OKLAHOMA CITY -- A series of earthquakes, three in all, hit near the town of Luther Sunday; the largest measured a 2.3 magnitude.
A fourth quake rattled near the town of Lawton.
Now, U.S. Geological Survey experts are providing new research to bolster claims that those earthquakes are caused by oil and gas drilling.
Their new study states spikes in earthquakes since 2001 near oil and gas drilling wells are almost certainly man-made.
But local scientists still call it an "intriguing coincidence."
"We haven't been able to scientifically establish that link," Austin Holland said, a Research Seismologist with the Oklahoma Geological Survey.
However, Holland said there is some evidence to support an earthquake link to hydraulic fracturing, known as "fracking."
That's when high-pressured fluid is drilled into the ground to fracture the rock and release hard-to-reach oil and natural gas.
Injection wells then pump the wastewater into the earth, often near fault lines.
"Then you put a little bit of pressure in there and relieve some of the friction and then you can cause it to slip more easily," Holland said.
The U.S. Geological Survey said earthquakes with a magnitude greater than 3.0 began increasing last decade when the number of oil and gas wells increased.
In 2009, there were 50 of those earthquakes; in 2010, there were 87.
Last year, that number jumped to 134 earthquakes across the central part of the U.S.
Holland said fracking in Garvin County in January last year seemed to trigger a small set of earthquakes.
When fracking stopped, the earthquakes stopped.
However, he said more studies are needed because a lot of earthquake activity is naturally occurring.
"Then we can learn how to prevent those earthquakes from happening and still be able to reach energy independence," he said.
In reaction to the study, Oklahoma Independent Petroleum Association (OIPA) President Mike Terry said, "Of course we're concerned. I mean, we live here too."
But Terry pointed out that when you slow drilling, there's a fuel price to pay.
He said more testing is needed before subsequent federal regulations hurt the energy industry.
"I'm sure there's an increase in (earthquake) activity somewhere else in the world where there's no oil and gas drilling or hydraulic fracturing right now," Terry said.
The Oklahoma Corporation Commission, which authorizes drilling permits, said any policy change will depend on what the Oklahoma Geological Survey determines.
Their research is ongoing.