manoj
2nd January 2005, 03:04 AM
Credits: The Hindu
Source: http://www.hindu.com/seta/2004/12/30/stories/2004123000111600.htm
Physics of the tsunami
The speed of the tsunami is governed by the water depth. Speed reduces and wave height increases as it approaches the shore.
BREEZE FLOWING across a lake or ocean can create wrinkles on the water surface and produce short waves restricted to shallow layer. Tides — high and low tides — that sweep the globe every day also produce waves.
But tsunamis that create killer waves are different for many reasons — they are not produced by gravitational pull of the moon but due to abrupt shifting of the sea floor and resultant vertical displacement of the overlying water. A tsunami is a series of ocean waves generated by such disturbances.
Bid to regain equilibrium
Waves are formed as the displaced water mass attempts to regain its equilibrium. And the size of the resultant tsunami waves is determined by the quantum of the deformation of the sea floor. More the vertical displacement, greater will be the size of the waves. As a rule, all earthquakes do not produce tsunamis.
To generate tsunamis, earthquakes must occur underneath or near the ocean, be large and create movements in the sea floor. The earthquake's magnitude, depth, fault characteristics and coincident slumping of sediments or secondary faulting also determine the size of the tsunamis.
They can be more aptly described as a series of waves of extremely long wavelength and long period generated in a body of water by an impulsive disturbance that displaces the water. Wind-generated waves usually have period (time between two successive waves) of five to twenty seconds and a wavelength (distance between two successive waves) of about 100 to 200 metres (300 to 600 ft).
Tsunamis can have a period in the range of ten minutes to two hours and a wavelength in excess of 500 km. It is because of their long wavelengths that tsunamis behave as shallow-water waves.
Inversely related
A wave is characterised as a shallow-water wave when the ratio between the water depth and its wavelength gets very small. And the rate at which a wave loses its energy is inversely related to its wavelength. Since tsunamis have a very large wavelength, in excess of 500 km, it will lose little energy as it propagates.
Hence in very deep water, a tsunami will travel at high speeds and travel great distances with limited energy loss. For example, when the ocean is more than 5000 metres deep, unnoticed tsunami travel about 890 km per hour, the speed of a jet airplane.
A tsunami at 1000 metres of water depth would travel at 356 km per hour speed. At 5039 metre water depth, it would travel at 800 km per hour and at 6000 m of water depth it would travel at 873 km per hour. So a tsunami travels at different speeds in the ocean; slow in shallow water and fast in deep water.
Though there are many places in the ocean that are deeper than 5000 metres, an average depth of 5000 metres is reckoned and hence an average speed of about 750 km per hour. Thus it can move from one side of the Pacific Ocean to the other side in less than a day.
But what in the first place provides the force needed to allow a tsunami to travel a long distance? Tsunamis are what are called long gravity waves. There are two interacting processes that allow these waves to propagate. The first is the slope of the sea surface, which creates a horizontal pressure force.
The second is the piling up (or lowering of sea surface) as water moves with different speeds in the direction that the wave form is moving. When these two processes have the right relationship in time, they create propagating waves.
As the tsunami crosses the deep ocean, its wavelength — distance from crest to crest — may be hundred kilometres or more and its amplitude — height from crest to trough — will be in the order of a few feet or less. They cannot be felt aboard ships nor can they be seen from the air in the open ocean.
Advance warning
Despite the great speed, they travel much slower than the seismic waves. Hence earthquake information is often available hours before the tsunamis are able to travel across the ocean. Tremors were felt around 6.30 am in Chennai; it took more than two and half hours to reach the shores.
As a tsunami leaves the deep water of the open sea and propagates into the more shallow waters near the coast, it undergoes a transformation. Its speed is affected. The speed of the tsunami is directly related to the water depth; it slows down as it enters the shallower continental shelf water.
The friction of the continental shelf floor slows down the front of the wave. As a result of this slowing down, the trailing waves pile onto the waves in front of them, like a rug crumpled against a wall creating a wave.
This results in decreasing the distance between individual waves in a process called `shoaling'. The conservation of energy requires that the amplitudes (height) of the waves grow larger as the waves slow down. The height of the wave rises up to 30 feet or more and the total energy of the tsunami remains a constant.
Shoaling effect
Because of this shoaling effect, a tsunami that remained imperceptible in deep water may grow in height as it reaches the shore. Aftermath of a tsunami attack is nothing but destruction.
All oceanic regions of the world can experience tsunamis, but they are much more frequent in the Pacific Ocean. Occurrence of large, destructive tsunamis is common because of the many large earthquakes along the margins of the Pacific Ocean.
