A 4.0 magnitude earthquake near a Youngstown, Ohio, natural gas well on New Year’s Eve has invigorated the public debate on the safety of hydraulic fracturing.
Activists there and elsewhere, such as in the United Kingdom, have called for a moratorium on the fracking process, increasingly widely used to extract gas from shale rock. In response, energy company officials have said their wells may not be to blame for nearby earthquakes.
So is fracking a dangerous source of earthquakes that should be halted? Or is it harmless when it comes to quakes? Scientists say the truth is somewhere in between, but generally believe fracking poses manageable earthquake risks.
Scientists began to understand that humans could cause small earthquakes in the 1960s, when the U.S. Army drilled a 12,000-foot well at the Rocky Mountain Arsenal, near Denver, to dispose of waste fluids. This created a series of unusual earthquakes in the area.
Later that decade, at an oil field in Rangely, Colo., tests were done and seismologists found that changes in the number of earthquakes recorded per year correlated with changes in the quantity of fluid injected into the ground.
“It’s important to recognize that the association between injection and triggered earthquakes has been known about for about 40 years,” said Mark Zoback, a Stanford University geophysicist.
But that doesn’t necessarily mean that hydraulic fracturing poses a significant public threat.
There are, after all, about 50,000 injection wells in the state of Texas, and only a handful of earthquakes a year.
There are four steps to the fracking process, which energy companies use to extract natural gas from shale rock formations that are thousands of feet beneath the surface.
First the well is drilled, and then highly pressurized fluid is injected to break up the rock. This increases the rate at which natural gas can be produced from the well.
At that point something must be done with the wastewater used to frack the well. To avoid contaminating nearby groundwater, it often is injected back, deep into the well.
It is this step, scientists believe, that can trigger earthquakes.
Although the world’s strongest earthquakes occur along the major fault lines, there are smaller, pre-existing faults all over.
The injection of water can change the pressure along these faults, causing them to slip and triggering small earthquakes, scientists say. Fracking, then, might cause an earthquake sooner than it would have occurred naturally.
But the process seems unlikely to amplify a tremor.
“My preliminary studies suggest you almost never get induced earthquakes that are bigger than the natural earthquakes in an area,” said Cliff Frohlich, a researcher at the University of Texas at Austin’s Institute for Geophysics.
That was the case even with the Youngstown quake.
As recently as 1998, there was a magnitude 4.5 earthquake near the city, according to the U.S. Geological Survey.
Although scientists say they have a basic understanding of fracking and earthquakes, more research is needed to put further limits on the risk posed by the process.
“I think it’s a manageable problem,” Frohlich said. “To me it seems that, especially if more research were done, it’s possible to establish some areas of the country where you could do almost any amount of injection.”
And in those areas where wastewater injection might be a seismological threat, he said, there may be other disposal options.
For now, then, it’s probably the safest practice to carefully monitor large injection wells in tectonic regions, scientists say.
Researchers hope to identify and prevent a situation in which fracking might cause severe seismicity problems.
This can be done through better imaging technologies to look at subsurface rock formations, as well as other monitoring of fracking wells, said Robert Stewart, a University of Houston geophysicist.
“Experience, research and case histories are required to try to find any rare and possibly untoward circumstances where hydrofracking could be undesirable,” he said.