Download How the shape of ocean floors can affect speed and height of tsunami

How the shape of ocean floors can affect speed and height of tsunami
By Nigel Hawkes
Times Online , December 27, 2004
Tsunami: Science
THE long arch of Sumatra and Java, sweeping like a crescent across the
north of the Indian Ocean, is one of the most active geological regions in the
world.
Close by, in the Sundra Strait between Sumatra and Java, is Krakatoa, the
scene in 1883 of one of the world’s most catastrophic volcanic eruptions,
when an entire island virtually disappeared.
Yesterday the region was the setting for an earthquake and tsunami more
frightening than anything dreamed up for a Hollywood disaster movie.
The cause lies deep beneath the ocean, where two of the great tectonic
plates that carry the continents around the Earth collide. The Indian-Australian
plate slides under the Philippine plate in a “subduction zone” 750 miles long
and more than 300 miles wide.
The movement is jerky, as the submerging plate tends to drag the upper plate
down with it. But as pressure builds, the upper plate breaks free, springing
back to its original position. Yesterday’s movement was just 16½ yards at a
depth six miles below the seabed, but it was more than enough.
The seabed lifted, displacing the ocean. The water had to find somewhere to
go, so began to flow outwards in a huge wave, or tsunami. Dr Roger Musson,
a seismologist with the British Geological Survey in Edinburgh, said that
similar events had been taking place in the region for several million years
and would happen again over the next few million years.
“The effect of the earthquake is like throwing a stone in a pond, except that
you are throwing it from below. You get the equivalent of a splash and water
is displaced with waves spreading outwards,” he said.
Over the ocean, the waves of a tsunami are small, probably no more than a
few centimetres to a metre high. Fisherman 20 miles out at sea barely notice
their passage.
Their speed depends on the depth of the water, but is typically several
hundred miles an hour. The deeper the water, the faster the waves travel and
at the bottom of the deepest ocean they can keep pace with a jet aircraft.
As yesterday’s tsunami approached the coasts of Sri Lanka, Indonesia, India,
Thailand, Malaysia and the Maldives, it slowed. The more it became
compressed, the more it grew in height. As it reached the shore it grew into a
monster.
“The scale of the waves will vary from place to place, depending on the
topography of the coast,” Dr Musson said.
“If you have a narrow inlet, the water stacks up and you get higher waves.
These are not normally more than one or two metres high but we are getting
reports of yesterday’s being up to ten metres.
“What does the damage is the speed and force of the water. It can be moving
at 300mph across the open ocean, and then slower as the water gets
shallower.
“By the time it hits the coast it will be doing tens of miles an hour. The water
will sweep in and collect everything in its way. It will drag it across the land
and then recede.
“The effect can be incredibly devastating. The land will be scoured of
everything that was previously there. They can wipe out whole villages,
leaving just gravel in their wake.”
The first signs of the earthquake were picked up at the British Geological
Survey’s Edinburgh headquarters just after 1am yesterday. It was clear that it
was a very big earthquake, measuring a magnitude of 8.9, the fifth largest
since 1900 and the largest since 1964.
Yesterday’s witness descriptions vary widely varied, depending on where they
came from. Tsunami is the Japanese word for “harbour wave”, reflecting the
fact that in a natural harbour or inlet the effects can be greatly magnified.
Small, steeply sloped islands suffer the least damage. Coral islands are often
protected by their barrier reefs. But low- lying islands suffer a far greater “runup” of the water, and much greater damage.
In the Pacific a tsunami warning system has been established. Seismic
stations, the US Geological Survey, and other international sources measure
earthquakes as they occur and, if they are in places where a tsunami might
result, issue warnings.
These include the predicted arrival times at places the tsunami is capable of
reaching. Local authorities are then responsible for implementing evacuation
plans, while radio and TV are used to warn the public to move to higher
ground.
International scientists have been calling for a tsunami warning system to be
set up by governments around the Indian Ocean, Dr Musson said.
“They have one in the Pacific. But governments have not invested in one for
the Indian Ocean, partly because events like this are far less common. You
need political will and governments (that are) prepared to stump up the cash.”
Dr Musson cautioned that the region was not yet out of danger. He said:
“There could be more tidal waves coming from earthquake aftershocks. The
aftershocks are still happening. There could be a 7 or a 7.5 still to come.”