These large, red water tanks supply water to a fracking operation in Weld County, Colo. Each tank, or unit, pumps water into the well bore under high pressure. The water is then combined with sand and chemicals to break apart dense rock and release the petroleum. 

(Photo: CPR/Lesley McClurg)

A new study published Thursday in the journal Science explains why the ground in central Oklahoma has been shaking a lot lately.

The research links a spike in earthquakes to large oil and gas wastewater injection wells in the region. It was conducted by Cornell University, University of Colorado Boulder and the U.S. Geological Survey.

Prior to 2008, Oklahoma averaged about two earthquakes per year with a magnitude of 3.0 or greater, according to the U.S. Geological Survey. In the first five months of this year there were 145. That 40-fold increase in seismic activity comes at the same time as the boom in fracking in Oklahoma, which can generate a large amount of wastewater that is often disposed of deep underground. 

Researchers analyzed earthquakes that shook central Oklahoma known as the Jones Swarm; this single swarm accounted for a fifth of all seismic activity in the region.

Oil and gas wastewater can seep into existing faults and pry apart the rocks causing an earthquake, according to CU Boulder's Matthew Weingarten, co-author of the paper. 

"The conventional analogy is an air hockey table where the puck doesn't slide when the air isn't on," Weingarten says. "But, as soon as you turn the air on the puck slides freely."

Data was collected from 89 injection wells,  including four wells that receive about a million barrels of wastewater a month. The results are unique because the Jones Swarm is relatively far, about nine miles, from the four high-rate injection wells. 

“The faults near the injection wells have too much friction to easily fail,” Weingarten says. “But you still have pressure changes far away from those wells and when those pressure changes encounter faults that are closer to failure, you have earthquakes."

Weingarten stresses that the geology and the situation in central Oklahoma is unique. CU Boulder is also studying aftershocks from the recent earthquakes near Greeley, in Weld County. But, it will be months or even years before hard data is available on that area.