Eleven minor earthquakes have hit the Cushing, OK, area over the past week, the strongest of which was a 4.5-magnitude temblor recorded by the U.S. Geological Survey (USGS) on Saturday.

According to USGS data, the first of the quakes occurred on Thursday, a 2.5-magnitude earthquake about 1.86 miles from Cushing, which is located in Payne County and is the delivery point for West Texas Intermediate (WTI) crude. Two earthquakes of 2.7 magnitude followed on Friday, at distances of about 1.24 and 2.49 miles from the city.

Seven earthquakes hit on Saturday, ranging from 2.5 to 4.5 magnitude, and from 1.86 miles to 4.35 miles from Cushing. The most recent temblor struck on Sunday, measuring 2.5 magnitude about 3.11 miles from the city.

Robert Noltensmeyer, emergency manager for the city of Cushing, told NGI's Shale Daily that Saturday's quakes all struck within two and a half hours, with the last one hitting early Sunday morning. As of Monday he had received no major reports of damage.

"It was just a lot of shake, a lot of shattered nerves," Noltensmeyer said Monday, adding that the 4.5-magnitude quake was the largest to hit the area in recent memory. "Having a quake that big really got people upset and nervous. I'm just really grateful that we didn't have any catastrophic damage. We could have."

Last month, the Oklahoma Corporation Commission (OCC) ordered two disposal wells to stop operations and for another three wells to reduce their disposal volume after analyzing disposal well and seismicity data in the Cushing area (see Shale DailySept. 21). All five wells were within a 10-mile radius of the city, and were classified by the OCC's Oil and Gas Conservation Division (OGCD) as "wells of interest" because they target the Arbuckle formation.

Last March, the OGCD issued a directive to operators of 347 disposal wells targeting the Arbuckle to prove they were not injecting oil and gas waste into basement rock below it. The directive was expanded to include an additional 211 disposal wells in July. That same month, the operators of another three wells agreed to change their operations (see Shale DailyJuly 29; July 20April 2).

Additional reductions were announced in August, after a swarm of 3.1-magnitude earthquakes near Oklahoma City (see Shale Daily, Aug. 18; Aug. 4).

Scientists with the Oklahoma Geological Survey (OGS) reported in April that the state was being hit by earthquakes at a rate about 600 times greater than historic background data (see Shale Daily, April 22). Seismicity increased from an average of one-and-a-half earthquakes of 3.0 magnitude or greater to about two-and-a-half such earthquakes every day in 2014.

According to the OGS, the earthquakes are primarily occurring within the crystalline basement. The majority of the state's approximately 900 injection wells target the Arbuckle formation, which closely overlies the crystalline basement.

Last fall, Oklahoma regulators shut in a disposal well that was thought to have been drilled too deep following a spate of seismic activity in the area (see Shale Daily,Oct. 31, 2014).