Eleven earthquakes hit the Dallas area this month, setting off a media uproar about whether hydraulic fracturing, or fracking, was to blame. No surprise there, given that more than 2,000 oil and gas wells operate near the region’s Balcones fault.

Earthquakes aren’t just a California thing anymore. The growing number of quakes across the United States—particularly in areas where they have been historically rare—is in lockstep with the fracking boom, which injects highly pressurized, chemical-laced water into subterranean rock formations to extract oil and gas.

The January quakes—the largest measured 3.6 in magnitude—totaled 26 in just the Dallas area in recent months. Since 2008, more than 100 temblors have struck North Texas.

Colorado, Oklahoma, Arkansas, and Ohio also have seen a dramatic rise in earthquakes directly proportional to the increased fracking activity, reports the U.S. Geological Survey.

In March 2014, the small town of Poland, Ohio, in one week experienced 77 quakes within a mile of a fracking operation.

A Miami University study published this month in Bulletin of the Seismological Society of America compared the Ohio quakes with local drilling data. Researchers concluded that wastewater from fracking injected underground had the potential to trigger unknown faults.

Scientists in 2013 determined that wastewater injection triggered the 5.7-magnitude quake that rattled Oklahoma two years earlier.

“During the fracking process, a large volume of water is injected into a rock layer with high pressure,” said Michael Brudzinski, a study author and a seismologist with Miami University in Ohio. “When that water is pulled out, it’s dirty with sediments, and we don’t want it to mix with our aquifers. So they find a layer of rock to host this water really deep down for a very long time, and that’s the idea behind wastewater injection.”

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Wastewater injection isn’t cheap, but it’s less expensive than completely treating the dirty water and recycling it, he said. The injections can awaken dormant faults, which trigger quakes.

In the town of Poland, fracking had occurred directly on top of small existing faults, which led to the quakes. Brudzinski suggested that officials increase the monitoring of seismic activity and map potential fault lines.

His team was already investigating a series of quakes that had occurred in Youngstown, seven miles north of Poland, and found that the volume of fracking wastewater injected underground was directly proportional to the number of quakes.

“Once they shut off injections, the quakes stopped,” Brudzinski pointed out. “The reason the quakes have become more prevalent now is because we are doing a lot more fracking now, so we’re doing more wastewater injections.”

How does that trigger an earthquake?

The wastewater injection doesn’t create a fault, but with unknown faults, the water gets inside the fault and pushes the two sides of the fault line apart. When that happens, it triggers a quake, Brudzinski said.

“Faults can lie dormant for thousands of years and not slip, but once they slip, they are active,” Brudzinski said. “So fracking is indirectly responsible for these quakes.”