The Hindu : National : Tsunamis return after 60 years

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Monday, Dec 27, 2004

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National

Tsunamis return after 60 years

By N. Gopal Raj

It is pell-mell near the Vivekananda Rock Memorial, Kanyakumari, where three passenger boats were washed ashore. - Photo: A. Shaikmohideen

THIRUVANANTHAPURAM, DEC. 26. Tsunamis returned to devastate India after an interval of more than 60 years. The tsunamis of 1941 and, before that, of 1881 were set off by earthquakes in the Andaman and Nicobar Islands.

Where the sea and the road merged. A scene on the Marina beach in Chennai. - AP

The islands are earthquake-prone as they are close to the zone where the section of the earth's crust carrying India and Australia is sliding below the section that holds Asia. The two sections are converging at an estimated rate of 5.4 cm a year, according to a paper in the scientific journal, Current Science, last year. As the sections slowly slip past one another, strains develop at various points and they are released in the form of earthquakes.

Today's undersea earthquake off Sumatra appears to have set off smaller quakes in the vicinity of the Andaman and Nicobar islands. At least one of them had a magnitude of over 7, according to data on the United States Geological Survey (USGS) website.

Trigger earthquakes?

Fish abandoned at the Visakhapatnam Fishing Harbour. - Photo: K.R. Deepak

The USGS data also showed that earthquakes in the Andaman and Nicobar islands had occurred up to 9 degrees northwards and 2 degrees westwards of the one in Sumatra. Tremors from the Sumatran earthquake might have caused "trigger earthquakes" at faults in and around the Andaman and Nicobar islands where strain had been building up, said C.P. Rajendran of the Centre for Earth Science Studies (CESS) here. Dr. Rajendran was the lead author of the Current Science paper.

The earthquake that occurred on June 26, 1941 is thought to have exceeded a magnitude of 8.5. "It is quite likely that the magnitude of this earthquake has been underestimated," the authors said in their Current Science paper. The quake caused extensive damage in the Andamans, including bringing down the central tower of the infamous Cellular Jail where many freedom fighters were imprisoned. The earthquake set off a tsunami that inundated the western coast of the Andaman Island and then hit the Indian east coast, destroying property and killing people.

The magnitude 7.5 earthquake of December 31, 1881 is thought to have occurred under the sea off the Car Nicobar Island. It too generated a tsunami.

Nearly a dozen earthquakes bigger than magnitude 5 have occurred in the region since 1973, with the one over magnitude 6 occurring in January 1983, according to the Current Science paper. But data from the country's tide gauges, available from the mid-1970s, do not indicate that any tsunamis had occurred, said Satish Shetye, Director of the National Institute of Oceanography at Dona Paula in Goa.

Although undersea earthquakes are the most common cause of tsunamis, submarine landslides, underwater volcanic eruptions and the large meteorities plunging into the sea can also set off these killer waves. A tsunami can race across the water at speeds of 500 to 1,000 km per hour. In the open ocean, however, the waves of a tsunami may be only 30 cm to 60 cm in height and can pass unnoticed. But as the tsunami nears a coastline and the water depth falls sharply, the waves slow down but gain in height.

Series of waves

A tsunami can be 10 to 20 m high when it hits the shore. One with waves almost half a kilometre high slammed into Alaska in 1958. Moreover, a tsunami is not one giant wave, but a series of waves that come ashore at intervals of 10 to 45 minutes.

With nearly 800 of them recorded between 1900 and 2001, the Pacific Ocean is where the most tsunamis occur, and 17 per cent of them are generated in or near Japan. As a result, countries in and around the Pacific established a Tsunami Warning System. Seismic stations pinpoint the earthquakes while seafloor pressure recorders detect tsunamis even one centimetre high and promptly pass this data over satellite. All this information is combined with extensive computer modelling to find out which places could be at risk.

Open sea tidal gauges would help in detecting if a tsunami had been generated, Dr. Shetye observed. Such gauges would be expensive and not easy to set up.

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