Sci-Tech

What caused the massive 2004 Indian Ocean tsunami

A megathrust quake caused the tsunami. Photo: R. Shivaji Rao

A megathrust quake caused the tsunami. Photo: R. Shivaji Rao  

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On December 26, 2004 morning walkers and fishermen residing along the eastern coast of India observed a strange phenomenon of the sea receding few hundred metres inside exposing parts of the coastal shelf. What they were observing was a precursor to the incoming gigantic tsunami waves generated due to a M 9.3 Great undersea earthquake off the coast of Banda Aceh, northern Sumatra.

This earthquake occurred along a thrust fault in the subduction zone where the Indian tectonic plate is going below the overriding Burmese plate. As a result, the ocean floor broke and there was a vertical displacement of about 15 to 20 meters along the fault causing large scale displacement of water and thus, generating tsunami waves.

This kind of large vertical displacement happened because the magnitude of the earthquake was greater than 9 and it occurred at a shallow depth of less than 30km below the ocean. Since 1900, only five earthquakes, worldwide have exceeded magnitude 9.0 and all of them occurred in subduction zones at shallow depths and broke the ocean floor with displacement of the order of greater than 10 metres generating gigantic tsunami waves.

These earthquakes which are referred to as “Megathrust” earthquakes were 1952 Kamchatka, Russia, 1960 Chile (the world’s greatest so far with M 9.5), 1964 Prince William Sound, Alaska, 2004 Sumatra and 2011 Tohoku, Japan. Tsunami waves are also known to have been generated by earthquakes of greater than 8.5 magnitudes but with lesser intensities.

The rupture of the M 9.3 Sumatra earthquake in 2004 began on a NW-SE trending thrust fault off the coast of Sumatra and then propagated in North-South direction along the Nicobar and Andaman islands to a length of 1200 km. Since, a large amount of tsunami energy propagates normal to the trend of the fault direction, the tsunami wave propagated in south-west and westerly direction in the Indian Ocean reaching Indian coast, Srilanka, Maldives and reaching up to eastern African coast lines.

Since the tsunami waves are long period waves with wave lengths of 200-250 kilometres, their height in the open sea ranges between few centimetres to a metre and cannot be distinguished by people travelling on a ship in an open sea. The speed of a tsunami wave is related to the depth of the ocean, greater the water depth higher the speed.

Typically, for an average ocean depth of 4 km, like in the Indian Ocean/Bay of Bengal region, the speed of the tsunami waves can go up to 720 km/h or about the speed of a jet airliner. As the tsunami waves approach the shore, the water depth becomes shallower, waves slow down, wavelength becomes shorter and the waves gain larger amplitude or heights and become destructive.

The average distance from Banda Aceh to the Indian eastern coast ranges between 1,800 and 2,000 km and hence, the travel time taken by the tsunami waves was about 150 minutes. The earthquake occurred at 06:28(IST) and the first waves were observed around 09:00 (IST) on the eastern Indian coast.

More than 16,000 people perished in the Andaman and Nicobar islands and along the eastern coast of India. Although the occurrence of tsunami across the world is well known, it was the first experience for the people of India. Earlier, there are records of tsunami waves touching the Indian shores in the West in 1945 and in the East in 1941.

But these waves were insignificant and less than 1 metre in height. But in the case of 2004 tsunami, the maximum run-up height of more than 5 metres was observed along the Nagapattinam region in Tamil Nadu and lateral inundation being up to 1 km at some places. Most of the loss of life and damage to property was within 500 metres of the shore and the local coastal topography played an important role in the inundation process.

The varying tsunami wave heights along the east coast from 2.5 metres at Devanaampatnam to 5.2 at Nagapattinam were due to the bathymetry of the coastline. The 2004 tsunami was also a wakeup call for the Indian earth scientists’ community to take up research on this new coastal hazard. The Indian scientists made great strides since then, by setting up a Tsunami Early Warning System at INCOIS, Hyderabad in 2007.

Since, then several successful warnings were given by the Centre whenever there was an undersea earthquake of any significance in the Indian Ocean. In terms of tsunami research several models were developed in which scenarios were created which will help in predicting the time of tsunami wave arrivals, their heights and inundation along the east and west coast of India in case of earthquakes occurrence in the two subduction zones in Markran, south of Pakistan in the west and Andaman and Nicobar and Sumatra in the east, identified to be sources of tsunami generation.

R.K. CHADHA

Chief Scientist, National Geophysical Research Institute, Hyderabad

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Printable version | Nov 14, 2019 1:34:29 PM | https://www.thehindu.com/sci-tech/what-caused-the-massive-2004-indian-ocean-tsunami/article6723085.ece

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