The Hindu : National : Second major quake confirms warnings

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Thursday, Mar 31, 2005

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National

Second major quake confirms warnings

By N. Gopal Raj

THIRUVANANTHAPURAM, MARCH 30. Ever since last December's earthquake off Sumatra, scientists had been expecting another major earthquake further south, like the one on Monday night. With the current state of scientific knowledge, however, it was impossible for them to predict when earthquakes would happen.

The December 26 quake, which unleashed a tsunami that devastated many Asian countries, occurred near the coast of northern Sumatra. Here, the section of the earth's crust (or `plate') carrying India and the Indian Ocean is being pushed beneath the small Burma Plate by the former's north-eastward movement. The movement of the two plates is not smooth and uniform. Instead, they can remain locked for long periods. Then, when the accumulated stress becomes too great, the fault — the contact area between the two plates — is ruptured, setting off an earthquake. While the Indian plate slips further down, the edge of the Burma plate is pushed upwards. This upward movement of the plate, if it happens below an ocean, can cause a tsunami.

`Considered dangerous'

Soon after the December 26 earthquake, Kerry Sieh of the Tectonics Observatory at the California Institute of Technology said that it was caused by a rupture of only the northernmost portion of the Sumatran section of the fault. Southern parts of the section had generated great earthquakes and the attendant large tsunamis in the past.

"Thus, other parts within the section of this fault should be considered dangerous over the next few decades," Mr. Sieh had warned.

In February this year, two scientists at the Northwestern University in the U.S. found that the December quake was three times more powerful than originally estimated, making it the second largest earthquake ever recorded. Seth Stein and Emile Okal also warned of a great earthquake on the Sumatran fault segments to the south and a resultant large tsunami.

Earlier this month, the scientific journal Nature published a paper by British scientists that spoke of significantly increased levels of stress in sections of the fault near Sumatra. Earthquakes cluster in space and time. "Where you have one earthquake you are likely to have others," said Prof. John McCloskey of the University of Ulster had said in a press statement.

While last night's earthquake off Sumatra was not unexpected, the question remains as to why such a powerful undersea quake did not trigger a tsunami. None of the Indian tide gauges, including those in the Andaman Islands, recorded a tsunami, Harsh K. Gupta, Secretary to the Centre's Department for Ocean Development, told The Hindu .

Although tsunamis had been recorded as a result of the recent quake, they were not destructive, said an agency report that quoted Laura Kong, director of the Hawaii-based International Tsunami Information Center.

A BBC report said seismologists believed a major tsunami had been averted because a much smaller area of the ocean floor moved this time.

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