Every so often, one large earthquake can trigger another. These "earthquake doublets" might happen because the first quake shifts stresses around in the Earth’s crust, triggering the second temblor, scientists say.

On Nov. 15, 2006 an 8.3 magnitude quake shook the Earth near the Kuril Islands, an archipelago off the southeast coast of Russia, and to the northeast of Japan. Within minutes, smaller quakes began shaking on the seaward side of the island chain. Then on Jan. 13, 2007 an 8.1 magnitude earthquake ripped through the upper portions of the Pacific plate to the east of the Kuril Islands.

The most recent quake took scientists by surprise, and a team of earthquake researchers has tried to piece together exactly happened. Their work is detailed in the Jan. 31 issue of the journal Nature.

Main shock, aftershock

When a large earthquake strikes, aftershocks that gradually diminish in strength typically follow the main shock for a few weeks or even months. But the two Kuril quakes were the same order of magnitude, so seismologists knew something different was going on.

"When you have a second event that's almost as big as the first event, well, that's completely different than the typical main shock-aftershock sequence," said study team member Thorne Lay of the University of California, Santa Cruz.

Earthquake doublets, such as the Kuril quakes, are known to happen, but are rare. And they differ not only in the comparative magnitudes of the first and second quakes, but in which part of a fault ruptures. Aftershocks of a big earthquake typically originate from the same place as the main shock did, but doublets will happen when a completely different section of a fault fails.

Or, in the case of the Kuril quakes, an entirely new fault ruptures. While the first quake emanated from a rupture in the subduction zone where the Pacific plate sinks under the North American plate, the second quake actually occurred as a break in the Pacific plate.

"That makes this doublet quite distinctive," Lay said. "We've never seen one like this, where you have a huge thrusting fault that triggers a huge extensional fault, or normal fault. That's not been observed to occur."

Pulling plates

What Lay and his co-authors think happened was that the first quake occurred just like any major thrusting earthquake, with the subducting plate lurching forward under the other plate. The second quake was a direct result of this sudden movement.

"[The earthquake] pulled on the Pacific plate as it was sinking and it snapped it basically," Lay told LiveScience.

Fractures in subducting plates have been known to happen before—many major quakes have resulted from this phenomenon off the coasts of Japan and Indonesia. Plates that are being pulled down into the Earth’s mantle undergo considerable stress, Lay says.

"In order to get the plate to go down—remember, it's a 100 kilometer [about 60 mile]-thick layer of stiff rock—it has to be kind of bent… and that means a lot of deformation is occurring," he explained.

Lay says that these plate-breaking quakes are a rarity compared with thrusting-fault earthquakes. For that reason, officials in earthquake-prone regions of the world don't plan for them. But despite their infrequent occurrences, they have incredible disaster potential, he adds.

"They're important, even though they're rare, because when they happen, they release a lot of high-frequency shaking," Lay said. "And that's because you're breaking fresh rock, and it snaps very violently versus grinding long and already worn-down contact [along a subduction zone]."

Lay says it's important for scientists to further probe when and why these anomalous quakes occur, so that vulnerable areas can be identified.