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Why tsunami stayed away
- Killer wave near Java would have taken 5 hours to hit shore

New Delhi, July 17: Within 15 minutes of the 7.2 magnitude undersea earthquake south of Java today, scientists at India’s tsumani watch centre in Hyderabad were peering at Indian Ocean sea level data relayed via satellite.

The earthquake triggered a tsunami featuring two-metre high waves in Java and 60-cm waves on Christmas Island, killing over 80 people and wrecking a beach resort.

But there was no sign of tsunami waves heading towards either the Andaman and Nicobar Islands or the Indian mainland, the scientists said.

“We didn’t see any significant sea level changes on the Indian side,” said Shailesh Nayak, the director of the Indian National Centre for Ocean Information Services. “There was nothing unusual several hours after the quake.”

The coastal belt along the Andamans and Bengal were put on high alert but no central drill was activated.

The location of the epicentre today ' 260 km south of Bandung in Java ' suggests that tsunami waves, had they moved towards India, would have taken four hours to strike the Andamans and five hours to strike the mainland.

Geologists and oceanographers said the absence today of a widespread tsunami of the class that wreaked havoc across South Asia on December 26, 2004, was due to differences in the mechanisms underlying the two triggering earthquakes.

Not all undersea earthquakes lead to tsunamis. The nature of the tsunami depends on the amount of water that the earthquake displaces through the movement of the crust.

“The greater the vertical movement of the crust, the larger the displacement of water, and the higher the chances of a tsunami,” said Shyam Rai, the head of seismology at the National Geophysical Research Institute in Hyderabad.

“A lateral or horizontal movement of the crust in an earthquake might not lead to a large tsunami,” Rai said. “The 2004 Indian Ocean tsunami had been triggered by a massive vertical shift of the crust that led to waves several metres high.”

Tsunami history suggests that the majority do not cause death or destruction. According to the Pacific Tsunami Warning Centre, during the 100-year period from 1901 through 2001, 796 tsunamis were recorded in the Pacific Ocean. Among these, 117 tsumanis caused casualties and damage close to the epicentre of the earthquake. Only nine resulted in widespread destruction.

India is setting up a tsunami warning centre that will rely on a network of undersea sensors and sea level measurement devices placed at strategic points in the Indian Ocean and along the coasts of the Andaman and Nicobar Islands and the mainland.

The process of testing sensors and the data collection and analysis is now under way, Prem Shankar Goel, the ocean development secretary, said today. The full-fledged tsunami warning system is expected to be ready by September next year.

Until then, the Hyderabad cell will serve as an interim tsunami watch centre, operating round the clock.

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