THE PHYSICS OF TSUNAMI
The disastrous Tsunami of December 2004 took most of us by surprise. Prior to that many of us were not even familiar with the name of Tsunami, and most of us did not even know how to pronounce it Tsunami is a Japanese word, in which ‘tsu’ means harbour and ‘nami’ means wave. The word is pronounced as soo-nah-mee or tsoo-nah-mee. After the 2004 calamity, the possibility of future tsunamis destroying our lives and the environment makes it necessary for us to understand how and why it occurs.
Television images of last year’s tsunami are still fresh in our minds. We know tsunami means the sea comes and hits the coast line in a series of waves that are immense in size and force. The waves that rise are generated due to a rapid displacement of water in the sea or even in a lake by earthquakes, landslides, volcanic eruptions or even meteorite collisions. The most common cause is, however, the occurrence of undersea earthquake, or an undersea land slide caused by an earthquake. The vertical displacement of the sea floor alongside plate boundaries of the earth’s crust can also cause tsunamis.
The movement of oceanic plates slipping below continental plates, which is known as seduction earthquakes can also cause large displacement of water. Studies have revealed that events that have an impact on the ocean like a falling meteorite or an explosive volcano throwing huge debris in the sea can also cause sudden and quick expansion of water.
Tsunamis are also referred to as tidal waves, though that’s a misnomer since tsunamis are not related to tides at all. Though tsunamis can have devastating effects on the coastal regions their occurrence are often not felt by ships sailing in the deep ocean. That is because it does not affect the sub-surface of the ocean, it simply involves a series of waves that assume the characteristics of a violent onrushing tide as they hit land.
Tsunamis are events that do not affect the surface water alone but its force can be felt deep under the ocean. They carry immense energy, propagate at high speeds and can travel great transoceanic distances. A tsunami can be felt long after the event that caused it occurred, so there may be several hours between its creation and its impact on a coast. The total energy of a Tsunami wave is spread over a large circumference as the wave travels, so the energy per linear meter in the wave decreases as the inverse power of the distance from the source.
A single tsunami event may involve a series of waves called a train. In open water, these waves have very long wavelengths that spread up to hundreds of kilometers and they seem colossal when compared to the wind generated waves on the ocean that have a mere wavelength of 150 meters.
The waves travel at high speed across the ocean ranging from 500 to 1,000 km/h. As the wave approaches land, because of the increasing shallowness of the sea the waves lose their speed and therefore begin to gather together with the wave-front becomes steeper and taller. While a person at the surface of deep water would probably not even notice the tsunami, the wave can increase to a height of 30 m or more as it approaches the coastline and compresses.
Tsunamis propagate outward from their source and diffract around landmasses. They may be asymmetrical and affect one direction more than the other. Though Tsunamis cannot be prevented or predicted with accuracy, the scientific community has developed a Tsunami warning system that studies under sea land and water movement in order to foresee any possible disaster occurring due to a Tsunami.
Sometimes animals can warn us against a Tsunami by their peculiar behaviour when they begun to disperse to higher areas. Certain tsunami prone countries like Japan have build tsunami resistant walls, floodgates and channels.