Mid-Atlantic Earthquake: A Sign of Things to Come? Probably Not

First come earthquakes, then come the aftershocks.

A Tuesday afternoon, rattling homes from Toronto to South Carolina. 

Tuesday evening, just after 8 p.m., the ground in Virginia shook again, registering about 4.2 on the Richter scale. Wednesday at 12:45 a.m. another, this one registering a 3.4.

So how long can the area expect to feel the after-effects of this unusually strong earthquake? 

“It depends on the area, what types of rocks are there," said David Brezinski, a geologist at the Maryland Geological Survey. “Normally aftershocks die out within a couple of weeks. But they can persist for a long time after [the initial quake].”

"It could," he said, "be a matter of weeks." Then again, “It could be months.”

Aftershocks occur as the ground underneath readjusts after an earthquake. They are usually much smaller than the main quake, Brezinski said. 

The data isn’t in yet, but Brezinski said it’s likely that this earthquake was the result of activity in an old fault, or fracture in the rock beneath the ground. By “old,” he said, Brezinski means anywhere from 20-200 million years.

Old faults aren't very active, he added, and "there's rarely any movement."

Faults that are more active--like those that lead to earthquakes on the West Coast--are younger, between 10,000 and 20,000 years old. “What we had cannot be compared to that," he said.  

According to the US Geological Survey, because the rocky structures on the East Coast are older and more settled, they absorb less shaking. That means the energy from an East Coast earthquake spreads farther than it would on the West Coast, where the rocks are newer and have more give.

As to why this earthquake at this time or whether this area can expect them more frequently, Brezinski said scientists won’t be able to reach any conclusions until they can determine the exact epicenter.

That means unrolling old geological maps in search of any faults in the area that had been identified in the past and studying the ground in the area to determine if there are rocks that are characterized by instability.

Sometimes, he said, geologists never definitively pinpoint a cause.

“There was a spat of small earthquakes in Columbia and Ellicott City in the early 90s,” he said. Seven or eight earthquakes in a row registered up to 2.7 on the Richter scale. 

“We never really determined exactly what the origin of the faulting or disturbance was,” he said. “This might be the same thing.”

At this point, as an individual event, Tuesday’s earthquake was a just a little out of the ordinary, but likely not cause for concern, according to Brezinski.

Although, he added, “This quake was larger than one would expect.

“The Eastern Seaboard is generally pretty stable, seismologically. It is pretty unusual to get quakes over a four (on the Richter Scale) … But totally surprising? Probably not.”