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Another Large Earthquake Off Coast Of Sumatra Likely

ScienceDaily (Dec. 4, 2008) — The subduction zone that brought us the 2004 
Sumatra-Andaman earthquake and tsunami is ripe for yet another large event, 
despite a sequence of quakes that occurred in the Mentawai Islands area in 
2007, according to a group of earthquake researchers led by scientists from the 
Tectonics Observatory at the California Institute of Technology (Caltech).

"From what we saw," says geologist Jean-Philippe Avouac, director of the 
Tectonics Observatory and one of the paper's lead authors, "we can say with 
some confidence that we're probably not done with large earthquakes in Sumatra."

The devastating magnitude 9.2 earthquake that occurred off the western coast of 
Sumatra on December 26, 2004—the earthquake that spawned a lethal tsunami 
throughout the Indian Ocean—took place in a subduction zone, an area where one 
tectonic plate dips under another, forming a quake-prone region.

It is that subduction zone that drew the interest of the Caltech-led team. 
Seismic activity has continued in the region since the 2004 event, they knew. 
But have the most recent earthquakes been able to relieve the previous 
centuries of built-up seismic stress?

Yes . . . and no. Take, for instance, an area just south of the 2004 quake, 
where a magnitude 8.6 earthquake hit in 2005. (That same area had also been the 
site of a major earthquake in 1861.) The 2005 quake, says Avouac, did a good 
job of "unzipping" the stuck area in that patch of the zone, effectively 
relieving the stresses that had built up since 1861. This means that it should 
be a few centuries before another large quake in that area would be likely.

The same cannot be said, however, of the area even further south along that 
same subduction zone, near the Mentawai Islands, a chain of about 70 islands 
off the western coasts of Sumatra and Indonesia. This area, too, has been hit 
by giant earthquakes in the past (an 8.8 in 1797 and a 9.0 in 1833). More 
recently, on September 12, 2007, it experienced two earthquakes just 12 hours 
apart: first a magnitude 8.4 quake and then a magnitude 7.9.

These earthquakes did not come as a surprise to the Caltech researchers. 
Caltech geologist and paper coauthor Kerry Sieh, who is now at the Nanyang 
Technological University in Singapore, had long been using coral growth rings 
to quantify the pattern of slow uplift and subsidence in the Mentawai Islands 
area; that pattern, he and his colleagues knew, is the result of stress 
build-up on the plate interface, which should eventually be released by future 
large earthquakes.

But was all that accumulated stress released in 2007? In the work described in 
the Nature letter, the researchers analyzed seismological records, remote 
sensing (inSAR) data, field measurements, and, most importantly, data gathered 
by an array of continuously recording GPS stations called SuGAr (for Sumatra 
Geodetic Array) to find out.

Their answer? The quakes hadn't even come close to doing their stress-reduction 
job. "In fact," says Ali Ozgun Konca, a Caltech scientist and the paper's first 
author, who did this work as a graduate student, "we saw release of only a 
quarter of the moment needed to make up for the accumulated deficit over the 
past two centuries." (Moment is a measure of earthquake size that takes into 
account how much the fault slips and over how much area.)

"The 2007 quakes occurred in the right place at the right time," adds Avouac. 
"They were not a surprise. What was a surprise was that those earthquakes were 
way smaller than we expected."

"The quake north of this region, in 2005, ruptured completely," says Konca. 
"But the 2007 sequence of quakes was more complicated. The slippage of the 
plates was patchy, and it didn't release all the strain that had accumulated."

"It was what we call a partial rupture," adds Avouac. "There's still enough 
strain to create another major earthquake in that region. We may have to wait a 
long time, but there's no reason to think it's over."

Their findings were published in a letter in the December 4 issue of the 
journal Nature.

Other authors on the paper include Anthony Sladen, Aron J. Meltzner, John 
Galetzka, Jeff Genrich, and Don V. Helmberger from Caltech; Danny H. 
Natawidjaja from the Indonesian Institute of Science (LIPI); Peng Fang and 
Yehuda Bock from the Scripps Institution of Oceanography in La Jolla; Zhenhong 
Li from the University of Glasgow in Scotland; Mohamed Chlieh from the 
Université de Nice Sophia-Antipolis in France; Eric J. Fielding from the Jet 
Propulsion Laboratory; and Chen Ji from the University of California, Santa 

The work was supported by funding from the National Science Foundation and the 
Gordon and Betty Moore Foundation.

Journal reference:

   1. . Partial rupture of a locked patch of the Sumatra megathrust during the 
2007 earthquake sequence. Nature, December 4, 2008

Adapted from materials provided by California Institute of Technology, via 
EurekAlert!, a service of AAAS.
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