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article imageThe 'myths' persist over human-caused seismic activity

By Karen Graham     Jun 9, 2015 in Science
Human-caused seismic activity creates "induced earthquakes." There are a number of myths surrounding the cause of these Earth tremors, and there are a number of misconceptions, too. One geologist tries to explain what is fact and what is fiction.
There has been a dramatic increase in the number of earthquakes caused by human activity, and most people believe these incidents are due to oil and gas production companies.
There is an increasing amount of evidence that says the earthquakes are caused by injecting large amounts of fluids from gas and oil injection sites deep into the Earth. This operation is what is supposed to be creating the induced earthquakes.
Seismologists have been studying these induced earthquakes, and one new study coming out online June 10 in the Seismological Research Letters focus section, tries to clear up some common misconceptions about the trend.
Ancient ocean brine and not the fracking itself is to blame
Guest editor Justin Rubinstein, a scientist with the U.S. Geological Survey, says the biggest misconception is that the induced seismicity is due to oil and gas recovery using hydraulic fracturing, or "fracking." Rubinstein explains that the induced seismicity is actually caused by the disposal of large amounts of wastewater.
The wastewater is mainly made up of ancient ocean brine that has been trapped in the rocks under the oil and gas deposits. Rubinstein says only a very small amount of induced seismicity comes from the fracking itself where fluid is injected under high pressure to break up rock formations.
The seismologist points out that there has been a huge increase wastewater disposal over the last 10 years in the U.S., especially in Oklahoma, where the amount of wastewater disposal doubled between 1999 and 2013.
"Wastewater disposal is expanding and waste fluids are being injected into new locations. There have been changes in production practices as well, so in some areas there is much more wastewater that needs to be disposed," Rubinstein noted.
Rubinstein says there are tens of thousands of wastewater disposal wells, specially-enhanced oil recovery, and hydraulic fracture wells in the country today, yet only a few dozen are responsible for induced earthquakes, and they are usually small, measuring less than 4.0 on the Richter scale.
In the central United States alone, there has been an increase from an average of 24 induced earthquakes between 1978 and 2008 to 193 induced earthquakes between 2009 and 2014, with 688 occurring in 2014.
Earthquakes are also being tracked in Canada, and interestingly, seismologists say that fracking may be the primary cause of induced seismicity in Canada, rather than wastewater disposal. David Eaton of the University of Calgary is the co-editor of the Focus section, and he has an explanation for this variant.
"There appear to be inter-regional differences between the U.S. and Canada," he noted, "but it's too early to say yet whether those reflect operational differences in the geological site conditions or if it simply reflects the focus of studies that have been completed to date."
But experts are continuing their studies in both countries and recommend a more proactive approach to induced seismicity. Another Focus section article by Randi Jean Walters and colleagues at Stanford University describes a possible work-flow protocol to reduce pre and post-injection risks at oil and gas sites. This would include seismic monitoring and an understanding of the area's past and present geology, which is certainly sensible.
More about humancaused seismic activity, Earthquakes, Fracking, induced earthquakes, Misconceptions
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