Oklahoma is a geologically active area. The rocks and faults that underlie the state reveal a history of significant seismic change. The occasional temblor has been known throughout state history to cause minor damage and rattle nerves.
The ongoing earthquake swarm that has shaken the state for the past four years has shaken Oklahomans and led to studies from organizations and universities throughout the region. But it is still unclear how rare the event is historically.
Despite the current swarm, five of the state’s 15 largest earthquakes occurred more than five years ago, including a 5.5 near El Reno in 1952 and a magnitude 4.9 in Bryan County in 1882.
What might be unusual about the spate of activity over the past five years is that it has included so many minor rattlers. But that is a difficult comparison considering how recently seismic monitors have been installed throughout this part of the country.
State’s first measuring device
The first seismograph in Oklahoma became operational in 1961. Before that, earthquakes were recorded only when they were felt. In most cases quakes below a magnitude 3.0 cannot be felt, and even those somewhat stronger can be felt only by those in the immediate vicinity.
In April 1952, Oklahoma experienced a 5.5 with an epicenter near El Reno. That quake shattered windows in downtown El Reno and was felt as far away as St. Louis and Austin, Texas.
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.”
Induced earthquake history
The country also has a long history with induced seismic activity.
Small earthquakes have been connected with filling Lake Mead behind the Hoover Dam, as well as other similar reservoirs throughout the country.
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.”
Earthquake magnitude scale
2.5 or less (minor): Usually not felt, but can be recorded by seismograph.
2.5 – 5.4 (light): Often felt, but only causes minor damage.
5.5 – 6.0 (moderate): Slight damage to buildings and other structures.
6.1 – 6.9 (strong): May cause a lot of damage in very populated areas.
7.0 – 7.9 (major): Major earthquake. Serious damage.
8.0 or greater (great): Great earthquake. Can totally destroy communities near the epicenter.