The fault that caused the 9.0 earthquake in Indonesia in December 2004 and the devastating tsunami that followed could still cause some big ruptures, U.S. researchers said Thursday.
Analysis of the damage of an 8.7 quake that followed in the same area three months later shows potential for large movements south of the 2004 and 2005 ruptures, said Richard Briggs and Kerry Sieh of Caltech.
"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," he said. "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 caused dramatic warping and uplift among the islands and coral atolls in the Sumatran archipelago, the researchers reported 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 high in some places.
This suggests the fault between the Australian and Sunda plates slipped by about 35 feet 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 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 that 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.