http://newsok.com/history-of-earthquakes-in-oklahoma/article/5335367/?page=2
The first documented earthquake caused by a disposal well was at the Rocky Mountain Arsenal northeast of Denver in 1961. The military used a deep disposal well to dump mustard gas and other waste fluids from arsenal operations. Shortly after disposal began, small earthquakes were felt in the area.
More than 1,300 mostly small quakes were recorded in the area between January 1963 and August 1967.
“You definitely could see a cause and effect between injections and when the earthquakes occurred,” said William W. Fleckenstein, professor and interim department head at the Colorado School of Mines. “When they stopped injecting, the earthquakes went away. When they started back up, the earthquakes started again.”
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Oklahoma also experienced a magnitude 3.8 in the area 11 months later and a 4.0 in 1956. It is possible that the state experienced a swarm similar to what it is experiencing now. But with no seismographs in the state, no Internet or Twitter to share reports and about 1 million fewer people in the area, such a swarm was not — and could not have been — reported.
The region also was seismically active well before oil and natural gas activity moved in.
El Reno lies on the Nemaha Ridge. The same structure experienced a 5.1 earthquake in Nebraska in 1877 and a 5.1 near Manhattan, Kan., in 1891.
“These earthquakes weren’t isolated,” said David Gordon, a retired U.S. Geological Survey seismologist who spent his career focusing on quakes in the central United States. “The whole area has a history of occasional earthquakes of that size.”
Gordon retired from the geological survey in 1994 and said he has not studied Oklahoma’s ongoing earthquake swarm. But he said other swarms have been documented in areas where oil and natural gas industry activity was not present.
“These swarms occur,” he said. “They occurred back East in places before there was any drilling. There is no cause that they know of. It’s just normal activity.”