SALT LAKE CITY, April 22 (UPI) -- Aftershocks of a magnitude-5.8 earthquake that rattled Virginia in 2011 may have been triggered by Hurricane Irene, researchers say.
While the frequency of aftershocks usually decreases with time, the rate of aftershocks following the Aug. 23, 2011, earthquake near Mineral, Va., increased sharply as Hurricane Irene passed by four days later, they said.
Scientists didn't notice the unusual pattern at first because the aftershocks were small and the hurricane produced a lot of seismic noise, seismologist Zhigang Peng of the Georgia Institute of Technology told a meeting of the Seismological Society of America in Salt Lake City.
"You have to use pattern-recognition techniques to detect small aftershocks buried by this noise," Peng said.
Peng and his colleagues analyzed seismic records from the days following the main earthquake to identify about 700 aftershocks, many more than had been previously reported using less-sensitive detection techniques, Nature reported Monday.
Researchers matched the aftershocks' timing to atmospheric-pressure readings in the earthquake zone during the storm's move up the U.S. East Coast to see if low pressure might have reduced forces on the earthquake fault enough to allow it to slip and create aftershocks.
The biggest increase in aftershocks didn't come when the barometric pressure was at its lowest, they found, but rather a spike in the rate occurred hours later as the storm receded.
While this may suggest the pressure change wasn't the trigger of aftershocks, the confirmed fact of an increase in their rate as the hurricane moved through the region should be indicator of the need for more such studies, Peng said.
"Basically, it's a single-case study," he said. "Hopefully we will do this for other events as well."
