Mega - Tsunami

By Tony Lucas

The East Coast of America faces a threat more devastating than the hurricanes which yearly ravage this area, more devastating than the tornadoes that annually ravage the Midwest.
A threat generated thousands of miles away but liable to impact on the 40 million people who inhabit and call the East Coast of America their home.

Over 70% of the earth is ocean, therefore it comes as no surprise that one of the greatest potentials for loss of life comes from an ocean generated threat, a Mega-Tsunami.

What is a Mega-Tsunami?
Basically it is giant waves, many metres high, generated not as ordinary tsunami but produced by massive landslides or volcanic collapse.

Ordinarily, tsunami are caused by undersea slides and seismic activity, in fact sea- quakes are responsible for 90% of all tsunami generated, as was the case with the magnitude 9+ quake which triggered the December 26 2004 tsunami.
To generate a seismic tsunami certain factors have to be right for the tsunami to form, the quake causing the tsunami has to be 6.5 on the Richter scale or greater, it has to be shallow, less than 30 kilometres deep beneath the earth's surface, and finally in order to form a tsunami, there has to be a vertical displacement (uplift) of the seabed to displace the water above the event.
Water depth also plays a part, as it has been found that tsunami generated in shallow water are not as powerful as those generated out at sea.
If all the above criteria are met a tsunami will form, if not there will be no tsunami formation. They are therefore moderately rare events.
The key factor in the formation is that large amounts of energy need to be conveyed into the water above to produce the energy needed to produce the wave.

Basic seismic tsunamis are limited in size. When a seismic event occurs on the ocean floor part of the fracture rises up. Even in the biggest of quakes this uplift can only be a maximum of at most 10 metres, in the Sumatra case the rise in seabed was 6 metres over a 1000 km stretch of the sea bottom. Once this uplift happens the water above is displaced by the rise in the seabed, this provides the energy for the production of the wave. However, the crest of the tsunami is proportional to the amount of water that was displaced. So a seaquake lifts a section of the seabed 10 metres the resulting tsunami will have a wave height proportional to this, in this case 10 metres.

This displacement is barely noticeable in the waters of the open sea, perhaps as just a slight ripple passing under a boat.
However, once the tsunami approaches shallower water and the inclined topography of the shore of a landmass the front waves slow, the rear waves continue on at the present speed building against the now slower front waves, and causes the energy of the wave to build and form a crest.
Tsunamis have a wavelength of 300 to 400 kilometres, so once they hit land they will keep surging for half an hour or more.

The energy build up in the tsunami is gradually dissipated as its edges across a landmass causing massive devastation by the kinetic energy stored within the power of the wave. Once inland, the water must then drain away again, carrying with it debris and bodies far out to sea. Bodies from such disasters are often washed ashore vast distances from where the devastation took place.

Large tsunamis have happened in the past and often with massive loss of life.
Papua New Guinea suffered a massive tsunami in 1998 and during the eruption of Karakatoa in 1883, when the volcano produce a caldera, the sea water rushed in to fill the void left by the explosion of the crater, this generated a phenomenon called a tsunami train, essentially one tsunami sized wave followed by another, the continued impacts lasting several hours, and most casualties of this eruption were not from the eruption itself but from the tsunamis the event generated. The waves from this explosive incident travelled halfway around the world and were responsible for death and destruction 10,000 kilometres from their point of origin.

However destructive these events have been a pale in comparison to a killer that even now lies waiting, the Mega-Tsunami.

These tsunami are caused by volcanic collapse, or more commonly, by massive landslides crashing into the sea with immense force.
Unlike ordinary tsunami they are not limited in size to a 10 meter maximum size. They can be in size; generally as big as the displacement of the material that create them.

They have happened before and will happen again.
The best examples of these Mega-Tsunami generating landslides can be found in the area of the Hawaiian Islands, where mapping of the sea floor shows at least 70 massive landslides, some having blocks over one kilometre long among the debris. The Mega-Tsunamis these created would have caused massive devastation over a very wide area.

The Cape Verde Islands also show signs of once having had a massive landslide and generating a Mega-Tsunami that slammed into Africa’s West Coast just an hour after its conception about 80,000 years ago.

Events such as these occur worldwide on average at between 25,000 to 100,000 years apart.

The next likely candidate for the generation of a Mega-Tsunami of the landslide kind in current times is an island in the Canary Islands group.

La Palma formed roughly 125,000 years ago; it rises 2 kilometres above sea level and plunges a further four kilometres below the sea to the seabed. At its widest point La Palma is 15 kilometres wide. It is an island with a steeply sloped topography, in some places having a slope angle of up to 30°.
Around 90,000 to 130,000 years ago a large piece of the northern part of the island disappeared into the ocean, this left a great semicircular crater 10 kilometres in diameter.

About 50 years ago part of Cumbre Vieja; the now active volcano on La Palma erupted. This caused a large fissure to open near the summit of the island; along a 3 kilometre stretch of this fissure the western side of the island dropped four metres towards the sea. The western flank of La Palma is now a 200 cubic kilometre piece of rock poised ready to plunge seaward and form a massive tsunami.

What would initiate such a cataclysm and what would be the global effects it would cause.

There are two scenarios that would initiate such an event; one is quite obvious the other is not so obvious.

Cumbre Vieja’s last eruption created the fissure and moved the initial land mass, another eruption could well see this section of the island plummet into the Atlantic waters.
The other lesser-known possibility is the result of water. If a period of prolonged rain occurred, due to an exceptionally wet season perhaps, the water would act as a lubricant barrier between the detached face of the island and the underlying solid foundation, the piece which is currently holding the dislodged section of the island in place. This film of water would allow the dislodged piece to slide over its foundation and onward to the waters of the Atlantic.

Whichever mechanism triggers its release this massive chunk of rock and debris plunges into the Atlantic, creating a 500 foot tidal wave which races across the Atlantic at speeds equivalent to a jet airliner.
After 4000 miles of travel the wave forms a lower but wider configuration.

Two to three hours after the initial collapse a wall of water 300 feet high slams into the West Coast of Africa. Towns and villages along with their inhabitants, along the coast, are literally wiped off the map.
An hour or so later a 30 foot high wave of destruction bears down on the south England Coast leaving coastal towns devastated and thousands homeless.

Five hours after the destructive events have occurred in southern England, the advancing wall of water approaches the Caribbean Islands. On account of the low topography of these Islands many are wiped clean of structures and inhabitants as a wall of water simply sweeps over them.

Finally, the East Coast of the United States is reached, the populace looking out to sea notice the water quickly recede, then on the horizon, a white foaming mass approaches, already it is too late for those witnessing this phenomena.
The wave slams into the coast and in a matter of minutes cities such as New York, Miami, Washington DC and Boston cease to exist. The death toll reaches tens of millions.
The wave sweeps up estuaries and rivers as far as 20 miles in land, destroying everything in its path.
The water recedes over many days carrying debris and bodies out to sea.
This is not the end of the threat however, bodies, human and animal, decompose resulting in outbreaks of diseases such as cholera.

For months after the initial landslide, far away in La Palma, the damage to property, person and the ecology is assessed.
Vast areas of the United States are now uninhabitable and will remain so for many years to come.
The financial strain put on the affected nations impacts on the world economy.

These events could well happen tomorrow, or perhaps not for the next 50 years, but eventually they will happen.
The governments of the at risk countries know it can happen, they know the inevitability and the risks, but as in the case of the United States how do you evacuate 40 million people in just nine hours without mass panic?