A series of earthquakes that hit Ohio last year were caused by fracking, experts have claimed.
Published in the Bulletin of the Seismological Society of America, researchers looked at five earthquakes that hit Poland Township, Ohio, in March 2014.
They discovered hydraulic fracturing had activated a previously unknown fault that triggered the seismic activity.
Fracking involves extracting gas and oil from shale rock by injecting a high-pressure water mix into the rock. The process normally creates micro-earthquakes far smaller than could be felt by humans.
Previous research has linked earthquakes with fracking. Last October, researchers connected 400 micro-earthquakes to fracking in Harrison County in Ohio between September and October 2013.
In July, another study found fracking was responsible for hundreds of earthquakes across Oklahoma. It found the rate of earthquakes increased from about one per year before the 2008 oil and gas boom to about 240 small earthquakes less than five years later.
Two earthquakes in the UK in 2011 were also found to have been caused by nearby fracking.
Larger tremors caused by fracking remain very rare but advances in seismic monitoring and the growth of the fracking industry has led to an increase in earthquakes over the last 10 years.
In the latest study, authors found earthquakes ranged from magnitude 2.1 to three and all occurred within 1km of a group of oil and gas wells run by Hilcorp Energy. The company was conducting fracking operations at the time.
Following the magnitude three earthquake, the Ohio Department of Natural Resources stopped operations due to the proximity of the tremor to one of the wells.
Study co-author Robert Skoumal said: "These earthquakes near Poland Township occurred in the Precambrian basement, a very old layer of rock where there are likely to be 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."
Speaking to IBTimes UK, he added: "The earthquakes occurred in the upper Precambrian basement, a very old layer of rock that we know is highly fractured. It is extremely difficult and expensive to identify these faults in the Precambrian basement prior to the identification of these small induced earthquakes.
"We know that both wastewater injection and hydraulic fracturing have activated pre-existing faults that were critically stressed by natural forces."
Researchers used seismic data looking for repeating signals similar to the Poland Township earthquakes that act as "seismic fingerprints". Findings showed 77 earthquakes between 4 to 12 March, one of which was felt by the local community.
The earthquakes were compared with the stimulation reports, which showed they coincided with fracking during specific stages. The authors also note industry activity at nearby wells did not produce earthquakes, suggesting the fault is fairly limited.
They said their technique could be used to monitor earthquakes induced by fracking in the future to find unknown faults.
"We just don't know where all the faults are located," Skoumal said. "It makes sense to have close cooperation among government, industry and the scientific community as hydraulic fracturing operations expand in areas where there's the potential for unknown pre-existing faults."
He said his team was looking at the different mechanisms that induce earthquakes.
"We know that both wastewater injection and hydraulic fracturing have activated pre-existing faults that were critically stressed by natural forces," Skoumal said. "In the Poland Township case, it appears that only a small portion of the operation was responsible for the earthquakes - this has also been observed in other cases of earthquakes induced by hydraulic fracturing."
Talking about the potential risk of larger earthquakes, he added: "The hazard of earthquakes induced by hydraulic fracturing is very low. Only a handful of felt earthquakes induced by hydraulic fracturing have been identified worldwide.
"We do not have any evidence to suggest that an induced earthquake will have any long-term impacts. In the case of the Poland Township sequence, when fracking stopped, the earthquakes stopped soon after. A similar scenario was observed in the Youngstown, Ohio sequence [which occurred 18 km away from the Poland Township sequence]. When the wastewater injection ceased, the number of earthquakes dramatically decreased."