The link between "fracking" and earthquakes was thrown into stark relief over the weekend when a magnitude 4.0 quake struck Youngstown, Ohio -- typically not a hot bed of noticeable seismic activity. The quake triggered shaking reportedly felt as as far away as Buffalo, N.Y., and Toronto.
The temblor struck Dec. 31 and was the latest and strongest of 11 minor-to-light quakes that have hit the region since March. The epicenters are clustered around a wastewater injection well for a hydraulic fracturing operation.
Understanding the potential effect hydraulic fracturing or related activities could have on local earthquake risks is one question some researchers hope to answer as they try to develop tools for communities.
Fracking allows energy companies to extract natural gas trapped in shale deposits deep underground. By injecting large quantities of pressurized fluids, drillers can break up the rock, releasing the gas for extraction.
One way to dispose of the waste fluids from the process is to inject them back into porous rock formations deep underground. But if pressurized fluids find their way into faults, the fluids can act like a hydraulic jack, separating locked sections enough to allow them to slip.
The goal of researchers is to provide local or state officials with a means of taking a company's injection plan, plus a knowledge of the rock formation the company plans to use as a dump, and get a rough estimate of the largest quake such a process might trigger.
Seismologist Arthur McGarr says he and his colleagues from the US Geological Survey have worked up such a tool and now are trying to refine it.
The biggest environmental concern around fracking has been groundwater pollution. But Ohio has also had problems with seismic activity. The string of quakes last year prompted the state to ban drilling new wastewater-injection wells within five miles of the well suspected of triggering the temblors. At the state's request, the well itself was shut down Dec. 24.
Ohio is not alone. In July, the Arkansas Oil and Gas Commission banned wastewater-injection wells from a 1,150-square-mile area overlying key shale deposits because of increased earthquake activity linked to the wells.
At issue is the effect fluids injected at high pressure can have on faults. The billion-year-old "basement" rock that underlies much of the eastern US is laced with faults, notes John Armbruster, a seismologist with the Lamont-Doherty Earth Observatory in Palisades, N.Y., who is gathering data on Youngstown's quivers.
In general, the concern is not about the fracking wells themselves. Fracking wells have been known to trigger tiny quakes, but they are typically too light to feel. These wells "are probably not going to be causing you any trouble" from an earthquake standpoint, he says. Any individual well injects fluid for only a day or two in order to release cooped-up gas.
But some portion of that fluid is recovered for later disposal. In the case of the suspect well in Ohio, it was "disposing the fluid from hundreds of fracking wells," he adds.
The disposal well aimed to flush those fluids into a broad, deep sandstone formation, where the fluids could spread in all directions. But drillers reportedly overshot the formation and sank the well some 300 feet into the basement rock, where the faults lurk.
In working up its first cut at a seismic-hazard tool related to fluid injection, Dr. McGarr's team looked at seven events since the 1960s in the US and Europe where fluid injections triggered quakes with magnitudes up to 5.0.
The team uncovered a link between the volume of fluid injected and a quake's magnitude. Every time the volume of fluids doubles, the quake magnitude increases by about 0.4.
So far, the model says nothing about the likelihood of a quake occurring -- just a maximum potential magnitude, given the key assumptions. The team is working to incorporate other variables, such as stress levels on known faults.