QUITMAN — Residents near Quitman have felt a little shaking going on lately. Since activity at several nearby natural-gas injection wells had ceased, the area near the border of Faulkner and Cleburne counties had only experienced four earthquakes since July 31, none of which was larger than magnitude 2.5 on the Richter scale.
Last week was a different story, however.
Two earthquakes were felt on the evening of Oct. 2, and five stronger quakes occurred on the morning of Oct. 5.
Scott Ausbrooks, geological supervisor for the U.S. Geological Survey, said the recent influx in quakes was not totally unexpected.
“We have been cautioning that just because they shut the wells down, earthquakes won’t automatically stop,” he said.
A temporary moratorium on the operation of injection wells was put into place in March when a magnitude 4.7 earthquake rattled the area. That quake was the largest the state had experienced in 35 years. The moratorium was made permanent in July, and the swarm of small earthquakes settled down.
Ausbrooks said the recent earthquakes are probably a result of pressure in the fault line moving north up the fault line.
The quakes earlier this year were much closer to Guy. As pressure moves north along the fault, the quakes move north as well, closer to Quitman, Ausbrooks said
“We have seen a dramatic decrease in the frequency of earthquakes,” Ausbrooks said. “During the swarm, we saw between four and five a day. Now we have gone a long time without any earthquakes at all.”
Ausbrooks said he and his team are researching a similar case in Denver, where injection wells were shut down after multiple earthquakes were registered.
“In Denver, it was almost seven years before the earthquakes totally stopped,” he said.
Ausbrooks isn’t concerned about the occurrence of the quakes nor their northward shift, even considering the coincidence that the seismic activity occurred after a well was recently plugged.
“[The week before the Oct. 2 earthquakes], they finished plugging that northern-most injection well,” he said. “It’s a stretch to imagine that plugging that well caused the earthquakes.”
However, Ausbrooks anda team will pursue his idea, researching it further in coming weeks.
After seismic activity settled down in the area, several testing instruments were removed, but Ausbrooks said they might be brought back and relocated to monitor the situation to the north.
As with any seismicactivity, Ausbrooks insists that residents should not be alarmed.
“We are aware of the situation, and we will continue to monitor it,” he said.
Staff writer Caroline Zilk can be reached at (501)244-4326 or czilk@arkansasonline.com.
River Valley Ozark, Pages 57 on 10/13/2011