Indonesia quakes triggered global spike in tremors - study

* First time jump in quakes observed after big tremor

* More quakes likely in area, with risks to Sumatra

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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