tsunami facts

Beginning of a tsunami wave
Sometimes plate boundaries abruptly deform and displace the overlying water vertically. Subduction earthquakes are efficient in generating a tsunami. In the 1940s, an earthquake measuring 7.8 on the Richter scale gave rise to a tsunami. Explosive volcanic action, landslides and impact events can also start a tsunami. The water wave may reach 50 to 150 meters and cover a height of 500 meters on local mountains. A "megatsunami" is caused by large landslides. The displaced water mass moves under the effect of gravity. This water radiates across the ocean similar to ripples in a pond.

Signs that a tsunami is approaching

• if an earthquake takes place near a body of water, it means that a tsunami will follow in a short time
• if the water along the shoreline recedes dramatically and exposes usually submerged areas it should be inferred that this is the trough of the tsunami and a crest will follow after a few seconds or minutes
• some large animals like elephants hear the noise of the tsunami and move in the opposite direction towards inland
• computer models can also foresee tsunami arrival and impact depending upon knowledge of the event that caused it and the shape of the oceanbed
• there is a loud roar similar to a train or aircraft

Tsunami warning system
Such systems comprise of two parts:

• a network of sensors to detect tsunamis
• a communications infrastructure to provide alarms for evacuation of coastal areas

There exist international and regional tsunami warning systems. The underlying principle used in both is that tsunamis move at a speed of 0.14 to 0.28 km/sec while seismic waves of 4 km/sec. Thus, when an earthquake is confirmed, there is sufficient time to predict a tsunami.

Reducing the effect of a tsunami

• Japan builds tsunami walls of 4.5 meters height in populated coastal areas
• floodgates and channels are built to redirect the water from the tsunami
• a tree cover is made on the shore

These measures slow down and moderates a tsunami. However, they cannot totally prevent the destruction and loss of life.

Records related to tsunami

• The maximum death toll due to tsunami has been 283,000 in 2004 in the Indian Ocean.
• The second largest has been 100,000 in 1755 in Portugal, Morocco and the United Kingdom.
• The third largest is 70,000 in 1908 in Italy.

In the Indian Ocean, an earthquake of 9.0 magnitude on the Richter scale took place. The epicenter was close to the west coast of the Indonesian island of Sumatra. The Earth's tectonic plates moved violently and displaced a large quantity of water. Powerful shock waves were sent in all directions. At some places, these waves reached a height of 9 meters. Within a span of some hours, killer waves hit the coasts of 11 Indian Ocean countries and devastated properties from Thailand to Africa.

Other tsunami facts
At the deepest point in the ocean, Tsunamis can have a speed of 600 mph. Close to the shore, this speed reduces to 30 to 40 mph. This energy of the wave's speed is transformed to increased height and sheer force. These waves can be as long as 100 kilometers and one hour apart. They can cross huge oceans without much loss of energy. Tsunamis can take place at any time in night or day. They can move up the rivers and streams that end up in the ocean. Tsunamis move faster than a human being.

Megatsunami
If the waves range from 40 meters to more than 100 meters, they are called as a "megatsunami". When they reach land, they acquire more height due to the force of impact. They are also called as "iminami" or "wave of purification"