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OKLAHOMA CITY – Oklahoma has never been known as earthquake country, with a yearly average of about 50 tremors, almost all of them minor. But in the past three years, the state has had thousands of temblors. This year has been the most active, with more than 2,600 so far, including 87 last week.

State officials say they are concerned, and residents accustomed to tornadoes and hail are now talking about buying earthquake insurance.

“I’m scared there’s going to be a bigger one,” said Mary Catherine Sexton who had objects knocked off her walls in a magnitude 4.5 earthquake on Saturday.

Just as unsettling in a state where more than 340,000 jobs are tied to the oil and gas industry is what scientists say may be causing many of the earthquakes: the widespread industry practice of disposing of billions of gallons of wastewater that is produced along with oil and gas, by injecting it under pressure into wells that reach permeable rock formations.

“Disposal wells pose the biggest risk,” said Dr. Austin Holland, the seismologist with the Oklahoma Geological Survey, who is studying the various clusters of earthquakes around the state.

Oklahoma has more than 4,000 disposal wells for waste from tens of thousands of oil and gas wells.

“Could we be looking at some cumulative tipping point? Yes, that’s absolutely possible,” Holland said. But there could be other explanations for the increase in earthquakes, he added.

Scientists have known for years that injection wells and other human activities can induce earthquakes by changing pressures underground. That can have the effect of “unclamping” old stressed faults so the rocks can slip past each other and cause the ground to shake.

The swarm of quakes has state regulators concerned, but cautious. The practice of hydraulic fracturing, or fracking – injecting liquid at high pressures into shale rock – causes very small tremors as the rocks break, releasing trapped oil or gas. The technique has also been linked to a few minor earthquakes – in Oklahoma about a year ago, and in England and British Columbia.