Scientists give the assessment¹ in this week's Nature journal.

 

They say their analysis of the tremors² - the biggest was a magnitude 8.7 - suggests major changes are taking place on the ocean floor that will eventually split the Indo-Australian plate in two.

 

It is not something that will happen soon; it could take millions of years.

 

"This is a process that probably started eight to 10 million years ago, so you can imagine how much longer it will take until we get a classic boundary," said Matthias Delescluse from the Ecole Normale Superieure in Paris.

 

Dr Delescluse is an author on one of three scholarly papers in Nature discussing the 11 April quakes.

 

Sumatra sits above the collision between the Indo-Australian plate and the Sunda plate.

 

These vast segments of the Earth's rigid³ outer shell are converging⁴ on each other at a rate of about 5-10cm/yr.

 

The elongated⁵ Indo-Australian, which comprises much of the Indian Ocean floor, dives under the Sunda, which carries the Indonesian island. 

 

It is friction⁶ at their boundary - the sticking and unsticking, and the sudden release of stored energy - that is at the root of so many violent quakes, such as the magnitude 9.1 event on 26 December 2004 that set off a catastrophic tsunami⁷.

 

But the 11 April 2012 tremors, although also immense in scale, did not have the same impact and generated no tsunami.

 

This can be explained by the nature of the faulting: so-called strike-slip, where rock moves horizontally either side of the line of breakage, as opposed to vertically⁸ in tsunamigenic thrust faults. 

 

The April tremors were also much further west, located directly on the Indo-Australian plate itself in an area of large-scale deformation⁹ and multiple faulting.

 

Dr Delescluse said it was evident that movement at the plate's ends was stressing the middle. 

 

"Australia already moves with respect to India, and India already moves with respect to Australia," he told the BBC World Service Science In Action Programme.

 

"They are separated by a lot of faults. And if you look on Earth today, between plates you have only one fault. So, the process we are talking about is how we go from several faults to only one fault. 

 

"That's the question - we don't know how long it takes to weaken one so that it localises all the deformation and the others stop being active. At the moment, a lot of faults in the Indian Ocean are active."

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