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Out on Oklahoma's flat prairie, Medford, population about 900, is the kind of place where people give directions from the four-way stop in the middle of town.

It seems pretty sedate, but it's not. "We are shaking all the time," says Dea Mandevill, the city manager. "All the time."

The afternoon I stopped by, Mandevill says two quakes had already rumbled through Medford.

"Light day," she laughs. But, she adds, "the day's not over yet; we still have several more hours."

Mandevill may be laughing it off, but Austin Holland, the state seismologist, isn't.

"I certainly regret starting smoking again, but there are some days when nicotine and coffee are about what get me through the day," he says. "As far as we know, this has never happened before."

Holland says that Oklahoma used to have, on average, one or two perceptible earthquakes a year. Now the state is averaging two or three a day. There were more magnitude 3 or greater tremors here last year than anywhere else in the continental United States, and the unprecedented spike in earthquakes has intensified.

Holland suspects that modern oil production techniques are triggering the jump in quakes. A few years back, companies figured out how to drill sideways through layers of shale, then break, or frack, the rock, releasing a torrent of oil.

The combination of fracking and horizontal drilling sparked a massive oil boom here, but the technique produces much more water than oil — tens of billions of gallons of very salty, toxic water. The only economical way to dispose of it, Holland says, is to force it deep into the earth.

"That pressure acts as a lubricant," he says. "It's not actually the water itself lubricating, but the pressure, and the best way to think about that is an air hockey table," with huge slabs of rock as the pucks.

Holland says injecting water near faults can deliver just enough lubricating pressure to set them in motion. It's called "induced seismicity."

The Prague earthquake hit the state four years ago. At magnitude 5.6, it was the strongest ever recorded in Oklahoma.

"It was coming from everywhere — I mean the walls, the roof," says Ryan Ladra, standing in his parents' battered house. "When it hit, it hit so violent and hard that we thought the house was coming down on top of us."

The Ladras' stone chimney collapsed, striking his mom, Sandra, who is suing companies that ran nearby wastewater injection wells.

But Kim Hatfield of the Oklahoma Independent Petroleum Association says he's not convinced there's a connection. He says oil companies have been pumping brine down wastewater injection wells for decades. More than 3,200 of the wells dot the state.

"You're going to find out that all tornadoes are close to injection wells as well," he says. "If a meteor strikes the state of Oklahoma, I'm going to guarantee it's going to be close to an injection well."

Still, evidence linking injection wells to earthquakes is building. And though oil industry wields enormous clout in Oklahoma, the agency regulating it is ramping up.

Matt Skinner, public information manager for the Oklahoma Corporation Commission, says that the agency has never denied a permit for a disposal well, but it has recently closed a few bad ones and is scrutinizing applications for new wells like never before.

"When we say we're doing everything we can, what we're really saying is, we're doing everything we know, today," Skinner says. "Tomorrow, we may know something more."

Mandevill says she worries about an earthquake rupturing the big natural gas pipeline here — but then beams while looking out over the new park the city recently built with oil boom tax money.

"We have a new swimming pool, splash pad, new sidewalks and a new basketball/tennis court," she says.

It illustrates the complex relationship between oil and earthquakes in Oklahoma.

"You put up with a few things falling off your walls, a few nights being woken up in the middle of the night with the shakes," she says. "Overall it's been good. I'll take the earthquakes for all the benefits that Medford's had so far."

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