for National Geographic News
Massive earthquakes—such as the magnitude 9 quake that sparked the deadly Indian Ocean tsunami in 2004—can set off smaller tremors around the globe, a new study shows.
Traditional aftershocks occur close to the time of the original earthquake—often within days or a few weeks—as the earth adjusts to changes caused by the slippage along the original fault.But smaller, more distant earthquakes can be triggered as low-frequency vibrations—somewhat like ocean swells—pass over faults. Such waves can't be felt by people standing on the ground.
It's too early to know if the recent deadly earthquake in China also triggered its own swarm of distant tremors, but "it wouldn't surprise me in the least," said study lead author Tom Parsons of the U.S. Geological Survey in Menlo Park, California.
Fault Stress
Vibrations from a big quake momentarily reduce the locking pressures on faults, allowing them to slip, Parsons said.
"That puts a temporary stress on a fault," Parsons added. "That's small, but [sometimes] appears to be enough to trigger an earthquake."
(Set off your own earthquake in an interactive simulation.)
To figure out how often this happens, Parsons and colleagues examined records of 15 major earthquakes with magnitudes of 7 or greater.
The scientists were able to correlate distant vibrations passing through various parts of the globe with the occurrence times of smaller, local earthquakes in 12 of the 15 events studied.