The Oklahoma Geological Society said Tuesday it is "very likely" that most of the state's recent earthquakes were triggered by the subsurface injection of wastewater from oil and natural gas drilling operations.
Geologists have been studying the cause of hundreds of earthquakes that have shaken the homes and the nerves of residents in central and north-central Oklahoma, where the pace of oil and gas drilling has accelerated in recent years.
A statement released by state geologist Richard D. Andrews and Dr. Austen Holland, state seismologist, said the rate of earthquakes and geographical trends around major oil and gas drilling operations that produce large amounts of wastewater indicate the earthquakes "are very unlikely to represent a naturally occurring process."
The society said the "primary suspected source" of the temblors is not the practice of hydraulic fracturing, or fracking, which is the practice of injecting fluid under high pressure to create cracks in deep-rock formations through which natural gas and oil will flow more freely during drilling. It said the source is more likely the injection in disposal wells of wastewater produced as a byproduct of fracking.
"The OGS considers it very likely that the majority of recent earthquakes, particularly those in central and north-central Oklahoma, are triggered by the injection of produced water in disposal wells," the statement said.
Earthquake activity in Oklahoma in 2013 was 70 times greater than the rate of earthquakes prior to 2008.
Geologists historically recorded an average of 1.5 earthquakes of magnitude 3 or greater each year. The state is now recording an average of 2.5 magnitude 3 or greater earthquakes each day, according to geologists.
The statement is the society's strongest since it began looking into the source of the state's earthquake swarm. In 2012, the society said it was possible that some earthquakes had been triggered by oil and gas operations but it was "unlikely that all of the earthquakes can be attributed to human activities."
Matt Skinner, a spokesman for the Oklahoma Corporation Commission which regulates the oil and gas industry, said the findings are part of the agency's evolving approach to earthquakes including earthquake risk assessments for all proposed disposal wells, limited permitting for wells in certain areas and requiring some well operators to mitigate earthquake risks.
But Skinner said the agency does not have the authority to issue a moratorium on wastewater disposal operations in the state.
Gov. Mary Fallin launched a new website to provide information about the growing frequency of earthquakes in the state and how state agencies are responding to protect homeowners.
Kim Hatfield, regulatory committee chairman for the Oklahoma Independent Petroleum Association, said the state's oil and gas industry supports additional study of Oklahoma's increased seismic activity.
"Oklahoma's oil and natural gas producers have a proven history of developing the state's oil and natural gas resources in a safe and effective manner," Hatfield said in a statement.