Hydraulic fracturing at two well pads in Mahoning County caused 77 small earthquakes last March along a previously unknown geologic fault, a new scientific study says.
The series of temblors included one quake of magnitude 3 -- rare in Ohio -- that was strong enough to be felt by neighbors, according to the study by three researchers from Miami University.
At the time of the quakes, only five were reported, ranging from magnitude 2.1 to 3.
The new research was published online Tuesday in the Bulletin of the Seismological Society of America. It will be printed in the February-March issue of the bulletin.
The peer-reviewed study of the quakes, which occurred in Poland Township southeast of Youngstown, appears to strengthen the link between small- and medium-sized earthquakes and both hydraulic fracturing (also known as fracking) and the use of injection wells for drilling wastes.
The report largely confirms what the Ohio Department of Natural Resources reported last spring, when it shut down the drilling in Mahoning County and later imposed a 3-mile drilling moratorium and stricter rules on drilling near geologic faults.
There were six wells on one pad and one on the second pad. No more fracturing has occurred in those wells, and all seven wells are producing, ODNR spokeswoman Bethany McCorkle said Tuesday.
No new quakes have been reported in eastern Ohio since March, she said.
Texas-based Hilcorp Energy Corp., the company operating the Mahoning County wells, declined comment on the new report.
"We are still in the process of reviewing this study and are not in the position to comment on any specific findings at this time," said Justin Furnace, director of external affairs for the company.
Shawn Bennett, of the Ohio Oil and Gas Association, said, "There is no reason for hysteria" regarding the new report. Ohio is working closely with researchers in other states on how "best to mitigate such events from happening in the future," he said.
In hydraulic fracturing, water, sand and certain chemicals are injected under pressure into underground laterals that can be more than 2 miles in length. The mixture pulverizes the black shale and frees natural gas and liquids that then can be pumped to the surface.
"This activity did not create a new fault, rather it activated one that we didn't know about prior to the seismic activity," said Robert Skoumal, who co-authored the study with Michael Brudzinski and Brian Currie.
The earthquakes occurred in what Skoumal called the Precambrian basement rocks, a very old layer where there are likely to be many pre-existing faults.
The researchers used a technique called template matching to sift through seismic data recorded by the Earthscope Transportable Array, a network of seismic stations. They searched for repeating signals similar to the known Poland Township earthquakes, and those signals were treated like seismic fingerprints.
The researchers identified 77 earthquakes between magnitude 1 and 3 that occurred from March 4 to March 12. Neighbors reported only one quake, the magnitude 3 quake, on March 10.
A magnitude 3 quakes produces vibrations similar to a passing truck.
Skoumal and his colleagues compared the identified earthquakes to well stimulation reports the ODNR released and found that the earthquakes coincided temporally and spatially with hydraulic fracturing at specific stages of the stimulation.
The seismic activity outlined a roughly vertical east-west fault within 0.62 miles of the well.
Drilling activities at nearby wells produced no seismicity, suggesting to the researchers 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," Skoumal said in prepared statement released with the study.
He suggested that template matching offers a cost-effective and reliable means to monitor seismicity induced by fracturing.
Fracturing or the use of injection wells have been linked to small earthquakes in other states, including Oklahoma, Texas and Colorado.
Earlier, researchers reported that there were 500 micro-quakes triggered by fracturing in late 2013 in Ohio's Harrison County.
The Mahoning and Harrison quakes are the first in Ohio to be linked to fracking.
(c)2015 the Akron Beacon Journal (Akron, Ohio)
