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Oklahoma is no stranger to natural disasters. The state is best known for tornados, but it might not be long before something else steals the spotlight: Earthquakes. As Joe Wertz of StateImpact Oklahoma reports, the number of quakes there has soared, including two more just today.

JOE WERTZ: Amanda Erwin says even on a clear day, she knows something's up when the thunder begins.

AMANDA ERWIN: The chandelier was swinging and the walls were rumbling and the bed was rumbling.

WERTZ: Her husband Keith says the earthquakes remind him of the artillery he used to hear growing up near a military base and when the sound of shaking fade, the game starts.

KEITH ERWIN: We just turn and look at each other and we play this game, what do you think it was? A 2.5. No, that had to have been a 3.0. It's a daily thing.

WERTZ: For the past three decades, Oklahoma averaged about 50 earthquakes annually, but in the last few years, it skyrocketed. In 2013, almost 3,000 quakes shook Oklahoma, the state's most seismically active year ever. The quakes are small and they're concentrated in the central part of the state right where the Erwin's live. As we talk in the living room of their suburban Oklahoma City home, a funny thing happens.

ERWIN: Was that one just now? Did you hear it? Did you hear that, that little rumble? OK. It might have been the workers.

WERTZ: The Erwins are on edge and they're not alone.

UNIDENTIFIED WOMAN: Oklahoma City, 9-1-1.

UNIDENTIFIED MAN: Yes. Did we just have an earthquake?

UNIDENTIFIED WOMAN: Yes, yes. We had an earthquake. Everything's OK now. Do you need police, fire or (unintelligible)?

UNIDENTIFIED MAN: No. I was just sitting here looking at the ball game and my butt started moving.

WERTZ: In October, the U.S. geological survey warned that Oklahoma's risk of earthquakes has increased 10-fold. The swarm of quakes includes Oklahoma's largest ever, a 5.6 magnitude temblor that struck east of Oklahoma City in 2011.

BILL LEITH: And that doesn't mean that there's going to be a large earthquake tomorrow or next month or next year even, but those probabilities are up very substantially.

WERTZ: That's Bill Leith with the U.S. Geological Survey. He says there's strong evidence linking Oklahoma's earthquakes to the state's large oil and gas industry. When they drill, toxic fluid from fracking and other types of drilling is injected deep underground. Cornell seismologist Katie Keranen says that can change pressures near fault lines.

KATIE KERANEN: And we can show that it's quite reasonable that water flowing from these wells is actually triggering these earthquakes.

WERTZ: While scientists say the quakes are likely connected to the wells, there's no proof. That's why regulators aren't considering new rules or laws. Oklahoma's official seismologist, Austin Holland, at the State Geological Survey, says oil and gas activity might trigger earthquakes, but it could just be a natural increase, too.

AUSTIN HOLLAND: But I don't think we can, at this point, attribute all the earthquakes to some sort of manmade cause.

WERTZ: In the fall, state regulators did force one well operator along the Oklahoma/Texas border to reduce injection volume and pressure after a series of nearby earthquakes. But the state might allow an experiment to let the company increase the injections to see if it does trigger more rumbling. Scientists, like Holland, would monitor the outcome.

HOLLAND: Or no earthquakes happen and we say, wow, that was the most amazing coincidence we've seen and, you know, we move on.

WERTZ: Back at the Erwin home, every little noise or rumble sends them scrambling for the iPad to see if the quake was real or a phantom.

ERWIN: You feel like you're playing Battleship. You look at the map and you see these little tings of all the different places where they hit.

WERTZ: Oklahoma is tornado alley, not earthquake country and residents here are slowly getting used to the shaking. But many Oklahomans like the Erwins, still feel rattled. For NPR News, I'm Joe Wertz in Oklahoma City.

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