Aug. 3 (UPI) -- Eight earthquakes were recorded Thursday in Oklahoma, a state at the heart of the shale oil and natural gas industry, the U.S. Geological Survey reported.
The largest of the tremors recorded by the USGS was a magnitude-4.2 event shortly before 3 a.m. local time in the town of Edmond, which is designated in a state area of interest. The Oklahoma Corporation Commission has ordered oil and gas operators to cut back on disposal well operations and closed some wells in response to increased seismic activity in those areas.
There's been no new well activity in the area in terms of drilling or hydraulic fracturing as of July, state regulators said after a recent outbreak. A statement from regulators, published by local KWTV News 9, said investigators were on the ground near the cluster of tremors in the Edmond area.
"The investigation is focused on oil and gas wastewater disposal wells that inject into the Arbuckle formation, the state's deepest formation," the statement read. "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."
Tremors are not associated with hydraulic fracturing in and of itself.
Oklahoma seismicity peaked in 2015, with more than 900 tremors of greater than magnitude-3 recorded. In January 2016, Gov. Mary Fallin approved $1.4 million in funding to expand efforts to address the issue.
Three of the eight events recorded Thursday by the USGS were less than magnitude-3.
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.
Federal data show Oklahoma's economy expanded by 1.3 percent during the fourth quarter, ending four straight quarters of contraction. State data on gross production taxes on oil and natural gas show collections of $41.6 million in June, up more than 60 percent from the previous year.