The largest recorded earthquake in Oklahoma history was likely triggered by the injection of wastewater from oil production into wells deep beneath the earth, according to a study published Tuesday in the scientific journal Geology.
The magnitude 5.7 earthquake, which struck in 2011 near Prague in central Oklahoma, is the largest and most recent of a number of quakes scientists have tied to wastewater injection from oil and natural gas production, raising new concerns about the practice.
Advanced methods of oil and gas drilling create massive amounts of toxic wastewater. For example, hydraulic fracturing, also known as fracking, uses high-pressure water to unlock natural gas from shale formations. Drillers also use water to force oil from wells that cannot be captured through traditional methods, part of a practice known as "enhanced oil recovery." (See related interactive: "Breaking Fuel from the Rock.")
The use of such methods has exploded in the United States in recent years, contributing to the domestic boom in shale gas and oil production. Much of the wastewater that emerges as a byproduct is pumped into wells beneath the earth's surface for disposal.
Although the controversial practice of fracking has been directly linked to at least two seismic events (small tremors in Garvin County, Oklahoma and Lancashire, England), the wastewater injection that follows fracking is much more likely to set the earth shaking. That's because injection wells receive far more water than fracking sites, said Katie Keranen, lead author of the Geology study. And unlike at fracking sites, the water is not removed. As pressure builds in these disposal wells, it pushes up against geological faults, sometimes causing them to rupture, setting off an earthquake. (See related blog post: "Tracing Links Between Fracking and Earthquakes.")
This is what most likely triggered the 2011 Oklahoma quake, according to the study. At the time of the earthquake, which damaged 14 homes and was felt as far away as Texas, there were three active wastewater injection wells—abandoned oil wells used for storage after oil drilling operations—within 1 mile (1.5 kilometers) of the site.
Keranen, an assistant professor of geology and geophysics at the University of Oklahoma, was at home at the time of the quake. Soon thereafter, she installed seismometers that recorded more than 10,000 aftershocks, which helped scientists estimate the area of the ruptured faults. The data showed that the initial rupture reached incredibly close to an active well—within 660 feet (200 meters)—and the majority of the aftershocks were located within the same level of sedimentary rock as the wastewater injection wells.
The study contends that the proximity of the quake to the active well, combined with rising wellhead pressure before the tremors and the relative lack of seismic activity preceding the event, suggest injection caused the quake. But it also says it is impossible to prove without a doubt. "Without question there is a strong likelihood that [the quake] was induced," Keranen said.
Luckily, the area is rural, and only two people were injured. "If this happened in a high-population center, we would expect a lot more damage," Keranen said. "This is something we should take seriously and help mitigate the risk of it happening again."
In addition to recording the largest quake linked to wastewater injection, the Geology study also shows that it can take decades for an injection well to spark an earthquake. In most documented cases, seismic activity begins within months after workers begin injecting wastewater into a well, and stops when the injection pressure is released, the study says. The 2011 Oklahoma earthquake, however, took place after wastewater injection had been occurring at the wells for more than 17 years.
Large earthquakes are rare in the United States east of the Rocky Mountains. However, the number increased dramatically after 2008, according to the study. The reason is unclear. However, a 2012 report by the National Academies of Science found the energy industry may be increasing the risk of earthquakes by injecting wastewater underground. (See related blog post: "Report Links Energy Activities to Higher Quake Risk.")
Fracking is one major cause of the increase in energy production wastewater. Although the process may not be the direct cause of the quakes, each drill site requires between 3 to 5 million gallons of water per frack, much of which is later disposed of underground. (See related story: "Water Demand for Energy to Double by 2035")
John Bredehoeft, a geological expert at the Washington State research firm Hydrodynamics Group, said scientists have long known that wastewater injection cause earthquakes. "There is no question about that anymore," he said.
But Bredehoeft, who held research and management positions during a 33-year career at the U.S. Geological Survey, said the overwhelming majority wastewater wells in the United States appear to be safe. The problem, he said, is scientists have no way of determining which of the roughly 30,000 wells are likely to trigger earthquakes.
"We don't know enough about the earth's crust to know where it will happen," Bredehoeft said. "Almost nowhere do we have enough data to do that."
Heather Savage, a research professor of geophysics at Columbia University, and a co-author of the Geology study, said increased data collection about wells could help prevent future earthquakes like the one that shook Oklahoma in 2011. "[The occurrence of human-induced earthquakes is] rare, but it is increasing. It's something we need to get ahead of," Savage said.
Despite the study's findings, some experts remain skeptical that wastewater injection caused the Oklahoma earthquake. A statement released by the Oklahoma Geological Survey in advance of Tuesday's study said its data show the earthquake was likely "the result of natural causes."