CORRECTION: The Associated Press erroneously described the geologic formation the well reached in an earlier version of this story. University of Colorado geology professor Anne Sheehan said the well came close to basement rock but probably did not penetrate it.

DENVER (AP) —The ground around a northern Colorado wastewater injection well has been relatively quiet for more than two months, offering hope that a 10-month string of more than 200 small earthquakes might have subsided.

The bottom 450 feet of the 10,800-foot-deep well was plugged with cement last year, and that might be keeping the wastewater – a byproduct of oil and gas wells – from seeping into fractures and triggering earthquakes, researchers and regulators say.

The newly shortened well is back in operation, and researchers say no quakes greater than magnitude 1 have been measured in a 7-mile radius around it since April 2.

Colorado is one of a handful of states grappling with earthquakes blamed on such wells, which inject wastewater deep underground because it’s too salty or contaminated to be poured into rivers or lakes. Similar problems have been reported in Oklahoma and Texas, as well as Alabama, Arkansas, Kansas, New Mexico and Ohio.

Wastewater injection is different from hydraulic fracturing, or fracking. Fracking is the practice of injecting high-pressure water, sand and chemicals into oil- or gas-bearing rock to increase the flow. Fracking is sometimes accompanied by “micro-earthquakes” that are usually too small to be felt, the U.S. Geological Survey says.

Injection wells push waste fluids deep underground, away from freshwater sources. Most of the waste is brackish water pumped to the surface along with oil and gas, but it can also contain discarded fracking chemicals.

A 3.2-magnitude earthquake radiated from the site of the injection well in Weld County about 65 miles north of Denver on May 31, 2014. The quake was felt some 40 miles away but no damage was reported. Researchers and the Colorado Oil and Gas Conservation Commission, which regulates the industry, eventually zeroed in on the well as the likely cause.

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“It’s hard to prove conclusively,” said Anne Sheehan, a geology professor at the University of Colorado who deployed six seismometers around the site after the first quake. “There were no earthquakes there before the well.”

The state ordered the operator, NGL Water Solutions, to shut the well down until the company came up with a fix. NGL decided to seal off the bottom of the well, said Stuart Ellsworth, engineering manager for the Oil and Gas Conservation Commission.

NGL didn’t respond to requests for comment.

The state allowed the company to resume injecting wastewater at a scaled-back rate once the well was shortened. Smaller earthquakes continued sporadically but on a long, uneven decline, Sheehan said.

“It goes up and down and up and down,” she said. “But the overall trend has been a strong decrease.”

Most of the subsequent quakes recorded there were under magnitude 2, and none equaled the 3.2 quake in May 2014, she said.

The state has allowed NGL to gradually increase the injection rate at the well.

Sheehan and Ellsworth are optimistic that shortening the well fixed the problem, but both say it’s too early to declare victory. Sheehan said similar fixes are being tried in Oklahoma, and the technique has potential elsewhere.

“Well, I think it’s the answer,” Ellsworth said. “It was a remediation effort that reduced the risk of future events. At least that’s what it looks like.”

Before it was plugged, the bottom of the Colorado well came very close to a brittle layer known as basement rock, and the wastewater might have flowed into fractures, Sheehan said. The wastewater had enough pressure to push the two sides of the cracks apart, allowing them to slip, the theory goes.

Injection wells are more likely to trigger earthquakes in basement rock because it is stiffer than sedimentary rock and better able to resist movement – unless something such as the injected wastewater makes it easier for it to slip, said Justin Rubinstein, a USGS seismologist.

Researchers have known for decades that injection wells can cause earthquakes, but the phenomenon is getting more attention because the number of quakes has increased dramatically in a swath of the central U.S. from Ohio across Colorado.

An average of 24 quakes of magnitude 3 or greater were recorded in that region every year from 1973 to 2008, but by 2014 it had risen to 688, the USGS says.

“The increase in seismicity is largely, if not entirely, induced by human activity,” Rubinstein said. Pinpointing the causes in specific areas would require more detailed study, he said.

Most injection wells don’t cause earthquakes, the USGS says.

By Dan Elliott, AP Writer

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