By Our Special Correspondent in Chennai
The Hindu
Source: http://www.hindu.com/seta/2004/12/30/stories/2004123000111600.htm
Physics of the tsunami
The speed of the tsunami is governed by the water depth. Speed reduces and wave height increases as it approaches the shore.
BREEZE FLOWING across a lake or ocean can create wrinkles on the water surface and produce short waves restricted to shallow layer. Tides — high and low tides — that sweep the globe every day also produce waves.
But tsunamis that create killer waves are different for many reasons — they are not produced by gravitational pull of the moon but due to abrupt shifting of the sea floor and resultant vertical displacement of the overlying water. A tsunami is a series of ocean waves generated by such disturbances.
Bid to regain equilibrium
Waves are formed as the displaced water mass attempts to regain its equilibrium. And the size of the resultant tsunami waves is determined by the quantum of the deformation of the sea floor. More the vertical displacement, greater will be the size of the waves. As a rule, all earthquakes do not produce tsunamis.
To generate tsunamis, earthquakes must occur underneath or near the ocean, be large and create movements in the sea floor. The earthquake's magnitude, depth, fault characteristics and coincident slumping of sediments or secondary faulting also determine the size of the tsunamis.
They can be more aptly described as a series of waves of extremely long wavelength and long period generated in a body of water by an impulsive disturbance that displaces the water. Wind-generated waves usually have period (time between two successive waves) of five to twenty seconds and a wavelength (distance between two successive waves) of about 100 to 200 metres (300 to 600 ft).
Tsunamis can have a period in the range of ten minutes to two hours and a wavelength in excess of 500 km. It is because of their long wavelengths that tsunamis behave as shallow-water waves.
Inversely related
A wave is characterised as a shallow-water wave when the ratio between the water depth and its wavelength gets very small. And the rate at which a wave loses its energy is inversely related to its wavelength. Since tsunamis have a very large wavelength, in excess of 500 km, it will lose little energy as it propagates.
Hence in very deep water, a tsunami will travel at high speeds and travel great distances with limited energy loss. For example, when the ocean is more than 5000 metres deep, unnoticed tsunami travel about 890 km per hour, the speed of a jet airplane.
A tsunami at 1000 metres of water depth would travel at 356 km per hour speed. At 5039 metre water depth, it would travel at 800 km per hour and at 6000 m of water depth it would travel at 873 km per hour. So a tsunami travels at different speeds in the ocean; slow in shallow water and fast in deep water.
Though there are many places in the ocean that are deeper than 5000 metres, an average depth of 5000 metres is reckoned and hence an average speed of about 750 km per hour. Thus it can move from one side of the Pacific Ocean to the other side in less than a day.
But what in the first place provides the force needed to allow a tsunami to travel a long distance? Tsunamis are what are called long gravity waves. There are two interacting processes that allow these waves to propagate. The first is the slope of the sea surface, which creates a horizontal pressure force.
The second is the piling up (or lowering of sea surface) as water moves with different speeds in the direction that the wave form is moving. When these two processes have the right relationship in time, they create propagating waves.
As the tsunami crosses the deep ocean, its wavelength — distance from crest to crest — may be hundred kilometres or more and its amplitude — height from crest to trough — will be in the order of a few feet or less. They cannot be felt aboard ships nor can they be seen from the air in the open ocean.
Advance warning
Despite the great speed, they travel much slower than the seismic waves. Hence earthquake information is often available hours before the tsunamis are able to travel across the ocean. Tremors were felt around 6.30 am in Chennai; it took more than two and half hours to reach the shores.
As a tsunami leaves the deep water of the open sea and propagates into the more shallow waters near the coast, it undergoes a transformation. Its speed is affected. The speed of the tsunami is directly related to the water depth; it slows down as it enters the shallower continental shelf water.
The friction of the continental shelf floor slows down the front of the wave. As a result of this slowing down, the trailing waves pile onto the waves in front of them, like a rug crumpled against a wall creating a wave.
This results in decreasing the distance between individual waves in a process called `shoaling'. The conservation of energy requires that the amplitudes (height) of the waves grow larger as the waves slow down. The height of the wave rises up to 30 feet or more and the total energy of the tsunami remains a constant.
Shoaling effect
Because of this shoaling effect, a tsunami that remained imperceptible in deep water may grow in height as it reaches the shore. Aftermath of a tsunami attack is nothing but destruction.
All oceanic regions of the world can experience tsunamis, but they are much more frequent in the Pacific Ocean. Occurrence of large, destructive tsunamis is common because of the many large earthquakes along the margins of the Pacific Ocean.
By Our Special Correspondent in Chennai
The Hindu