Storing massive amounts of carbon dioxide underground in an effort to combat global warming may not be easy to do because of the potential for triggering small to moderate earthquakes, according to Stanford geophysicist Mark Zoback.

While those earthquakes are unlikely to be big enough to hurt people or property, they could still cause serious problems for the reservoirs containing the gas.

 "It is not the shaking an earthquake causes at the surface that creates the hazard in this instance, it is what it does at depth," Zoback said. "It may not take a very big earthquake to damage the seal of an underground reservoir that has been pumped full of carbon dioxide."

Carbon dioxide is a major cause of a global warming. In many countries, including the United States, China and India, the majority of carbon dioxide is produced by coal-burning power plants and refineries. Keeping some of that carbon dioxide from entering the atmosphere by storing it underground could reduce the amount of warming.

The other complication, Zoback said, is that for sequestration to make a significant contribution to reducing carbon dioxide emissions, the volume of gas injected into reservoirs annually would have to be almost the same as the amount of fluid now being produced by the oil and gas industry each year. This would likely require thousands of injection sites around the world.

"Think about how many wells and pipelines and how much infrastructure has been developed to exploit oil and gas resources over the last hundred years," he said. "You need something of comparable scale and volume for carbon dioxide sequestration."

A continent crisscrossed by ancient faults

The problem with potential earthquakes arises, Zoback said, because the interior of the continent is crisscrossed by ancient faults that are often poised to fail – what he calls "a state of failure equilibrium" – because of the immense tectonic forces acting on them.

Those forces drive the huge tectonic plates across the surface of the globe and trigger the occasional violent upheavals on faults along the plate margins, such as the San Andreas fault zone. Those tectonic forces exist even in the interior of the country, far from California and the San Andreas.

The interior may seem quite stable viewed on a human timescale, but that is only because the rate at which the intraplate faults are failing is quite slow, Zoback said.

"So, in that context, when we start perturbing the system by changing fluid pressure [as we inject massive amounts of carbon dioxide into the subsurface], we have the potential for activating faults," he said. 

Zoback emphasized that any earthquake triggered by injecting gas would have happened anyway, because the fault was going to fail eventually.  "You are just advancing the time at which the earthquake occurs," he said. But the quakes would still be potential hazards to the reservoirs.