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Mega-Quake Study Offers Good News (and Bad)

Larry O'Hanlon, Discovery News

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Not an Anomaly

March 6, 2008 -- The average frequency of catastrophic, magnitude 9 or greater earthquakes around the world is at about three per century, according to a new study.

That's despite the fact that there have been five such quakes in the last half century -- a cluster that includes the deadly 2004 rupture offshore of northern Sumatra.

The new estimate is lower than previous ones, but there is a dark side to the new study: It removes certain limits on where such quakes can occur.

Now it looks like any place where one plate of the Earth is being actively shoved under another -- what's called a subduction zone -- can experience a catastrophic magnitude 9 earthquake.

Previously, different subduction zones were thought to pose greater or lesser risks of magnitude 9 quakes, depending on the speeds at which the plates were colliding, and their stiffness and age. That popular hypothesis evaporated on Dec. 26, 2004.

"The Sumatra-Andaman earthquake happened in one of the least likely places," said seismologist Robert McCaffrey of New Zealand's government research agency, GNS Science. The hypothesis failed. "It was quite humbling."

McCaffrey decided to reexamine the risks of the largest earthquakes without biasing his simulation with the known recent history of giant earthquakes or with the failed theory that was supposed to predict the danger of any given subduction zone.

When he ran his simulation, it popped out the three-per-century figure, as well as the new possibility that any subduction zone posed a danger for these mega-quakes -- albeit not a very frequent danger.

McCaffrey published his new analysis on mega-thrust quakes in the March issue of the journal Geology.

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