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The sudden vertical motion of the seabed by the earthquake displaced massive volumes of water, resulting in a tsunami that struck the coasts of the Indian Ocean. A tsunami which causes damage far away from its source is sometimes called a "teletsunami", and is much more likely to be produced by vertical motion of the seabed than by horizontal motion. The tsunami, like all others, behaved very differently in deep water than in shallow water. In deep ocean water, tsunami waves form only a small hump, barely noticeable and harmless, which travels at very high speed (faster than a jet plane); in shallow water near coastlines, a tsunami slows down to the speed of a car but forms large destructive waves. Two radar satellites that happened to be overhead at the right moment recorded two wavefronts 500–850 km apart with a height of 50 cm. These are the first such observations ever made (http://www.newscientist.com/article.ns?id=dn6854). According to Tad Murty, vice-president of the Tsunami Society, the total energy of the tsunami waves was about five megatons of TNT (20 petajoules). This is more than twice the total explosive energy used during all of World War II (including the two atomic bombs), but still a couple of orders of magnitude less than the energy released in the earthquake itself (http://www.canada.com/montreal/montrealgazette/news/story.html?id=2257b78c-3897-4594-ad86-18c0eb661bea). Because the 1,200 km of faultline affected by the quake was in a nearly north-south orientation, the greatest strength of the tsunami waves was in an east-west direction. Bangladesh, which lies at the northern end of the Bay of Bengal, had very few casualties despite being a low-lying country. Coasts that have a land mass between them and the tsunami's location of origin are usually safe; however, tsunami waves can sometimes diffract around such land masses. Thus, the Indian state of Kerala was hit by the tsunami despite being on the western coast of India, and the western coast of Sri Lanka also suffered substantial impacts. Also distance alone is no guarantee of safety: Somalia was hit harder than Bangladesh despite being much farther away. Due to the distances involved, the tsunami took anywhere from fifteen minutes to seven hours (for Somalia) to reach the various coastlines (see travel time maps: (http://tsun.sscc.ru/tsulab/20041226trt.htm) ). The northern regions of the Indonesian island of Sumatra were hit very quickly, while Sri Lanka and the east coast of India were hit roughly two hours later. Thailand was also struck about two hours later, despite being closer to the epicentre, because the tsunami travelled more slowly in the shallow Andaman Sea off its western coast. 2.1)Signs and warnings

Despite a lag of up to several hours between the earthquake and the impact of the tsunami, nearly all of the victims were taken completely by surprise: there is no tsunami warning system in the Indian Ocean to detect tsunamis, and equally importantly, warn the general populace living around the ocean quickly. Tsunami detection is not easy because while a tsunami is in deep water it has a very low height and a network of sensors is needed to detect it. Setting up the communications infrastructure to issue timely warnings is an even bigger problem (http://www.newscientist.com/article.ns?id=dn6839). Scientists were also hampered by the fact that the initial estimate for the magnitude of the earthquake was 8.1. The determination that the earthquake had actually been much stronger (and the resulting tsunami much larger) was not made until after the tsunami had already struck. Tsunamis usually occur in the Pacific Ocean due to earthquakes in the "Ring of Fire", and an effective tsunami warning system has long been in place there. Although the extreme western edge of the "Ring of Fire" extends into the Indian Ocean (the point where this earthquake struck), no warning system exists in that ocean due to the rarity of tsunamis there — the last major one was caused by the Krakatoa eruption of 1883. In the aftermath of the disaster there is a new awareness of the need for a tsunami warning system for the Indian Ocean. The UN aims to have a working East Asia and Southeast Asia early warning system within a year (http://www.un.org/apps/news/story.asp?NewsID=12932). Some have even proposed creating a unified global tsunami warning system, to include the Atlantic Ocean and Caribbean. 2.2)Unfamiliarity with warning signs

The first warning sign of a possible tsunami is the earthquake itself; however tsunamis can strike thousands of miles away, where the earthquake is only felt weakly or not at all. Also, in the minutes preceding a tsunami strike the sea often recedes temporarily from the coast. People in Pacific regions are more familiar with tsunamis and often recognise this phenomenon as a sign to head for higher ground. However, around the Indian Ocean, this rare sight reportedly induced people, especially children, to visit the coast to investigate and collect stranded fish on as much as 2.5 km of exposed beach, with fatal results(http://www.npr.org/templates/story/story.php?storyId=4246573). One of the few coastal areas to evacuate ahead of the tsunami was on the Indonesian island of Simeulue, very close to the epicentre. Island folklore recounted an earthquake and tsunami in 1907 and the islanders fled to inland hills after the initial shaking — before the tsunami struck (http://www.timesonline.co.uk/article/0,,18690-1422835,00.html). On Maikhao beach in northern Phuket, Thailand, a 10 year old British girl named Tilly Smith had studied tsunamis in geography class at school and recognised the warning sign of the receding ocean. She and her parents warned others on the beach, which was evacuated safely (http://www.telegraph.co.uk/news/main.jhtml?xml=/news/2005/01/01/ugeog.xml

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