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 that causes damage far away from its source is sometimes
called ateletsunami 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 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 per 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 metres (80 ft) when coming ashore along large stretches
of the coastline, rising to 30 metres (100 ft) in some areas when traveling
inland.
Radar satellites
recorded the heights of tsunami waves in deep water: at two hours after the
earthquake, the maximum height was 60 centimetres (2 ft). These are the first
such observations ever made. Unfortunately these observations could not be used
to provide a warning, since the satellites were not built for that purpose and
the data took hours to analyze.
According to Tad Murty,
vice-president of the Tsunami Society, the total energy of the tsunami waves
was equivalent to about fivemegatons of TNT (20 petajoules). This is more than
twice the total explosive energy used during all of World War II (including the
twoatomic 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 (1.2 mi) inland.
Because the 1,600 km
(1,000 mi) fault 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 epicenter.
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
state of Kerala was hit by the tsunami despite being on the western coast of
India, and the western coast of Sri Lanka suffered substantial impacts. Distance
alone was no guarantee of safety, as 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 to reach the coastlines. 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 struck about two hours
later despite being closer to the epicentre, because the tsunami traveled 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 Showa
Base recorded oscillations of up to a metre (3 ft 3 in), with disturbances
lasting a couple of days.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). At
Manzanillo, Mexico, a 2.6 m (8 ft 6 in) crest-to-trough tsunami was measured.
As well, the tsunami was large enough to be detected in Vancouver, British Columbia,
Canada. 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 theorized that the tsunamis were focused and directed at
long ranges by the mid-ocean ridges which run along the margins of the
continental plates.
