Quake swarm rattles OK

A magnitude 4.0 earthquake shook central Oklahoma shortly after 10 p.m. Friday, the U.S. Geological Survey reported.

The quake followed two other earthquakes -- a 2.8 and a 2.6 magnitude -- that were centered near Langston just minutes earlier and four other earthquakes ranging from 2.7 to 2.9 magnitude near Guthrie, Yale and Crescent on Friday afternoon and evening, the Geological Survey reported.

That makes seven earthquakes of magnitude 2.6 or greater recorded in the state on Friday.

Since January 2009, more than 200 magnitude 3.0 or greater earthquakes have hit central Oklahoma,— marking a significant rise in the frequency of the seismic events, the Geological Survey reported. A study released last year shows that one to three magnitude 3.0 or greater earthquakes occurred yearly in Oklahoma from 1975 to 2008 but that the average grew to around 40 earthquakes per year from 2009-2013.

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"It's important to understand that it's not the fracking process that causes these earthquakes," said Bill Leith, the earthquake hazards program coordinator with the U.S. Geological Survey in Reston, Va., who labels the quake swarm affecting central Oklahoma as "unusual."

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