May 5, 2014
The U.S. Geological Survey and Oklahoma Geological Survey, issued a joint statement reporting 145 earthquakes of magnitude 3.0 or greater have occurred in Oklahoma since January 2014. The previous annual record, set in 2013, was 109 earthquakes. The largest earthquake in Oklahoma history, a magnitude 5.7, occurred in Nov 2011, near Prague.
So, what's causing the earth to shake in the middle of the United States? Scientists have suggested that disposal wells, used to dispose waste from oil and gas drilling operations (also known as "fracking") could be the cause for the recent spike of seismic activity.
As of Jan 2013, more than 10,000 underground injection wells were active in Oklahoma. About 6,000 of these wells are a type of injection well used for enhanced oil recovery. The remaining 4,000 are disposal wells used to store drilling waste. Most of these disposal wells store waste between 10,000-20,000 feet underground.
Source: Oklahoma Corporation Commission
The 5.7 magnitude earthquake that hit near Prague, was preceded by a 5.0 quake that hit a day earlier. This fore shock occurred near an active wastewater disposal well, along the Wilzetta fault. A new study confirms that small man-made earthquakes can trigger damaging quakes.
Earthquakes linked to fracking are mainly caused by the fluid disposal in deep wells; injecting water, sand and chemicals into the Earth to crack open rock and then remove oil and gas. The millions of gallons of waste fluids are usually pumped back into the Earth via deep wastewater injection wells. The wastewater can lubricate or jack open fractures and faults, triggering earthquakes.
Researchers suggested that the first earthquake primed the fault for another, larger earthquake which hit the next day. The study was published March 7 in the Journal of Geophysical Research.