Horizontal hydraulic fracturing, or fracking, has been surrounded by controversy since the first well was drilled more than a decade ago. On one hand, harnessing the expanse of natural gas trapped deep beneath the earth's surface has caused an economic boom. It provides jobs, reduces dependency on foreign energy and has helped lower the cost of electricity in many areas. However, environmentalists have concerns with how the drilling process is affecting the planet. Fracking uses billions of gallons of water each year and forces chemical-laden cocktails into the earth's surface. But now fracking is shaking things up even further. A new report shows that it's the cause of earthquakes.

The small farming town of Greenbrier, Arkansas, is situated on the Fayetteville Shale formation, a site where fracking companies flocked in the midst of drilling innovation about 10 years ago. But as more wells were drilled, the small community experienced some unexpected consequences. Between 2010 and 2011, Greenbrier, a city not known for seismic activity, had more than 1,000 earthquakes.

The majority of these earthquakes were small. The largest quake — a magnitude of 4.7 on the Richter scale — was considered moderate. Though Greenbrier was free of serious destruction from these small-scale earthquakes, the volume of these tremors was enough to get the scientific community interested.

The U.S. Geological Survey sent its seismologists to Greenbrier to investigate the cause of this strange earthquake activity. They found that the quakes were most likely triggered by the injection of fracking wastewater into deep disposal wells. The finding prompted the Arkansas Oil and Gas Commission to shut down several fracking wells, and the earthquake problem subsided.

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More than a dozen residents of Greenbrier have filed lawsuits against three fracking companies in the area: BHP Bilton, Chesapeake Operating Inc. and Clarita Operating LLC. However, Clarita Operating LLC filed for bankruptcy in 2011 and was dropped from the lawsuit.

As of August 28, five of the Greenbrier residents accepted settlements of undisclosed amounts from BHP Bilton and Chesapeake Operating. These cases are the first to attempt to legally link fracking to earthquakes. If any of the remaining cases makes it to a jury, it could open the door for further litigation against drilling, including not only fracking but traditional drilling for oil and gas as well as geothermal generation.

The U.S. Geological Survey has said that earthquake activity has been discovered in only a handful of the 30,000 injection wells across the country. In the case of Greenbrier, it appears a few fracking wells were drilled near a fault line, the Guy-Greenbrier fault, which scientists didn't find until after the fracking boom.

Because of these findings, the Arkansas Oil and Gas Commission placed a permanent moratorium on all new injection wells, protecting about 1,200 square miles around the newly discovered fault. The commission also requires any new wells to be 1 to 5 miles away from known faults.

On a national level there are no regulations that relate earthquakes to fracking injection sites. However, the U.S. Environmental Protection Agency is working on addressing the issue due to similar quake activity in central and eastern states.

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