January 2005

Predicting Tsunamis

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The tsunami that struck South Asia on 26th December caused waves to rise to a height of 9 metres at some places and was undoubtedly one of the most devastating tsunamis in recent history. The earthquake measuring 9.0 on the Richter scale struck at 6.58 a.m and is believed to have led to a sequence of 15 quakes across Andaman and Nicobar islands. It is believed to be the fourth most destructive tsunami in history. The basic mechanism behind tsunamis is the displacement of a large volume of water, which also displaces the sea floor. Just as in the case of earthquakes on land, tsunamis are dictated by plate tectonics. A displacement of the tectonic plates will always generate a wave at the surface of the ocean – known as tsunami. In the present case, the tectonic plates are estimated to have been displaced by as much as 30 metres.

Image courtesy: New York Times online


Actually, a tsunami is not a single wave, but a series of waves and is therefore, sometimes called a 'wave train'. In the case of the tsunami that struck on December 26, 2004 too, there were three waves, which struck at an interval of approximately 20 minutes. Usually, the first tsunami wave is the least destructive too, as is corroborated by survivors of tsunami waves in Andaman and Nicobar Islands. Tsunami waves can cross entire oceans without losing their intensity and energy and can rattle up speeds upto 800 km per hour. Survivors of tsunamis say that the sound of a gushing tsunami is quite similar to that of a freight train. Tsunamis, as is by now widely known, are 'harbour waves'. The word is essentially made up of two Japanese words – 'tsu', meaning harbour and 'nami' meaning waves. Though tsunamis were generally considered to be a Pacific phenomenon, the tsunami that struck on December 26, 2004, has proved that natural phenomena cannot be restricted to a single area or region. Tsunamis have often erroneously been termed as 'tidal waves'. The reason behind is the fact that tsunamis can be caused even by disturbances other than seismic, such as landslides taking place under the water, meteoric disturbances and also of human activity.

Tsunamis can be generated by earthquakes, volcanic eruptions, nuclear explosions and major landslides or even by the impact of meteorites. Earthquakes occurring on the ocean floor, measuring more than 6-6.5 on the Richter scale, usually cause tsunamis. Interestingly, an earthquake is in itself the first natural warning of an impending tsunami. However, tsunamis themselves are determined not by the magnitude of the earthquake, but by the type of fault through which the earthquake is generated.

Harbour waves 'Tsunami'
Image courtesy: ABC news online

Is tsunami prediction really possible?
This really is million-dollar question, as of today. Though tsunamis do follow a certain pattern, it is not uniform for all regions and areas. As the epicenters of most of the tsunamigenic earthquakes take place deep below in the oceans, an accurate measurement of its geophysical and geochemical precursors, which could act as a warning for an impending tsunami, is a difficult and indeed a far-fetched proposition, given the technology existing as of today.

If proper parameters of historic tsunami determinism are drawn up, it is possible to predict a reasonable time frame for the recurrence of a tsunami. However, even then, given the fact that earthquakes on lands too can be predicted only with a reasonable degree of accuracy, accurate prediction of a tsunami is still some distance away. Of course, the loss of life and property can be minimised.

To accurately predict a tsunami, various parameters of the source of the tsunami have to be drawn up in 'real time'. The time lag between the prediction of a tsunami based on a study of these parameters would effectively leave very little time to act on the warning of an impending tsunami. Seismic parameters – viz the earthquake's magnitude, the depth of the water, parameters of the fault line, besides a host of other factors like data connected to the sea level have to be taken into account.

Tsunamis have been a rare occurrence in the Indian Ocean, as the seismic activity is relatively lesser than in the Pacific Ocean. There have been only seven instances of earthquake-triggered tsunamis near Indonesia, Pakistan and one in the Bay of Bengal in the last 100 years.

It is in this context that the demand for aligning with the Pacific Tsunami Warning Centre (PTWC) based in Ewa Beach, Hawaii, has to be understood in the proper context. Though it has been pointed out by the national and international media that the colossal damage inflicted on India as a result of tsunami could have been avoided, had India joined the PTWC, what is being ignored is the fact that Thailand and Indonesia, two of the other nations which were devastated by the tsunami are signatories to the PTWC. Despite a three-hour warning of the impending tsunami, the fate of Thailand and Indonesia is equally bad, if not worse than that of India. Surprisingly, there has been little or no talk of 'Project Thrust' – a low cost and a reliable regional programme aimed at those countries that have inadequate or an absence of PTWC.

Moreover, what holds good for the Pacific Ocean system may not necessarily hold good for the Indian Ocean. Researchers at the 'Laboratoire de Geosciences Marines and Centre de Recherches Petrogaphiques et Geochemiques' have reported that the earth's mantle in the Indian Ocean is entirely different in its chemical composition from that of the Pacific and Atlantic Oceans. The mantle of the Indian Ocean is characterized by a lack of titanium. Titanium is highly resistant to heat with a melting temperature as high as 1668



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