LITTLE ROCK, Ark. -- The number and strength of earthquakes in central Arkansas have noticeably dropped since the shutdown of two injection wells in the area, although a state researcher says it's too early to draw any conclusions.
"We have definitely noticed a reduction in the number of earthquakes, especially the larger ones," said Scott Ausbrooks, geohazards supervisor for the Arkansas Geological Survey. "It's definitely worth noting."
The Center for Earthquake Research and Information recorded around 100 earthquakes in the seven days preceding the shutdown earlier this month, including the largest quake to hit the state in 35 years – a magnitude 4.7 on Feb. 27. A dozen of the quakes had magnitudes greater than 3.0. In the days since the shutdown, there have been around 60 recorded quakes, with only one higher than a magnitude 3.0. The majority were between magnitudes 1.2 and 2.8.
The two injection wells are used to dispose of wastewater from natural-gas production. One is owned by Chesapeake Energy, and the other by Clarita Operating. They agreed March 4 to temporarily cease injection operations at the request of the Arkansas Oil and Gas Commission.
The commission said preliminary studies showed evidence potentially linking injection activities with nearly 1,000 quakes in the region over the past six months.
But Ausbrooks said it's too soon to tell if the decline in quakes is directly related to the injection well closures, adding that the drop could just be a normal low period of the swarm cycle.
"Either way, I wouldn't expect (the earthquakes) to quit immediately," he said. "If there is a relationship, the seismic activity could go on for weeks, months or even years."
Chesapeake Energy has said it does not believe there is a connection between the injection wells and the area's seismic activity.
A six-month moratorium on new injection wells in the area took effect in January to allow time to determine what relationship, if any, there is between the wells and the earthquakes.
The Fayetteville Shale, an organically-rich rock formation underlying the region, is a major source of natural gas in Arkansas. Drillers free up the gas by using hydraulic fracturing or "fracking," which requires injecting pressurized water to create fractures deep in the ground. The two injection wells at issue dispose of "frack" water when it can no longer be re-used by injecting it into the ground.
The state's Oil and Gas Commission will reconsider the issue at a meeting March 29 when both sides will get to testify.