Another rare case of fracking-caused earthquakes has jolted Ohio.
A new study connects some 400 micro-earthquakes in Harrison County, near the town of Canton, to hydraulic fracturing wells. The three wells operated from September through October 2013 in the Utica Shale. Ten of the quakes registered between magnitude 1.7 and magnitude 2.2, but the tremors were too deep to cause damage or to be easily felt by people, according to the study, published today (Oct. 14) in the journal Seismological Research Letters.
The new study is the second report this year of fracking-linked earthquakes from drilling in the Utica Shale. The shale is a rock formation that is deeper and closer than the Marcellus Shale to the crystalline basement rocks where faults are more common. In March, scientists with Ohio's Department of Natural Resources (ODNR) shut down drilling at seven Utica Shale gas wells in Poland Township after fracking triggered two small earthquakes. The ODNR now requires monitoring of seismic activity at fracking sites near known fault lines, and reducing the flow of water if earthquakes begin to occur.
The Harrison case is one of the few scientifically documented incidents of hydraulic fracturing causing earthquakes on a fault, said lead study author Paul Friberg, a seismologist and owner of Instrumental Software Technologies Inc. (ISTI). Harrison County is the fifth documented case in the world, Friberg said. Other locations of earthquakes caused by fracking include Oklahoma; the United Kingdom; British Columbia, Canada; and Ohio's Poland Township. [7 Ways the Earth Changes in the Blink of an Eye]
Fracking involves pumping large volumes of water, sand and chemicals into underground shale or other rocks, such as coal. The pressure forces open the rocks, allowing trapped oil and gas to escape.
Within the oil and gas industry, hydraulic fracturing is known to cause earthquakes, but the tremors are usually so small that seismometers barely wiggle in response. The micro-earthquakes from fracturing rocks often register as negative magnitude 1 to negative magnitude 3. (The magnitude scale is logarithmic. On a seismogram, a wiggle of 20 millimeters, or 0.8 inches, corresponds to a magnitude 2 earthquake, and a wiggle of 0.02 millimeters is magnitude minus 1.)
"Fracking earthquakes pose no real hazard, because they are so small in the majority of cases," Friberg told Live Science in an email interview.
The Harrison County quakes struck less than 1 mile (1.4 kilometers) below the horizontal wells. Shaking started just 26 hours after fracking began on Sept. 29, 2013. Nearly 190 earthquakes hit during a 39-hour period on Oct. 1 and 2.
The quakes tapered off after the fracking was completed on the wells, the study reports.
Because the earthquakes line up in an east-west direction in ancient crystalline rocks beneath the Utica Shale, Friberg and his co-authors think the fracking activated a small, unknown fault. The fracking water could have "greased" the fault, unclamping the fracture and allowing it to slip.
Since 2008, shale gas drilling has been linked to earthquakes from Oklahoma to Ohio, but in almost all cases, the quakes are tied to wastewater disposal wells. Fracking produces millions of gallons of wastewater, which is pumped back underground and stored in deep wells to protect groundwater.
Though Ohio is one of the few states to monitor wells for earthquake activity, many of the small faults triggered by injection wells or fracking have never been previously identified by scientists.
"Ohio has been very proactive in installing seismometers throughout eastern Ohio to better analyze seismic data as it relates to oil and gas activity. If the data conclusively shows a probable correlation to a felt event, ODNR has and will continue to take the appropriate steps necessary to ensure public health and safety is protected," said Bethany McCorkle, ODNR spokeswoman.
Editor's note: This story was updated to clarify that Paul Friberg said that this was a rare case of hydraulic fracturing causing earthquakes on a fault, not of felt earthquakes.
Email Becky Oskin or follow her @beckyoskin. Follow us @livescience, Facebook & Google+. Original article on Live Science.
