Aug. 4 (UPI) -- The U.S Geological Survey recorded a cluster of tremors early Friday in a part of Oklahoma on a watch list for activity related to shale oil and gas operations.
Four tremors were reported as of 5:45 a.m. EDT by the USGS, the largest of which was a magnitude-3.3 event about three miles outside the town of Edmond. At least eight quakes were recorded Thursday, with Edmond experiencing a magnitude-4.2 event.
One of the U.S. states with a significant amount of shale oil and natural gas, a study from the USGS found the disposal of oil and gas-related wastewater is the "primary reason" for an increase in seismic activity in central states like Oklahoma. That process is different from hydraulic fracturing.
State orders related to an area of interest near Edmond translate to a volume reduction of wastewater injections for wells under state jurisdiction. The order was mandatory and related in large part to fault data coming out of the Oklahoma Geological Survey and the USGS.
The Oklahoma Corporation Commission and the Oklahoma Geological Survey sent investigators to the area after Thursday's outbreak.
"The investigation is focused on oil and gas wastewater disposal wells that inject into the Arbuckle formation, the state's deepest formation," the commission stated. "The earthquakes have been clustered close together in an area where there is a known fault. There are no Arbuckle disposal wells at or very close to the location."
Oklahoma accounts for as much as 5 percent of the total national output of crude oil, making it one of the more significant oil producers in the nation. It's the fifth-largest shale natural gas producer in the country.
Oklahoma seismicity peaked in 2015, with more than 900 tremors of greater than magnitude-3 recorded.