Fracking activities are the most likely cause of the series of earthquakes in Youngstown, Ohio, two years ago, new research published in the journal Geophysical Research Letters has found.

Youngstown is located on the Marcellus Shale, one of the current hubs of U.S. fracking industry. Prior to drilling operations close to the town there had never been an earthquake recorded there, at least not since 1776, when data collection started. However, when an injection well began storing wastewater from fracking sites deep underground in December 2010, quakes started to occur. Over the course of one year more than 100 earthquakes were detected, the Huffington Post reported.

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Records reveal that the first quake happened less than two weeks after pumping began, said researcher Won-Young Kim, a seismologist at Columbia University's Lamont-Doherty Earth Observatory. The strongest earthquake, with a magnitude of 3.9, occurred on Dec. 31, 2011, and led to the shutdown of the well. Researchers have linked the quakes to fracking wastewater injection ever since they occurred and the latest study confirms that this is the most likely cause.

However, authors of the study pointed out that not all wastewater wells were likely to cause earthquakes and this was an exception rather than the norm. Overall, there are 177 wells in Ohio that are comparable in size to the one suspected of triggering seismic activities, but none of them has ever been linked to earthquakes.