A "very small number" of disposal wells are behind a swarm of earthquakes that have plagued Oklahoma since 2009, and embroiled fracking in a new controversy, according to a study released on Thursday.
In new research conducted by geologists from Cornell University, the University of Colorado and the U.S. Geological Survey (USGS), the surge in Oklahoma's tremors may be linked to "a small number of exceedingly high-rate" wastewater injection wells. These repositories are the primary method of dumping water used in hydraulic fracturing (fracking), and are seen by scientists as a force multiplier behind an exponential increase in quakes across the country.
The study used geological models to show how migration of wastewater from key wells in the state may be the culprit behind the largest swarm of earthquakes.
Although tremors have been felt in states involved in or close to the shale boom, The Sooner State has been the epicenter of a cluster of tremors since the shale boom began. Oklahoma recorded nearly 100 earthquakes of at least 2.5 magnitude in the last month, according to USGS data, and just this week experienced two quakes of at least 3.0 on the Richter scale.
Although there are thousands of wells across the country, "four of the highest rate wells likely induced 20 percent" of the seismic activity within the central U.S. between 2008 and 2013, the study said. Because shale rock already contains fluid, introducing wastewater ratchets up the pressure underground, creating seismic events.
Injecting wastewater into the earth can induce earthquakes at distances of about 19 miles from the site of the wells, the report added, noting that Oklahoma's seismic swarms have contributed 45 percent of the larger earthquakes in the U.S. during the five-year period.
Calling the dramatic rise in tremors a "primary challenge" for the rapid expansion of shale development in the world's largest economy, lead researcher Katie Keranen called on shale producers to avoid injecting wastewater near major faults, and bolster monitoring of tremors.
Meters "show how much pressure is in the reservoir," Keranen told CNBC in an interview. "If you see it rising, you know you're putting in more (water) that can't be diffused away. They need to do a better job of putting more sensors near these wells, but they're not cheap."
The report is significant, given that it comes in the wake of a decision by Ohio regulators to impose a limited fracking moratorium. In April, officials for the very first time drew an explicit link between earthquakes and the shale extraction activity—even as scientists and the USGS draw a distinction between wastewater disposal and fracking.
According to the USGS, there were almost 450 recorded quakes of magnitude 3.0 or larger during 2010-2013, or more than 100 a year on average. That compares with an average of 20 per year in the preceding 30 years. In March, the agency said a 5.7 earthquake in Oklahoma—near the epicenter of neighboring Texas' Eagle Ford shale drilling—in 2011 was "unintentionally human induced."
Ohio has more than 300 shale wells that churned out about 100 billion cubic feet of natural gas and nearly 4 million barrels of oil last year, according to the state's Department of Natural Resources.
The controversy surrounding wastewater injection is one of several reasons cited by opponents, who have vocally supported fracking restrictions in places like California and New York. Both states sit on top of prolific but as yet untapped shale plays.
—By CNBC's Javier E. David