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.
As for why McCaffrey's model came up with fewer magnitude 9 quakes than have been seen in the last 50 years, that's just a matter of the sample size, he said.
"One of the analogies is that you go into a friend's house and their baby is crying the entire ten minutes you are there," McCaffrey explains. You leave with the impression that the baby cries all the time. However, it might also be that you just happened in during the baby's only fit all day. The same goes for watching really big, rare earthquakes.
"We've been just watching this process that has thousands of years in a cycle," said McCaffrey.
Yet the seismograph was only invented about 100 years ago, and historical records noting magnitude 9 earthquakes only extend back about 300 years, at best.
So whatever patterns we see are not likely to represent a long-term average.
"His argument is, anywhere you have a subduction zone you can't exclude the possibility," said seismologist Seth Stein of Northwestern University. "What that means is when you think of giant ocean tsunamis, you can't count on where they will come from."
In other words, said Stein, McCaffrey's paper is bad news for tsunami emergency planners because it increases the number of disaster possibilities.
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