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Why didn't the quake trigger a tsunami?

By P Rajendran in New York
Last updated on: March 29, 2005 23:55 IST
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Following the earthquake that hit Indonesian island of Nias on Monday night, authorities had issued tsunami warnings.

Fortunately, no major tsunami followed.

Why did the December 26 earthquake result in the killer tsunami while the one on Monday  produce one that, at its highest, did not crest 25 centimeters?

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Experts claim that an undersea earthquake can produce a tsunami if the local plates that form the earth's crust there move up or down sharply, causing water to shift suddenly.

If the recent quake resulted essentially in plates rubbing parallel to each other -- and not lifting or falling -- it could no more than cause local damage and some large-scale agitation of the water, the experts explain.

The location and depth of the epicentre of the quake, the magnitude, the amount of sediments available, the depth of the waters around the quake zone and the movement of sediments underwater, can all play a role.

The United Nations Educational, Scientific and Cultural Organisation International Tsunami Information Centre describes the worst tsunamis as being generated from large, shallow earthquakes with an epicentre or fault line near or on the ocean floor.

According to the US Geological Survey, even though the epicentres of both the December quake and the current one were 30 km deep, the current one was 8.7 on the Richter scale while the earlier one measured 9. Given that the scale is a logarithmic one, making 9.0 on the scale 10 times stronger than 8.0, the difference is significant.

But most seismometers measure only higher frequencies of energy; but most energy in an earthquake comes from low frequency sources. So what is measured on the Richter scale cannot quite explain the effects seen.

While the December quake ruptured almost a quarter of a million square kilometres, it is not clear if the current one covered anywhere as much ground.

Slippage of heavy sediments - often in the form of landslides -- can also cause water to move forward in a rush. This is one of the primary reasons for large tsunamis after even a small earthquake.

Also read:

Fresh quake hits Indonesia
8.7 quake rocks Indian Ocean off Indonesia
Centre withdraws tsunami warning

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P Rajendran in New York