The April 11 earthquake off Sumatra was unusually large and, like a few other recent large temblors, triggered small quakes during the 3 hours it took for seismic waves to travel through Earth's crust.
However some faults weren't rattled enough by the seismic waves to fail immediately but were primed to break up to six days later, researchers at the University of California, Berkeley, reported Wednesday.
Seismologists said their analysis found five times the expected number of quakes during the six days following the April 11 quake and aftershock.
The findings should be a warning to those living in seismically active regions worldwide that the risk from a large earthquake could persist -- even on the opposite side of the world -- for more than a few hours, researchers said.
"Until now, we seismologists have always said, 'Don't worry about distant earthquakes triggering local quakes,'" earth and planetary science Professor Roland Burgmann said.
"This study now says that, while it is very rare -- it may only happen every few decades -- it is a real possibility if the right kind of earthquake happens."
It is possible the East Indian Ocean quake triggered a cascade of smaller, undetectable quakes on distant faults that led to larger ruptures later on, Burgmann said.