The April 11 quake, felt from India to Australia and throughout South Asia and Southeast Asia, was caused by at least four undersea fault ruptures southwest of Sumatra, Indonesia, within a 2-minute, 40-second period, they said.
Writing in the journal Nature, scientists from the University of Utah and University of California, Santa Cruz, said the magnitude-8.7 quake, and an 8.2 quake 2 hours later, were part of the breakup of the Indian and Australian subplates along a yet-unclear boundary beneath the Indian Ocean west of Sumatra and southeast of India.
The breakup was part of a process that started roughly 50 million years ago and will go on for millions more, they said.
"We've never seen an earthquake like this," study co-author Keith Koper, a geophysics professor at the University of Utah, said. "This is part of the messy business of breaking up a plate. This is a geologic process. It will take millions of years to form a new plate boundary and, most likely, it will take thousands of similar large quakes for that to happen."
Seismic analysis shows the quakes "involve rupture of a very complex network of faults, for which we have no documented precedent in recorded seismic history," the researchers wrote in the Nature article.