The Arkansas Geological Survey says it is stepping up its monitoring of seismic activity in central Arkansas after dozens of small earthquakes in the region.
Six minor quakes were recorded Friday near Quitman, the latest of more than 50 temblors in October. The Friday tremors began with a 2.0-magnitude quake around 7:45 a.m. and peaked with a 2.5 quake later in the morning.
The shaking follows more than 1,000 earthquakes centered between Guy and Greenbrier from September 2010 to July of this year, when the Arkansas Oil and Gas Commission ordered four gas disposal wells shut down and voted to create a large moratorium area in which no future disposal wells could be drilled.
Geologists said the activity - which involves injecting pressurized liquid into the ground - was likely contributing to the shaking.
The quakes between Guy and Greenbrier tailed off significantly in August, but more quakes began occurring in October closer to Quitman. It sits about 10 miles northeast of Guy.
That distance is potentially concerning, warned Scott Ausbrooks, geohazards supervisor for the Arkansas Geological Survey. The Quitman quakes are occurring along the same line as the Guy-Greenbrier ones, but there's a miles-long gap between Guy and Quitman in which no quakes have been recorded.
That might suggest that the tension is building, possibly leading up to a larger quake centered on that gap along the fault. A similar-sized gap occurred on the southern end of the Guy-Greenbrier fault and it in February unleashed a 4.7-magnitude quake, the largest of any of the recent tremors.
Anytime there is a gap in seismicity along an active fault, it "bears watching," Ausbrooks said.
"I'm not saying that's what it is," he said. "I don't want to scare people and I don't want to say it's going to happen. It's just something we're looking at."
The 4.7 tremor was felt across Arkansas and in neighboring states, though no injuries or major damage was reported.
It and the other hundreds of smaller quakes that rumbled the area helped convince the Oil and Gas Commission to take action. So why is the shaking continuing if the drilling has stopped?
Ausbrooks has a few ideas, though he said the phenomenon is still very much under investigation. Officials have already placed one temporary quake-monitoring station near Center Ridge and he said another will likely be placed elsewhere in the region in the next two weeks.
"We're kind of beefing back up because of this renewed seismicity," he said
Ausbrooks said the shaking could be purely natural. Earthquakes continued for years after drilling ended at the Rocky Mountain Arsenal in Colorado, where an injection well is thought to have initially triggered seismic activity.
The continuing shaking could also be related to ongoing drilling activity about 8 miles outside of Quitman, Ausbrooks said, though he noted that's a significant distance. Or it might be tied somehow to the plugging of one of those four shuttered wells.
Ausbrooks said he couldn't preliminarily pinpoint how the plugging of a well would induce quakes, but the shaking in Quitman started just days after the northernmost of the four closed wells was plugged.
"Right now, we'll just say it's a coincidence," he said.
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