Return to story
By RUSTY DENNEN
Until a few months ago, aftershocks from the Aug. 23, 2011, earthquake were unnerving and regular occurrences.
Those in the vicinity of Mineral in Louisa County, where the magnitude-5.8 quake was centered--and where aftershocks are concentrated--felt some shaking and heard that now-familiar dishes-rattling sound.
But the last aftershock large enough to be noticed, according to the U.S. Geological Survey, was probably the magnitude-2.4 bump on July 31. Since then, there have been others, but most of those registered only on sensitive seismic instruments.
Aftershocks are smaller earthquakes that follow a main shock.
"The frequency certainly has been declining," Wright Horton, a research geologist with the USGS in Reston, said Tuesday. Since the July aftershock, the largest was magnitude 1.8, "and that's on the threshold of not being able to feel anything."
Virginia's second-largest earthquake hit at 1:51 p.m. on a Tuesday 14 months ago, knocking the two nuclear reactors at North Anna Power Station offline and causing damage to structures from Mineral to Washington. It was felt from Georgia to Canada.
Geologists said at the time that aftershocks would continue for a while, and that has been the case. The largest one, a magnitude-4.5 jolt, hit on Aug. 25, 2011, two days after the earthquake, and was widely felt in the area. It gave Virginians their first encounter with multiple aftershocks in modern times.
The state's largest earthquake ever was a magnitude-5.9 tremor in Giles County in 1897. Historical accounts of that quake reported long-lasting aftershocks.
More recently, a magnitude-3.2 aftershock on March 31 of this year triggered an "unusual event" designation at North Anna. An unusual event is the second-lowest of four emergency classifications.
In the year between the 2011 earthquake and the first anniversary this summer, there were more than 450 aftershocks, according to the USGS.
Geologists are mapping the aftershocks to gain a better understanding of the structure of complex faults crisscrossing the region.
Horton says most of the seismic monitoring equipment rushed to central Virginia by geologists after the quake has been removed.
But he added, "What's most unusual is that we left the instruments in place as long as we did."
That's because large earthquakes are rare in the East, and aftershocks here are only beginning to be understood.
For months, geologists worked on the ground to better understand the forces that caused the earthquake, and to study scores of aftershocks.
Beginning in March, they took to the air to map ground features with a laser system known as lidar, which stands for light detection and ranging. The data from those aerial surveys are still being analyzed.
"We're in the process of making it public as quickly as we can," Horton said.
That could happen prior to the Geological Society of America conference in Charlotte, N.C., in early November.
Meantime, Horton says, though the number of aftershocks has subsided significantly, they're probably not over just yet.
"It's still possible that over the next year people may be occasionally feeling one," he said. "It's even possible to have one up to magnitude 4."
Rusty Dennen: 540/374-5431
This year, Southeastern states, including Virginia, are participating for the first time because of the magnitude-5.8 earthquake in Louisa County
Locally, the University of Mary Washington will participate in the ShakeOut.
At 10:18 a.m., UMW's Area Warning System will be tested and a text alert will be sent.
Mary Morrison, director of news and public information
"So many people were unprepared last time," she said. "This is a reminder that it can happen here."
Read more at shakeout.org/southeast