A new study links earthquakes in Poland Township to hydraulic fracturing that activated a previously unknown fault.

According to research published online by the Bulletin of the Seismological Society of America, the induced seismic sequence included a rare 3.0 magnitude earthquake that could be felt by residents of the area.

Last March, a series of five recorded earthquakes, ranging from magnitude 2.1 to 3.0, occurred near a group of oil and gas wells operated by Hilcorp Energy, which was conducting active hydraulic fracturing operations at the time. As a result, the Ohio Department of Natural Resources halted operations at the Hilcorp well on March 10.

Hydraulic fracturing, or fracking, is a method for extracting gas and oil from shale rock by injecting a high-pressure water mixture directed at the rock to release the oil and gas trapped inside. The process of fracturing the rocks normally results in micro-earthquakes much smaller than humans can feel.

Researchers say that it remains rare for hydraulic fracturing to cause larger earthquakes that are felt by humans. However, due to seismic monitoring advances and the increasing popularity of hydraulic fracturing, the number of earthquakes – felt and unfelt– associated with hydraulic fracturing has increased in the past decade.

“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,” said Robert Skoumal who co-authored the study with Michael Brudzinski and Brian Currie at Miami University in Ohio. “This activity did not create a new fault, rather it activated one that we didn't know about prior to the seismic activity.”

Using a technique called template matching, the researchers sifted through seismic data recorded by a network of seismic stations, looking for repeating signals similar to the known Poland Township earthquakes, which were treated like seismic “fingerprints.” They identified 77 earthquakes with magnitudes from 1.0 and 3.0 that occurred between March 4 and 12 in the Poland Township area.

The local community reported feeling only one earthquake, the magnitude 3.0, on March 10. Skoumal and his colleagues compared the identified earthquakes to well stimulation reports, released in August 2014 by the ODNR, and found the earthquakes coincided with hydraulic fracturing at specific stages of the stimulation.

According to the Ohio Oil and Gas Association (OOGA), well stimulation is the process of applying hydraulic force, using water, to induce and extend tiny fissures into a reservoir rock. OOGA compares the process to creating a drainage ditch that penetrates horizontally into the oil and gas bearing reservoir rock.

"So, this is a warning flag for this part of the world. There is something in the basement and maybe we shouldn't be injecting waste fluids. We shouldn't be fracking so close to these faults," said Dr. Ray Beiersdorfer, Y.S.U. Geology professor.

The seismic activity outlined a vertical fault within one kilometer of the well. Industry activities at other nearby wells produced no seismic activity, suggesting to the authors that the fault is limited in extent.

“Because earthquakes were identified at only the northeastern extent of the operation, it appears that a relatively small portion of the operation is responsible for the events,” said Skoumal, who suggests the template matching technique offers a cost-effective and reliable means to monitor seismicity induced by hydraulic fracturing operations.

“We just don't know where all the faults are located,” said Skoumal. “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.”

According to its website, the Seismological Society of America is an international scientific society devoted to the advancement of seismology and its applications in understanding and mitigating earthquake hazards and in imaging the structure of the Earth. Founded in 1906 in San Francisco, the Society now has members throughout the world representing a variety of technical interests: seismologists and other geophysicists, geologists, engineers, insurers and policy-makers in preparedness and safety.