From 1975 to 2008, only a handful of magnitude-3.0 earthquakes or greater occurred yearly in Oklahoma. But the average grew to around 40 annual earthquakes from 2009 to 2013, seismologists said in the report on the uptick of quake activity.
Since 2009, more than 200 magnitude-3.0 or greater earthquakes have hit the state's midsection, according to the Geological Survey. Many have been centered near Oklahoma City, the most populous part of the state.
Scientists are not sure why seismic activity has spiked, but they are studying the phenomena. One theory is that the shaking could be related to wastewater disposal from oil and gas drilling operations that rely on hydraulic fracturing, or "fracking."
Some researchers and environmental groups have long suspected fracking can have seismic repercussions because it forces millions of gallons of water, sand and other materials deep underground to free pockets of fossil fuels.
The energy industry has repeatedly insisted that the practice is safe. Scientists researching the theory say it's not clear yet whether fracking can trigger significant quakes.
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