Sumatra Megaquake
Defied Theory
Boulder, Colo. -- The risks of Sumatra-style megaquakes 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 didnt 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 didnt 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
dont know how to predict the size of quakes, he said, and we
should not discount that there will be surprises