Seismologists（地震学家） have known for years that the Indo-Australian plate of Earth's crust is slowly breaking apart, but they saw it in action last April when at least four faults broke in a magnitude-8.7 earthquake that may be the largest of its type ever recorded. The great Indian Ocean quake of April 11, 2012 previously was reported as 8.6 magnitude, and the new estimate means the quake was 40 percent larger than had been believed, scientists from the University of Utah and University of California, Santa Cruz, report in the Sept. 27 issue of the journal Nature.

The quake was caused by at least four undersea fault ruptures（断层断裂） southwest of Sumatra, Indonesia, within a 2-minute, 40-second period. It killed at least two people, and eight others died from heart attacks. The quake was felt from India to Australia, including throughout South Asia and Southeast Asia.

If the four ruptures were considered separate quakes, their magnitudes would have been 8.5, 7.9, 8.3 and 7.8 on the "moment magnitude" scale used to measure the largest quakes, the scientists report.

The 8.7 main shock broke three faults that were parallel but offset from each other -- known as en echelon（雁列式的，梯形） faults -- and a fourth fault that was perpendicular to and crossed the first fault.

The new study concludes that the magnitude-8.7 quake and an 8.2 quake two 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 -- a process that started roughly 50 million years ago and that will continue for millions more.

"We've never seen an earthquake like this," says study co-author Keith Koper, an associate professor geophysics and director of the University of Utah Seismograph Stations. "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."

All four faults that broke in the 8.7 quake and the fifth fault that ruptured in the 8.2 quake were strike-slip faults, meaning ground on one side of the fault moves horizontally past ground on the other side.

The great quake of last April 11 "is possibly the largest strike-slip earthquake ever seismically recorded," although a similar size quake in Tibet in 1950 was of an unknown type, according to the new study, which was led by two University of California, Santa Cruz, seismologists: graduate student Han Yue and Thorne Lay, a professor of Earth and planetary sciences. The National Science Foundation funded the study.

The 8.7 jolt also "is probably the largest intraplate（内陆板块） [within a single tectonic plate of Earth's crust] ever seismically recorded," Lay, Yue and Koper add. Most of Earth's earthquakes occur at existing plate boundaries.

The researchers cannot be certain the April great quake was the largest intraplate quake or the largest strike-slip quake because "we are comparing it against historic earthquakes long before we had modern seismometers（地震仪）," says Koper.