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The rate of 3.0-or-larger earthquakes in Oklahoma jumped by about 50 percent since last year alone—and the increase is even more dramatic if you look at Oklahoma's longterm quake history. The earliest year for which there are reliable USGS records is 1978, so we'll start there.

For 30 years—from 1978 until 2008—Oklahoma experienced an average of two earthquakes per year that measured 3.0 of bigger. But then something crazy happened. In 2009, the number of earthquakes began to shoot up. And it kept climbing. "People thought oh this might be a swarm of earthquakes, where you get a series of small quakes that build up to a bigger one then dies off," Williams said. "But this has just gone on and on. It's over a much broader area. We're not even calling it a swarm anymore. It's surprising."

Last year there were 109 earthquakes of 3.0 or bigger in Oklahoma—a record high. But by one-third of the way through this year, Oklahoma had already logged 145 earthquakes of that magnitude. Looking at these numbers, scientists believe there's a significant chance the state could see a damaging magnitude 5.5 (or bigger) quake next. Last month, Oklahoma made headlines when it experienced seven earthquakes—most strong enough to knock dishes off shelves, the largest measuring at a magnitude of 4.3—over the course of a single weekend. Officials are now developing an emergency earthquake plan for the state, where there has been a 500 percent increase in the purchase of earthquake insurance in three years, according to local TV station KFOR.

"It's an unprecedented situation in Oklahoma state history," Williams told me. "And it's kind of a seismological rule of thumb that when you have certain number of earthquakes in a region over time, there's a relationship between the rates of smaller earthquakes and the rates of larger ones. We're applying that rule to Oklahoma."

Scientists say the connection between increased seismic activity and fracking is clear, but there's still a lot we don't understand. And that's because there's a lot about fracking that we don't actually know. We don't know the pressure at which wastewater is injected, and we don't know how deep into the ground wastewater is injected—it could be hundreds of feet, or it could be miles. "The depth of injection matters," Williams said. "If it's really deep, the pore pressure changes affect where faults have more energy to release... But the operators of these wells aren't required to produce this information in detail."

There are also questions about which chemicals are used in the fracking process. Companies might use different materials in the fluids that help break up rock and extract petroleum. And without fully understand why fracking causes earthquakes when it does, we also can't figure out why it doesn't always cause them. Correlation, of course, isn't causation. Why does fracking seem to be linked to an uptick in earthquakes in some places but not in others?