Analysis of the damage of a quake that followed in the same area three months later shows potential for large movements south of the 2004 and 2005 ruptures, Richard Briggs and Kerry Sieh of the California Institute of Technology said.
''This southern part is very likely about ready to go again,'' Sieh said in a statement.
''It could devastate the coastal communities of southwestern Sumatra, including the cities of Padang and Bengkulu, with a combined population of well over a million people. It could happen tomorrow, or it could happen 30 years from now, but I'd be surprised if it were delayed much beyond that.'' The aftershock of the 2004 quake killed more than 2,000 Indonesians. It was one of the most powerful earthquakes recorded, with a magnitude of 8.7.
It caused dramatic warping and uplift among the islands and coral atolls in the Sumatran archipelago, the researchers report in Friday's issue of the journal Science.
Sieh, Briggs and colleagues used satellite data and measurements of the low-tide line in corals before and after the March 2005 aftershock to show it caused strips of uplift nearly 10-feet (3-meters)-high in some places.
This suggests the fault between the Australian Plate and the Sunda plate slipped by about 35 feet (11 meters) under the Sumatran archipelago.
They also found a place, which they called a locked segment, on the northwestern edge of the March 2005 quake rupture that separates it from the 1,000-mile (1,600-km) December 2004 rupture.
The little segment under the island of Simeulue that separates the two quake ruptures might act as a barrier that limits how far a crack spreads, the researchers said.
Previous work by the Caltech group and their Indonesian colleagues found another locked segment south of this point, which has not broken since a magnitude 9.0 earthquake in 1833.
They suggest this could be the next area to go.
''We were fortunate to have installed the geodetic instruments right above the part that broke,'' Sieh said in a statement.
''This is the closest we've ever gotten to such a large earthquake with continuously recording GPS (global positioning system) instruments.'' The 2005 earthquake did not trigger a tsunami comparable to the one that followed the 2004 event, partly because it caused a smaller rupture and in part because it lay more under land than under water.
And, because they rose during the earthquake, the islands escaped the small tsunami that did follow, the researchers said.
REUTERS SK RN0547