Swarm. Seismic records since 1990 show that the southwestern coast of Sumatra, Indonesia, has been a hotbed of earthquake activity.

U.S. Geological Survey

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.

On 26 December 2004, the second-largest earthquake on record struck the southwestern coast of the Indonesian island of Sumatra, as a massive portion of the Indian Ocean plate suddenly slipped underneath the Indonesian plate (ScienceNOW, 20 May 2005). The temblor, which registered 9.2 on the local-magnitude scale, set off a tsunami that spread across the Indian Ocean, wrecking coastal communities and killing more than 200,000 people. Three more major quakes--including one with a magnitude equal to that of the 1906 San Francisco earthquake--have since struck the same vicinity, along with hundreds of relatively minor shakes. None of these triggered a tsunami, but the question residents of the region desperately want to know is, "Are the big quakes over?"

The answer, according to a U.S.-Indonesian team, is probably not. 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.

The paper's strength "lies in the authors' great efforts to integrate very diverse type of data" in a way that hasn't been done before, says geologist Jian Lin of the Woods Hole Oceanographic Institution in Massachusetts. The work points out that "just because an area has just had a large earthquake, it doesn't mean that all the [energy] has been released," adds seismologist Arthur Frankel of the U.S. Geological Survey in Denver, Colorado. "The seismic hazard can still be significant," says Frankel. And Seismologist Nano "Leonardo" Seeber of the Lamont-Doherty Earth Observatory in Palisades, New York, says the paper's findings about the "patchiness" of fault rupture behavior can help scientists understand more about tsunami generation--a critical issue if another major quake is indeed on its way.