What caused the third largest earthquake ever measured, and the resulting 2004 Indian Ocean Tsunami?

 

The continents and oceans on earth are part of the earth’s upper layer, or Crust. The Crust is made of solid rock about 5 km – 80 km thick. But the Crust is not one single piece of rock. It is divided into separate “tectonic plates”. And each tectonic plate floats on the layer below the Crust – the Mantle. Tectonic plates can move apart from or into each other, or slip past one another, or break up. This is a constant process. And since the plates are constantly moving it also means that continents and oceans are also not static, but float with the tectonic plates on which they lie.

The plate on which India lies, the Indian plate, is part of the larger Indo-Australian plate. This plate is moving North-North East, at about 6 cm per year. To the north and east of the Indian plate is the Eurasian plate. The Indian plate started colliding with the Eurasian plate about 55 million years ago, and the collision is still continuing. This collision is causing the denser Indian plate to “subduct” (sink) under the lighter Eurasian plate. The deformation of the Earth’s Crust caused by this collision is the reason why the Himalayas were formed. The Himalayas are the northern boundary of this collision.

(By Alataristarion - https://commons.wikimedia.org/wiki/File:Tectonic_plates_boundaries_detailed-en.svg, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=39882586)

(Orogeny means Mountain Formation)

In the east the Indian plate is colliding with the Burma Plate and Sunda Plate (both part of the Eurasian plate). The boundary of this collision is marked by the “Sunda Trench”, a 3,200 km long trench on the ocean floor with a maximum depth of 7,290 metres. The Sunda Trench stretches from the Lesser Sunda Islands, past Java, around the southern coast of Sumatra to the Andaman Islands.

The Indo Australian plate collides with the Eurasian Plate in the north, and with Burma plate and Sunda plate (both of which are part of the Great Eurasian plate) in the east

As the Indian plate sinks deeper beneath the Sunda plate the increasing temperature and pressure drive volatile material out of the subducting plate. This material rises up, causing partial melting and formation of magma. The rising magma passes through the crust and exits it through volcanoes in the Sunda Arc. Sudden explosive release of large quantity of volatile material can also cause the collision boundary to rupture, leading to an earthquake.

On 26 December 2004, around 8 am local time, the constantly building up pressure caused a rupture which started off shore near Bandar Aceh in Sumatra, causing an earthquake measuring 9.2-9.3 on the Richter Scale. About 30 km below the surface, an estimated 1,600 km of fault surface slipped (or ruptured) an average of about 15 metres along the entire subduction zone.

The slip took place in two phases over several minutes. In the first phase a rupture about 400 km long and 100 km wide occurred - the largest rupture ever known to have been caused by an earthquake. The rupture began off the coast of Aceh in Sumatra, and travelled north west at about 2.8 km/second for about 100 seconds. After about another 100 seconds, the rupture continued northwards towards the Andaman and Nicobar Islands for about 5 minutes, this time at a slower speed of 1 km/second. It stopped only when it reached an area where the fault type changes from subduction to strike-slip (the two plates slide past one another in opposite directions).

The sea floor at the collision boundary moved westward by about 10 metres, but, importantly, also rose by an average of 5 metres. The sudden vertical rise of the seabed displaced massive volumes of water, resulting in a tsunami that struck the coasts of the Indian Ocean. In deep ocean water, the tsunami waves formed only a low, broad hump, barely noticeable and harmless, travelling at high speed. But in shallow water near coastlines, the tsunami slowed down to only tens of kilometres per hour. Shallow coastal areas also meant that the wave height increased dramatically near shorelines. 20 minutes after the earthquake, tsunami waves 24-30 m tall ravaged Banda Aceh. Waves as much as 10 m tall rammed into Sri Lanka, and as much as 5 m tall washed the eastern coast of India about 1.5-2 hours after the earthquake. The Tsunami also reached western Australia, Thailand, Bangladesh, Myanmar, and the African coast from Somalia to South Africa. Small tsunamis were measured even in the Americas, and in Antarctica.

The tsunami took 5 hours to reach Western Australia, 7 hours to reach the Arabian Peninsula, and about 11 hours to reach the South African coast.

(Source: NOAA Official website, [1]., Public Domain, https://commons.wikimedia.org/w/index.php?curid=31919348)

At least 2,28,000 people are estimated to have died due to the Tsunami. The bulk of the deaths were in Indonesia. But over 30,000 people died in Sri Lanka, and more than 10,000 in India.

The shift of mass and the release of energy in the earthquake was so massive that it altered earth's rotation. Models suggested the earthquake shortened the length of a day by 2.68 microseconds, due to a decrease in the oblateness of the Earth. It also caused the Earth to "wobble" on its axis by 1-2 inches.

A positive after effect of the 2004 earthquake however, was the setting up of the Indian Ocean Tsunami Warning System. This system has been effective in issuing early warnings during several earthquakes since then, and has saved many lives.