WASHINGTON — The natural gas industry uses hydraulic fracturing, called fracking, in which millions of gallons of water, sand and chemicals are injected deep underground to break up rock.
While that may sound like it could cause an earthquake, the typical energy released in tremors triggered by fracking “is the equivalent to a gallon of milk falling off the kitchen counter,” Stanford University geophysicist Mark Zoback said.
The magnitude-5.6 quake that rocked Oklahoma three miles underground had the power of 3,800 tons of TNT, which is nearly 2,000 times stronger than the 1995 Oklahoma City bombing.
In Oklahoma, home to 185,000 drilling wells and hundreds of injection wells, the question of man-made seismic activity comes up quickly. But so far, federal, state and academic experts say readings show that the Oklahoma quakes were natural.
“There's a fault there,” U.S. Geological Survey seismologist Paul Earle said. “You can have an earthquake that size anywhere east of the Rockies. You don't need a huge fault to produce an earthquake that big. It's uncommon, but not unexpected.”
But there's a reason people ask whether the quakes are man-made rather than from the shifting of the Earth's crusts.
In the past, earthquakes have been linked to energy exploration and production, including from injections of enormous amounts of drilling wastewater or injections of water for geothermal power, but those quakes have been too small to cause much damage.
Still, scientists would like to know whether human activity can trigger a larger event. The National Academy of Sciences is studying the seismic effects of energy drilling and mining and will issue a report next spring.
“This is an area of active research,” said Rowena Lohman, a Cornell University seismologist.
Hydraulic fracturing has been practiced for decades but has grown rapidly in recent years as drillers have learned to combine it with horizontal drilling to tap enormous reserves of natural gas and oil in the United States.
About 5 million gallons of fluid are used to fracture a typical well. That's typically not nearly enough weight and pressure to cause more than a tiny tremor.