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Could novel technique to curb global warming also trigger earthquakes?

A report finds that injecting carbon dioxide into underground rock formations, while a potential means of fighting global warming, could increase stresses on faults, leading to earthquakes. 

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If the goal is to sequester 1 billion tons a year of CO2 globally by 2050, and utilities were to aim only at the most-suitable formations, some 3,500 such sites would have to be uncovered and at spots convenient enough to be economical, Zoback and Stanford colleague Steven Gorelick found in their analysis. Some 85 sites a year would have to come on line between now and 2050 to meet that goal.

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Yet by some estimates, 3.5 billion tons a year would need to be sequestered to reach emissions goals countries have been discussing internationally to curb global warming.

It might be possible to find large-scale formations that could serve an entire region, the researchers say. One such formation lies beneath the southern border of Indiana and Illinois. But the formation borders on a fault zone known for quakes that have reached magnitude 7.

If 100 million tons of CO2 were injected into the formation each year through 2050, the sequestered carbon would place a large amount of pressure on the fault zone. The 100 million ton figure is a small fraction of the amount power plants in the region emit today, the researchers estimate.

The other alternative is to try to sequester the carbon close to the plants. But during the past few decades, increasingly dense arrays of earthquake sensors have shown that earthquakes originate deep in the interiors of continents, far away from the boundaries between Earth's vast plates of crust usually associated with earthquakes and volcanoes. Such sensors also have picked up quakes from oil and gas recovery efforts. Last year, waste-water injection appears to have triggered at least three earthquakes with magnitudes ranging from 4.0 near Youngstown, Ohio; and 4.7 near Guy, Ark.; to 5.3 near Trinidad, N.M. 

Many of the faults within a continent's interior are on the verge of snapping, but don't for long periods because it takes so long for strain to build up in a continent's interior. Forcing fluid CO2 into the wrong formations could unlock a fault and allow it to rupture.

The tough faults to find are not the long ones with large, obvious offsets, even though deep underground. They are the smaller faults with shorter displacements, but still capable of creating a quake powerful enough to compromise a repository.

"I agree that these risks are serious," writes Ruben Juanes, a geophysicist at the Massachusetts Institute of Technology who studies the flow of fluids through soil and rock, in an e-mail. Researchers currently have no models to forecast the effect that injecting fluids below ground could have on earthquake activity. And with no carbon-capture-and-storage project yet operating at the large scales needed to offset carbon dioxide emissions, no one has field experience on which to draw to characterize the risk.

On the other hand, he continues, the US hosts some 30,000 waste-water wells, and only eight examples exist where the use of a well triggered a moderate earthquake.

The notion that the technique could founder on the shoals of earthquake hazards is a bit premature, he suggests. Still, he says, the paper "points to the need for more research" on the issue – one that is "an important, but well-known concern."

Zoback's and Dr. Gorelick's calculations are not meant to be taken as a nail in the coffin for sequestration, Zoback emphasizes, but rather a starting point for discussion about getting a better handle on the risks potential repositories face from earthquakes as the formations receive CO2.

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