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Originally published Thursday, April 14, 2011 at 10:01 PM

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Series of big quakes mean more on the way?

First came 2004's killer quake and tsunami off Sumatra. Then a year ago, Chile was rocked almost as hard. Now Japan is reeling from its biggest quake in recorded history. The juxtaposition of three of the world's most powerful earthquakes within a span of seven years has some scientists asking the question: Are we in an age of megaquakes?

Seattle Times science reporter

First came 2004's killer quake and tsunami off Sumatra. Then a year ago, Chile was rocked almost as hard. Now Japan is reeling from its biggest quake in recorded history.

The juxtaposition of three of the world's most powerful earthquakes within a span of seven years has some scientists asking the question: Are we in an age of megaquakes?

Few corners of the world are more keen to know the answer than the Pacific Northwest, where an offshore fault is cocked to unleash a quake and tsunami on a par with the magnitude 9.0 that devastated Japan last month.

Most geologists are skeptical that monster quakes come in clusters. But some are convinced they do — though no one can explain how quakes separated by several thousand miles could be linked.

On its face, the lineup of very large quakes over the past century seems to fall into two peaks of activity: Between 1950 and 1965, when there were seven earthquakes of magnitude 8.5 or greater; and 2004 to now, with three megaquakes (and two big aftershocks) so far.

The nearly four decades preceding the disaster off Sumatra (one of the main islands in Indonesia) didn't see a single great earthquake, U.S. Geological Survey scientist Charles Bufe said Thursday, from the Seismological Society of America's annual meeting, in Memphis, Tenn.

When Bufe, of Denver, and his USGS colleague David Perkins analyzed the clustering, they concluded it's not coincidental when it comes to the small subset of truly monstrous quakes: those of magnitude 9.0 or greater.

"It's very statistically significant," Bufe said. "We think we're in an increased hazard situation for these very large earthquakes."

If they're right, Bufe and Perkins estimate there's a 63 percent chance that another magnitude 9.0 or greater quake will strike somewhere in the world within the next six years. If monster quakes strike randomly, the chance is about 24 percent.

The random interpretation is backed by several other scientists who subjected the earthquake records to more extensive statistical tests.

"Seismologists are in a very difficult situation, because we can't run experiments," said Andrew Michael, of the USGS Science Center in Menlo Park, Calif. The best they can do is parse the existing data in multiple ways, then compare it against random scenarios to tease out any meaningful patterns.

It's well known that quakes beget other quakes, Michael pointed out. Japan has been rattled by hundreds of aftershocks since the main event on March 11.

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A big quake on one segment of a fault can also prime adjacent segments to rupture, as happened twice after the magnitude 9.1 Indian Ocean quake in 2004. And large quakes can trigger small quakes more than 1,000 miles away. A magnitude 7.9 earthquake in Alaska touched off rattles and rearranged the plumbing in geyser fields in Yellowstone National Park in 2002.

But could a megaquake in the Indian Ocean lead to the rupture of a giant fault off the coast of Chile more than five years later and 9,000 miles away?

Michael doesn't think so.

When aftershocks were factored out, he and his colleagues found no pattern in the timing of quakes of magnitudes 7.5 to 8.5 that couldn't be explained by random fluctuation. There have been only five magnitude 9 or greater quakes over the past century — far too few to draw any conclusions, he said.

Michael also found no evidence that a higher number of quakes occur in the five years following giant quakes.

Aaron Velasco, of the University of Texas, El Paso, examined quakes of magnitude 7.0 or greater, and found there's a limit to their geographical reach in terms of setting off other destructive quakes. Big quakes do spawn daughter quakes of moderate size, 5.0 or greater, but not beyond about 600 miles, he said.

With only about 100 years of data and an understanding of earthquakes that's far from complete, it's important to keep asking questions about possible links between megaquakes, said Rick Aster, outgoing president of the seismological society.

The conventional wisdom can turn out to be wrong, as it did when the faults off Japan and Indonesia fractured with unexpected force.

"We used to think many of these subduction-zone segments were not capable of magnitude 9," said Aster, of the New Mexico Institute of Mining and Technology. "That confidence has really been shattered by these earthquakes."

Scientists who drill sediment cores and dig trenches looking for evidence of ancient quakes in the Pacific Northwest do see signs of clustering through time.

The shallow faults that crisscross the Puget Sound region appear to have gone through active and quiet periods. The same is true of the Cascadia Subduction Zone, the menacing fault where the ocean floor dives under the continent off Washington, Oregon and Northern California.

Chris Goldfinger, of Oregon State University, finds evidence there for three to four earthquake clusters in the last 10,000 years on one fault segment.

"The clusters are real," he wrote in an email. "They are not just a random product, and therefore they mean something."

He doesn't yet know what, or whether it has any bearing on global earthquake clusters.

A plausible explanation for the way megaquakes might link up over great distances would give scientists a better starting point for future analyses, Michael said.

The Japanese quake sent small vibrations around the globe and tilted the Earth's axis several inches.

"After a very big earthquake, we have the Earth ringing like a bell ... and this can go on for days, weeks, months," Bufe said.

The motions are tiny, but widespread. Big earthquakes also deform the taffylike mantle layer that underlies the planet's rocky crust, and those ripples spread slowly.

Maybe these slight motions can further weaken a fault that's already on the edge, Bufe speculated.

Michael and others remain unconvinced.

But geologists can't really explain how earthquakes like the one in Alaska in 2002 trigger smaller quakes as far away as Yellowstone, Velasco pointed out.

"We still have a long way to go as far as understanding the earthquake process."

Sandi Doughton: 206-464-2491 or sdoughton@seattletimes.com

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