In 150 years of European settlement—admittedly a short time, in geologic terms—Azle, Texas, never experienced an earthquake. The region has small faults, but there'd been nothing to trigger them. Then oil companies moved in, and between November 2013 and January 2014, the United States Geological Survey recorded more than two dozen quakes. The tremblors were too small to cause real damage, but many were strong enough for folks to feel, prompting protest from the townspeople.
Now, a new analysis supports what many locals already believed: Oil and gas drilling practices are to blame. The analysis, conducted by university and federal geologists, presents evidence for the involvement of extraction practices and affirmation against other hypotheses—say, that the earthquakes are natural and unlinked to human activity. "To us, the most likely explanation, given the results, is that oil and gas activity plays the biggest role in causing earthquakes in this region," says Matthew Hornbach, the earth scientist at Southern Methodist University who led the study.
Outside scientists agree that's a likely cause for Azle's quakes, although it's always impossible to rule out a natural tremblor. "I'm pretty convinced they're right," says Jean-Philippe Avouac, a geophysicist at the University of Cambridge in the United Kingdom. "They did the best they could to analyze this case example, but they are missing some information—probably things that regulators should ask for."
"The most likely explanation, given the results, is that oil and gas activity plays the biggest role in causing earthquakes in this region."
Among the data Hornbach and his team identified as missing: underground pressures that companies regularly collect, but don't have to make publicly available. In addition, geologists say it's time Texas installs sensitive seismometers around Azle. Seismometers are common in California, but not elsewhere in the United States, because other regions never had California's tendency to quakes—until now.
In the past few years, a growing body of evidence has linked oil and gas industry practices with earthquake elicitation. Careful monitoring of the Earth's movement around Azle—and other oil and gas drilling sites where people have noticed increased shaking—would help scientists predict how often larger quakes will come. It could also help them gather the data they need to answer critical questions, such as how large of an earthquake is possible in a given region. So far, Azle has hosted quakes of about magnitude 3. That's strong enough for people to feel, but not enough to cause serious damage to buildings. In Prague, Oklahoma, earthquakes linked to a practice called wastewater injection have reached magnitude 5.6, destroying four houses and bringing down the stone turret of a local college. It's possible for wastewater injection, which occurs in Azle, to trigger earthquakes stronger than magnitude 3s. But nobody knows whether that could happen in Azle specifically given how different its geological makeup is to Prague's.
It's important to note that science actually shows that hydraulic fracturing, or fracking, is unlikely to cause large earthquakes. Fracking, the practice of injecting water into wells to stimulate them to produce more oil and gas, is linked to quakes of magnitude 2 or less. When it comes to earthquakes, it's the other practices an oil or gas company might employ that really matter: Wastewater injection, which is when companies dispose of the toxic liquid byproducts by storing them deep underground; and brine production, which occurs when drillers remove high volumes of briny, underground water in their search for oil.
"All of us in the research community do feel this is a hazard that can be managed."
Wastewater injection and brine production—both common industry practices—alter the pressure underneath the Earth. If they're able to reach a fault, those pressure changes can trigger earthquakes.
With more science, however, geologists believe people will be able to live with the Earth-shaking of oil wells. "All of us in the research community do feel this is a hazard that can be managed," says Bill Ellsworth, a U.S. Geological Survey scientist who also worked on the Azle study. Oil companies could perhaps inject wastewater at a slower rate to reduce the likelihood of earthquakes, Ellsworth says. But exactly what rate will make a difference needs more data to answer.