The Oklahoma Geological Survey said Tuesday that 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 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 survey said the "primary suspected source" of the temblors is not hydraulic fracturing, or fracking, which is the practice of injecting fluid under high pressure to create cracks in deep-rock formations so 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 before 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 earthquakes of magnitude 3 or greater each day, according to geologists.
The statement is the survey's strongest since it began looking into the source of the state's earthquake swarm. In 2012, the survey 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."
The U.S. Geological Survey and various scientific studies have previously linked the practice of injecting wastewater produced by oil and gas drilling to earthquakes in Oklahoma.
After the report's release, Democratic State Rep. Cory Williams called for a moratorium on oil and gas wastewater disposal wells in 16 counties that the Oklahoma Corporation Commission, which regulates the oil and gas industry, has identified as "areas of interest" because of earthquake activity.
But a spokesman for the agency, Matt Skinner, said the commission does not have the authority to issue a moratorium on wastewater disposal operations.
Skinner said the survey's 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.
The Sierra Club of Oklahoma has expressed concern about the possibility of environmental disasters if earthquakes rupture aging construction pipelines that transport oil and natural gas across Oklahoma. A spokesman for the environmental group did not immediately return a telephone calling seeking comment.
Kim Hatfield, regulatory committee chairman for the Oklahoma Independent Petroleum Assn., said the state's oil and gas industry supports additional study of Oklahoma's increased seismic activity.