Oklahoma sits in the stable interior of the continent. The Oklahoma Historical Society's account of the state's earthquakes notes that apart from the Meers Fault in Comanche County, which broke the surface in a very large earthquake about 1,200 to 1,300 years ago, the ground has seen almost no significant tectonic activity since the Pennsylvanian and Permian periods. Before 2011 the largest recorded earthquake was a magnitude 5.5 near El Reno on April 9, 1952, which put a fifty-foot crack in the State Capitol.
November 5, 2011
At 10:53 that Saturday night a magnitude 5.6 earthquake, later revised to 5.7, struck near Sparks in Lincoln County, a few miles from Prague. A smaller foreshock had shaken the area earlier that day. US-62 buckled in three places, fourteen homes were damaged, a chimney fell, and in Shawnee one turret of Benedictine Hall at St. Gregory's University collapsed. Two people had minor injuries. The quake was felt as far as Tennessee and Wisconsin, and weather radar caught birds, bats and insects rising into the air the moment it hit.
The Oklahoma Geological Survey placed it on the Wilzetta Fault, a fifty-five-mile strike-slip zone running from central Pottawatomie County to western Creek County, where two blocks of crust slide past each other far from any plate edge. Several active wastewater disposal wells stood near the epicenter.
Water from the oil fields
Oil in Oklahoma comes up with large volumes of salt water, and the water goes back down disposal wells. The U.S. Geological Survey says hydraulic fracturing itself accounts for only one or two percent of the state's induced earthquakes, and that less than a tenth of the water injected is spent fracturing fluid. The rest is brine produced with the oil. Most of the water from two of the big oil plays is put into the Arbuckle Group where it rests on the granite basement, the same rock that holds fresh water in the Arbuckle Mountains.
In 2013 a University of Oklahoma team published a paper in the journal Geology linking the Prague sequence to injection, and in 2016 the USGS named pressure on faults from cumulative wastewater injection as the primary cause. By then the swarm had grown. The state averaged about two magnitude 3 earthquakes a year from 1978 to 2008, recorded twenty in 2009, and had more than five hundred in 2014, over three times as many as California.
After Pawnee
On September 3, 2016, a magnitude 5.8 struck near Pawnee on a buried fault that no one had mapped. The Pawnee Nation declared an emergency and closed buildings until they could be checked, the Corporation Commission ordered thirty-seven disposal wells near the epicenter shut, and the Environmental Protection Agency closed thirty-two more that stood too close to the new fault. In March 2017 the Nation sued oil companies in its own tribal court. Closures and cuts in injection followed across the state, and in 2017 Oklahoma had less than half as many magnitude 3 earthquakes as in 2016.