[meteorite-list] A Last Wave Goodbye

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:50:26 2004
Message-ID: <200204121924.MAA16967_at_zagami.jpl.nasa.gov>

http://www.guardian.co.uk/Archive/Article/0,4273,4391088,00.html

A last wave goodbye

An asteroid landing in the ocean would produce the tsunami to
end them all, says Duncan Steel

Duncan Steel
Guardian (United Kingdom)
April 11, 2002

Throw a stone into a pond and the ripples take just seconds to spread to the
banks. Drop an asteroid into the Atlantic and you get much the same effect, but
on a grander scale. Hours later, the coastlines are deluged.

When the possible consequences of cosmic impacts on our planet are discussed,
people take comfort from the fact that 70% of the Earth's surface is ocean. In
reality, we would not want a small asteroid to land in the open sea: it would cause
more damage and kill more people.

Consider a lump of space rock 200 metres in diameter, colliding with our planet
at 12 miles per second, a typical speed. As it is brought to an abrupt halt, it
releases its kinetic energy in an explosion equivalent to 600 megatons of TNT, 10
times the yield of the most powerful nuclear weapon tested (underground). The
asteroid vaporises, and the rapidly expanding shock wave carries away its
energy. For a land impact, the area devastated would be about 10,000 square
miles: everything within a 60-mile radius would be obliterated. Beyond that, the
destruction would still be substantial.

However, the implications for humanity would likely be worse if the open ocean
were struck instead. An asteroid landing in an ocean produces a phenomenal
tsunami (often mistakenly called a "tidal wave"). Even though only about 10% of
the energy of the impactor would be transferred to the tsunami, such waves are
effective at carrying that energy over the large distances to the coastlines. They
therefore cause destruction over a much wider area than is the case for a land
impact. In the latter, the interaction between the blast wave and the irregularities
of the ground (hills, buildings, trees) limits the area damaged, but on the ocean,
the wave propagates until it runs into something.

Paradoxically, ships at sea are little affected by tsunamis. They simply ride the
waves that move outward from the epicentre at well over 100mph. On the open
ocean, a major tsunami may be only 10 to 50cms high; it is the run-up that
occurs as the wave reaches the continental shelf that causes the wave height to
increase markedly, and they can penetrate miles inland.

During the past century, several significant tsunamis have swept across the
Pacific, provoked by undersea landslips, earthquakes or volcanic eruptions. A
major rumble off the coast of Chile in 1960 induced a series of waves that killed
more than a thousand people on the mainland. The waves reached Hawaii 15
hours later, drowning more than 60 people. As the main wave smacked Hilo
harbour it was over 10 metres high. Seven hours later, it killed hundreds in
Japan. In July 1998, an earthquake-generated tsunami in Papua New Guinea
swept more than 2,000 coastal dwellers to their deaths.

Dr Steven Ward, who works in the Institute of Geophysics and Planetary
Physics at the University of California at Santa Cruz, modelled a 200-metre
asteroid slamming into the Atlantic about 700 miles west of the tip of Cornwall.
The transient crater formed in the three-mile deep ocean is about four miles wide
and a mile-and-a-half deep. It's like throwing a big rock into a vast pond,
although at hypervelocity. One hour later, the outermost edge of the wave has
spread about 300 miles from the epicentre. The greatest wave amplitude is
further back, because the tsunami continues to be boosted by the water
oscillating up and down at the epicentre. To see what happens, over a much
shorter time scale, drop a sugar cube into a cup of coffee.

After two hours, the tsunami is just reaching the south-western tip of Ireland.
The distortion of the wave from circularity is clear, due to the crossing of the
continental shelf, where the water is much shallower. This slows the wave, but
considerations of conservation of momentum say that it must get taller. This is
called "shallow water shoaling". Three hours post-impact, the west coast of
Ireland is inundated: goodbye Galway and Donegal, although Limerick and Cork
may survive, shielded by their natural harbours. In this scenario, Ireland provides
a singular service to the rest of the British Isles, bearing the brunt of the
tsunami. Five to six hours after the strike, Cornwall, Devon and south Wales are
hit, as are the Western Isles of Scotland, but the ports lining the Irish Sea
escape the worst.

Further south, the news is not so good. The shallow waters of the English
Channel cause the wave to elevate, and seaside resorts to Brighton and beyond
would be swept away. One must hope that eight hours - the time the tsunami
would take to reach the Isle of Wight - are enough to organise an evacuation.
Shortly after, the wave would penetrate between Dover and Calais, and the
dykes of the Netherlands would provide little resistance. With no offshore shield,
the coasts of France, Spain and Portugal are pummelled by the tsunami.

The shadowing effect provided by islands is most obvious in the case of the
Azores. When a tsunami hits a coast, it ramps up in height by a factor that
depends upon the profile of that coastline and, in particular, the gradient of the
sea bottom. That factor may be only two or three, but it may be more than 10.
The latter would mean that waves 10 metres in amplitude on the open sea would
attain a height of 100 metres as they hit land. We are not talking of a simple
flood, then. We are talking about a coastline swept clean.

How often does such an event occur? Asteroids around 200 metres in size strike
the Earth about once every couple of thousand years. The Atlantic has about a
one-in-10 chance of being the next target.

A parting thought. Last time I checked my house insurance, I was covered for
tsunami damage. Trouble is, tsunamis were defined in the small print as being
large sea-waves produced by sub-oceanic volcanoes, landslips or earthquakes.
Asteroid impacts don't count, it seems.

· Duncan Steel conducts research on asteroids at the University of Salford.
                     
Received on Fri 12 Apr 2002 03:24:28 PM PDT