Quake follows scientists’ predictions

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 MSNBC.com. "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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