ananyo writes "First came reports of earthquakes caused by hydraulic fracturing and the reinjection of water during oil and gas operations. Now U.S. scientists are reporting tremors may have been caused by the injection of carbon dioxide during oil production. The evidence centers on a sudden burst of seismic activity around an old oil field in the Permian Basin in northwest Texas. From 2006 to 2011, after more than two decades without any earthquakes, seismometers in the region registered 38 tremors, including 18 larger quakes ranging from magnitude 3 to 4.4, scientists report in the Proceedings of the National Academy of Sciences. The tremors began just two years after injections of significant volumes of CO2 began at the site, in an effort to boost oil production. 'Although you can never prove that correlation is equal to causation, certainly the most plausible explanation is that [the tremors] are related to the gas injection,' says Cliff Frohlich, a seismologist at the University of Texas Institute for Geophysics in Austin, who co-authored the study."