April Sumatra quakes signal Indian ocean plate break-up

By Jonathan Amos
Science correspondent, BBC News

image captionThe 11 April quakes created panic but there were few deaths (mostly from heart attacks)

The sequence of huge earthquakes that struck off the coast of Sumatra in April may signal the creation of a new tectonic plate boundary.

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

They say their analysis of the tremors - 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 rigid outer shell are converging on each other at a rate of about 5-10cm/yr.

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

It is friction 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 tsunami.

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 vertically 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 deformation 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."

All around the world

In a second Nature paper, Thorne Lay, of the University of California, Santa Cruz, and colleagues provide some fascinating detail on this interplay of faults and how they ruptured on 11 April. Their seismic analysis indicates at least four faults were involved in the main 8.7 event, which lasted about 160 seconds.

Three of the faults were parallel but offset from each other; the fourth was perpendicular to and crossed the first fault.

The 8.7 jolt "is probably the largest intraplate (within a single tectonic plate of Earth's crust) ever seismically recorded," Prof Lay's team remarked.

The third Nature paper describes how this historic quake triggered other tremors around the world.

This effect has been noted before, but Dr Fred Pollitz, from the US Geological Survey, and co-workers were surprised by the delays involved.

Dr Pollitz told the BBC: "For the vast majority of earthquakes, you can expect an aftershock zone not to really go beyond [1,000km].

"But it's also known that very large mainshocks - like the Japanese event we had last year, the magnitude 9 event off north-east Japan - can trigger earthquakes around the world. Most of these triggered events are small and they occur instantly as the seismic waves from the large event are passing by.

"But this April 2012 earthquake triggered many larger and potentially damaging earthquakes around the world, and with a time delay of hours or up to several days. That effectively extended the aftershock zone to the entire globe."

Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos

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