Was Oklahoma's earthquake the result of fracking?

Oklahoma normally had about 50 earthquakes a year until 2009, when the number soared. Last year there were 1,047.

Time - Oklahoma isn't California-this is a state that is usually pretty seismically stable. . .  Which begs the question-has something changed to make the Sooner State unstable? Perhaps something like hydraulic fracturing?

Fracking-causing small fractures in the Earth miles beneath the surface with explosives in order to tap trapped oil and gas deposits-is common in Oklahoma, one of the centers of the fossil fuel extraction industry. And it's not hard to wonder whether injecting millions of gallons of water, sand and chemicals deep underground in order to break up rock might worsen existing faults or even trigger a tremor.

There's some evidence that fracking may induce some minor tremors. . . But [fracking induced quakes in England and earlier this year in Oklahoma were much smaller than the 5.6-magnitude temblor on November 5, which released the equivalent energy three miles underground of the explosion of 3,800 tons of TNT. Can fracking really trigger a quake that big?

Experts say it's unlikely, simply because fracking is so much less powerful.

"There's a fault there," said U.S. Geological Survey seismologist Paul Earle. "You can have an earthquake that size anywhere east of the Rockies. You don't need a huge fault to produce an earthquake that big. It's uncommon, but not unexpected."

Huffington Post -The number and strength of earthquakes in central Arkansas have noticeably dropped since the shutdown of two injection wells in the area, although a state researcher says it's too early to draw any conclusions.

"We have definitely noticed a reduction in the number of earthquakes, especially the larger ones," said Scott Ausbrooks, geohazards supervisor for the Arkansas Geological Survey. "It's definitely worth noting."

The Center for Earthquake Research and Information recorded around 100 earthquakes in the seven days preceding the shutdown earlier this month, including the largest quake to hit the state in 35 years - a magnitude 4.7 on Feb. 27. A dozen of the quakes had magnitudes greater than 3.0. In the days since the shutdown, there have been around 60 recorded quakes, with only one higher than a magnitude 3.0. The majority were between magnitudes 1.2 and 2.8.

But Ausbrooks said it's too soon to tell if the decline in quakes is directly related to the injection well closures, adding that the drop could just be a normal low period of the swarm cycle.