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GRELEY, Colo. (CBS4) – An earthquake occurred in Greeley for the first time in more than forty years last month, and now seismologists are taking a close look at the oil and gas development in the area.

In three decades of growing corn on her Weld County farm, Judy Dunn never thought she’d feel an earthquake.

“I thought that there was a possibility that maybe one of the gas wells had exploded,” Dunn said.

But an earthquake is exactly what caused her house to shake on May 31. The quake also has seismologists shaking their heads.

“We have no idea why this earthquake occurred at the moment,” University of Colorado researcher William Yeck said.

And not just because the area doesn’t usually see them.

“There are many wastewater injection wells in this area. Some of them are very high volume,” Yeck said.

Wastewater injection wells take in fluid related to oil and gas drilling, including left over water from hydraulic fracturing, or “fracking.” Yeck says a big part of finding out the answer starts with the question of where it began.

There are five different seismic monitoring devices set up across Weld County. All of them are about 15 kilometers from where they think the earthquake started, and the goal is not only to find where it started, but also how deep.

“The closest wastewater injection well to the earthquake is injecting, is injecting about 3.25 kilometers deep,” Yeck said.

The Colorado Oil and Gas Conservation Commission says it’s still too early to tell. In a statement to CBS4 they said, “This is an important matter and we have only preliminary information at this time. We need to see the facts and further investigation lead to better understand this event.”

“There’s been numerous cases where earthquakes have been linked to wastewater injection,” Yeck said.

But Yeck says only the data and time will tell whether or not the earthquake was human caused.

“The fact of the matter is we don’t have the data at the time to say one way or the other … and that’s why we are out here,” he said.

Researchers say it will take several weeks to collect all the data and up to a year before they can determine if wastewater injection caused the earthquake.