Shallow M 5.6 earthquake strikes Oklahoma

A magnitude 5.6 earthquake struck 15 km northwest of Pawnee, Oklahoma, and abut 74 miles northeast of Oklahoma City, at 7:03 AM CDT today (12:03 UTC), the US Geological Survey reports.


Structural damage to buildings near 6th & Harrison in Pawnee (Fox-23 via Twitter)

A security camera at Cushing High School recorded about 20 seconds of shaking during the earthquake, but damage was largely limited to historic buildings downtown and a few bricks coming loose or ceilings cracking. The quake reportedly startled the football coaching staff who were reviewing game film from last night’s victory over Mannford.

The epicenter was near an area where state and federal regulators had stepped in to limit wastewater disposal, ABC News and CNN reported. An earthquake occurred in the same region in November 2011. Gov Mary Fallin has ordered the disposal wells, linked to fracking and other industries, to be shut down.

“This is a mandatory directive,” CNN quoted her as saying. The US Environmental Protection Agency is also investigating, since although fracking itself probably doesn’t cause earthquakes, pressure that builds up in the wells used for the disposal of toxic wastewater that comes with the oil and gas the industry gets from fracking might cause earthquakes.

“All of our actions have been based on the link that researchers have drawn between the Arbuckle disposal well operations and earthquakes in Oklahoma,” said Matt Skinner, spokesman for the Oklahoma Corporation Commission, according to ABC News. “We’re trying to do this as quickly as possible, but we have to follow the recommendations of the seismologists, who tell us everything going off at once can cause an (earthquake).”

The quake was felt as far away as Arizona, according to news reports. NBC News reported that people in Kansas City and St Louis, Chicago, Memphis, Little Rock, Des Moines, and in Arkansas and southwest Texas all reported feeling the earthquake.

The US Geological Survey analyzed the earthquake, saying it occurred as the result of shallow strike-slip faulting but far from any tectonic plate boundaries. That is, it occurred within the North America plate.

Scientists speculate that the tectonic plate may have ruptured on either a left-lateral fault striking east-southeast or on a right-lateral fault striking north-northeast. Data from the seismographs tends to suggest the fault has a southwest-to-northeast orientation, but geologists admitted that it’s “very difficult to correlate earthquakes to specific faults in the region and identification of the fault responsible for this event will require further study and fieldwork.”

“Most earthquakes in North America east of the Rockies occur as faulting within bedrock, usually miles deep,” the survey writes about this earthquake. “Few earthquakes east of the Rockies, however, have been definitely linked to mapped geologic faults, in contrast to the situation at plate boundaries such as California’s San Andreas fault system, where scientists can commonly use geologic evidence to identify a fault that has produced a large earthquake and that is likely to produce large future earthquakes.”

The bedrock in this region of north-central Oklahoma tends to be hard, which could explain why a 5.6-magnitude earthquake caused relatively little damage.

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Paul Katula
Paul Katula is the executive editor of the Voxitatis Research Foundation, which publishes this blog. For more information, see the About page.