Tsunami characteristics
Tuesday, 13. June 2006, 01:22:51
Animation of the tsunami caused by the earthquake showing how the tsunami radiated from the entire length of the 1,200 kilometer (750 mi) rupture.
The sudden vertical rise of the seabed by several metres during 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.[26]
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 generally travels at a very high speed of 500 to 1,000 km/h (310 to 620 mph); in shallow water near coastlines, a tsunami slows down to only tens of kilometres an hour but in doing so forms large destructive waves. Scientists investigating the damage in Aceh found evidence that the wave reached a height of 24 m (80 ft) when coming ashore along large stretches of the coastline, rising to 30 m (100 ft) in some areas when travelling inland.[27]
Radar satellites recorded the heights of tsunami waves in deep water: at two hours after the earthquake, the maximum height was 60 cm (2 ft). These are the first such observations ever made. However, these observations could not have been used to provide a warning, because the satellites were not intended for that purpose and the data took hours to analyse.[28][29]
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. In many places the waves reached as far as 2 km (1.24 mi) inland.[30]
Because the 1,200 km (745.6 mi) of faultline affected by the earthquake 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 relatively near the epicentre. It also benefited from the fact that the earthquake proceeded more slowly in the northern rupture zone, greatly reducing the energy of the water displacements in that region.
Coasts that have a landmass between them and the tsunami's location of origin are usually safe; however, tsunami waves can sometimes diffract around such landmasses. 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.
Because of the distances involved, the tsunami took anywhere from fifteen minutes to seven hours (for Somalia) to reach the various coastlines.[31][32] 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 90 minutes to 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.
The tsunami was noticed as far as Struisbaai in South Africa, some 8,500 km (5,300 mi) away, where a 1.5 m (5 ft) high tide surged on shore about 16 hours after the earthquake. It took a relatively long time to reach this spot at the southernmost point of Africa, probably because of the broad continental shelf off South Africa and because the tsunami would have followed the South African coast from east to west. The tsunami also reached Antarctica, where tidal gauges at Japan's Syowa Base recorded oscillations of up to a meter, with disturbances lasting a couple of days.[33]
Some of the tsunami's energy escaped into the Pacific Ocean, where it produced small but measurable tsunamis along the western coasts of North and South America, typically around 20 to 40 cm (7.9 to 15.7 in).[34] At Manzanillo, Mexico, a 2.6 m (8.5 ft) crest-to-trough tsunami was measured. This puzzled many scientists, as the tsunamis measured in some parts of South America were larger than those measured in some parts of the Indian Ocean. It has been theorised that the tsunamis were focused and directed at long ranges by the mid-ocean ridges which run along the margins of the continental plates.
Casualties in historical context
This earthquake was the fourth most powerful earthquake recorded since 1900, and the confirmed death toll is just under 200,000 due to the ensuing tsunami. The deadliest earthquakes since 1900 were the Tangshan, China earthquake of 1976, in which at least 255,000 were killed; the earthquake of 1927 in Xining, Qinghai, China (200,000); the Great Kanto earthquake which struck Tokyo in 1923 (143,000); and the Gansu, China, earthquake of 1920 (200,000). The deadliest known earthquake in history occurred in 1556 in Shaanxi, China, with an estimated death toll of 830,000, though figures from this time period may not be reliable.[41]
The 2004 tsunami is the deadliest in recorded history. Prior to 2004, the deadliest recorded tsunami in the Pacific Ocean was in 1782, when 40,000 people were killed by a tsunami in the South China Sea.[42] The tsunami created by the 1883 eruption of Krakatoa is thought to have resulted in 36,000 deaths. The most deadly tsunami between 1900 and 2004 occurred in 1908 in Messina, Italy, on the Mediterranean Sea, where the earthquake and tsunami killed 70,000. The most deadly tsunami in the Atlantic Ocean resulted from the 1755 Lisbon earthquake, which, combined with the toll from the actual earthquake and resulting fires, killed over 100,000.
The 2004 earthquake and tsunami seem to be the deadliest natural disaster since either the 1976 Tangshan earthquake or the 1970 Bhola cyclone, or could conceivably exceed both of these. Because of uncertainty over death tolls, it might never be known for sure which of these natural disasters was the deadliest.
Human component in magnitude of damage
Source: Wikipedia
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