Richmond earthquake “not likely” an aftershock from 2011 Virginia quake

296Earthquake22The 3.2 magnitude earthquake that rattled Richmond and surrounding areas of the Mid-Atlantic last night (at 9:47 p.m.) was likely its own, new earthquake – not an aftershock of the much more powerful 5.8-magnitude earthquake that struck August 23, 2011 and damaged the Washington Monument.

Last night’s quake, its epicenter 32 miles west of Richmond or 8 miles southwest of Powhatan, lies in a different area of the central Virginia seismic zone from the 2011 quake, whose epicenter was in Mineral, Va.

“This is pretty far south of that plate [from the 2011 quake] – about 60 kilometers [37 miles] south: not likely an after shock,” said Paul Caruso, a geophysicist at the U.S. Geological Survey’s National Earthquake Information Center in Denver.

The USGS has received over 1,600 reports from witnesses who felt last night’s quake, but no damage has been reported.  Typically, it takes an earthquake of magnitude 5 or higher to cause damage.  Magnitude 3 earthquakes are associated with “weak” shaking.

While a minor quake, Capital Weather Gang readers felt shaking as far away as Arlington, Annapolis, and Potomac.  South of the epicenter, shaking was felt as far southwest as Lynchburg, according to USGS.

Areas where last night's earthquake was felt as of 10 a.m., May 22. (USGS)

Areas where last night’s earthquake was felt. (USGS)

 

By comparison, the 2011 quake was felt “from central Georgia to central Maine and west to Detroit, Michigan and Chicago, Illinois,” writes USGS.

In addition to its much greater range, the 2011 quake was thousands of times more powerful than last night’s relatively minor tremor.

“Every time you go up 1 magnitude, there’s a 100 times more energy released,” said USGS’ Caruso.  “If you go from from 3.2 to 5.2, it’s 10,000 times the energy released.”

Sometimes when an earthquake strikes, it is a foreshock, portending a bigger tremor to come.

But Caruso doesn’t think there’s much to worry about. “We can’t predict the future, so no one can say,” he said. “But there’s very little likelihood of something bigger occurring.”

“Even if it was a foreshock, we wouldn’t expect anything significant [to follow], magnitude 6 or larger, say,” Caruso added.

 

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