Oklahoma’s 5.7 tremor is biggest fracking-induced earthquake yet

compartments once filled with oil, the pressure to keep the fluid going down had to be ratcheted up. As pressure built up, a known fault — known to geologists as the Wilzetta fault — jumped. “When you overpressure the fault, you reduce the stress that’s pinning the fault into place and that’s when earthquakes happen,” said study coauthor Heather Savage, a geophysicist at Columbia University’s Lamont-Doherty Earth Observatory.

The amount of wastewater injected into the well was relatively small, yet it triggered a cascading series of tremors that led to the main shock, said study co-author Geoffrey Abers, also a seismologist at Lamont-Doherty. “There’s something important about getting unexpectedly large earthquakes out of small systems that we have discovered here,” he said. The observations mean that “the risk of humans inducing large earthquakes from even small injection activities is probably higher” than previously thought, he said.

Hours after the first magnitude 5.0 quake on 5 November 2011, University of Oklahoma seismologist Katie Keranen rushed to install the first three of several dozen seismographs to record aftershocks. That night, on 6 November, the magnitude 5.7 main shock hit and Keranen watched as her house began to shake for what she said felt like twenty seconds. “It was clearly a significant event,” said Keranen, the Geology study’s lead author. “I gathered more equipment, more students, and headed to the field the next morning to deploy more stations.”

The release notes that Keranen’s recordings of the magnitude 5.7 quake, and the aftershocks that followed, showed that the first Wilzetta fault rupture was no more than 650 feet from active injection wells and perhaps much closer, in the same sedimentary rocks, the study says. Further, wellhead records showed that after thirteen years of pumping at zero to low pressure, injection pressure rose more than 10-fold from 2001 to 2006, the study says.

The Oklahoma Geological Survey has yet to issue an official account of the sequence, and wastewater injection at the site continues. In a statement responding to the paper, Survey seismologist Austin Holland said the study showed the earthquake sequence could have been triggered by the injections. He said, however, that “it is still the opinion of those at the Oklahoma Geological Survey that these earthquakes could be naturally occurring. There remain many open questions, and more scientific investigations are underway on this sequence of earthquakes and many others within the state