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At the American Geophysical Union meeting in San Francisco last week, scientists presented the latest evidence tying the disposal of wastewater from shale gas hydrofracking to increased earthquakes.

Gas guzzler: A drilling rig near the Eagle Ford shale formation in Texas.

Some U.S. states, including Oklahoma, Texas, and Colorado, have experienced a significant rise in seismic activity over the last few years, coinciding with a boom in fracking—a process that forces gas from hard-to-reach underground deposits by injecting water and chemicals into shale rock. Fracking produces huge quantities of wastewater that is typically disposed of in deep wells. But the degree to which the disposal of wastewater from fracking operations has caused the unusual seismic activity is still up for debate among scientists.

The question matters because most states don’t consider earthquake risk when allowing gas drilling companies to dispose of large volumes of chemical-laden drilling water.

Hydrofracking produces far more wastewater than conventional oil and gas drilling. So how to dispose of this waste safely is becoming a bigger question as fracking expands.

Scientists believe that wastewater injection wells, which are often the cheapest disposal option for drilling companies, are the main quake culprit. Today, 90 percent of fracking wastewater in the U.S. is disposed of in injection wells, according to the Natural Resources Defense Council, an environmental group.

Since 2010, Oklahoma residents have felt more than 250 of them—many more than the one to three reported each year in previous decades.

The largest ever in the state’s history, a magnitude 5.7 quake in 2011 that damaged some 200 buildings, was “likely caused by fluid injection,” concluded University of Oklahoma, Columbia University, and U.S. Geological Survey scientists in presenting their data at the conference. They used aftershocks to map fault patterns and show how pressure built over time as water from fracking was disposed of as close as 250 meters away from the resulting quakes.

Other USGS scientists looked at a series of earthquakes since 2001 in Colorado and New Mexico’s Raton Basin (including a 5.3 earthquake last year). They also presented their results this week, and concluded that these quakes were the result of wastewater injections. The likelihood that the rise in the rate of larger earthquakes, greater than magnitude 3.0, would occur naturally is extremely low, the study said.

The larger picture is complicated, however, because even before hydrofracking became common in the last decade, oil and gas drillers and mining companies have used tens of thousands of injection wells in these regions.

Austin Holland, a seismologist with the Oklahoma Geological Survey, says that clearly some of the recent earthquakes could be caused by wastewater disposal from fracking activities. But his data on Oklahoma suggests that “no changes in oil and gas activities are immediately apparent that could explain the dramatic rise in earthquake rates.”

University of Texas at Austin geophysicist Cliff Frohlich said at the meeting that it’s difficult to pinpoint the causes of the trend because most studies have looked at individual earthquakes, rather than doing broader surveys. His study of a portion of the Dallas-Forth Worth area, in the Barnett Shale, found that nearly all earthquakes were within two miles of high-volume injection wells. However, many other wells seemed to trigger no quakes at all.

Discerning the reasons for these differences will be important for officials who are considering regulation, Frohlich says. Regulations could reduce the risk of earthquakes by more closely dictating where wells can be located and how they are built and used. Ohio, for example, put in place a moratorium on new wastewater injection permits after a series of earthquakes last year, but recently started approving them again after putting more standards in place.

There are also alternatives for wastewater disposal, such as processing the water at existing or dedicated treatment plants. The U.S. EPA is now evaluating national standards for treating water in this way, and this option may be more often considered if earthquake concerns continue to grow, and if scarcer supplies make water itself a more valuable resource, Frohlich says.

Manmade earthquakes, triggered by underground injections, have been known about for a long time, at least since the 1960s, when an Army waste disposal well triggered an earthquake that caused major damage in Denver.

Overall, however, experts seem to believe the “seismic hazard” in many shale gas states is growing: “The future probably holds a lot more in induced earthquakes as the gas boom expands,” says USGS Earthquake Science Center researcher Art McGarr. 

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