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On December 26, 2004, at 7:58, a megathrust earthquake of moment magnitude 9.3 occurred off the west coast of the Indonesian island of Sumatra. The uplift of the ocean floor, estimated to be 7.0-10 meters, created a tsunami that affected Indonesia, Thailand, Malaysia, Myanmar, Bangladesh, Sri Lanka, India, Seychelles, Maldives, all the way to Somalia, Tanzania, and Kenya. The impact of the earthquake and tsunami event was catastrophic: loss of life exceeded 300,000 with an estimated 1.5 million made homeless.

What is tsunami?

According to the Encyclopedia Britannica, a tsunami is “a seismic sea wave or tidal wave that is caused by an earthquake that occurs less than 30 miles beneath the seafloor and has a magnitude greater than 6.5 on the Richter scale.” Tsunami is a Japanese word which means “harbor wave” due to the devastating impacts of these waves on Japanese coastal communities. It is a series of waves as a result of the rapid displacement of a body of water like an ocean or sea. The term tsunami is often mistakenly interchanged with the term tidal wave, but this is inaccurate because tsunami is not caused by tidal action and oceanographers and geologists discourage the use of this term. However, the term tsunami is no more accurate since its occurrence is not limited to harbors.

Causes

Tsunamis are primarily caused by large earthquakes, underwater landslides, or underwater volcanic eruption, causing the sea floor to move and displace the large body of water in the surface. Asteroid impacts can also generate a tsunami, similar to what happened in the Jurassic period. Conducting  nuclear weapon tests at sea also has the potential to cause a tsunami. To cause a large tsunami, the earthquake must occur under the ocean or near it, it must be large, and must generate vertical movements of the sea floor.

What happens in a tsunami

Following the disturbance in the ocean floor, a tremendous amount of the earthquake’s energy is transferred to the entire water column above it. This is how seismic waves differ from ocean swells and wind-driven waves: the latter only affect the top layer of the body of water. From the source area, waves behave like ripples, traveling outward in all directions  at very high speeds. A tsunami can travel at speeds of 450 mph to 600 mph in the open ocean. In deep water the waves travel unnoticeably, with high speed, long wave length, and low amplitude.

But as the waves come closer to the coast, their amplitude (height) increases and their speed decreases. When they reach the coast, they are likely to travel at about 10 to 15 mps (this is considerably faster compared to normal wind-driven waves) and the time between successive wave crests varies from 5 to 90 minutes. Some tsunamis are not felt by people because their energy is released so gradually.

Large tsunamis can destroy an entire town or village. People, beaches, houses, infrastructures, trees, and anything else in the waves’ path are pushed inland. As they lose energy, waves begin to return back or recede into the ocean. Those who have survived the first surge are pulled back into the ocean . While the wave recedes into the ocean, another group of waves may come on land as well. Areas within a mile of the coast and those not more than 25 feet above sea level are at greatest risk.

Effects

The effects of a large tsunami is devastating. According to official figures available in the middle of January 2005, at least 160,000 people died in Indonesia, 31,000 in Sri Lanka, 10,000 in India and 5,000 in Thailand in the December 2004 tsunami. The indescribable human tragedy is compounded by the economic consequences. The short-term economic effects are the destruction of buildings, natural resources,  and infrastructures.

More alarming is the long-term economic impact of the tsunami on the disaster areas. Water supply in affected areas can be disrupted as  bridges carrying water pipes across the various rivers and canals may collapse. Lack of access to clean water may have not only economic consequences but also health effects as well. Tsunamis can also destroy the tourism industry and the livelihood of the affected communities. Thus, job creation is a major area of concern following devastating tsunamis.

 

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