Scientists have found more evidence that injecting wastewater from fossil fuel production underground can lead to earthquakes.
A new study in the journal Geology has tied a string of unusual earthquakes in central Oklahoma to the pumping of wastewater into abandoned oil wells deep underground, including a magnitude 5.7 earthquake near Prague on November 6, 2011.
Felt as far off as Milwaukee, more than 800 miles away, the quake – the biggest ever recorded in Oklahoma – destroyed 14 homes, buckled a federal highway and left two people injured.
The magnitude 5.7 quake near Prague was preceded by a 5.0 shock and followed by thousands of aftershocks with small earthquakes continuing to be recorded in the area.
The recent boom in US energy production has produced massive amounts of wastewater, which is used both in fracking, which cracks open rocks to release natural gas, and in coaxing petroleum out of conventional oil wells.
In both cases the brine and chemical-laced water has to be disposed of, often by injecting it back underground elsewhere, where it has the potential to trigger earthquakes. The water linked to the Prague quakes was a byproduct of oil extraction at one set of oil wells, and was pumped into another set of depleted oil wells targeted for waste storage.
What made this quake swarm unusual is that wastewater had been pumped into abandoned oil wells nearby for 17 years without incident, but in the study the researchers hypothesize that as wastewater replenished 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," says study co-author 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, says 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 says.
The observations mean that "the risk of humans inducing large earthquakes from even small injection activities is probably higher" than previously thought, he says.
Hours after the first magnitude 5.0 quake on November 5 University of Oklahoma seismologist Katie Keranen rushed to install the first three of several dozen seismographs to record aftershocks.
That night, on November 6, the magnitude 5.7 main shock hit and Keranen watched as her house began to shake for what she says felt like 20 seconds.
"It was clearly a significant event," says Keranen, the study's lead author. "I gathered more equipment, more students, and headed to the field the next morning to deploy more stations."
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.
Further, wellhead records showed that after 13 years of pumping at zero to low pressure, injection pressure rose more than 10-fold from 2001 to 2006.
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, but he was cagy about attributing blame.
He said: "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 of Oklahoma."
Hydrofracking itself is not implicated in significant earthquakes as the amount of water used is usually not enough to produce substantial shaking, but scientists have linked a rising number of quakes in normally calm parts of Arkansas, Texas, Ohio and Colorado to below-ground injection.
In the last four years, the number of quakes in the middle of the United States jumped 11-fold from the three decades prior, the authors of the Geology study estimate and last year, a group at the US Geological Survey also attributed a remarkable rise in small to mid-size quakes in the region to humans.
The risk is serious enough that the National Academy of Sciences, in a report last year called for further research to "understand, limit and respond" to induced seismic events but despite these studies, wastewater injection continues near the Oklahoma earthquakes.
The risk of setting off earthquakes by injecting fluid underground has been known since at least the 1960s, when injection at the Rocky Mountain Arsenal near Denver was suspended after a quake estimated at magnitude 4.8 or greater struck nearby – the largest tied to wastewater disposal until the one near Prague, Okla.
The Wilzetta fault system remains under stress, the study's authors say, yet regulators continue to allow injection into nearby wells.
Ideally, injection should be kept away from known faults and companies should be required to provide detailed records of how much fluid they are pumping underground and at what pressure, says Keranen
The study authors also recommend sub-surface monitoring of fluid pressure for earthquake warning signs.
“There should be careful monitoring in regions where you have injection wells and protocols for stopping pumping even when small earthquakes are detected," says Abers.