Another Big One for Indonesia?

Bad news for the survivors of the 2004 Indian Ocean tsunami: Neither the giant earthquake that triggered the killer wave nor the hundreds of smaller temblors that followed have exhausted the area's pent-up seismic energy, a new analysis reveals. The potential remains for a quake large enough to set off another devastating inundation.

The researchers have been poring over an array of landscape and subterranean measurements taken with global positioning system instruments, satellite radar, and seismic records of the area, all collected beginning before the 2004 temblor. The team also gathered information on coral-reef growth patterns and other historical geologic records. They then plugged all the data into a model that computed the amount of energy released by the two major quakes in 2007 and compared that with the strength of two previous giant quakes, which hit the same area in 1797 and 1833. The team reports tomorrow in Nature that the 2007 temblors released only about 25% of the energy that has built up since 1833.

"There must be quite a lot of elastic energy stored that will need to be released by new earthquakes at some point," says earth scientist and co-author Jean-Philippe Avouac of the California Institute of Technology's Tectonics Observatory in Pasadena. Avouac explains that when a major quake releases built-up stresses between geologic plates, it does so unevenly, so that some parts of the fault stay locked up and still under stress--trapping some of the fault's seismic energy.