More Quakes Recorded in North-Central Arkansas

February 21, 2011

An earthquake powerful enough to be felt 200 miles away struck Greenbrier, Ark., about 2 a.m. on Feb. 18, when most residents were asleep. Many probably never woke up.

Greenbrier Fire Chief Jim Sutterfield said he was one of those who slept through it. The earthquake didn’t damage any buildings or cause any injuries, and Sutterfield said earthquakes have become so common in the area that residents are used to them.

“My wife said it shook our windows in the house,” Sutterfield said. “We’ve had about 18 to 20 earthquakes nearly every day this week.”

The 4.3 magnitude quake was the largest one so far in what seismologists are calling the Guy-Greenbrier swarm – a slew of small earthquakes occurring in the state’s north-central region. It is also the third-largest earthquake since a similar grouping of tremors in the 1980s called the Enola swarm.

The earthquake shook the ground with enough force to register on an automated seismograph at Keiser in east Arkansas, said Scott Ausbrooks, geohazards supervisor for the Arkansas Geological Survey.

Scientists closely monitor seismic activity along the New Madrid fault that stretches from northeast Arkansas to southern Illinois. A series of massive earthquakes occurred along the fault during the winter of 1811-12, and seismologists say they expect the region to be hit again by a major quake.

But Steve Horton, research scientist at the Center for Earthquake Research and Information at the University of Memphis, near the New Madrid fault, said the recent quakes 200 miles west in Greenbrier and other parts of north-central Arkansas aren’t related to activity in the New Madrid Zone.

In northern Faulkner County, 29 earthquakes were recorded from midnight on Feb. 16 to late on Feb. 18 They ranged in magnitude from 1.8 to 4.3. More than 60 quakes have been recorded in the entire region, according to CERI.

Seismic researchers say there is no need for the people of Greenbrier and Guy to be overly concerned.

“So far it’s been normal when compared with previous swarms,” Ausbrooks said. “This (area) hasn’t seen a whole lot of activity, so it’s kind of like the domino effect.”

But Horton, at CERI, said earthquake activity in the area at all is not exactly a normal occurrence.

“Swarm activity anywhere in the east is not normal activity,” he said. “In the west, the (tectonic) plate boundaries are where you normally have earthquakes, but we don’t know what the cause in the east is. It’s an unusual event that’s getting the attention of a lot of scientists.”

Horton said researchers from CERI and the state geological agency installed equipment in north-central Arkansas in September to get a better understanding of the situation, particularly whether there is a connection between seismic activity and nearby injection wells that gas-exploration companies use to dispose of liquid waste by pumping it into the earth.

“The earthquakes in that area started after those two disposal wells actually went online and started injecting last summer,” he said. “We don’t know if there is a correlation yet, but we think we are close to an answer.”

More than a dozen earthquakes shook portions of Arkansas on Feb. 18 alone, including the one of magnitude 4.3.

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