Sumatra megaquake defied theory
- From: "George" <george@xxxxxxxxxxxxxxxxxxxxx>
- Date: Tue, 28 Mar 2006 05:38:17 GMT
http://www.eurekalert.org/pub_releases/2006-03/gsoa-smd032706.php
Boulder, Colo. - The risks of Sumatra-style mega-quakes around the world
have been sorely misjudged, say earth scientists who are re-examining some
of the pre-December 2004 assumptions scientists made about such rare
events.
For more than two decades geologists had thought that the largest quakes,
of magnitude 9 and greater, happen when a young tectonic plate is
subducted, or shoved quickly, under another plate. But the Great
Sumatra-Andaman earthquake of 26 December 2004 didn't match that pattern at
all. The Indian Plate is middle-aged and moving at a middling rate, which
throws into question the estimated quake dangers at other similar
quake-prone zones near Japan, in the Pacific Northwest, Chile, Alaska, and
elsewhere.
"We didn't expect such a big earthquake in that location," said Emile Okal
of Northwestern University. Okal is slated to speak about how the
Sumatra-Andaman quake calls into question theoretical assumptions made
about other similar dangers zones worldwide and especially in South America
on Thursday, 6 April, at Backbone of the Americas - Patagonia to Alaska.
The meeting is co-convened by the Geological Society of America and
Asociación Geológica Argentina, with collaboration of the Sociedad
Geológica de Chile. The meeting takes place 3-7 April in Mendoza,
Argentina.
Previous to the catastrophic 26 December 2004 earthquake, the theory about
how subduction zones generate quakes was straightforward, says Okal. It
boiled down to age and speed. Where an older, colder and therefore denser
slab of crust is being pushed slowly under another plate, "It will want to
sink," he said. As a result there's not a lot of stress building up to
cause large quakes.
At the other end of that same spectrum are subduction zones where young,
buoyant crust is being forced quickly under another plate. The rate of
"convergence" and the fact that the young crust resists sinking causes lots
of stress to build up and results in much larger quakes.
"So you could take a map of all the subduction zones of the world and look
at it," said Okal. "The red areas were ones with younger, faster moving
crust and the blue areas were older, slow moving crust."
The theory seemed tidy enough and could be verified somewhat by dating the
crust, measuring the rates entire tectonic plates seemed to be moving at,
and estimating the power of past quakes from historical accounts. According
to the theory, the Sumatra subduction zone was capable of no more than a
magnitude 8 earthquake, Okal explains.
"The cold shower we got was Sumatra," said Okal. "We have a 9.3 on our
hands. You got a point that violates the plan outrageously."
Fortunately, says Okal, the science of plate tectonics has made great
strides since the 1980s and the danger map now can be greatly refined and
reassessed. For instance, where once researchers looked to the centers of
plates to see how fast they may be colliding at the edges, Global
Positioning System technology now allows geophysicists to track specific
movements and deformation in the actual subduction zone.
The result is that some places may be at greater risk of large quakes, and
others may be at lesser risk. "Suddenly there are points moving up and down
when you reassess them," he said.
It's been discovered, for instance, that despite being one of the best
big-quake factories on the planet, the convergence of the Nazca Plate and
the South American Plate on the Pacific Coast of South America is happening
at a significantly slower rate than previously thought, says Okal.
On the other hand, some subduction zones have quakes that do not directly
express the subduction - and so have inflated the apparent risk of a large
event. One example is in the Caribbean where, besides a subduction zone,
there are quakes that occur along strike-slip, San Andreas-type, faults.
These faults accommodate sideways movement in the collision zone instead of
the blunt shoving of one plate under another. "So we are reassessing this
whole area," said Okal. There is also the matter of how subduction zones
let loose their built-up energy. They can break along small segments,
together or individually. One segment might produce a moderate quake. But
if four or five segments all go at once you get a colossal release of
seismic energy, like that seen when the Sumatra-Andaman zone "unzipped" for
800 miles (nearly 1,300 kilometers) on that terrible day 15 months ago.
"The bottom line is that we have to be very humble," said Okal. We don't
know how to predict the size of quakes, he said, and we should not discount
that there will be surprises.
.
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