Earthquakes that have shaken Oklahoma communities in recent months have damaged homes, alarmed residents and prompted lawmakers, seismologists and regulators to investigate what's behind the tremors — and what can be done to stop them. (Sue Ogrocki/AP)

Pumping wastewater from natural gas drilling sites into wells buried deep underground is probably why Oklahoma is experiencing more small earthquakes than California, a new study says.

Like previous research, the study says hydraulic fracturing for gas cannot be directly linked to increased seismic activity, but the injection of wastewater from drilling at disposal sites creates fluid pressure below the surface that can trigger earthquakes of magnitude 3 or higher, said its lead author, Katie Keranen, an assistant professor of seismology at Cornell University in New York.

The research, published Thursday in the journal Science, underscores earlier findings by the U.S. Geological Survey and agencies that study seismic activity in several states. It stands to “help improve the scientific discussion and understanding of what’s occurring in Oklahoma,” said Austin A. Holland, a research seismologist for the Oklahoma Geological Survey.

State geologists have suspended or halted wastewater injection in several states, including Colorado and Ohio, after earthquakes rocked areas near wells. Oklahoma County, near a fault line that runs through the center of the state, experienced only six earthquakes over eight years starting in 2000, according to the Oklahoma Geological Survey. But in 2009, there were 31. In the following 15 months, there were 850.

Hydraulic fracturing, known as fracking, is a controversial process in which high-pressure blasts of water mixed with a chemical cocktail are injected into wells to fracture rock and unlock trapped natural gas.

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The practice creates millions of gallons of wastewater. Some fracking wastewater is trucked away, and some is pumped into disposal wells drilled into porous rock, such as limestone, below the surface layer. According to Oklahoma state records, 10,000 wastewater wells were active in the state last year.

Much of the research centered on a sprawling natural gas drilling operation in the small town of Jones, the site of one of the state’s highest-volume disposal wells, about 15 miles north of Oklahoma City.

Jones often experiences seismic swarms, a sequence of earthquakes that strike in a short time period, the study says. “Seismic swarms within Oklahoma dominate the recent seismicity in the central and eastern United States, contributing 45 percent of magnitude 3 and larger earthquakes between 2008 and 2013. No other state contributed more than 11 percent,” the study says.

The study advises regulators to adhere to best practices that “include avoiding wastewater disposal near major faults,” followed by close monitoring, Keranen said.

In spite of a rising number of studies pointing to the role of wastewater injection in producing earthquakes, the oil and gas industry has pushed back against the findings, saying hydraulic fracturing and the disposal of wastewater have been proved safe.

Energy in Depth, a research group started by the Independent Petroleum Association of America, says on its Web site that “hydraulic fracturing has been catching some unmerited headlines lately” as a result of questions about seismic activity.

“Despite what you may have heard, hydraulic fracturing has been a safe and proven technology for decades and does not pose a major risk of inducing felt seismic events,” the statement says.

View Graphic A Cornell University study found that a dramatic increase in earthquakes in Oklahoma is likely attributable to the injection of wastewater from hydraulic fracturing into underground pools in the state.

EID quotes Mark Zoback, a Stanford University geophysicist who said such that “microseismic events affect a very small volume of rock” and compared the resulting trembles to “a gallon of milk falling off a kitchen counter.”

The federal government’s National Research Council, also quoted, said that wastewater injection “does pose some risk” of inducing earthquakes, but that “very few events have been documented over several decades relative to the large number of disposal wells in operation.”

But research is building. In May, the USGS issued a statement about increased seismic activity in Oklahoma — 183 earthquakes that were magnitude 3 or stronger from October 2013 to April 14, 2014 — and warned that the “likelihood of future, damaging earthquakes has increased for central and north-central Oklahoma.”

Holland said in a television news interview last year that he had a “gut feeling” the seismic activity is natural but that he also was keenly aware of the peer-reviewed research by Keranen and four other authors: Matthew Weingarten, a University of Colorado geologist; Geoffrey A. Abers, a Columbia University seismologist; Barbara A. Bekins, a research hydrologist at USGS; and Shemin Ge, a geologist at the University of Colorado at Boulder.

“It complements our work that we are currently doing or that has been published in the past,” Holland said of the study in an e-mail Wednesday. “We are currently quite busy monitoring seismicity in the state and completing research of our own.”

Keranen said her research found that earthquakes do not happen near the disposal wells; they often rumble eight to nine miles away.

A former professor at the University of Oklahoma, she said she felt a magnitude-5.7 earthquake, one of the strongest in the state’s recorded history, in 2011. The researchers embarked on the study in October because “it was likely induced,” she said, and “we wanted to be sure what was happening over this broader region.

“I think it intrigues people,” Keranen said. “Any time that there’s an area of the world that has low seismic activity and now has an eruption is very puzzling.”