The U.S. Geological Survey (USGS) has taken its first stab at quantifying the hazard from earthquakes associated with oil and gas development. The assessment, released in a preliminary report today, identifies 17 areas in eight states with elevated seismic hazard. And geologists now say that such induced earthquakes could potentially be large, up to magnitude 7, which is big enough to cause buildings to collapse and widespread damage.
The new bull’s-eyes on the map, regions such as central Oklahoma, have short-term hazards that are comparable to the those in traditional earthquake states, like California, says Mark Petersen, chief of the USGS National Seismic Hazard Mapping Project in Golden, Colorado. “These earthquakes are occurring at a higher rate than ever before and pose a much greater threat to people living nearby,” he says. “This report represents our first step in identifying and quantifying the ground shaking from induced earthquakes.”
Geoscientists have known for decades that the injection of fluid can increase pressures within the pores of deep rock formations, pushing faults that are already critically stressed by forces in Earth’s crust past the snapping point. But the phenomenon has been brought to the fore by an extraordinary rise in small earthquakes across parts of the central United States. That surge has coincided in time and place with the boom in unconventional oil and gas extraction such as hydraulic fracturing, or "fracking," in which high-pressure fluid is injected into the ground to break up the underlying rock and release trapped gas or oil. In most cases, the earthquakes are not due to fracking itself, which is usually completed in hours or days. Rather, the culprit is typically wastewater disposal, where high volumes of water extracted in oil and gas operations is reinjected into deep basement rocks, where the bigger and more dangerous faults lie.
So far, the largest induced earthquake in the United States has been the 2011 magnitude-5.6 earthquake in Prague, Oklahoma, which damaged dozens of buildings. But geoscientists now say there is no reason why oil and gas operations couldn’t end up triggering something much larger. “There are certainly faults large enough to produce a magnitude 7,” says Justin Rubinstein, a geophysicist at USGS in Menlo Park, California, and a co-author of the new report. “We can’t rule this out.”
In July 2014, USGS published its most recent update to its national seismic hazard map, which is incorporated into engineering codes for buildings and bridges. The hazard is expressed in terms of the probability of exceeding a certain level of shaking in 50 years—not only because the hazard in places like California is not expected to change much over that time period, but also because 50 years—the typical life span of a building—is a useful period of time for engineers.
But that map ignored the threat of induced earthquakes, precisely because the frequency of these earthquakes was likely to change over the course of 50 years. Economic or policy conditions could affect the frequency of induced earthquakes, Petersen says. In short order, wastewater wells could be drilled in new locations, the falling price of oil could shut down certain operations, or regulators could change their policies in certain regions. To account for that uncertainty, the new map describes probabilities over an extremely short, 1-year interval.
The report is a preliminary study that is going to be revised later this year, Petersen says. But William Ellsworth, a geophysicist at USGS in Menlo Park, suggests that it could still be useful—for instance, for state transportation departments trying to prioritize which bridges should be singled out for repairs or retrofits first.
Science agencies and regulators finally seem to be taking induced earthquakes seriously. For a long time, the Oklahoma Geological Survey was reluctant to link the earthquakes to oil and gas operations. But in a statement released on 21 April, the agency now says that it is “very unlikely” that the surge in earthquakes represents a natural process. At a teleconference the following day, held in conjunction with unveiling the new USGS report, Austin Holland, the state seismologist at the Oklahoma Geological Survey in Norman, went even further, saying, “The vast majority of these [earthquakes], we suspect, are from waste water disposal.”