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Earthquakes are synonymous with California to most Americans, but West Coasters might be surprised to learn they’re far from the new center of the seismic landscape in the U.S.

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Oklahoma recorded more than three times as many earthquakes as California in 2014 and remains well ahead in 2015. Data from the U.S. Geological Survey shows that Oklahoma had 562 earthquakes of magnitude 3.0 or greater in 2014; California had 180. As of Jan. 31, Oklahoma recorded 76 earthquakes of that magnitude, compared with California’s 10.

According to the Advanced National Seismic System global catalog, in 2014, Oklahoma even beat Alaska, the nation’s perennial leader in total earthquakes, though many small events in remote areas go unrecorded there.

In California, earthquakes always have been relatively common, but in Oklahoma, they were much more rare—at least until 2009.

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And though Oklahoma has had the most dramatic increase in earthquakes, other states such as Kansas, Texas, Ohio and Colorado also are seeing more “induced seismicity”—earthquakes likely triggered by human activity.

So far, earthquakes have proven much more deadly on the West Coast. In California, 63 people were killed in the 1989 Loma Prieta quake, and about 60 died during the Northridge event in 1994. None of the recent Midwest quakes have resulted in deaths. But the changes in the Midwest are so significant that the country’s top experts are being forced to fundamentally rethink their approach to seismic risk.

“A fundamental assumption of the old maps was that everything stays the same. Now we’re challenged with the earthquakes rates varying with time,” said William Ellsworth, a seismologist for the U.S. Geological Survey who is part of a group studying new ways to understand the hazards of induced earthquakes. “Everybody realized that this was an appropriate thing to do because they didn’t know what to do.”

Numerous studies agree that wastewater disposal from hydraulic fracturing, or fracking, is a major factor in increasing seismic activity.

Massive amounts of water, along with other fluids, are used in the fracking process to break rocks, release natural gas and push it to the surface. That water, along with brine pulled from those rocks, comes back to the surface and has to go somewhere. So drillers inject it into underground disposal wells. In Oklahoma, more than 50 billion gallons of wastewater went into disposal wells in 2013 alone, according to the Oklahoma Geological Survey. The fracking process itself also has been linked to earthquakes, though those generally have been smaller than those associated with wastewater disposal.

The areas facing new earthquakes share a significant burst of drilling activity and pre-existing geologic faults. Heavily drilled areas such as North Dakota’s Bakken formation, which aren’t located near faults, have not seen the same increase in earthquakes.

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The largest Midwest quake in recent years—a magnitude 5.7—was centered near Prague, Oklahoma, on Nov. 5, 2011. The city of 2,400 people is about 2 miles east of the Wilzetta fault system, and in 2011, it had several active wastewater disposal wells nearby.

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