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Since 2008, Oklahoma, once a seismically quiet state, has experienced a swarm of earthquakes. Now it is the quake centre of the US, with more activity this year than even California.
Suspicions that the tremors are connected with Oklahoma’s rapidly expanding oil and gas industry have been confirmed by a study published in the journal Science. A team of geophysicists has shown that the tremors are due to huge volumes of wastewater from oil and gas extraction being pumped into disposal wells.
New extraction technologies generate far more water than traditional methods. Most emerge in “dewatering” production systems from reservoirs deep underground. Having pumped out millions of gallons of polluted water, operators need to inject it back into the earth. The resulting increase in pressure underground can trigger seismic activity.
Although the oil and gas industry has denied any connection between large-scale wastewater injection and tremors, the study found a strong statistical link between four high-volume disposal wells in Oklahoma and quakes as far away as 35km.
Combining precise maps of local earthquake swarms with a hydrogeological model, the researchers found that the underground pressure waves from four wells near Oklahoma City were correlated closely with the times and places of seismic events. The wells pump about 500,000 cubic metres of water a month into rock strata more than 2km deep.
So far this year Oklahoma has experienced about 190 earthquakes above the detectable threshold of magnitude 3 – more than twice as many as California, which had been the most seismically active US state. None has yet caused serious casualties, though a magnitude 5.7 in 2011 was responsible for some structural damage. The big unanswered question is whether continued activity could trigger a really destructive tremor.
Oil and gas are so important for the Oklahoman economy that some citizens may feel that even the threat of a major quake is a price worth paying. But the researchers say it should be possible to continue with wastewater injection in a much safer way. The Science study found that most injection wells have no seismic impact. “Our results … suggest that adherence to standard best practices may substantially reduce the risk of inducing seismicity,” says Katie Keranen of Cornell University, lead author.
Wastewater injection is not the same as hydraulic fracturing or fracking, in which high pressure water is used to break up rock strata to release oil or gas, but the two processes are similar enough for further analysis of the Oklahoma data also to shed light on the issue of whether and to what extent fracking causes earthquakes.