Shortly before midnight on Aug. 23, 2011, residents of Trinidad, Colo. and surrounding communities were startled when the ground started shaking beneath them, knocking bricks and stones loose from buildings. Fortunately, no one was injured. As far as earthquakes go, the 5.3 event and the aftershocks that followed were relatively mild.
Nevertheless, the Trinidad quake raised anxiety for another reason. The U.S. Geological Survey eventually concluded that it probably was a man-made quake, caused by the disposal of waste water produced by the oil and gas industry.
Similarly, scientists have linked disposal of oil-gas industry waste water to increased seismic activity in states ranging from Texas to Ohio. That's raised additional worries about one of the sources of that waste water -- fracking, the controversial process in which water, sand and chemicals are injected into the earth at high pressure to crack rock formations and reach deposits of natural gas and oil.
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While the fracking boom in recent years has provided an economic boost to the United States and increased its energy independence, some worry that there's a potentially catastrophic downside, if the process adds to the waste water that's lubricating earthquake faults.
And while most of the quakes linked to waste water injection wells have been small to moderate in intensity, some worry that one eventually could trigger a major quake that might seriously damage buildings and important infrastructure, and endanger people as well.
In oil-and-gas-rich Oklahoma, for example, where the rate of quakes increased by 50 percent between 2013 and 2014, the U.S. Geological Survey and state officials issued a May 2014 alert to residents that the state had an increased risk of a 5.5 magnitude quake or greater, and pointed to waste water injection wells as a likely explanation for the heightened seismic risk. (From Michigan Technological University, here's a chart explaining what risks are posed by various magnitudes.)
It's been known for a long time that humans could induce earthquakes by pumping fluids underground. Back in 1962, the U.S Army injected toxic waste fluids into a deep well at the Rocky Mountain Arsenal northwest of Denver, but then stopped after the area was rattled over five years by more than 1,500 quakes, including one that shook chandeliers at the state Capitol and forced legislators to take cover.
But those worries have risen because of fracking. Seismologists' big concern is not the fracking process itself, but what what operators do with the enormous quantities of waste water that flows back out of the well afterward. Some of that water, which is salty and contaminated with chemicals used in the fracking process, is treated and reused in fracking. But much of it is too contaminated for reuse and has to be trucked or piped to other sites, where it is injected into storage wells that are drilled thousands of feet deep into the Earth.
There are about 30,000 such waste water injection wells across the nation. USGS research geophysicist Justin Rubinstein, who worked on the study of the 2011 Colorado quake, explains that injection wells have a potentially much greater seismic impact than the fracking process itself, because fracking wells tend to be short-lived. Injection wells, in contrast, may last for years and receive a far greater quantity of water, and not just from fracking.
Rubinstein says that in Oklahoma, for example, the biggest portion of what goes into them is "produced water" that naturally exists underground, and comes up along with the oil that's sucked out of both conventional and fracked wells.
"If you drill a well, and it's productive, saltwater is going to come up with the oil," he explains. "This is going to happen whether you fracked or not."
While most of the wells don't seem to be presenting a seismic hazard, others are located in worrisome places.
The prime example is California, where a just-released study shows that fracking has been used in half of the state's new oil and gas wells over the past decade. The state, which is crisscrossed with a complex system of large and small faults, has more than 1,500 active waste water wells, according to a March 2014 report issued by Earthworks, the Center for Biological Diversity and Clean Water Action. According to the environmental groups, more than half of those wells are within 10 miles of an earthquake fault, and six percent of them -- 87 wells -- are within one mile.
Andrew Grindberg, a spokesman for Cleanwater Action, says that two major California population centers -- Los Angeles and Bakersfield -- have waste water injection wells that are close to earthquake faults. "This isn't something we want to be playing with," he says.
In a panel at the Seismological Society of America's May 2014 annual meeting in Anchorage, several prominent earthquake researchers said that disposal of fracking waste water may also pose a serious risk in parts of the Southwest and Midwest where faults have not been mapped as extensively as California's.
And while it was once believed that injection wells only had the potential to produce small quakes, that was shattered by the 2011 Colorado quake and a subsequent 5.7 magnitude quake in Oklahoma, which a study by Cornell University geophysicist Katie Keranen and colleagues linked to decades of injection of waste water from conventional oil drilling.
Another panelist, Gail Atkinson, an earth sciences professor at the University of Western Ontario, warned that it was unclear how big of a quake could be triggered by injection of waste water. But the effects of a such an induced quake might be magnified, because such quakes occur relatively close to the surface -- only 1 to 2 miles deep -- compared to most naturally-occurring quakes, which typically occur 6 miles down.
Atkinson warned that such man-made quakes pose "a significant and as-yet-unquantified risk" to the integrity of critical infrastructure, such as major dams.
Atkinson said that in addition to waste water disposal, the fracking drilling process itself also poses an earthquake risk -- but only for smaller quakes, with a magnitude of up to 4.
The risk might be reduced if the oil and gas industry is able to reduce the amount of fracking waste water that it needs to put into injection wells. One company has developed a system that uses gelled propane instead of water. Because the fluid merges into the oil and gas being extracted, it could eliminate the need to drain away waste water. Friction-reducing additives also eventually could allow operators to keep reusing the same water in wells, instead of having to dispose of it.