The Enid News and Eagle, Enid, OK

Local and State News

March 7, 2014

Study: Strongest Oklahoma earthquake may have been human-induced

ENID, Okla. — The largest earthquake in recorded Oklahoma history may have been human-induced, researchers with U.S. Geological Survey say in a new study published this week.

The magnitude 5.7 earthquake of Nov. 6, 2011, which was felt in Enid, may have been triggered by wastewater injection, according to the study. If that is true, it would be the largest human-caused earthquake associated with wastewater injection. The quake’s epicenter was 21 miles north-northeast of Shawnee at a depth of about 1.7 miles.

The quake damaged 14 homes, injured two people and buckled a highway.

USGS released the findings Thursday in a news release on its website,

According to the study, researchers said a human-induced magnitude 5.0 earthquake near Prague on Nov. 5, 2011, may have triggered the larger earthquake the next day.

“The observation that a human-induced earthquake can trigger a cascade of earthquakes, including a larger one, has important implications for reducing the seismic risk from wastewater injection,” said USGS seismologist and coauthor of the study Elizabeth Cochran.

Historically, earthquakes in the central United States have been uncommon. Yet in 2011 alone, numerous moderate-size earthquakes occurred in Colorado, Texas, Oklahoma, Ohio and Arkansas, according to USGS. Many of those earthquakes occurred near waste-water injection wells, and some have been shown to be caused by human activities, according to USGS.

The 2011 Oklahoma earthquake sequence included the Nov. 6 magnitude 5.7 earthquake that ruptured a part of the Wilzetta fault system, a complex fault zone about 124 miles in length near Prague, according to USGS. Less than 24 hours before, the magnitude 5.0 foreshock occurred near active waste-water disposal wells, and was linked in a previously published study to fluid injection in those wells. The earthquakes have not been directly linked to hydrofracturing, according to USGS.

In a study published last year in the journal “Geology,” the strong earthquakes were unusual because “wastewater had been pumped into abandoned oil wells nearby for 17 years without incident,” according to The Earth Institude at Columbia University’s website.

“In the study, researchers hypothesize that as wastewater replenished compartments once filled with oil, the pressure to keep the fluid going down had to be ratcheted up,” according to the website. As pressure built up, 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, according to the Columbia University website.

The research published this week suggests the foreshock, by increasing stresses where the magnitude 5.7 mainshock ruptured, may have triggered the mainshock, which in turn, triggered thousands of aftershocks along the Wilzetta fault system, including a magnitude 5.0 aftershock on Nov. 8, 2011, according to USGS. All three earthquakes of magnitude 5.0 and greater along the Wilzetta fault exhibited strike-slip motion at three locations along the fault, suggesting three separate portions of the Wilzetta fault system were activated, according to USGS.

The wastewater pumped into the injection wells is a byproduct of hydrofracturing, or fracking. Water and chemicals are pumped into the ground, where they crack open rocks to release natural gas and oil. The wastewater then is disposed of by pumping it under pressure back into the ground.

The paper, “Observations of Static Coulomb Stress Triggering of the November 2011 M5.7 Oklahoma Earthquake Sequence,” by D.F. Sumy, E.S. Cochran, K.M. Keranen, M. Wei, G.A. Abers, from the University of Southern California, USGS, Cornell University, Brown University, and the Lamont Doherty Earth Observatory at Columbia University, was published in the “Journal of Geophysical Research” this week.

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