July 12, 2005— There's reason to believe that spats of strong tremors like those last month in California can be triggered by larger earthquakes thousands of miles away, but it's darned hard to prove, seismologists say.

Take the Sumatra-Andaman megaquake of Dec. 26 and another giant quake there in March: the Earth is still ringing from those shocks, seismologists said.

So is it possible all that seismic energy bouncing around in the planet could loosen up faults in California that could have caused the series of earthquakes, measured at 4.9 on the Richter scale?

"The question is, 'If you have very big quakes, what can happen worldwide?'" said seismologist Susan Hough of the U.S. Geological Survey.

Hough has a special interest in remotely triggered earthquakes, but warned that the topic is controversial among seismologists.

The problem is, even if remote-triggering is real, it's not a simple matter to show cause and effect. That's because we just don't know enough about the mechanics of how quakes are triggered, said Hough. So that leaves seismologists wading through statistical probabilities that distant quakes are connected, not the stuff on which modern seismologists prefer to build their theories.

"There's this history in seismology of people making a lot out of patterns (of earthquakes)," Hough cautioned. "It's easy to make a lot out of nothing."

Theoretically, there are at least two sorts of remotely triggered earthquakes, said Hough.

"One is where you're shoving real estate around," she said.

That's when a quake changes stresses inside its local piece of crust and therefore applies new stresses to an adjacent pieces of the Earth's crust, causing shifting and quakes nearby. The effects can be on the order of hundreds of miles.

"Such long-range triggering phenomena have been documented for the 1992 Landers (California) earthquake, the 2001 Denali (Alaska) earthquake and others," said seismologist Thorne Lay of the University of California at Santa Cruz and the Incorporated Research Institutions for Seismology (IRIS).

The second mechanism for remotely triggered quakes is less certain. It's where the seismic energy sent out through the Earth by a monster quake jostles fault zones thousands of miles away, triggering slips and jolts.

But from Sumatra to California?

"We are talking changes in activity two-thirds of the way around the world, distant from a large event six months after," said Lay. "(It's) very tough to validate any statistical correlation or physical mechanism operating over that distance and time scale."

That doesn't mean it can't happen. It's just really hard to study.

"These big earthquakes are profound events in the physics of the planet," Hough said. And if they are, in fact, causing distant faults to slip and it can be proven, it might reveal something about how earthquakes start, a vital missing clue in the quest for one of seismology's Holy Grails: earthquake prediction, she said.

As for all the rattling in California, no one can yet point to a single cause, said Hough. But if it was triggered by another quake, she said, her money would be on the June 13 magnitude 7.8 quake at Tarapaca, Chile. And like any wager, "it gets speculative" she said.

Still, said Hough optimistically, the fledgling field of earthquake interactions could be a revolution in the making. "There's a lot of work that hasn't been done yet," she said.