KUSA - Scientists are investigating whether a rare 3.4 magnitude earthquake near Greeley, Colorado this weekend may have been caused by the disposal of fracking fluid.
The quake was centered in an area of Weld County located near four underground injection sites, in which used fracking fluid is forced deep underground as a method of disposal. The red star marks the epicenter of the earthquake.
"I think we have a good reason to suspect there may be a link," said Shemin Ge, a hydrologist with the University of Colorado. "We're still looking into it."
Ge says there are several injection wells very close to the epicenter of the earthquake.
"One of them is relatively high volume," Ge said.
Ge is part of a team of scientists that are responding to the Greeley quake by placing a series of seismometers in the area to get more detailed data.
A team from the University of Colorado at Boulder was sent out to scout locations for the measurement devices on Monday. Landowners will need to give permission before portable seismometers are put on their property.
The seismometers will be able to measure any aftershocks - and how deep they are. That could help answer the question many want to know.
"Natural earthquakes tend, generally, to be a little deeper - maybe like 10 miles, deep in the earth's crust," said Anne Sheehan, seismologist and professor at the University of Colorado at Boulder Department of Geological Sciences. "I mean that can certainly vary a lot. But if it's really shallow then we would be more suspicious [of fracking being the cause]."
The Centennial state doesn't have very many seismometers right now. According to Sheehan, the ones in Colorado are located in Idaho Springs, Red Feather Lakes, Kit Carson, County, Creede, Divide and Sand Dunes. There are also seismometers near Trinidad, Mesa Verde and Dinosaur.
"It's not very many for a state of this size," Sheehan said. "It's in part because we have not had a lot of earthquakes before. California, Nevada and Utah have good reason to have seismic networks because they get a lot of earthquakes."
This map shows the number of seismometers (seismic stations) in the United States.
Colorado is not known as an earthquake hot spot.
"Lately there have been more earthquakes in southern Colorado near Trinidad, in particular," Sheehan said. "And some of us have been wondering why there have not been many earthquakes in Weld County, given the amount of oil and gas production and injection, which in some places can induce earthquakes."
Sheehan is quick to point out that in some places fracking has not induced earthquakes.
"So we've kind of wondered," Sheehan explained. "And so since there's now an earthquake that's happened, we're very interested in trying to better understand: 'Is this just a one-off event or is this the beginning of a sequence?'"
HYDRAULIC FRACTURING PROCESS
The actual process of hydraulic fracturing (fracking) causes very minor seismic activity by its nature, but it is nearly always too subtle to be noticed.
By contrast, forced injection disposal of wastewater has been demonstrated to cause earthquakes on several past occasions.
Seismologists believe an increase in this method of dealing with used fracking fluid may be to blame for a spike in quakes in parts of the country that are traditionally less seismically active.
"If we can make a fault weak, it's possible that those forces would cause the fault to suddenly rupture, producing an earthquake," said Bill Ellsworth, who studies the effect of fluid injection.
Forced deep into bedrock, wastewater can add pressure and also lubricate layers of rock, causing faults to slip.
While there must be pre-existing tension for that to happen, it does not necessarily mean that the forced injection merely sped along a quake that was imminent anyway.
"These faults may not have moved in hundreds of millions of years," Ellsworth said. "These earthquakes may never have happened on their own."
Denver played a critical role in discovering the link between forced injection wells and induced earthquakes.
In the mid 1960s, Denver experienced a series of quakes believed to have been caused by a 12,000-foot deep wastewater disposal well at Rocky Mountain Arsenal.
It culminated in a 5.3 magnitude quake on August 9, 1967 that remains the strongest recorded earthquake in Denver's history, after which the well was decommissioned and the Army began to remove the wastewater it had pumped below the surface.
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