Hidden Faults Explain Earthquakes in Fracking Zones

Oklahoma, Ohio and Arkansas have experienced an unusually large number of earthquakes in recent years.

The shaking is rising at the same time that oil and gas production have increased. But other states that are hotbeds for new drilling have stayed seismically quiet, such as Montana, North Dakota and South Dakota.

Now, two new studies explain why some regions of the country are rattling more than others. In Oklahoma, hidden faults beneath the surface are primed to pop, reports a study published Jan. 27 in the journal Geophysical Research Letters. Some of these faults were previously unknown and threaten critical structures, such as huge oil-storage facilities, said lead study author Daniel McNamara, a U.S. Geological Survey research geophysicist based in Golden, Colorado. But the geology underlying Montana and the Dakotas is more benign, with fewer faults near their breaking point, according to a study published Feb. 10 in the journal Seismological Research Letters. [Top 10 Ways to Destroy Earth]

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The majority of the faults are perfectly aligned to slip under pressure transmitted through the continent from faraway tectonic plate boundaries, the study reports. Oklahoma's buried rock layers are squeezed in an east-west direction from forces at the Mid-Atlantic Ridge, San Andreas Fault and Juan de Fuca Ridge, McNamara said. But the underground faults identified in the study tilt to the northwest or northeast, creating an angle of about 30 to 40 degrees from this regional pressure.

"This is the optimal orientation for producing strike-slip faulting," McNamara told Live Science. If the faults were more steeply tilted in either direction, it would be harder to force them to jerk apart. (Imagine mashing together two bricks that point toward noon on a clock if your hands are at 9 p.m. and 3 p.m. Now rotate those two bricks toward 1:30 p.m. and squeeze them the same way — they slide against each other more easily.)

Oklahoma's faults are left over from pushing and pulling that occurred in North America some 300 million years ago. These faults are being awakened by oil and gas drillers who are injecting fluids into the deep underground rocks above the layers that host the faults, according to several previous studies..These fluids reduce pressure on the faults just enough for the rocks to slip apart, triggering earthquakes.

Despite the huge escalation in Oklahoma's seismicity, some 30,000 wastewater disposal wells operate in the United States without triggering damaging earthquakes. For instance, North Dakota is the second-largest crude oil producer in the United States, but recorded only nine earthquakes between September 2008 and May 2011, according to the new Seismological Research Letters study. And the study authors attribute just one of those nine quakes to oil and gas production.

While less is known about the geology underlying North Dakota because it is so seismically quiet, the study authors suggest that local geology plays a role. The faults and regional pressure are less favorably aligned to trigger earthquakes. However, the amount of fluids injected into underground rock layers in the Dakotas could also be important, the researchers said. Oklahoma has some of the highest-volume injection wells in the country.

Becky Oskin covers Earth science, climate change and space, as well as general science topics. Becky was a science reporter at Live Science and The Pasadena Star-News; she has freelanced for New Scientist and the American Institute of Physics. She earned a master's degree in geology from Caltech, a bachelor's degree from Washington State University, and a graduate certificate in science writing from the University of California, Santa Cruz.