A new study, released by the Seismological Society of America on Monday, has confirmed that a series of small earthquakes experienced in Ohio were triggered by fracking activity. This seismic sequence, which took place in March 2014, comprised five recorded earthquakes, ranging from magnitude 2.1 to 3.0.

One of these events was a rare “felt” earthquake, meaning it was large enough to be felt by people in nearby towns, although it didn’t pose any risk and didn’t cause damage. Given the fact that the events took place within one kilometer (0.6 miles) of a group of oil and gas wells, state officials decided to halt operations two days after the 3.0 quake hit. Since then, scientists have been scouring through seismic data to determine whether the fracking activity was to blame, and the results have now been published in the Bulletin of the Seismological Society of America.

Fracking, or hydraulic fracturing as it is correctly termed, is a technique designed to extract natural gas and oil from shale rock layers deep within the Earth. It involves drilling deep into the Earth’s surface, reaching depths of one to two miles or more, before blasting the rock with a high-pressure water mixture. This opens up new cracks and channels in the rock from which trapped gas can escape up the well bore.

It’s well known that fracking triggers tiny tremors; it’s a pretty explosive process, but usually these are far too small to be felt by humans in the surrounding area. It’s therefore quite unusual for the activity to have triggered a felt earthquake in Ohio. However, as explained in a press release from the SSA, seismic monitoring advances and an increase in the popularity of fracking have meant that the number of earthquakes—both felt and unfelt—associated with the activity has started to increase in recent years.

Scientists were able to conclude that fracking was indeed the cause of the series of earthquakes experienced last year in Ohio by trawling through data recorded by a network of stations called the Earthscope Transportable Array. In total, they found 77 earthquakes occurred between March 4 and 12 in the Poland Township area, ranging in magnitude from 1.0 to 3.0. These were then compared to well stimulation reports, which revealed that the events matched up with fracking activities at the Hilcorp well in terms of both timing and location.

So what was it about Hilcorp’s activities that caused such seismic activity? It seems that it was just a case of bad luck due to the fact that the company was fracking in a very old layer of rock which probably possesses many pre-existing faults. “This activity did not create a new fault, rather it activated one that we didn’t know about prior to the seismic activity,” explained study co-author Robert Skoumal. “We just don’t know where all the faults are located.” As a result, the Ohio Department of Natural Resources has now made tougher regulations in regards to fracking near known fault lines and other areas of potential seismic activity.

[Via SSA, Forbes and Live Science]