An 8.7 magnitude earthquake that struck west of Indonesia on April 11 this year was the biggest of its kind ever recorded, confirming suspicions that a giant tectonic plate is breaking up, according to a new study.

The quake previously was reported as 8.6 magnitude. The new estimate means it was 40 per cent stronger, said scientists from the University of Utah and the University of California.

The 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 sub-plates along a yet-unclear boundary beneath the Indian Ocean west of Sumatra and southeast of India.

“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,” said study co-author Keith Koper.

The 8.7 jolt also “is probably the largest intra-plate [within a single tectonic plate of Earth’s crust] ever seismically recorded”, researchers added.

It was caused by at least four undersea fault ruptures southwest of Sumatra, Indonesia, within a two-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.

The findings are published in the journal Nature.