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 a
ordinary tsunami but produced by massive landslides or volcanic
collapse.
Ordinarily, tsunamis 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 create 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
Sumatra the seabed rose 6 metres over a 1,000 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.
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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 more shallow 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
causing 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.
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Large tsunamis have happened in the past and often with a 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
lasted 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 pale in comparison to a killer that even now lies waiting, the Mega-Tsunami.
These tsunamis are caused by volcanic collapse, or more commonly, by
massive landslides crashing into the sea with immense force.
Unlike ordinary tsunamis they are not limited in size. They can be 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-Tsunamis 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.
In my next
article we shall look at the events that will eventually initiate the
Mega-Tsunami and threaten the East Coast of America and its 40 million
residents.
Tags:
tsunami disasters earth 
