PASADENA, Calif. - Regions of the Earth previously thought to be immune to giant earthquakes might actually be at high risk of experiencing them, according to a Caltech study released Wednesday.

Researchers studied the Dec. 26, 2004, Sumatra-Andaman earthquake in the Indian Ocean, which accounted for last winter's devastating tsunami, and concluded that previous ideas about where giant earthquakes are likely to occur need to be revised. Based on their data, researchers determined that the Sumatra-Andaman earthquake had a magnitude of 9.15, making it the third-largest earthquake in the past 100 years.

"This earthquake didn't just break all the records, it also broke some of the rules," said Kerry Sieh, a Caltech geology professor and one of the study's authors.

The earthquake occurred on the Sunda subduction megathrust, a giant earthquake fault. Previously, researchers believed megathrusts could only produce giant earthquakes if the oceanic plate was young and buoyant.

The oceanic crust at the site of the 2004 earthquake is old and dense, and the relative motion between the plates is slow. "For all these reasons, received wisdom said that the giant 2004 earthquake should not have occurred," said Jean-Philippe Avouac, a Caltech geology professor.

"But it did, so received wisdom must be wrong. It may be, for example, that a slow rate of motion between the plates simply causes the giant earthquakes to occur less often, so we didn't happen to have seen any in recent times -- until 2004," Avouac said.

Other subduction zones that were not previously considered to be a risk, but may need to be reassessed, include the Ryukyu Islands between Taiwan and Japan and the Caribbean from Trinidad to Barbados and Puerto Rico.

Better monitoring systems with continuously recording GPS stations should be installed in some subduction zones to assess their seismic potential, Sieh said.

The report will appear in Thursday's edition of the journal Nature.