Fluids injected into wells lubricate faults and increase slippage.

Natural-gas extraction, geothermal-energy production and other activities that inject fluid underground have caused numerous earthquakes in the United States, scientists report today in a trio of papers in Science1–3.

Most of these quakes have been small, but some have exceeded magnitude 5.0. They include a magnitude-5.6 event that hit Oklahoma on 6 November 2011, damaging 14 homes and injuring two people, says William Ellsworth, a seismologist at the US Geological Survey in Menlo Park, California, and the author of one of the papers1.

He says that the annual number of earthquakes record at magnitude 3.0 or higher in the central and eastern United States has increased almost tenfold in the past decade — from an average of 21 per year between 1967 and 2000 to a maximum of 188 in 2011. A second study2, led by Nicholas van der Elst, a seismologist at Columbia University's Lamont-Doherty Earth Observatory in Palisades, New York, finds that at least half of the magnitude-4.5 or larger earthquakes that have struck the interior United States in the past decade have occurred near injection-well sites.

Ellsworth suspects that increased activity at natural-gas wells has altered stresses in earthquake-susceptible areas by increasing the fluid pore pressure in underground rock, lubricating pre-existing faults so they are more likely to rupture.

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