May 19, 2005
Thursday

Nearly six months after the massive Indian Ocean earthquake that generated a deadly tsunami, the planet is still "ringing like a bell," scientists say.

The worst devastation and more than 300,000 deaths occurred in parts of Indonesia, Sri Lanka and Thailand closest to the 800-mile-long crust rupture. But the event was so powerful that no point on Earth was left undisturbed, according to new reports on the quake and its aftermath published online Thursday by the journal Science.

"The Earth is still ringing like a bell today." - Roland Burgmann, University of California-Berkeley. "Even among seismologists, we call this a monster earthquake." - Thorne Lay, University of California-Santa Cruz.

The Dec. 26 quake was the second-largest ever recorded by instruments, and the third-most-deadly in human history. The energy released was equivalent to the amount of energy consumed by the entire United States for six months, said Roger Bilham, a University of Colorado-Boulder seismologist who has studied historic earthquakes in the region and wrote a perspective article on the new findings.

The seafloor's uplifting displaced more than 30 cubic kilometers of water, generating a tsunami that was recorded in oceans worldwide and leaving global sea level about .1 millimeter higher due to the smaller capacity of the Indian Ocean.

"No point on Earth remained undisturbed at the centimeter level," Bilham said.

"The Earth is still ringing like a bell today" because a huge chunk of its surface has been distorted, said Roland Burgmann, a professor of earth and planetary science at the University of California-Berkeley and co-author of one of the papers.

Using data on ground movements from global positioning system stations in the region, Burgmann and colleagues from India and the U.S. Geological Survey modeled the fault motions that would have produced the ground movement. They determined that the initial earthquake was both stronger and slower than previously thought.

The scientists concluded that the quake was probably a magnitude 9.15 - twice as strong as the original 9.0 estimate - and that much of the slippage along the northern part of the fault near the Andaman Islands took place more than half an hour after the initial rupture off Sumatra and continued for as long as three hours afterward.

Bilham noted that if not for the slow movement at the northern end of the quake, there would have been much more damage on the coasts of India, Myanmar and Thailand, and cautioned that additional shifts along the same fault lines could bring more devastation in the near future, particularly at the southern end of the zone.

Meanwhile, a team led by Thorne Lay of the University of California-Santa Cruz, who also heads a consortium called the Incorporated Research Institutions for Seismology, concluded that the quake might even have been a magnitude 9.3.

Lay said that the ground shook 100 times harder than it did during Northern California's 6.9 Loma Prieta temblor in 1989. "Even among seismologists, we call this a monster earthquake," he said.

The December quake and a March 28 secondary rupture along the same fault near the island of Nias just to the south are the largest events seen since deployment of a global network of digital broadband seismometers began in 1997, which the Lay team used.

This network showed that after the first event, the ground moved up and down more than 3-1/2 inches in Sri Lanka, 1,000 miles from the epicenter, and by more than 1 centimeter (0.4 inch) everywhere else on the planet within 21 minutes, as seismic waves spread.

Researchers noted that the quake's shock waves seemed to travel to the northwest from Sumatra, with seismometers in Russia picking up the approach at a higher pitch, while similar instruments in Australia picked up the waves from the quake at a lower frequency as the active part of the quake moved away from them. This Doppler-like effect of seismic waves was similar to the change in sound of a roaring fire engine or train as it approaches or moves away.

Another group of researchers used models based on the normal quivering of the planet, called free oscillations, to measure the quake's development. These scientists will eventually study how the seismic waves moved through Earth to better understand the makeup of the interior.

"Just like thumping a watermelon to hear if it is ripe, after a big earthquake thumps our planet, we measure the Earth's natural tones from seismograms to detect properties of the Earth's deep mantle and core," said Jeffrey Park, a professor of geology and geophysics at Yale University.

On the Net:

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