Record Number of Oklahoma Earthquakes Caused by Oil Industry Wastewater, Says Study
Over less than a decade, the state of Oklahoma has changed from being a seismic dead zone to an earthquake hotspot. Perhaps most alarming is the fact that these earthquakes are man-made.
In 2008, the state recorded only one earthquake with a magnitude of 3 or greater, which is big enough to be felt. In 2015, however, the number of earthquakes of a similar strength rose a peak of 903 incidents.
"Oklahoma has been transformed from a seismic dead zone to a hotspot in less than a decade," wrote Thomas Gernon, the co-author of a new study that used computer models to determine why the problem is not getting better despite regulations.
As first confirmed in a 2013 study, this rise was caused more than 10,000 injection wells littered across the state, including both oil recovery wells and wells used to dispose of the saltwater that is an unwanted byproduct of the oil and gas extraction process.
"Subsurface data indicate that fluid was injected into effectively sealed compartments, and we interpret that a net fluid volume increase after 18 yr of injection lowered effective stress on reservoir-bounding faults," it reads. "Significantly, this case indicates that decades-long lags between the commencement of fluid injection and the onset of induced earthquakes are possible, and modifies our common criteria for fluid-induced events. The progressive rupture of three fault planes in this sequence suggests that stress changes from the initial rupture triggered the successive earthquakes, including one larger than the first."
The wastewater is injected deep underground, at around .6 to 1.2 miles, well below the level of fresh groundwater supplies in order to avoid contamination. That's the only good news. The bad news is that since 2011, this process has infused an average of 2.3 billion barrels of the stuff into layers of sedimentary rock per year.
The new study, meanwhile, seeks to understand just how complex the seismic jumps have become, and why they are not decreasing in frequency despite new regulations for wastewater dumping. The researchers created a Bayesian Network computer model to show that the more saltwater is injected into the Earth, and the deeper it goes, the larger the resulting earthquake.
"The injection is even more likely to cause earthquakes at depths where layered sedimentary rocks meet the crystalline 'basement' rocks below them, at depths of between 1km and 6km (0.6 and 3.7 miles)," writes Gernon. "This shouldn't be surprising because we know that when fluid gets into the spaces within rocks it can considerably weaken them. Pressurized fluids can effectively lubricate ancient fault zones that may have locked-in stresses built up over time, making them more prone to failure."
He adds that science must research better methods for wastewater disposal if this problem is every to subside. Meanwhile, scientists predict a dramatic earthquake spike this year.