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2004
Indian Ocean earthquake
From Wikipedia, the free encyclopedia. |
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Animation
of the tsunami caused by the earthquake. (See also the full-length version)
Note: Tsunamis have very low height while travelling over deep ocean, and ocean-going
vessels in their path do not usually notice them. High waves only occur when shallow water
is reached.
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The 2004 Indian
Ocean earthquake was an undersea earthquake that occurred at 00:58:53 UTC (07:58:53 local
time) on December 26, 2004. The earthquake generated tsunamis that were among the
deadliest disasters in modern history. At a magnitude of 9.0, it was the largest
earthquake since the 9.2 magnitude Good Friday Earthquake off Alaska in 1964, and tied for
fourth largest since 1900.
The earthquake originated in the Indian Ocean just north of Simeulue island, off the
western coast of northern Sumatra, Indonesia. The resulting tsunamis devastated the shores
of Indonesia, Sri Lanka, India, Thailand, and other countries with waves of up to 15 m (50
feet) high, even reaching the east coast of Africa, 4500 km (2,800 miles) west of the
epicenter.
At least 155,000 people are known to have died as a result of the tsunamis and the count
is still taking place. The true final toll may never be known due to bodies swept out to
sea, but it is likely to be higher than the current estimate. Relief agencies warn of the
possibility of more deaths to come as a result of epidemics because of poor sanitation,
but the threat of starvation seems now to have been averted [1]
(http://news.bbc.co.uk/2/hi/asia-pacific/4157947.stm). The plight of the many affected
people and countries prompted a widespread humanitarian response.
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| Quake
characteristics |
 The earthquake was initially reported as 6.8 on the Richter scale. On the
moment magnitude scale, which is more accurate for quakes of this size [2]
(http://earthquake.usgs.gov/faq/meas.html), the earthquake's magnitude was first reported
as 8.1 by the U.S. Geological Survey. After further analysis, this was increased to 8.5,
8.9, and finally to 9.0 [3] (http://earthquake.usgs.gov/recenteqsww/Quakes/usslav.htm).
Since 1900, the only earthquakes recorded with a greater magnitude were the 1960 Great
Chilean Earthquake (magnitude 9.5), the 1964 Good Friday Earthquake in Prince William
Sound (9.2), and the March 9, 1957 earthquake [4]
(http://neic.usgs.gov/neis/eq_depot/usa/1957_03_09.html) in the Andreanof Islands (9.1).
The only other recorded earthquake of magnitude 9.0 was in 1952 off the southeast coast of
Kamchatka [5] (http://neic.usgs.gov/neis/eq_depot/world/1952_11_04.html). (See Top 10
earthquakes (http://neic.usgs.gov/neis/eqlists/10maps_world.html)). Each of these
megathrust earthquakes also spawned tsunamis (in the Pacific Ocean), but the death toll
from these was significantly lower—a few thousand for the worst one — probably because
of the lower population density along the coasts near affected areas and the much greater
distances to more populated coasts.
The hypocenter was at 3.316?N, 95.854?E, some 160 km (100 miles) west of Sumatra, at a
depth of 30 km (18.6 miles) below mean sea level (initially reported as 10 km). This is at
the extreme western end of the Ring of Fire, an earthquake belt that accounts for 81
percent of the world's largest earthquakes [6]
(http://earthquake.usgs.gov/faq/hist.html#1). The earthquake itself (apart from the
tsunamis) was felt as far away as Bangladesh, India, Malaysia, Myanmar, Thailand,
Singapore and the Maldives.
The earthquake was unusually large in geographical extent. An estimated 1200 km (750
miles) of faultline slipped about 15 m (45 ft) along the subduction zone where the India
Plate dives under the Burma Plate. The slip did not happen instantaneously but took place
in two phases over a period of several minutes. Seismographic data indicates that the
first phase involved the formation of a rupture about 400 km long and 100 km wide, located
30km beneath the sea bed. The rupture proceeded at a speed of about 2 km/s, beginning off
the coast of Aceh and proceding north-westerly over about 100 seconds. A pause of about
another 100 seconds took place before the rupture continued northwards towards the Andaman
and Nicobar Islands. [7]
(http://www.guardian.co.uk/life/feature/story/0,13026,1383675,00.html)
The India Plate is part of the great Indo-Australian Plate, which underlies the Indian
Ocean and Bay of Bengal, and is drifting northeast at an average of 6 cm/year (2
inches/year) (or 20 m (60 feet) per 330 years; i.e. this earthquake moved this fault 330
years worth). The India Plate meets the Burma Plate (which is considered a portion of the
great Eurasian Plate) at the Sunda Trench. At this point the India Plate subducts the
Burma Plate which includes the Nicobar Islands, the Andaman Islands and northern Sumatra.
The India Plate slips deeper and deeper beneath the Burma Plate until the increasing
temperature and pressure turns the subducting edge of the India Plate into magma which
eventually pushes the magma above it out through the volcanoes (see Volcanic arc). This
process is interrupted by the locking of the plates for several centuries until the build
up of stress causes their release resulting in a massive earthquake and tsunami. The
volcanic activity that results as the Indo-Australian plate subducts the Eurasian plate
has created the Sunda Arc.
As well as the sideways movement between the plates, the sea bed is estimated to have
risen by several metres, triggering devastating tsunami waves. The tsunamis did not
originate from a point source, as mistakenly depicted in many illustrations of their
spread, but radiated outwards along the entire 1200 km length of the rupture. This greatly
increased the geographical area over which the waves were observed, reaching as far as
Mexico and Chile.
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