this hot spot. Section C hosted a talk recently where the existence of hotspots was questioned.  The evidence provided by the Yellowstone plume and its chain of precedent volcanic fields seems incontrovertible to me: furthest away in the south west and oldest is the McDermitt VF at 16.1 million years, followed in a straight line by the Owyhee-Homboldt VF at 13.8my, the Bruneau-Jarbridge VF at 12.5my, the Picabo VF at 10.2, the Heise at 6.5, plus two more at 4.3 and 1.2my, finishing with the Yellowstone at 0.6my. In fact, the whole situation for the American North-West looks rather precarious as the Juan de Fuca and Gorda Plates slip under the North American Plate, rather than sliding past it as the Pacific Plate does to the south, or the Queen Charlotte transform fault to the north.

Before venturing northwards out of Washington state and over the border into Canada it is well worth having a look as the US Geological Survey’s website at  http://vulcan.wr.usgs.gov/Volcanoes/Baker/framework.html There are plenty of informative links here to keep you browsing for quite some time. Examining the situation up from Vancouver to Queen Charlotte’s Island there are a number of active and dormant volcanoes associated less with the subduction arc that characterizes the Cascades and more with hot spots and rifting; in fact a mirror image of the situation to the south of the Cascades.  It’s only until you reach the Yukon-Alaska border area that vulcanicity associated with subduction is once again characterized. Here, the largest volcano is the dormant Mount Churchill which last erupted around the time when King Ethelbald of the Mercians was killed in a mutiny which led to Offa succeeding the throne. It may seem a long time ago since squabbling Anglo-Saxons were the news of the day but geologically this is nothing.

There is method in the madness of taking a roundabout tour to Alaska. The Horizon ‘megastunami’ documentary referred to earlier began with a reference to two events which took place approximately fifty years ago in Glacier Bay National Park, southern Alaska. The actual location was Lituya Bay, a narrow body of water fed by three glaciers. In 1958 an earthquake dislodged a huge amount of material at the head of the inlet, which in turn created a tsunami that rode over a headland to a height of 1,700 feet above sea level.  The details of this and a preceding tsunami in the bay which took place five years earlier can be found at the ‘Horizon’ website where there is a transcript of the Wave of Destruction’ programme:  http://www.bbc.co.uk/science/horizon/2000/mega_tsunami_transcript.shtml

Locomotive swept inland by the 1964 Alaskan tsunami

The Lituya Bay tsunami was a local affair in comparison with the 1964 Alaskan tsunami. I remember this well because I was studying A-level geography and geology at the time and the descriptions and pictures of the event have stuck in my mind. In particular was the rather grainy film footage shot by a Russian trawlerman; this showed the sea receding prior to the tsunami run up. This was the very feature preceding the destructive waves that hit Thailand and adjacent countries that alerted ten-year old Tilly Smith of Oxshott in Surrey; she had recently studied earthquakes and their effects at school and was able to warn over a hundred people to get off the beach area and escape to the safety of high ground. Modest Tilly said that the credit should go to her teacher, Andrew Kearney, of Danes Hill Preparatory School!

Finally, there are innumerable websites from which more information can be gleaned, though the one I’d recommend starting with is the website of a very clever chap, Dr. George Pararas-Carayannis:   http://www.smallshop.com/science/tsunami.htm (site no longer available as of Mar 2011). There’s another very important website too and you’ll find that at  http://www.dec.org.uk/  if you haven’t done so already.

Graham Stocks