WASHINGTON—The pressure was building deep underground in the Indian Ocean off the coast of Indonesia, and scientists knew disaster could be coming. They warned of a potential earthquake that could trigger the second catastrophic tsunami in three months.
So on Monday, when a massive 8.7-magnitude quake shook the region—just as it was forecast to do two weeks ago—the world waited for yet another killer tsunami.
But by the end of the day it hadn't materialized.
People fled the coast and sought higher ground, as they should have. And scientists gazed at readings from the few tide gauges in the Indian Ocean region. They found a small tsunami on two remote islands, but nothing more.
Hours passed and the all-engulfing waves that once killed hundreds of thousands of people never seemed to appear.
Hundreds of people did die, nonetheless, but most perished from damage produced by the quaking ground. Based on an early analysis, Monday's quake was tied for the seventh strongest earthquake ever registered since modern measuring devices recorded earthquakes.
Scientists caution that failing to find evidence of a killer tsunami Monday doesn't prove it didn't occur. Even in an era of satellite imaging and instant news, a tsunami may have struck an area with such force that the devastation may take hours to be noticed.
After studying the scene for more than six hours Monday, scientists are somewhat baffled.
"The mystery is why there was not a large tsunami right near" the site of the quake, said Frank Gonzalez, who heads the National Oceanic and Atmospheric Administration's tsunami inundation mapping efforts.
Monday's quake demonstrates how much science has learned about what causes the earth to shake, but how little it knows about how a tsunami develops. Compounding the problem of detection is the lack of monitoring devices in the Indian Ocean.
Scientists have two theories about what happened Monday. Either no tsunami was produced, or one was formed but headed out to sea and away from populated areas.
Last year's 9.0 Indian Ocean earthquake ruptured the ocean floor in a way that pushed a tsunami at nearly the worst possible direction—toward the most vulnerable areas with the most people. But the ground may have split Monday in a different way—either in direction or exact location—that kept the waves and their deadly effects to a minimum, said Eric Geist, a U.S. Geological Survey geophysicist in Menlo Park, Calif.
One possibility is that the earthquake split along a fault plane that was more horizontal instead of vertical, according to an early estimate out of Harvard, said USGS geologist Brian Atwater. A tsunami occurs when a ridge thrusts up and pushes water upward. Monday's quake may not have displaced any water.
It's also possible that the waves generated went southwest—away from populated areas, Geist said. The rupture could have been in shallower areas that wouldn't have caused a big tsunami, he said. An NOAA computer model showed a similar pattern.
"It's amazing, considering that those epicenters were not that far apart," Geist said. "We'll just have to wait and see."
USGS oceanographer Bruce Jaffe, who's leading a team of scientists already en route to the area to study last year's tsunami, said it's still too early to determine why nothing may have happened. He said it's still a good possibility that there was a devastating tsunami, but the news hasn't reached the rest of the world yet.
"My experience with tsunamis, especially larger tsunamis, is that it takes a while to find out about it," Jaffe said. "No news is not good news."
But if the tsunami part of what happened is still murky, the earthquake science isn't.
Monday's temblor was a classic "trigger earthquake."
A major earthquake reduces stress on one part of a fault but massively increases stress in other parts. The pressure builds down the fault line. A similar situation happened in Turkey in the late 1990s.
In the March 17 issue of Nature, John McCloskey of the University of Ulster in Ireland pinpointed two areas off Sumatra, near where the Dec. 26 quake occurred, that were ripe for a major earthquake.
Australian government seismologist Phil Cummins also warned of future quakes, emphasizing the need for an Indian Ocean tsunami warning system. The quake came before the area could get tsunami monitoring gear up, something that's still about a year away, he said.
With Monday's quake, scientists will be watching stress in the same fault line, only farther south.
"We do have to be concerned about the rest of Sumatra," Cummins said. "It's going to go at some point within the next 50 years."
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(c) 2005, Knight Ridder/Tribune Information Services.
GRAPHICS (from KRT Graphics, 202-383-6064): 0050328 Quake detail, 20050328 Quake locations, 20050328 Indonesia quake
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