OKLAHOMA CITY, Nov. 14 (UPI) -- Following a cluster of events near Oklahoma shale basins, data from the U.S. Geological Survey show nearly two dozen small tremors in the last week.
A magnitude-2.5 event recorded midday Sunday in Medford, near the northern Oklahoma border with Kansas, was one of 22 smaller tremors recorded in the state in the last week. USGS data show the largest event was a magnitude-3.7 quake reported in Medford on Wednesday.
Oklahoma Gov. Mary Fallin last week issued a declaration of emergency after a magnitude-5.0 event struck Cushing, the central U.S. storage hub for crude oil.
"Two years ago, we established the coordinating council on seismicity," she said. "Regulators continue to target areas for additional scrutiny that are experiencing increased seismic activity, which has led to the shutting down of disposal wells or reducing the volume of disposal wells and flow pressure in known fault lines where we believe there is a correlation to earthquakes."
According to the Oklahoma Geological Survey, the area around Cushing has been an active zone for seismic events since 2015. The Oklahoma Corporation Commission said areas in and around the so-called Arbuckle formation are prone to seismic events and directives have evolved to limit the disposal of oil- and gas-related wastewater tied to shale operations in the state.
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 itself.
"Most of the wastewater comes not from hydraulic fracturing operations, but rather from producing wells," the state commission explained in a summary statement. "The water exists in the producing formation and comes up with the oil and natural gas."
Oklahoma is one of the most significant producers of crude oil in the United States, accounting for about 4 percent of the nation's total. It hosts some of the largest deposits of shale oil in the country.