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Tsunamis are some of the most devastating natural disasters known to man. Think of a flood with its source being an ocean and you can grasp a little of how much devastation tsunamis can create. For most people who live in land the greatest threat is from overflowing rivers and creeks. Normally extraordinarily heavy rainfall causes rivers and and other waterways to overflow. The excess water creates deadly currents and sweep away people, causing them to drown. It also does a lot of damage in the initial surge and then with standing water. A tsunami has all of these detrimental effects plus the added destructive power crashing waves.
As you many know a tsunami is caused by a strong earthquake on the ocean bed. The vibrations travel through the water traveling sometimes thousands of kilometers. If you were on the water or deep sea diving in SCUBA gear you would not notice much probably just rough waves or a momentarily strong downward pull if you were underwater. However, a tsunami gains its true destructive power as it approaches land. The water level becomes shallower causing the waves caused by the earthquake to compress and combine. This is what creates the massive and destructive waves that cause so much destruction.
Imagine over several tons worth of water either falling on you or surging towards you. You would have a better chance at the Running of the Bulls. The waves not only sweep people away, but can also destroy even well built structures. The costs to human life can also be devastating. The deadliest tsunamis in recorded history was the Christmas tsunamis of 2004 in the Indian Ocean. On December 24, 2004, a massive 9.2 earthquake occurred of the island of Sumatra. It created a deadly series of tsunamis that swept Indonesia, India, Madagascar, and Ethiopia. The death toll was estimated to be in the neighborhood of 300,000 to 350,000. This was one of the greatest losses of life due to a major natural catastrophe in modern history.
The immediate destruction is only the beginning of the damage. After the waters retreated there was the elevated risk of disease created by stagnant and contaminated water. Since most tsunamis occur south of the Equator and In the Pacific this only raises the risk of disease further.
There can also be more interesting effects that deal solely with scientific curiosity. The Christmas tsunami was so powerful it actually sped up the rotation of the Earth reducing the length of its sidereal day. The earthquake that spawned it also caused the Earth to vibrate all over by as much as 1 cm.
If you enjoyed this article there are several others that you will enjoy on Universe Today. There is a great article on how GPS can be used to predict tsunamis. There is also an article about famous earthquakes.
There are also great resources online. There is PBS Nova has web page that is a companion site to the documentary it did on the Christmas Indian Ocean Tsunamis. The USGS also has a great article that analyzes the tsunamis.
You can also check out Astronomy Cast. Check out the Questions episode for Nov. 10 2008. It has an interesting take on killer asteroids.