Video: Indonesia quake science

By Alan Boyle Science editor
updated 3/28/2005 9:28:01 PM ET 2005-03-29T02:28:01

Seismologists who have been studying the devastation left behind by the Dec. 26 quake and tsunami warned just two weeks ago that another big earthquake was likely to hit the Indian Ocean area. So when an 8.7 magnitude earthquake hit southwest of the Indonesian island of Sumatra on Monday, scientists were quick to spring into action.

Less than an hour after the tremor was recorded at 11:09 a.m. ET, the U.S. National Oceanic and Atmospheric Administration's Pacific Tsunami Warning Center raised the alert. While the Hawaii-based center has no monitoring instruments capable of sensing a tsunami in the Indian Ocean, it called on authorities in the region to "take immediate action."

"This action should include evacuation of coasts within 1,000 kilometers of the epicenter and close monitoring to determine the need for evacuation further away," the advisory said. Alerts were quickly sounded in areas as far away as Thailand and Singapore. In contrast, it took hours after the Dec. 26 earthquake for the tsunami warning to get out, and by that time it was mostly too late.

Quake ranks high
At 8.7 magnitude, Monday's earthquake was roughly half the strength of the December quake, which measured 9.0 on the logarithmic magnitude scale. Monday's event ranked as "one of the largest earthquakes in the past century," said Kerry Sieh, a tsunami researcher at the California Institute of Technology. Additional aftershocks followed Monday's main jolt, said George Choy, a spokesman for the U.S. Geological Survey's National Earthquake Information Center.

Ready to rumble?Although scientists cannot predict exactly when seismic events will occur, Monday's quake was not unexpected. In the March 17 issue of the journal Nature, researchers at the University of Ulster-Coleraine in Northern Ireland reported that stress was building in the Sumatran subduction zone as well as in the adjacent Sunda Trench. They warned that the stress was likely to be released in another seismic event.

The faults are part of the Pacific "ring of fire," where continental plates grind against each other and spark periodic seismic shocks. Three months ago, a dramatic shift in the ocean floor along the Sumatra fault set off the waves inundating coastal areas in Indonesia, Thailand, Sri Lanka, India and even as far afield as Somalia.

The Dec. 26 quake set off international calls for better monitoring of Indian Ocean quakes, as well as better plans for emergency response to tsunamis. Officials have been discussing ways to beef up the international tsunami monitoring system, but little action has been taken on that front so far.

"The Indian Ocean still does not have the tide level gauges or the detection buoys that we enjoy in the Pacific," NOAA spokesman Greg Romano told

Why no killer wave?
Even though Monday's quake was weaker than the Dec. 26 shock, researchers and emergency officials nevertheless braced for a large-scale tsunami — a killer wave that never came.  Instead, only a "small tsunami" was reported in Australia's Cocos Islands, south of Sumatra. Coastal tide gauges there rose only 4 inches (10 centimeters), about a third of the rise recorded after the Dec. 26 quake.

One reason for the smaller wave may have to do with the geometry behind Monday's seismic shock. Researchers said the bulk of the energy from Monday's quake was apparently focused toward the southwest — more southerly than the course taken by the Dec. 26 tsunami.

Eddie Bernard, director of NOAA's Pacific Marine Environmental Laboratory in Seattle, said the southward course was predicted by the lab's computer models for tsunami directionality. He noted that the epicenter was southwest of the Sumatran coastline — close to the islands of Nias and Simeulue, which absorbed the brunt of the quake.

"Tsunami directionality is greatly influenced by the coastline," Bernard told "If it reflects and concentrates its energy, it will head southwest."

John Armbruster, a seismologist at Columbia University's Lamont-Doherty Earth Observatory, speculated that the direction of the seismic thrust may have minimized the wave disturbance. "The amount of motion up and down of the ocean floor was not so great and perhaps was more to the side," he told NBC News.

Even without a substantial tsunami, the earthquake had a deadly impact. Hundreds of people were killed on the island of Nias when their homes collapsed on them.

Researchers agreed that there was still more shaking in store for the region. "There will be earthquakes in that area, to be sure," Bernard said. "Exactly when they will occur is the question."

Armbruster said the shocks in December and on Monday could be part of a larger pattern of activity along the Sumatra fault, trending toward the southeast. "There is a significant chance of this extending even further," he said.

Tsunami awareness in U.S.
As was the case for the Dec. 26 tsunami, Monday's event had virtually no chance of significantly affecting Hawaii or the West Coast of the United States.

Coincidentally, officials in Alaska have been preparing for the first statewide test of a broadcast tsunami alert system on Wednesday. "We've never done this before, and we need to, because if there ever were a tsunami, we'd want people to be notified," said Tracey Lake, a spokeswoman for the National Weather Service.

While the test was still set to go forward, Lake said it could conceivably be postponed if there were more Indian Ocean aftershocks — which might lead emergency officials to the conclusion that they were dealing with a real tsunami rather than a practice run.

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