* First time jump in quakes observed after big tremor
* More quakes likely in area, with risks to Sumatra
By David Fogarty
SINGAPORE, Sept 27 (Reuters) - Two massive quakes off Indonesia's Sumatra island in April showed something scientists have long been trying to prove - they can trigger a swarm of more tremors around the globe.
The latest findings could eventually help efforts to predict some earthquakes.
Over the six days after the quakes hit on April 11, the number of tremors of magnitude greater than 5.5 jumped nearly fivefold globally, lead author Fred Pollitz of the U.S. Geological Survey and colleagues found in a study published in the journal Nature on Thursday.
"This is the first time that we've seen these remote aftershocks at this magnitude and to this extent," said Kerry Sieh, director of the Earth Observatory of Singapore, author of a number of studies on earthquake hazards in the Sumatra region.
The first quake on April 11 was 8.7 magnitude, nearly as large as the one which triggered a massive tsunami and nuclear disaster in eastern Japan just over a year earlier.
The second one was also huge, at 8.2. Both occurred under the Indian Ocean several hundred kilometres west of Sumatra, in a zone where the Indo-Australian tectonic plate is slowly tearing itself apart.
"The Pollitz paper shows that you can get significant, potentially destructive-sized earthquakes thousands of kilometres away from a mid-8 earthquake," Sieh, who was not part of the current study, told Reuters.
LOVE WAVES
Pollitz and colleagues examined a range of seismic data, with particular focus on so-called Love-wave radiation, surface waves that move in a snake-like motion and can travel right across the globe.
All the global aftershocks were located right along the four main lines of the Love-wave radiation that began at the epicentre of the main April 11 quake.
In future, studying such data could improve calculations about the increased likelihood of earthquakes after a major quake, said Sieh.
Scientists rushed to analyse the April quakes because they occurred in one of the world's most seismically active zones.
The 2012 quakes are believed by most seismologists to be the largest strike-slip tremors ever recorded, and all the more noteable for not triggering a large tsunami.
Strike-slip faults slide horizontally when they fracture, rather than triggering a sudden uplift of the sea floor like the 9.2 quake that struck off nearby Aceh province in northern Sumatra in December 2004, or last year's 9.0 quake off Japan. Both triggered deadly tsunamis. (Editing by Jonathan Thatcher)
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