Web address:
http://www.sciencedaily.com/releases/2008/12/
081203131042.htm
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
Barbara.
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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California Institute of Technology (2008, December 4). Another Large Earthquake
Off Coast Of Sumatra Likely. ScienceDaily. Retrieved December 4, 2008, from
http://www.sciencedaily.com /releases/2008/12/081203131042.htm
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