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Magnitude 5.7 earthquake was due to 'manmade seismicity'

27 March 2013

A new study points to wastewater injection as the culprit for a sizeable Oklahoma earthquake back in 2011.

A new study in the journal Geology is the latest to tie a string of unusual earthquakes - in this case, in central Oklahoma - to the injection of wastewater deep underground. Researchers now agree that the magnitude 5.7 earthquake near Prague, Oklahoma, on November 6, 2011, may also be the largest ever linked to wastewater injection.

Felt as far off as Milwaukee in the state of Wisconsin, some 800 miles away, the earthquake (the biggest ever recorded in Oklahoma) destroyed 14 homes, damaged a federal highway and left two people injured. And small earthquakes continue to be recorded in the area.

The recent boom in US energy production has produced massive amounts of wastewater. The water is used both in hydraulic fracturing (hydrofracking) for shale gas acquisition, and in coaxing crude oil residues from conventional 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 earthquakes 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.

US scientists have linked a rising number of earthquakes in normally calm parts of Arkansas, Texas, Ohio and Colorado to below-ground injection. In the last four years, the number of earthquakes in the middle of the United States are estimated to have jumped eleven-fold from the three decades prior.

Last year, a group at the US Geological Survey also attributed a remarkable rise in small- to mid-size earthquakes in the region to human activity. The risk is serious enough that the US National Academy of Sciences has called for further research to "understand, limit and respond" to induced seismic events. Despite these studies, wastewater injection continues near the site of the Oklahoma earthquakes.

The magnitude 5.7 earthquake near Prague was preceded by a 5.0 shock and followed by thousands of aftershocks. What made the swarm unusual is that wastewater had been pumped into abandoned oil wells nearby for 17 years without incident. In the study, researchers hypothesize that as wastewater replenished compartments once filled with oil, the pressure required to maintain fluid injection had to be increased.

As pressure built up, a known geological fault (the 'Wilzetta') suffered a displacement. Study co-author, Heather Savage, a geophysicist at Columbia University's Lamont-Doherty Earth Observatory says that when you overpressure a fault, you reduce the stress that's pinning it in place and that's when earthquakes happen.

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, according to co-author and Lamont-Doherty seismologist, Geoffrey Abers. "There's something important about getting unexpectedly large earthquakes out of small systems that we have discovered here," he said. The observations seem to indicate that the risk of inducing large earthquakes from even small injection activities is probably higher than previously thought.

Hours after the first magnitude 5.0 earthquake on November 5 2011, University of Oklahoma seismologist and study lead author, Katie Keranen installed the first three of several dozen seismographs to record the aftershocks. That night, on Novemebr 6, the magnitude 5.7 main shock hit and Keranen watched as her house began to shake for what she said felt like 20 seconds.

Keranen's recordings of the magnitude 5.7 earthquake, 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 ten-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 emphasises, 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."

Nevertheless, Keranen suggests that 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. The study authors also recommend sub-surface monitoring of fluid pressure for earthquake warning signs.

Further research is needed but as a minimum requirement, Abers recommends careful monitoring in regions where there are injection wells, and protocols in place for stopping pumping even when small earthquakes are detected.

In a recent op-ed in the Albany Times Union (a New York state newspaper), Abers argued that New York should consider the risk of induced earthquakes from fluid injection in weighing whether to allow hydraulic fracturing to extract the state's shale gas reserves.

Les Hunt

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