Tue August 19, 2014
Injection Well Earthquakes Feel Weaker Than Natural Ones, New Study Suggests
While a growing chorus of scientific research has linked Oklahoma’s recent spike in earthquake activity to oil and gas industry disposal wells, a new study suggests such artificial earthquakes are less intense than naturally occurring temblors.
The peer-reviewed paper appears in the October 2014 edition of the Bulletin of the Seismological Society of America and was authored by U.S. Geological Survey geophysicist Susan Hough, who found that people reported less shaking from earthquakes linked to fluid injection than naturally occurring earthquakes of similar magnitude.
Hough studied 11 earthquakes in five states that occurred between 2011 and 2013 that are “generally acknowledged or suspected” of being triggered by injection wells. Four Oklahoma quakes were analyzed — including the 5.7-magnitude that shook near Prague in 2011, the state’s largest on record — as well as other high-profile earthquakes recorded in other states, including headline-making temblors in Youngstown, Ohio, and Greenbriar, Ark.
Hough analyzed user-submitted data from the USGS’s “Did You Feel It?” reporting system and found that while a natural and injection-induced quake might generate similar numbers recorded on earthquake sensors, people say they feel different levels of shaking.
For those close to the epicenter — within about 6 miles or so — residents reported little difference between the two types of quakes. Beyond that distance though, the artificial quakes feel, on average, about 16 times less intense than naturally quakes, the paper suggests.
A “4.8 induced quake felt like a 4.0 quake,” Hough tells the Associated Press’ Seth Borenstein:
Sometimes the difference is even bigger. Hough said a 5.3 August 2011 artificial quake in Trinidad, Colorado, actually felt like a 4.0 quake, which is about 90 times weaker, based on the thousands of responses in the “Did You Feel It” survey system.
“The hazard of these earthquakes is lower than what you’d expect,” Hough said. “It’s not that there’s no hazard, it’s just that it’s a little better than you might think.”
The artificial quakes may have less energy — only after 6 miles away — because the fault is lubricated by the injected wastewater, making it easier to slip and do so more smoothly in less of a herky-jerky motion, Hough theorized. Also these faults can be slipping with less pent-up energy than they would have if they slipped naturally years later.
But induced quakes are shallow, which means the shaking has to travel less distance to buildings nearby, said seismologist Steve Horton of the University of Memphis, who wasn’t part of the study. He and others said Hough’s study made sense and could change how researchers look at quake intensity.
In a phone interview with StateImpact, Hough says the new data could provide additional clarity to state regulators as they consider new rules for injection wells.
“Distance makes a big difference, so a lot of the reported ground-shaking could be reduced if injection wells were located away from populated areas,” she says.