This weekend's quakes registered between 2.6 and 4.3 on the Richter scale. Last month, Oklahoma passed California as the state with the most earthquakes, and scientists believe the increase is man-made.
In a joint statement earlier this year, the US Geological Survey and the Oklahoma Geological survey noted that the changes in earthquake rates "do not seem to be due to typical, random fluctuations in natural seismicity rates."
The statement continued: "The analysis suggests that a likely contributing factor to the increase in earthquakes is triggering by wastewater injected into deep geologic formations. This phenomenon is known as injection-induced seismicity, which has been documented for nearly half a century, with new cases identified recently in Arkansas, Ohio, Texas and Colorado."
So the earthquakes are linked not to the oil- or gas-extraction process, but to wastewater disposal, in which large amounts of water are injected underground where they can lubricate fault lines for 20 miles.
According to a study published this month in the journal Science, that injection process sends "a wave of water pressure coursing through the subsurface. The pressure can reduce forces acting to keep faults locked and trigger earthquakes."
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While the link is not definitive, the U.S. Geological Survey has begun researching and mapping man-made earthquakes.
"It's pretty clear high-volume pumping is having an impact on the natural system," said study co-author Geoff Abers, a Cornell University geophysicist. "Modern waste disposal wells can trigger earthquakes."
At a town meeting in Edmond, Oklahoma at the end of June, some citizens called for a moratorium on injection wells.
But state officials said a moratorium would make it more difficult to study the quakes, according to reporting in the Oklahoman. State Geological Survey seismologist Austin Holland told the crowd that stopping the use of injection wells "would not be recommended from a scientific standpoint because that would rob researchers of valuation data that could help them figure out how to prevent earthquakes."
“You want to study us like animals,” one man said in response. “Do you want a 7.0 (earthquake) that leveled Haiti to occur in the middle of Edmond?”